230989501
POLICY OPTIONS FOR STABILIZING GLOBAL CLIMATE
\
DRAFT
REPORT TO CONGRESS
Volume I: Chapters I-VI
United States Environmental Protection Agency
Office of Policy, Planning, and Evaluation
February 1989
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DISCLAIMER
This draft is being circulated for review and comment and does not
necessarily reflect the official position of the U.S. Environmental Protection
Agency. Mention of trade names does not constitute an endorsement.
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SUMMARY TABLE OF CONTENTS
EXECUTIVE SUMMARY (Bound under separate cover)
VOLUME 1
CHAPTER I: INTRODUCTION 1-1
CHAPTER II: GREENHOUSE GAS TRENDS IM
CHAPTER HI: CLIMATIC CHANGE HI-1
CHAPTER IV: HUMAN ACTIVITIES AFFECTING TRACE GASES AND CLIMATE IV-1
CHAPTER V: THINKING ABOUT THE FUTURE V-l
CHAPTER VI: SENSITIVITY ANALYSES VI-1
VOLUME II
CHAPTER VII: TECHNICAL OPTIONS FOR REDUCING GREENHOUSE GAS EMISSIONS . VIM
CHAPTER VIII: POLICY OPTIONS VHI-1
CHAPTER IX: INTERNATIONAL COOPERATION TO REDUCE GREENHOUSE
GAS EMISSIONS IX-1
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DETAILED TABLE OF CONTENTS
EXECUTIVE SUMMARY
(Bound under separate cover)
INTRODUCTION 1
Congressional Request for Reports 1
Previous Studies 2
Goals of this Study 4
Approach Used to Prepare this Report 6
Limitations 9
HUMAN IMPACT ON THE CLIMATE SYSTEM 11
The Greenhouse Gas Buildup 11
The Impact of Greenhouse Gases on Global Climate 17
SCENARIOS FOR POLICY ANALYSIS 19
Defining Scenarios 19
Scenarios with Unimpeded Emissions Growth 22
The Impact of Policy Choices 29
Sensitivity of Results to Alternative Assumptions 45
EMISSIONS REDUCTION STRATEGIES BY ACTIVITY 54
Energy Production and Use 56
Industrial Activity 66
Changes in Land Use 71
Agricultural Practices 75
THE NEED FOR POLICY RESPONSES 77
A Wide Range of Policy Choices 78
The Timing of Policy Responses 79
FINDINGS 83
I. Uncertainties regarding climatic change are large, but there is a growing consensus
in the scientific community that significant global warming due to anthropogenic
greenhouse gas emissions is probable over the next century, and that rapid climatic
change is possible 83
II. Measures undertaken to limit greenhouse gas emissions would decrease the magnitude
and speed of global warming, regardless of uncertainties about the response of the
climate system 85
III. No single country or source will contribute more than a fraction of the greenhouse
gases that will warm the world; any overall solution will require cooperation of many
countries and reductions in many sources 87
IV. A wide range of policy choices is available to reduce greenhouse gas emissions while
promoting economic development, environmental, and social goals 89
111
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DETAILED TABLE OF CONTENTS (Continued)
VOLUME 1
CHAPTER I
INTRODUCTION
INTRODUCTION 1-2
The Earth's Climate and Global Change 1-2
CONGRESSIONAL REQUEST FOR REPORTS 1-3
Goals of this Study 1-4
Report Format 1-5
THE GREENHOUSE GASES 1-8
Carbon dioxide 1-9
Methane 1-9
Nitrous oxide 1-12
Chlorofluorocarbons 1-12
Other gases influencing composition 1-13
PREVIOUS STUDIES 1-13
Estimates of the Climatic Effects of Greenhouse Gas Buildup 1-14
Studies of Future CO2 Emissions 1-15
Studies of the Combined Effects of Greenhouse Gas Buildup 1-20
Major Uncertainties 1-22
Conclusions From Previous Studies 1-23
CURRENT NATIONAL AND INTERNATIONAL ACTIVITIES 1-26
National Research and Policy Activities 1-26
International Activities 1-27
REFERENCES 1-29
CHAPTER II
GREENHOUSE GAS TRENDS
FINDINGS II-2
INTRODUCTION II-5
CARBON DIOXIDE II-7
Concentration History and Geographic Distribution II-7
Mauna Loa II-8
Ice-core Data II-9
GMCC Network 11-10
IV
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DETAILED TABLE OF CONTENTS (Continued)
Sources and Sinks 11-14
Fossil Carbon Dioxide 11-14
Biospheric Cycle - ' 11-16
Ocean Uptake 11-17
Chemical and Radiative Properties/Interactions 11-18
METHANE 11-22
Concentration History and Geographic Distribution 11-22
Sources and Sinks 11-24
Chemical and Radiative Properties/Interactions 11-29
NITROUS OXIDE 11-30
Concentration History and Geographic Distribution 11-30
Sources and Sinks 11-32
Chemical and Radiative Properties/Interactions 11-35
CHLOROFLUOROCARBONS (CFCs) 11-36
Concentration History and Geographic Distribution 11-36
Sources and Sinks 11-37
Chemical and Radiative Properties/Interactions 11-39
OZONE 11-40
Concentration History and Geographic Distribution 11-40
Sources and Sinks 11-43
Chemical and Radiative Properties/Interactions '. 11-44
OTHER FACTORS AFFECTING COMPOSITION 11-45
Global Tropospheric Chemistry 11-46
Carbon Monoxide 11-47
Nitrogen Oxides 11-48
Stratospheric Ozone and Circulation 11-49
CONCLUSION 11-50
REFERENCES 11-59
CHAPTER III
CLIMATIC CHANGE
FINDINGS III-2
INTRODUCTION III-4
CLIMATIC CHANGE IN CONTEXT IH-6
CLIMATE FORCINGS III-8
Solar Luminosity Ill-12
Orbital Parameters 111-13
Volcanoes 111-13
Surface Properties Ill-14
The Role of Greenhouse Gases Ill-14
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DETAILED TABLE OF CONTENTS (Continued)
Internal Variations 111-15
PHYSICAL CLIMATE FEEDBACKS 111-15
Water Vapor - Greenhouse Ill-17
Snow and Ice Ill-17
Clouds 111-19
BIOGEOCHEMICAL CLIMATE FEEDBACKS 111-20
Release of Methane Hydrates 111-20
Oceanic Change 111-22
Ocean Chemistry 111-23
Ocean Mixing 111-23
Ocean Biology and Circulation HI-24
Changes in Terrestrial Biota 111-25
Vegetation Albedo 111-25
Carbon Storage 111-26
Other Terrestrial Biotic Emissions 111-26
Summary 111-27
EQUILIBRIUM CLIMATE SENSITIVITY 111-28
THE RATE OF CLIMATIC CHANGE 111-31
CONCLUSION 111-35
REFERENCES 111-37
CHAPTER IV
HUMAN ACTIVITIES AFFECTING TRACE GASES
AND CLIMATE
FINDINGS IV-2
INTRODUCTION IV-5
HISTORICAL OVERVIEW OF POPULATION TRENDS IV-5
Global Population Trends IV-7
Population Trends by Region IV-7
Industrialized Countries IV-10
Developing Countries IV-10
ENERGY CONSUMPTION IV-12
History of Fossil-Fuel Use IV-13
Current Energy Use Patterns and Greenhouse Gas Emissions IV-18
Emissions bv Sector IV-20
Fuel Production and Conversion IV-25
Future Trends IV-27
The Fossil-Fuel Supply IV-29
Future Energy Demand IV-29
VI
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DETAILED TABLE OF CONTENTS (Continued)
INDUSTRIAL PROCESSES IV-31
Chlorofluorocarbons, Halons, and Chlorocarbons IV-33
Historical Development and Uses IV-33
The Montreal Protocol IV-37
Landfill Waste Disposal IV-40
Cement Manufacture IV-43
LAND USE CHANGE IV-45
Deforestation IV-47
Biomass Burning IV-50
Wetland Loss IV-51
AGRICULTURAL ACTIVITIES IV-55
Enteric Fermentation In Domestic Animals IV-55
Rice Cultivation IV-56
Use of Nitrogenous Fertilizer IV-61
IMPACT OF CLIMATIC CHANGE ON ANTHROPOGENIC EMISSIONS IV-63
REFERENCES IV-67
CHAPTER V
THINKING ABOUT THE FUTURE
FINDINGS V-2
INTRODUCTION V-4
APPROACH TO ANALYZING FUTURE EMISSIONS V-5
Production V-7
Consumption V-ll
SCENARIOS FOR POLICY ANALYSIS V-13
Scenarios with Unimpeded Emissions Growth V-17
Scenarios with Stabilizing Policies V-21
ANALYTICAL FRAMEWORK V-22
Energy Module V-25
Industry Module V-26
Agriculture Module V-26
Land Use and Natural Source Module V-27
Ocean Module V-27
Atmospheric Composition and Temperature Module V-28
Assumptions '. V-29
Population Growth Rates V-29
Economic Growth Rates V-29
Oil Prices V-30
Limitations V-30
Vll
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DETAILED TABLE OF CONTENTS (Continued)
SCENARIO RESULTS V-33
Energy Sector V-33
End-use Consumption V-33
Primary Energy Supply V-39
Greenhouse Gas Emissions From Energy Production and Use V-44
Comparison to Previous Studies V-45
Industrial Processes V-56
Halocarbon Emissions V-56
Emissions From Landfills and Cement V-59
Changes in Land Use V-60
Agricultural Activities V-63
Total Emissions V-64
Atmospheric Concentrations V-71
Global Temperature Increases V-76
Comparison with General Circulation Model Results V-81
Relative Effectiveness of Selected Strategies V-82
CONCLUSIONS V-82
REFERENCES V-87
CHAPTER VI
SENSITIVITY ANALYSES
FINDINGS VI-3
INTRODUCTION VI-12
ASSUMPTIONS ABOUT THE MAGNITUDE AND TIMING OF GLOBAL
CLIMATE STABILIZATION STRATEGIES VI-12
No Participation by the Developing Countries VI-13
Delay in Adoption of Policies VI-17
ASSUMPTIONS AFFECTING RATES OF TECHNOLOGICAL CHANGE VMS
Availability of Non-Fossil Technologies VI-18
Cost and Availability of Fossil Fuels VI-22
High Coal Prices VI-22
Alternative Oil and Natural Gas Supply Assumptions VI-24
Availability of Methanol-Fueled Vehicles VI-29
ATMOSPHERIC COMPOSITION: COMPARISON OF MODEL RESULTS TO ESTIMATES
OF HISTORICAL CONCENTRATIONS VI-30
ASSUMPTIONS ABOUT TRACE-GAS SOURCES AND STRENGTHS VI-34
Methane Sources VI-35
Nitrous Oxide Emissions From Fertilizer VI-38
Anhydrous Ammonia VI-38
NgO Leaching From Fertilizer VI-39
N2O Emissions From Combustion VI-39
vui
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DETAILED TABLE OF CONTENTS (Continued)
UNCERTAINTIES IN THE GLOBAL CARBON CYCLE VI-41
Unknown Sink In Carbon Cycle VI-43
Amount of CO2 From Deforestation VI-44
Alternative CO2 Models of Ocean Chemistry and Circulation VI-47
ASSUMPTIONS ABOUT CLIMATE SENSITIVITY AND TIMING VI-50
Sensitivity of the Climate System VI-50
Rate of Heat Diffusion VI-53
ASSUMPTIONS ABOUT ATMOSPHERIC CHEMISTRY: A COMPARISON OF MODELS . . . VI-54
Model Descriptions VI-56
Assessment Model for Atmospheric Composition VI-56
Isaksen Model VI-57
Thompson et al. Model VI-58
Results from the Common Scenarios VI-59
EVALUATION OF UNCERTAINTIES USING AMAC VI-67
Atmospheric Lifetime of CFC-11 VI-67
Interaction of Chlorine with Column Ozone VI-69
Sensitivity of Tropospheric Ozone to CH4 Abundance VI-69
Sensitivity of OH to NOX VI-71
BIOGEOCHEMICAL FEEDBACKS VI-72
Ocean Circulation VI-72
Methane Feedbacks VI-73
Combined Feedbacks VI-75
REFERENCES VI-78
VOLUME II
CHAPTER VII
TECHNICAL OPTIONS FOR REDUCING GREENHOUSE GAS EMISSIONS
PART ONE: ENERGY SERVICES VII-27
TRANSPORTATION SECTOR VII-32
Near-Term Technical Options: Industrialized Countries VII-36
Light-Duty Vehicles VII-38
Freight Transport Vehicles VII-49
Aircraft VII-52
Control of NO and CO Emissions from Mobile Sources VII-53
Near-Term Technical Options: Developing Countries VII-55
Fuel-Efficiency Improvements VII-57
Improving Existing Vehicles VII-58
Alleviating Congestion and Improving Roads VII-58
Alternative Modes of Transportation VII-59
Alternative Fuels VII-60
Near-Term Technical Options: Soviet Bloc Countries VII-61
ix
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DETAILED TABLE OF CONTENTS (Continued)
Summary of Near-Term Technical Potential in the Transportation Sector VII-62
Long-Term Potential in the Transportation Sector VII-63
Urban Planning and Mass Transit VII-63
Alternative Fuels VII-65
Expanded Use of Emerging Technologies VII-66
RESIDENTIAL/COMMERCIAL SECTOR VII-67
Near-Term Technical Options: Industrialized Countries VII-71
Improvements in Space Conditioning VII-71
Indoor Air Quality VII-80
Lighting VII-81
Appliances VII-83
Near-Term Technical Options: Developing Countries VII-83
Increasing Efficiency of Fuelwood Use VII-85
Substituting More Efficient Fuels VII-87
Retrofit Efficiency Measures for the Modern Sector VII-88
New Homes and Commercial Buildings VII-89
Near-Term Technical Options: Soviet Bloc Countries VII-90
Summary of Near-Term Technical Potential in the Residential/Commercial Sector .... VII-91
Long-Term Potential in the Residential/Commercial Sector VII-92
INDUSTRIAL SECTOR VII-93
Near-Term Technical Options: Industrialized Countries VII-98
Accelerated Efficiency Improvements in Energy-Intensive Industries VII-98
Aggressive Efficiency Improvements of Other Industries VII-100
Cogeneration VII-101
Near-Term Technical Options: Developing Countries VII-102
Technological Leapfrogging VII-103
Alternative Fuels VII-104
Retrofit Energy Efficiency Programs VII-105
Agricultural Energy Use VII-106
Near-Term Technical Options: Soviet Bloc Countries VII-107
Summary of Near-Term Technical Potential in the Industrial Sector VII-111
Long-Term Potential in the Industrial Sector VII-112
Structural Shifts VII-112
Advanced Process Technologies VII-113
Non-fossil Energy VII-115
PART TWO: ENERGY SUPPLY VII-116
FOSSIL FUELS VII-117
Refurbishment of Existing Powerplants VII-121
Clean Coal Technologies and Repowering VII-122
Cogeneration VII-123
Natural Gas Substitution VII-124
Natural Gas Use At Easting Powerplants VII-124
Advanced Gas-Fired Combustion Technologies VII-125
Natural Gas Resource Limitations VII-127
Additional Gas Resources VII-130
Emission Controls VII-132
NO.. Controls VII-132
C02 Controls VII-133
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DETAILED TABLE OF CONTENTS (Continued)
Emerging Electricity Generation Technologies VII-134
Fuel cells VII-134
Magnetohydrodynamics (MHD) VII-136
BIOMASS VII-137
Direct Firing of Biomass VII-138
Charcoal Production VII-140
Anaerobic Digestion VII-141
Gasification VII-142
Liquid Fuels From Biomass VII-143
Methanol VII-143
Ethanol VII-145
Other VII-146
SOLAR ENERGY VII-146
Solar Thermal VII-147
Parabolic Troughs VII-147
Parabolic Dishes VII-149
Central Receivers VII-149
Solar Ponds VII-150
Solar photovoltaic VII-150
Crystalline Cells VII-152
Thin-Film Technologies VII-153
Multi-Junction Technologies VII-154
ADDITIONAL PRIMARY RENEWABLE ENERGY OPTIONS VII-155
Hydroelectric Power VII-155
Industrialized Countries VII-155
Developing Countries VII-157
Wind Energy VII-158
Geothermal energy VII-159
Ocean Energy VII-162
NUCLEAR POWER VII-165
Nuclear Fission VII-165
Nuclear Fusion VII-170
ELECTRICAL SYSTEM OPERATION IMPROVEMENTS VII-171
Transmission and Distribution VII-171
Superconductors VII-172
Storage Technologies VII-173
Types of Storage Technologies VII-174
HYDROGEN VII-176
PART THREE: INDUSTRY VII-178
CFCs AND RELATED COMPOUNDS VII-178
Technical Options For Reducing Emissions VII-182
Chemical Substitutes VII-182
Engineering Controls VII-184
Product Substitutes VII-185
xi
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DETAILED TABLE OF CONTENTS (Continued)
Summary of Technical Potential VII-187
METHANE EMISSIONS FROM LANDFILLS VII-187
Methane Recovery '. VII-190
Recycling and Resource Recovery VII-192
CO2 Emissions From Cement Production VII-193
PART FOUR: FORESTRY VII-195
FORESTS AND CARBON EMISSIONS VII-195
DEFORESTATION VII-197
TECHNICAL CONTROL OPTIONS VII-202
Reduce Demand for Forest Land and Products VII-206
Option 1: Slow Deforestation by Introducing Sustainable Forest Use Systems . . . VII-209
Option 2: Substitute Sustainable Agriculture for Swidden Forest Practices VII-210
Option 3: Reduce Demand For Other Land Uses That Have Deforestation
As A Byproduct VII-217
Option 4: Increase Conversion Efficiencies Of Technologies Using Fuelwood . . . VII-218
Option 5: Decrease Production of Disposable Forest Products VII-218
Substitute durable wood or non-wood products for high-volume disposable
uses of wood VII-219
Expand recycling programs for forest products VII-220
Increase Supply of Forested Land and Forest Products VII-220
Option 1: Increase Forest Productivity: Manage Temperate Forests For
Higher Yields VII-220
Option 2: Increase Forest Productivity: Improve Natural Forest Management
of Tropical Little-Disturbed And Secondary Forests VII-222
Option 3: Increase Forest Productivity: Plantation Forests VII-224
Option 4: Improve Forest Harvesting Efficiency VII-228
Option 5: Expand Current Tree Planting Programs in the Temperate Zone .... VII-229
Option 6: Reforest Surplus Agricultural Lands VII-231
Option 7: Reforest Urban Areas VII-234
Option 8: Afforestation for Highway Corridors VII-235
Option 9: Reforest Tropical Countries VII-236
Obstacles to Large-Scale Reforestation in Industrialize^ Countries VII-241
Obstacles to Reforestation in Developing Countries VII-243
Summary of Forestry Technical Control Options VtI-244
PART FIVE: AGRICULTURE VII-249
RICE CULTIVATION VII-252
Existing Technologies and Management Practices .' VII-253
Emerging Technologies VII-257
Research Needs and Economic Considerations VII-258
USE OF NITROGENOUS FERTILIZER VII-259
Existing Technologies and Management Practices VII-260
Management Practices That Affect NoO Production VII-260
Technologies that Improve Fertilization Efficiency VII-262
xii
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DETAILED TABLE OF CONTENTS (Continued)
Emerging Technologies VII-263
Research Needs and Economic Considerations VII-264
ENTERIC FERMENTATION IN DOMESTIC ANIMALS VH-265
Methane Emissions from Livestock VII-268
Existing Technologies and Management Practices VII-269
Emerging Technologies VII-272
Research Needs and Economic Considerations VII-273
CHAPTER VIII
POLICY OPTIONS
FINDINGS VIII-2
INTRODUCTION VIII-6
INTERNALIZING THE COST OF CLIMATE CHANGE RISKS VIII-8
Evidence of Market Response to Economic Incentives: Energy Pricing VIII-9
Financial Mechanisms to Promote Energy Efficiency VIII-14
Creating Markets for Conservation VIII-16
Limits to Price-Oriented Policies VIII-18
REGULATIONS AND STANDARDS VIII-22
Existing Regulations that Restrict Greenhouse Gas Emissions VIII-23
Regulation of Chlorofluorocarbons VIII-24
Energy Efficiency Standards VIII-25
Air PoUution Regulations Vffl-28
Waste Management VIII-29
Utility Regulation VTII-31
Existing Regulations that Encourage Emissions Reductions VIII-35
RESEARCH AND DEVELOPMENT VIII-39
Energy Research and Development VIII-40
Global Forestry Research & Development VIII-45
Research to Eliminate Emissions of CFCs VIII-46
INFORMATION AND TECHNICAL ASSISTANCE PROGRAMS VIII-47
CONSERVATION EFFORTS BY FEDERAL AGENCIES Vffl-50
STATE AND LOCAL EFFORTS VHI-52
PRIVATE SECTOR EFFORTS VIII-57
COMPLEMENTARY STRATEGIES TO REDUCE GREENHOUSE GAS EMISSIONS VIII-59
IMPLICATIONS OF POLICY CHOICES AND TIMING VIII-63
Xlll
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DETAILED TABLE OF CONTENTS (Continued)
SENSITIVITY TESTS OF THE EFFECT OF ALTERNATIVE POLICIES ON
GREENHOUSE GAS EMISSIONS: RISK TRADE-OFFS VHI-67
Policies That Increase Greenhouse Gas Emissions VHI-69
Policies Designed to Reduce Greenhouse Gas Emissions VIII-76
Conclusions From the Sensitivity Tests VIII-78
REFERENCES VIII-83
CHAPTER IX
INTERNATIONAL COOPERATION TO REDUCE GREENHOUSE GAS EMISSIONS
FINDINGS IX-2
INTRODUCTION IX-4
THE CONTEXT FOR POLICIES INFLUENCING GREENHOUSE GAS EMISSIONS IN
DEVELOPING COUNTRIES IX-5
Economic Development and Energy Use IX-7
Oil Imports, Capital Shortages, and Energy Efficiency IX-13
Greenhouse Gas Emissions and Technology Transfer IX-17
STRATEGIES FOR REDUCING GREENHOUSE GAS EMISSIONS IX-18
International Lending and Bilateral Aid IX-20
U.S. Bilateral Assistance Programs IX-21
Policies and Programs of Multilateral Development Banks IX-24
New Directions IX-31
REDUCING GREENHOUSE GAS EMISSIONS IN EASTERN BLOC NATIONS IX-33
U.S. LEADERSHIP TO PROMOTE INTERNATIONAL COOPERATION K-36
Restricting CFCs to Protect the Ozone Layer IX-36
International Efforts to Halt Tropical Deforestation IX-38
Ongoing Efforts Toward International Cooperation IX-42
REFERENCES IX-45
xiv
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LIST OF FIGURES
Executive Summary (bound under separate cover)
Page
1 Carbon Dioxide Concentrations at Mauna Loa and Fossil Fuel CO2 Emissions . . 3
2 Structure of the Atmospheric Stabilization Framework 8
3 Greenhouse Gas Contributions to Global Warming 12
4 Impact of CO? Emissions Reductions on Atmospheric Concentrations 14
5 Atmospheric Concentrations 25
6 Realized Warming: No Response Scenarios 27
7 Realized Warming: No Response and Stabilizing Policy Scenarios 31
8 Stabilizing Policy Strategies: Decrease in Equilibrium Warming
Commitment 34
9 Rapid Reduction Strategies: Additional Decrease in Equilibrium Warming
Commitment 37
10 Share of Greenhouse Gas Emissions by Region 41
11 Increase in Realized Warming When Developing Countries Do Not
Participate 43
12 Increase in Realized Warming Due to Global Delay in Policy Adoption 44
13 Accelerated Emissions Cases: Percent Increase in Equilibrium Warming
Commitment 46
14 Impact of Climate Sensitivity on Realized Warming 48
15 Activities Contributing to Global Warming 55
16 Primary Energy Supply by Type 58
17 CO2 Emissions From Deforestation 73
VOLUME I
Chapter I
1-1
1-2
Carbon Dioxide Concentrations at Mauna Loa and Fossil Fuel CO2
Emissions 1-10
Impact of CO2 Emissions Reductions on Atmospheric Concentrations 1-11
Chapter II
2-1 Greenhouse Gas Contributions to Global Warming
2-2 Carbon Dioxide Concentration
2-3 CO2 Atmospheric Concentrations by Latitude
2-4 The Carbon Cycle
2-5 Gas Absorption Bands :
2-6 Methane Concentration
2-7 Current Emissions of Methane by Source
2-8 Nitrous Oxide Concentrations
2-9 Temperature Profile and Ozone Distribution in the Atmosphere
II-6
11-11
11-12
11-15
11-20
11-23
11-25
11-31
11-41
xv
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LIST OF FIGURES (Continued)
Page
Chapter III
3-1 Surface Air Temperature III-7
3-2 Oxygen Isotope Record From Greenland Ice Cores III-9
3-3 Carbon Dioxide and Temperatures Records From Antarctic Ice Core 111-10
3-4 Oxygen Isotope Record From Deep Sea Sediment-Cores III-ll
3-5 Global Energy Balance 111-16
3-6 Equilibrium Temperature Changes from Doubled CO2 Ill-18
3-7 Greenhouse Gas Feedback Processes 111-21
Chapter IV
4-1 Regional Contribution to Greenhouse Warming — 1980s IV-6
4-2 Regional Population Growth - 1750-1985 IV-8
4-3 Global Energy Demand by Type - 1950-1985 IV-14
4-4 CO, Emissions Due to Fossil Fuel Consumption - 1860-1985 IV-16
4-5 Global Commercial Energy Demand by Region IV-17
4-6 1985 Sectoral Energy Demand by Region IV-19
4-7 Potential Future Energy Demand IV-32
4-8 Historical Production of CFC-11 and CFC-12 IV-36
4-9 CFC-11 and CFC-12 Production/Use for Various Countries IV-39
4-10 CO2 Emissions from Cement Production - 1950-1985 IV-44
4-11 Cement Production in Selected Countries -- 1951-1985 IV-46
4-12 Net Release of Carbon from Tropical Deforestation - 1980 IV-48
4-13 Wetland Area and Associated Methane Emissions IV-53
4-14 Trends in Domestic Animal Populations - 1890-1985 IV-57
4-15 Rough Rice Production - 1984 IV-59
4-16 Rice Area Harvested - 1984 IV-60
4-17 Nitrogen Fertilizer Consumption - 1984/1985 IV-64
Chapter V
5-1 Total U.S. Energy Consumption per GNP Dollar - 1900-1985 V-8
5-2 Consumption of Basic Materials V-10
5-3 Population by Region V-19
5-4 Structure of the Atmospheric Stabilization Framework V-23
5-5 Geopolitical Regions For Climate Analyses V-24
5-6 End-Use Fuel Demand by Region V-34
5-7 End-Use Electricity Demand by Region V-35
5-8 Share of End-Use Energy Demand by Sector V-38
5-9 Primary Energy Supply by Type V-40
5-10 Share of Primary Energy Supply by Type V-41
5-11 Energy Demand for Synthetic Fuel Production V-42
5-12 Emissions of Major CFCs V-58
5-13 CO2 Emissions from Deforestation V-62
5-14 CO2 Emissions by Type V-66
5-15 Share of CO2 Emissions by Region V-68
5-16 CH4 Emissions by Type V-69
xvi
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LIST OF FIGURES (Continued)
Page
5-17 Share of CH4 Emissions by Region V-70
5-18 Atmospheric Concentrations V-72
5-19 Realized and Equilibrium Warming V-77
5-20 Relative Contribution to Warming by 2100 V-80
5-21 Stabilizing Policy Strategies: Decrease in Equilibrium Warming Commitment . . . V-83
Chapter VI
6-1 Increase in Realized Warming When Developing Countries Do Not
Participate VI-16
6-2 Increase in Realized Warming Due to Global Delay in Policy Adoption VI-19
6-3 Availability of Non-Fossil Energy Options VI-21
6-4 Impact of 1% Per Year Real Escalation in Coal Prices VI-23
6-5 Impact of Higher Oil Resources On Total Primary Energy Supply VI-26
6-6 Impact of Higher Natural Gas Resources on Total Primary Energy Supply VI-28
6-7 Realized Warming Through 1985 VI-32
6-8 Increase in Realized Wanning Due to Changes in the Methane Budget VI-37
6-9 Change in Atmospheric Concentration of N2O Due to Leaching VI-40
6-10 Change in Atmospheric Concentration of N2O Due to Combustion VI-42
6-11 Impact on Realized Warming Due to Size of Unknown Sink VI-45
6-12 CO2 From Deforestation Assuming High Biomass VI-46
6-13 Impact of High Biomass Assumptions on Atmospheric Concentration of CO2 . . . VI-48
6-14 Comparison of Different Ocean Models VI-51
6-15 Impact of Climate Sensitivity on Realized Warming VI-52
6-16 Change in Realized Warming Due to Rate of Ocean Heat Uptake VI-55
6-17 Regional Differences for Urban Areas With Different Emissions of CO and
NO VI-64
6-18 OH and Ozone Perturbations in the Isaksen and Hov Model VI-66
6-19 Sensitivity of Atmospheric Concentration of CFC-11 to Its Lifetime VI-68
6-20 Change in Realized Warming Due to Rate of Interaction of CLx With
Ozone VI-70
6-21 Increase in Realized Warming Due to Change in Ocean Circulation VI-74
6-22 Increase in Realized Warming Due to Methane Feedbacks VI-76
6-23 Increase in Realized Warming Due to Change in Combined Feedbacks VI-77
xvu
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LIST OF FIGURES (Continued)
Page
VOLUME II
Chapter VII
7-1 Current Contribution to Global Warming VII-20
7-2 Global Energy Use by End-Use VD-28
7-3 Secondary Energy Consumption by Region VII-30
7-4 End-Use Energy Demand by Sector VII-33
7-5 Transportation Energy Use by Region VII-35
7-6 Components of Transportation Energy Use in the OECD: 1985 VII-37
7-7 U.S. Residential/Commercial Energy Use VII-68
7-8 Residential/Commercial Energy Use by Region VII-70
7-9 Industrial Energy Use by Region VII-97
7-10 Electricity Utility Demand by Fuel Type VII-118
7-11 Average Fossil Powerplant Efficiency VII-120
7-12 Strategies for Improving Efficiency of Biomass Use VII-139
7-13 Basic Solar Thermal Technologies VII-148
7-14 Photovoltaic Electricity Costs VII-151
7-15 Nuclear Capital Costs VII-167
7-16 Industrial Process Contribution to Global Warming VII-179
7-17 Emissions of Major CFC's ' VII-180
7-18 CH4 Emissions by Type VII-188
7-19 Movement of Tropical Forest Lands Among Stages of Deforestation and Potential
Technical Response Options VII-198
7-20 Population Growth, Road Building, and Deforestation in Amazonia VII-201
7-21 Model Agroforestry Farm Layout, Rwanda VII-214
7-22 Agricultural Practices Contribution to Global Warming VII-250
7-23 Trace Gas Emissions From Agricultural Activities VII-251
Chapter VIII
8-1 Energy Intensity Reductions, 1973-1983 VIII-11
8-2 U.S. Electricity Demand and Price VIII-15
8-3 Cost of Driving Versus Automotive Fuel Economy VIII-21
8-4 U.S. Carbon Monoxide Emissions VIII-30
8-5 Changes in U.S. Renewable Energy R&D Priorities Over Time VIII-42
8-6 Cost of Potential Residential Electricity Conservation
in Michigan by 2000 VIII-55
8-7 U.S. Energy Consumption By Fuel Share VIII-66
8-8 Atmospheric Response to Emissions Cutoff VIII-68
8-9 Actual and Projected U.S. Coal Production VIII-70
8-10 Accelerated Emissions Cases: Percent Increase in Equilibrium Warming
Commitment VIII-74
8-11 Rapid Reduction Strategies: Additional Decrease in Equilibrium Warming
Commitment VIII-79
xvui
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LIST OF FIGURES (Continued)
Page
Chapter IX
9-1 Greenhouse Gas Emissions By Region IX-6
xix
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LIST OF TABLES
Executive Summary (bound under separate cover)
2
3
4
5
6
7
Approximate Reductions in Anthropogenic Emissions Required to Stabilize
Atmospheric Concentrations at Current Levels
Overview of Scenario Assumptions
Current and Projected Trace Gas Emissions Estimates
Scenario Results for Realized and Equilibrium Warming
Examples of Policy Responses by the Year 2000
Sensitivity Analysis: Impact on Realized Warming and Equilibrium Warming
Key Global Indicators for Energy and COo
Major Chlorofluorocarbons, Halons, and Chlorocarbons: Statistics and Uses .
Page
15
21
24
33
39
50
61
68
VOLUME I
Chapter II
2-1 Radiative Forcing for a Uniform Increase in Trace Gases From Current Levels . . 11-21
2-2 Trace Gas Data 11-51
Chapter IV
4-1 Regional Demographic Indicators IV-9
4-2 Emission Rate Differences by Sector IV-21
4-3 End-Use Energy Consumption Patterns for the Residential/Commercial
Sectors IV-24
4-4 Carbon Dioxide Emission Rates for Conventional and Synthetic Fuels IV-28
4-5 Estimates of Global Fossil-Fuel Resources IV-30
4-6 Major Halocarbons: Statistics and Uses IV-34
4-7 Estimated 1985 World Use of Potential Ozone-Depleting Substances IV-38
4-8 Refuse Generation Rates in Selected Cities IV-42
4-9 Land-Use: 1850-1980 IV-49
4-10 Summary Data on Area and Biomass Burned IV-52
4-11 Nitrous Oxide Emissions by Fertilizer Type IV-62
Chapter V
5-1 Overview of Scenario Assumptions V-14
5-2 Economic Growth Assumptions V-18
5-3 Key Global Indicators V-46
5-4 Comparison of No Response Scenarios and NEPP V-48
5-5 Comparison of Stabilizing Policy Scenarios and ESW V-49
5-6 Summary of Various Primary Energy Forecasts for the Year 2050 V-51
5-7 Comparison of Energy-Related Trace-Gas Emissions Scenarios V-55
5-8 Trace Gas Emissions V-65
5-9 Comparison of Estimates of Trace-Gas Concentrations in 2030 and 2050 V-75
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LIST OF TABLES (Continued)
Page
Chapter VI
6-1
6-2
6-3
6-4
6-5
Impact of Sensitivity Analyses on Realized Warming and Equilibrium Warming
Comparison of Model Results to Concentrations in 1986
Low and High Anthropogenic Impact Budgets for Methane
Comparison of Emission Estimates for EPA #2, RCW and SCW Cases
Comparison of Results from Atmospheric Chemistry Models for the Year 2050
Compared to 1985
VI-7
VI-33
VI-36
VI-60
VI-61
VOLUME II
Chapter VII
7-1 Key Technical Options by Region and Time Horizon VII-19
7-2 High Fuel Economy Prototype Vehicles VII-39
7-3 Actual New Passenger Car Fuel Efficiency VII-40
7-4 Summary of Energy Consumption and Conservation Potential With
Major Residential Equipment VII-84
7-5 Reduction of Energy Intensity In the Basic Materials Industries VII-95
7-6 Energy Intensities of Selected Economies VII-108
7-7 Innovation in Steel Proudction Technology VII-110
7-8 Total U.S. Gas Reserves and Resources VII-128
7-9 COo Scrubber Costs Compared to SO? Scrubber Costs VII-135
7-10 Estimates of Worldwide Geothermal Electric Power Capacity Potential VII-160
7-11 Capacity of Direct Use Geothermal Plants in Operation - 1984 VII-163
7-12 Geothermal Powerplants On-Line as of 1985 VII-164
7-13 Major Forestry Sector Strategies for Stabilizing Climate Change VII-203
7-14 Potential Forestry Strategies and Technical Options to Slow Climate Change .... VII-207
7-15 Comparison of Land Required for Sustainable Swidden Versus Agricultural
Practices VII-212
7-16 Potential Carbon Fixation and Biomass Production Benefits from Agroforestry
Systems VII-216
7-17 Natural and Managed Tropical Moist Forest Yields VTI-223
7-18 Productivity Increases Attributable to Intensive Plantation Management VII-227
7-19 Summary of Major Tree Planting Programs in the U.S VII-230
7-20 Estimates of CRP Program Acreage Necessary to Offset CO, Production from
New Fossil Fuel-Fired Electric Plants, 1987-96, by Tree Species or Forest Type . . VII-233
7-21 Estimates of Forest Acreage Required to Offset Various CO? Emissions Goals . . VH-239
7-22 Comparison of Selected Forest Sector Control Options: Preliminary Estimates . . VII-245
7-23 Overview of Two Social Forestry Projects Proposed to Offset CO2 Emissions of
a 180-MW Electric Plant in Connecticut VII-248
7-24 Water Regime and Modern Variety Adoption for Rice Production in Selected
Asian Countries VII-256
7-25 Average Meat Yield Per Animal VII-267
Chapter VIII
8-1 Energy Intensity of Selected National Economies, 1973-85 VIII-12
8-2 Payback Periods in Years for Appliances, 1972-1980 VIII-20
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LIST OF TABLES (Continued)
Page
8-3 Appliance Efficiency Improvements Required by Law VIII-26
8-4 Cogeneration Facilities VIII-34
8-5 Erodible Acreage Available to Offset CO2 Emissions From Electricity
Production VIII-37
8-6 Government Efficiency Research and Development Budgets in OECD Member
Countries, 1986 VIII-41
8-7 Federal Energy Expenditures and Cost Avoidance, FY1985-FY1987 VIII-51
8-8 Scenario Results for Realized and Equilibrium Warming VIH-82
Chapter IX
9-1 1985 Population and Energy Use Data From Selected Countries IX-8
9-2 Efficiency of Energy Use in Developing Countries: 1984-1985 IX-10
9-3 Potential for Electricity Conservation in Brazil IX-12
9-4 Net Oil Imports and Their Relation to Export Earnings for Eight Developing
Countries, 1973-1984 IX-14
9-5 Annual Investment in Energy Supply as a Percent of Annual Total Public
Investment (Early 1980s) IX-15
9-6 World Bank Estimate of Capital Requirements for Commercial Energy in
Developing Countries, 1982-1992 IX-16
9-7 U.S. AID Forestry Expenditures by Region IX-23
9-8 World Bank Energy Sector Loans in 1987 IX-26
9-9 Expenditures of Multilateral and Bilateral Aid Agencies in the Energy Area .... IX-27
9-10 World Bank Energy Conservation Projects: Energy Sector Management
Assistance Program (ESMAP) Energy Efficiency Initiatives IX-30
9-11 Energy Use in the Soviet Union and Eastern Bloc IX-34
9-12 Countries Responsible for Largest Share of Tropical Deforestation IX-41
xxii
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ACKNOWLEDGEMENTS
This report would not have been possible without the tireless efforts of the primary chapter
authors:
Executive Summary Daniel Lashof
Dennis Tirpak
Chapter I. Introduction Joel Scheraga
Irving Mintzer
Chapter II. Greenhouse Gas Trends Inez Fung
Michael Prather
Chapter III. Climatic Change Daniel Lashof
Alan Robock
Chapter IV. Human Activities Affecting Trace Gases and Climate Barbara Braatz
Craig Ebert
Chapter V. Thinking About the Future Daniel Lashof
Leon Schipper
Chapter VI. Sensitivity Analyses Craig Ebert
Chapter VII. Technical Control Options Paul Schwengels (Energy Services)
Michael Adler (Renewable Energy)
Dillip Ahuja (Biomass)
Kenneth Andrasko (Forestry)
Lauretta Burke (Agriculture)
Craig Ebert (Energy Supply)
Joel Scheraga (Energy Supply)
John Wells (Halocarbons)
Chapter VIII. Policy Options Alan Miller
Chapter IX. International Cooperation to Reduce Greenhouse Gas Emissions Alan Miller
Jayant Sathaye
Appendix A. Model Descriptions William Pepper
Appendix B. Scenario Definitions Craig Ebert
Appendix C. Results of Sensitivity Analyses Craig Ebert
xxm
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Model integration was coordinated by William Pepper and Craig Ebert, with assistance from
Rossana Florez. Models and/or analysis were prepared by Irving Mintzer; Jayant Sathaye, Andrea
Ketoff, Leon Schipper, and Sharad Lele; Klaus Frohberg and Phil Vande Kamp; Richard Houghton;
Berrien Moore, Chris Ringo, and William Emmanuel; Michael Prather; Ivar Isaksen, Terje Berntsen,
and Sverre Solberg; and Anne Thompson.
Document integration was coordinated by Craig Ebert and Barbara Braatz. Editorial assistance
was provided by Susan MacMillan. Technical, graphics, and typing assistance was provided by
Courtney Dinsmore, Katey Donaldson, Donald Devost, Michael Green, Karen Zambri, Judy Koput,
Donna Whitlock, Margo Brown, and Cheryl LaBrecque.
Literally hundreds of other people have contributed to this report, including the organizers and
attendants at four workshops sponsored by EPA to gather information and ideas, and dozens of
formal and informal reviewers. We would like to thank this legion for their interest in this project,
and apologize for not doing so individually. Particularly important comments were provided by,
among others, Thomas Bath, Deborah Bleviss, Gary Breitenbeck, William Chandler, Robert Friedman,
Howard Geller, James Hansen, Tony Janetos, Stan Johnson, Julian Jones, Michael Kavanaugh,
Andrew Lacis, Michael MacCracken, Elaine Matthews, William Nordhaus, Steven Piccot, Marc Ross,
Stephen Schneider, Paul Steele, Pieter Tans, Thomas Wigley, Edward Williams, and Robert Williams.
This work was conducted within EPA's Office of Policy Analysis, directed by Richard
Morgenstern, within the Office of Policy Planning and Evaluation, administered by Linda Fisher.
Technical support was provided by the Office of Research and Development, administered by Eric
Brethauer.
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CHAPTER I
INTRODUCTION
INTRODUCTION 1-2
The Earth's Climate and Global Change 1-2
CONGRESSIONAL REQUEST FOR REPORTS 1-3
Goals of this Study 1-4
Report Format 1-5
THE GREENHOUSE GASES 1-8
Carbon dioxide 1-9
Methane 1-9
Nitrous oxide 1-12
Chlorofluorocarbons 1-12
Other gases influencing composition 1-13
PREVIOUS STUDIES 1-13
Estimates of the Climatic Effects of Greenhouse Gas Buildup 1-14
Studies of Future CO2 Emissions 1-15
Studies of the Combined Effects of Greenhouse Gas Buildup 1-20
Major Uncertainties 1-22
Conclusions From Previous Studies 1-23
CURRENT NATIONAL AND INTERNATIONAL ACTIVITIES 1-26
National Research and Policy Activities 1-26
International Activities 1-27
REFERENCES 1-29
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INTRODUCTION
The Earth's Climate and Global Change
The greenhouse effect is a natural phenomenon that plays a central role in determining the
Earth's climate. Sunlight passes through the atmosphere and warms the Earth's surface. The Earth
then radiates infrared energy, some of which escapes back into space. But certain gases (known as
greenhouse gases) that occur naturally in the atmosphere absorb most of the infrared radiation and
emit some of this energy back toward the Earth, warming the surface. This effect is, to a great
extent, responsible for making the Earth conducive to life. In its absence, the Earth would be
approximately 30°C colder.
Concerns about the greenhouse effect arise out of apprehension that anthropogenic (man-made)
emissions of greenhouse gases will further warm the Earth. Greenhouse gases — primarily carbon
dioxide (CO2), methane (CH4), nitrous oxide (N2O), chlorofluorocarbons (CFCs), and tropospheric
ozone (O3) — are produced as by-products of human activities. When these gases are emitted into
the atmosphere and their concentrations increase, the greenhouse effect is compounded. The result
is an increase in mean global temperatures.
There is scientific consensus that increases in greenhouse gas emissions will result in climate
change (Bolin et al., 1986; NAS, 1979, 1983, 1987; WMO, 1985). The Council on Environmental
Quality concluded in 1981 that the potential long-term risks of societal disruption caused by increased
atmospheric concentrations of CO2 (aside from the other greenhouse gases) are significant. However,
considerable uncertainty exists with regard to the ultimate magnitude of the warming, its timing, and
the regional patterns of change. In addition, there is great uncertainty about changes in climate
variability and regional impacts.
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CONGRESSIONAL REQUEST FOR REPORTS
EPA has studied the effects of global warming for several years. The goal of its efforts has been
to use the best available information and models to assess the effects of climatic change and to
evaluate policy strategies for both limiting and adapting to such change.
In 1986, Congress asked EPA to develop two reports on global warming. In one of these studies
Congress directed EPA to include:
"An examination of policy options that if implemented would stabilize current levels of
atmospheric greenhouse gas concentrations. Tliis study should address the need for and
implications of significant changes in energy policy, including energy efficiency and development
of alternatives to fossil fuels; reductions in the use of CFCs; ways to reduce other greenhouse
gases such as methane and nitrous oxide; as well as the potential for and effects of reducing
deforestation and increasing reforestation efforts."
These issues are the focus of this report.
This report differs from most previous studies of the climate change issue in that it is primarily
a policy assessment. Although some aspects of the relevant scientific issues are reviewed, this
document is not intended as a comprehensive scientific assessment. A recent review of the state of
the science is contained in the U.S. Department of Energy's State of the Art series (MacCracken and
Luther, 1985a, 1985b; Strain and Cure, 1985; Trabalka, 1985).
Congress also asked EPA to prepare a companion report on the health and environmental effects
of climate change in the U.S., which would examine the impact of climate change on agriculture,
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forests, and water resources, as well as on other ecosystems and society. In response to the latter
request, EPA produced its report entitled, Tlie Potential Effects of Global Climate Change on the
United States (Smith and Tirpak, 1989). That report provides insights into the ranges of possible
future effects that may occur under alternative climate change scenarios, and establishes qualitative
sensitivities of different environmental systems and processes to changes in climate. The report also
examines potential changes in hydrology, agriculture, forestry, and infrastructure in the Southeast,
Great Lakes, California, and Great Plains regions of the United States.
Goals of this Study
Congress presented EPA with a very challenging task. From a policy perspective, it is not
enough to know how emissions would have to change from current levels in order to stabilize the
atmosphere. Instead, policy options must be evaluated in the context of expected economic and
technological development and the uncertainties that prevent us from knowing precisely how a given
level of emissions will impact the rate and magnitude of climate change. It is also necessary for the
scope of this study to be global and the time horizon to be more than a century, because of the long
lags built into both the economic and climatic systems (we chose 1985-2100 as the time frame for
the analysis). Predictions with such a scope cannot be attempted, but scenarios can be developed
to explore policy options.
Based on these considerations EPA established four major goals:
• To assemble data on global trends in emissions and concentrations of all
major greenhouse gases and activities that affect these gases.
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• To develop an integrated analytical framework to study how different
assumptions about the global economy and the climate system could
influence future greenhouse gas concentrations and global temperatures.
• To identify promising technologies and practices that could limit greenhouse
gas emissions.
• To identify policy options that could influence future greenhouse gas
concentrations and global warming.
To achieve these goals EPA conducted an extensive literature review and data gathering process.
The Agency held several informal panel meetings, and enlisted the help of leading experts in the
governmental, non-governmental, and academic research communities. EPA also conducted five
workshops, which were attended by over three hundred people, to gather information and ideas
regarding factors affecting atmospheric composition and options related to greenhouse gas emissions
from agriculture and land-use change, electric utilities, end-uses of energy, and developing countries.
Experts in NASA, the Department of Energy, and the Department of Agriculture were actively
engaged.
Report Format
The structure of this report is designed to answer the following questions in turn: What is the
greenhouse effect? What evidence is there that the greenhouse effect is increasing? How will the
Earth's climate respond to changes in greenhouse gas concentrations? What activities are responsible
for the greenhouse gas emissions? How might emissions and climate change in the future? What
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TERMINOLOGY OF CLIMATE CHANGE
An attempt has been made throughout this report to avoid technical jargon, yet some
specialized terminology is inevitable. The specialized terms used in this Report are defined
below.
Climate System
The interactive components of our planet which determine the climate. This includes the
atmosphere, oceans, land surface, sea ice, snow, glaciers, and the biosphere. Climate change
can be measured in terms of any part of the system, but it is most convenient to use surface
air temperature as a measure of climate, since it is the parameter for which we have the best
record and it is most directly relevant to the component of the biosphere that we know best -
- humans.
Radiative Forcing (also called "external forcing," 'forcing," or "perturbation")
A change imposed on the climate system (as opposed to generated by the internal dynamics
of the climate system) that modifies the radiative balance of die climate system. Examples
include: changes in the output of the sun or the orbit of the Earth about the sun, increased
concentrations of particles in the atmosphere due to volcanoes or human activity, and increased
concentrations of greenhouse gases in the atmosphere due to human activity. Radiative forcing
is often specified as the net change in energy flux at the tropopause (W/m2) or the
equilibrium change in surface temperature in the absence of feedbacks (°C).
Climate Feedbacks
Processes that alter the response of the climate system to radiative forcings. We distinguish
between physical climate feedbacks and biogeochemical climate feedbacks. Physical climate
feedbacks are processes of the atmosphere, ocean, and land surface, such as increases in
water vapor, changes in cloudiness, and decreases in land- and sea-ice accompanying global
warming. Biogeochemical feedbacks involve changes in global biology and chemistry, such as
the effect of changes in ocean circulation on carbon dioxide concentrations and changes in
albedo from shifts in ecosystems. The impact of climate feedbacks is generally measured in
terms of their effect on climate sensitivity. Positive feedbacks increase climate sensitivity, while
negative feedbacks reduce it.
Climate Sensitivity (or equilibrium sensitivity)
The ultimate change hi climate that can be expected from a given radiative forcing. Climate
sensitivity is generally measured as the change in global average surface air temperature when
equilibrium between incoming and outgoing radiation is reestablished following a change in
radiative forcing. A common benchmark, which we use in this report, is the equilibrium
temperature increase associated with a doubling of the concentration of carbon dioxide front
preindustrial levels. The National Academy of Sciences has estimated that this sensitivity is
in the range of 1_5-4.50C, with a recent analysis suggesting 1.5-5.5°C; a reasonable central
uncertainty range is 2-4°C.
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TERMINOLOGY OF CLIMATE CHANGE
(continued)
Transient Response
The time-dependent response of climate to radiative forcing. Climate responds gradually to
changes in radiative forcing, primarily because of the heat capacity of the oceans. The
transient mode is characterized by an imbalance between incoming and outgoing radiation,
Given the changing concentrations of greenhouse gases the Earth's climate mil be in a
transient mode for the foreseeable future. Most GCMs (see below), however, have so far
examined equilibrium conditions because transient effects are much more difficult to analyze.
Albedo
The fraction of incoming solar radiation that is reflected back into space.
Flux
How per unit time per unit area. The flow can be of energy (e.g., watts per square meter
[W/m*|) or mass (e.g.» grams per square meter per day {g m'2
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Policy Options for Stabilizing Global Climate -- Review Draft Chapter I
technologies are available for limiting greenhouse gas emissions? And what domestic and
international policy options, if implemented, would help to stabilize global climate?
This chapter provides a general introduction to the climate change issue and reviews selected
previous studies. Chapter II discusses the greenhouse gases, their sources and sinks, chemical
properties, current atmospheric concentrations and distributions, and related uncertainties. Chapter
III relates the greenhouse gases to the process of climatic change. Once this link is made, Chapter
IV examines those human activities that affect trace-gas emissions and ultimately influence climate
change. Chapter V discusses the scenarios developed for this report to assist us in thinking about
possible future emissions and climate change. Chapter VI then presents sensitivity analyses of the
modeling results. Chapter VII gives a detailed description of existing and emerging technologies that
should be considered in the formulation of a comprehensive strategy for mitigating global warming.
Chapter VIII outlines domestic policy options, and the concluding chapter (Chapter DC) discusses
international mechanisms for responding to climate change.
THE GREENHOUSE GASES
Congress presented EPA with an extremely challenging task. Once emitted, greenhouse gases
remain in the atmosphere for decades to centuries. As a result, if emissions remained constant at
1985 levels, the greenhouse effect would continue to intensify for more than a century. Carbon
dioxide concentrations would reach 440-500 parts per million (ppm) by 2100, compared with about
350 ppm today, and about 290 ppm 100 years ago. CFC concentrations would increase by more than
a factor of three from current levels, while nitrous oxide concentrations would increase by about 20%,
and methane concentrations might remain roughly constant. Indeed, in many cases drastic cuts in
emissions would be required to stabilize atmospheric composition.
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Carbon dioxide
Carbon dioxide (CO2) is the most abundant and single most important greenhouse gas in the
atmosphere. Its concentration has increased by about 25% since the industrial revolution. Detailed
measurements since 1958 show an increase from 315 to 350 parts per million (ppm) by volume
(Figure 1-1). These data clearly demonstrate that human activities are now of such a magnitude as
to produce global consequences. Current emissions are estimated at 5.5 billion tons of carbon (Pg
C) from fossil-fuel combustion and 0.4-2.6 Pg C from deforestation.1 Most of this CO2 remains in
the atmosphere or is absorbed by the ocean. Even though only about hah0 of current emissions
remain in the atmosphere, currently available models of CO2 uptake by the ocean suggest that
substantially more than a 50% cut in emissions is required to stabilize concentrations at current
levels (Figure 1-2).
Methane
The concentration of methane (CH4) has more than doubled during the last three centuries.
Methane, which is currently increasing at a rate of 1% per year, is responsible for about 20% of
current increases in the greenhouse effect. Of the major greenhouse gases, only CH4 concentrations
can be stabilized with modest cuts in anthropogenic emissions: a 10-20% cut would suffice to
stabilize concentrations at current levels due to methane's relatively short atmospheric lifetime
(assuming that the lifetime remains constant, which may require that hydrocarbon and carbon
monoxide emissions be stabilized).
1 One billion tons of carbon = 1015 grams of carbon = 1 petagram of carbon (Pg C).
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Chapter I
FIGURE 1-1
CARBON DIOXIDE CONCENTRATIONS AT MAUNA LOA
AND FOSSIL FUEL C02 EMISSIONS
2000
I U 5 0 1900 1963 1964 I860 1961) 1871) 1972 1074 1976 1870 1 0 H 0 1862 I9B4 188C 1808
Figure 1-1. The solid line depicts monthly concentrations of atmospheric CO2 at Mauna Loa
Observatory, Hawaii. The yearly oscillation is explained mainly by the annual cycle of photosynthesis
and respiration of plants in the northern hemisphere. The steadily increasing concentration of
atmospheric CO? at Mauna Loa since the 1950s is caused primarily by the CO2 inputs from fossil
fuel combustion (dashed line). Note that CO2 concentrations have continued to increase since 1979,
despite relatively constant emissions; this is because emissions have remained substantially larger than
net removal, which is primarily by ocean uptake. (Sources: Keeling, 1983, pers. communication;
Komhyr et al., 1985; NOAA, 1987; Conway et al., 1988; Rotty, 1987, pers. communication.)
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Chapter I
FIGURE 1-2
IMPACT OF C02 EMISSIONS REDUCTIONS
ON ATMOSPHERIC CONCENTRATIONS
(Parts Per Million)
500
475 \-
326
1965 2000
2025 2050
YEAR
2075
2100
Figure 1-2. The response of atmospheric CO2 concentrations to arbitrary emissions scenarios based
on two one-dimensional models of ocean CO2 uptake. See Chapter VI for a description and models.
(Sources: Hansen et al, 1984; Lashof, 1988; Siegenthaler, 1983).
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Nitrous oxide
The concentration of nitrous oxide (N2O) has increased by 5-10% since preindustrial times.
Nitrous oxide is currently increasing at a rate of 0.25% per year, which represents an imbalance of
about 30% between sources and sinks. Assuming that the observed increase in N2O concentrations
is due to anthropogenic sources and that natural emissions have not changed, then an 80-85% cut
in anthropogenic emissions would be required to stabilize N2O at current levels.
Chlorofluorocarbons
Chlorofluorocarbons (CFCs) were introduced into the atmosphere for the first time during this
century. The most common species are CFC-12 (CFjCy and CFC-11 (CFC13), which had
atmospheric concentrations in 1986 of 392 and 226 parts per trillion (ppt) by volume, respectively.
While these concentrations are tiny compared with that of CO2, each additional CFC molecule has
as much as 20,000 times more impact on climate, and CFCs are increasing very rapidly—more than
4% per year since 1978. A focus of attention because of their potential to deplete stratospheric ozone,
the increasing concentrations of CFCs also account for about 15% of current increases in the
greenhouse effect. For CFC-11 and CFC-12, cuts of 75% and 85%, respectively, of current global
emissions would be required to stabilize concentrations. However, in order to stabilize stratospheric
chlorine levels ~ of particular concern for stratospheric ozone depletion ~ a 100% phaseout of fully-
halogenated compounds (those that do not contain hydrogen) and a freeze on the use of methyl
chloroform would be required.
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Other gases influencing composition
Emissions of carbon monoxide (CO), nitrogen oxides (NOJ, and other species, in addition to
the greenhouse gases just described, are also changing the chemistry of the atmosphere. This change
in atmospheric chemistry alters the distribution of ozone and the oxidizing power of the atmosphere,
changing the atmospheric lifetimes of the greenhouse gases. If the concentrations of the long-lived
gases were stabilized, it might only be necessary to freeze emissions of the short-lived gases at current
levels to stabilize atmospheric composition.
PREVIOUS STUDIES
Evidence that the composition of the atmosphere is changing has led to a series of studies
analyzing the potential magnitude of future greenhouse gas emissions. A few of these studies have
carried the analysis further, making projections of the timing and severity of future global warming.
The first generation of these studies focussed principally on energy use and CO2 emissions. (See,
for example, Arrhennius, 1896; NAS, 1979; Clark et al, 1982; IIASA, 1983; Nordhaus and Yohe, 1983;
Rose et al., 1983; Seidel and Keyes, 1983; Edmonds and Reilly, 1983b, 1984; Legasov, et al. 1984;
Goldemberg et al., 1985, 1987; and Keepin et al., 1986). Subsequent studies have recognized that
other radiatively-active trace gases significantly amplify the effects of CO2. (See, for example, Lacis
et al., 1981; Ramanathan et al., 1985; Dickinson and Cicerone, 1986; WMO, 1985; and Mintzer, 1987).
In the following sections, some of the most important of these earlier analyses are reviewed in order
to provide a basis for comparison with this study.
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Estimates of the Climatic Effects of Greenhouse Gas Buildup
The first serious analysis of the effect of increasing CO2 concentrations on global warming was
conducted by the Swedish chemist Svante Arrhennius (1896). Arrhennius, concerned about the
rapidly increasing rate of fossil-fuel use in Europe, recognized that the resulting increase in the
atmospheric concentration of CO2 would alter the thermal balance of the atmosphere. Using a
simplified, one-dimensional model, Arrhennius estimated that if the atmospheric concentration of CO2
doubled, the surface of the planet would warm by about 5°C. (The expected equilibrium climate
change associated with a doubling of CO2 has become a benchmark. That is, many studies examine
the consequences of greenhouse gas increases with a total warming effect equivalent to that from a
doubling of the concentration of CO2.)
In 1979, a study by the U.S. National Academy of Sciences (NAS) evaluated the impact on global
climate of doubling the concentration of CO2 relative to the preindustrial atmosphere (NAS, 1979).
The NAS study concluded that the planet's surface would be 1.5-4.5°C warmer under such conditions.
Subsequent re-evaluations by NAS (1983, 1987) as well as the "State-of-the-Art" report issued by the
Department of Energy (MacCracken and Luther, 1985a) have reaffirmed this estimate.
Recent work by Dickinson (1986) suggests that the effects of a greenhouse gas buildup radiatively
equivalent to doubling the preindustrial concentration of CO2 might warm the planet to a greater
extent than had previously been expected. Focusing on the uncertainties in current understanding
of atmospheric feedback processes, Dickinson estimated that the warming effect of such a buildup
was likely to be between 1.5° and 5.5°C. Dickinson's "best guess" was that the actual equilibrium
wanning would be between 2.5° and 45°C.
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Studies of Future CO2 Emissions
For the next eighty years after Arrhennius issued his warning, little additional scientific attention
was directed toward understanding the factors that contribute to future greenhouse gas emissions.
By the mid-1970s, measurements of atmospheric CO2 concentrations at Mauna Loa begun by Keeling
during the International Geophysical Year (1957-1958) provided indisputable evidence of a long-term
increasing trend (see Figure 1-2), while the oil embargo of 1973 and the nuclear power debate
focussed attention on future energy supplies. Increasing interest was placed on the problems of
projecting future global energy use and on estimating the resulting CO2 emissions.
A major international study of future energy use was conducted by the International Institute for
Applied Systems Analysis (IIASA, 1981 and 1983). Employing an international group of almost 200
scientists, the IIASA team developed a set of computer models to estimate regional economic growth,
energy demand, energy supply, and future CO2 emissions. Although the models were never
completely integrated, the first phase of the IIASA study produced two complete scenarios of global
energy use. The IIASA low scenario generated CO2 emissions of about 10 petagrams of carbon per
year (Pg C/yr) in 2030. The IIASA high scenario projected emissions of about 17 Pg C/yr in 2030.
In the second phase of the IIASA study a third scenario was outlined, emphasizing increased use of
natural gas. In this third scenario, CO2 emissions in 2030 were only about 9.4 Pg C/yr.
In 1983 Edmonds and Reilly, two U.S. economists, developed a detailed partial equilibrium
model to investigate the effects of alternative energy policies and their implications for future CO2
emissions (Edmonds and Reilly, 1983a). This model disaggregates the world into nine geopolitical
regions. It offers a highly detailed picture of the supply side of the world's commercial energy
business but only limited detail on the demand side. It considers nine primary and four secondary
forms of commercial energy (including biomass grown on plantations) but ignores non-commercial
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uses of biomass for fuel. Using explicit assumptions about regional population changes and economic
growth and combining them with assumptions about technological change and the costs of extracting
various grades of fuel resources in each region, the model calculates supply and demand schedules
for each type of fuel.
For their first major report, Edmonds and Reilly (1983b) developed a Base Case energy future
for the period 1975 to 2050. In this scenario, CO2 emissions in 2050 were approximately 26.3 Pg
C/yr. The authors generated several other scenarios in this study that reflected the effect of various
taxes imposed on fuel supply and use. These taxes reduced CO2 emissions by varying amounts, with
emissions in some scenarios falling as low as 15.7 Pg C/yr in 2050. In 1984 Edmonds and Reilly
produced a new set of scenarios for the U.S. Department of Energy by varying other key parameters
in the model (Edmonds and Reilly, 1984). In these new scenarios, CO2 emissions in 2050 vary from
about 7 to 47 Pg C/yr, with a new "Base Case" value of about 15 Pg C/yr. The principal force
contributing to the difference between the results of the two studies conducted by Edmonds and
Reilly is the higher coal price applied in the second study.
A number of other studies have used the Edmonds-Reilly (ER) model to project future energy
use and CO2 emissions. The most important of these were studies conducted by the U.S.
Environmental Protection Agency (Seidel and Keyes, 1983) and Rose et al. (1983). The EPA study
used the ER model to generate 13 scenarios for the period 1975-2100, which were used as a basis
for investigating whether actions taken now to reduce fossil-fuel consumption could significantly delay
a future global warming. Six baseline and seven policy-driven scenarios were investigated in this
study. The scenarios generated in the EPA study projected CO2 emissions in 2050 at levels of 10-
18 Pg C/yr. The authors concluded from these scenarios that the timing of a 2°C warming is not
very sensitive to the effects of the energy policies they tested.
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Rose and his colleagues at the Massachusetts Institute of Technology (MIT) also used the ER
model to study the effect of various energy policy options on the timing and extent of future CO2
emissions (Rose et al., 1983). Eleven scenarios were investigated, covering the period from 1975 to
2050 and incorporating a much wider range of assumptions and policies than those tested in the EPA
study. Rose et al. studied the effects of increased energy efficiency, increased fossil-fuel prices, higher
nuclear energy supply costs and a moratorium on building nuclear plants, lower photovoltaic costs,
higher oil prices, and a cutoff of oil imports from the Middle East. The MIT study went beyond the
ER model results to provide detailed estimates of the materials required for construction and
operation of energy facilities in each scenario. In the MIT scenarios, emissions of CO2 in 2050
ranged from less than 3 to about 15 Pg C/yr. The most important new conclusion of Rose et al.
was that a feasible "option space exists in which the CO^climate problem is much ameliorated"
through energy policy choices and improvements in technology.
In 1983 the National Academy of Sciences completed a Congressionally-mandated study to
evaluate, among other things, the effects of fossil-fuel development activities authorized by the Energy
Security Act of 1980 (NAS, 1983). One of the chapters in this study, authored by energy economists
Nordhaus and Yohe, used a compact model of global economic growth and energy use to analyze
CO2 emissions between 1975 and 2100 (Nordhaus and Yohe, 1983). Unlike the partial equilibrium
approach employed in the ER model, the Nordhaus and Yohe (NY) model used a generalized Cobb-
Douglas production function to estimate future energy demand. In this approach global GNP is
estimated as a function of assumptions about average rates of change in labor productivity,
population, and energy consumption. Demand for energy is separated into two categories, fossil and
non-fossil. Projections of CO2 emissions (based on the weighted average release rate from fossil
fuels) were used as inputs to a simple airborne fraction model of the carbon cycle.
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The NY analysis used an approach called "probabilistic scenario analysis," to evaluate the effects
on CO2 emissions of alternative assumptions used in the model. The results of 1000 cases were
examined. The CO2 emissions trajectories in these cases were presented as percentiles in the overall
distribution among the 1000 scenarios. Using this approach to uncertainty analysis, Nordhaus and
Yohe concluded that the 50th percentile for carbon emissions in 2050 was approximately 15 Pg C/yr.
The 95th percentile case suggested that emissions hi 2050 would likely be less than 26 Pg C/yr, while
the 5th percentile case indicated that emissions would likely be greater than 5 Pg C/yr.
The probabilistic approach was subsequently applied to the more detailed ER model using Monte
Carlo analysis (Edmonds et al., 1986; Reilly et al., 1987). The results of this analysis suggest a larger
total range of uncertainty and a substantially lower median emissions estimate compared with the
Nordhaus and Yohe (1983) results. When the likely correlations between model parameters are
taken into account Edmonds et al. obtain emissions of 7.7 Pg C/yr in 2050 for the 50th percentile
case with 5th and 95th percentile bounds of 2.3 and 58.1 Pg C/yr, respectively. Note that the median
result is about half of the Base Case scenario obtained in earlier analysis by Edmonds and Reilly
(1984).
In 1984 Legasov et al. published one of a continuing series of Soviet analyses of future global
energy use and its environmental implications. Legasov et al. analyzed two scenarios in which energy
demand reaches 6 and 20 kilowatts per capita by the end of the next century. Annual per capita
energy consumption is treated as a logistic function, approaching these levels asymptotically in 2100.
Assuming a global population of 10 billion persons, the minimal variant implies a global energy
demand of 60 terawatts (TW), about six times the current level by 2100.2 CO2 emissions in this
scenario follow a bell-shaped trajectory, peaking at about 133 Pg C/yr in 2050.
2 1 terawatt = 1012 watts = 31.5xl018 joules per year = 31.5 exajoules (EJ) per year = 29.9
Quadrillion British Thermal Units (Quads) per year.
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Goldemberg and his colleagues have used a completely different approach to projecting future
energy use and its consequences for CO2 emissions (Goldemberg et al., 1985, 1987, 1988). The
Goldemberg et al. analysis is based on an end-use oriented approach to evaluating the demand for
energy services, rather than the availability of energy supply. Based on detailed studies of energy
demand in four countries (U.S., Sweden, India, and Brazil), Goldemberg and his colleagues developed
a scenario of future energy requirements in both industrialized and developing countries. Although
the study does not represent a forecast of future energy demand, it provides an "existence proof,"
demonstrating the feasibility of a world economy that continues to grow while consuming much less
energy than it would if historical trends continue.
Emphasizing the potential to improve the efficiency of energy supply and use, per capita energy
demand in the industrialized countries is cut by 50% in the Goldemberg et al. scenarios. During the
same 40-year period, per capita demand for energy in the developing countries grows by about 10%,
with commercial fuels displacing traditional biomass fuels at a rapid and increasing rate. Global
energy demand remains essentially constant in the base case with CO2 emissions in 2020 of 5.9 Pg
C/yr, only about 5% higher than today's level.
A limitation of the Goldemberg et al. studies is that the impact of market imperfections and the
rate of capital stock turnover are not fully addressed. Nonetheless, these studies, along with the Rose
et al. analysis, demonstrate that economic growth can be decoupled from increases in CO2 emissions.
Experience over the last 15 years in the U.S., Western Europe, and Japan suggests that this
conclusion is correct.
A study by Keepin et al. (1986) reviewed and re-evaluated the range of previous energy and CO2
projections, including those summarized here. It concluded that the feasible range for future energy
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in 2050 was somewhere between about 10 and 35 TW, with CO2 emissions between 2 and 20 Pg
C/yr.
Studies of the Combined Effects of Greenhouse Gas Buildup
In the last few years a number of analysts have investigated the combined effects on global
surface temperature of a buildup of CO2 and other trace gases. Preliminary analysis of the impact
of concentration increases during the 1970s was presented by Lacis et al. (1981) and estimates of
future impacts were included in Seidel and Keyes (1983). A seminal article by Ramanathan et al.
(1985) focused attention on the subject. This study used a one-dimensional radiative-convective model
to estimate the impact of a continuation of current trends in the buildup of more than two dozen
radiatively active trace gases between 1980 and 2030. Ramanathan and his colleagues calculated an
expected value for the equilibrium warming of about 1.5°C over this period, with a little less than
hah0 of that amount due to the buildup of CO2 alone. (The Ramanathan et al. analysis included the
effects of water-vapor feedback, but not the other known feedback mechanisms; see Chapter III.)
The most important conclusion of the analysis by Ramanathan et al. is that, if current trends continue
and uncertainties in the future emissions projections are accounted for, the warming effects of the
non-CO2 trace gases will amplify the warming due to the buildup of CO2 alone by a factor of
between 1.5 and 3.
In 1986, Dickinson and Cicerone extended the work of Ramanathan et al. to evaluate a range
of trace-gas scenarios covering the period from 1985 to 2050. Using the radiative-convective model
developed by Ramanathan et al., and considering a range of emissions growth rates for the most
important greenhouse gases, Dickinson and Cicerone (1986) estimated that equilibrium global average
surface temperatures would rise at least 1°C and possibly more than 5°C by 2050, when the full range
of atmospheric feedback processes was considered.
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Each of the analyses described above was based on the assumption that historical trends in the
growth of greenhouse gas emissions continue for the next 40-50 years. Mintzer (1987) has developed
a model to consider the alternative: that policy and investment choices made in the next several
decades will substantially alter the growth rates of future emissions. Mintzer's analysis uses a
composite tool called the Model of Wanning Commitment to link future rates of economic growth
to the increasing atmospheric concentrations of carbon dioxide, nitrous oxide, chlorofluorocarbons,
methane, and tropospheric ozone. The results are reported as the date of atmospheric commitment
to a warming equivalent to doubling preindustrial CO2 concentrations and as the magnitude of
warming commitment in 2075.
Mintzer's initial analysis considered four policy-driven global scenarios, including a Base Case
representing a continuation of current trends. All four scenarios support a global population of about
10 billion people and the same levels of regional economic growth. Most recent analyses, including
the ones cited above and Mintzer's Base Case, indicate that a continuation of current trends would
lead to a warming commitment equivalent to doubling the preindustrial concentration of CO2 by
about 2030. In Mintzer's Base Case, by 2075, the planet is committed to an eventual warming of
about 3-9°C. Alternatively, in the High Emissions case, policies that increase coal use, spur
deforestation, extend the use of the most dangerous CFCs and limit improvements in energy
efficiency, will accelerate the onset of the "doubled CO2 equivalent" atmosphere to about 2010 and
commit the planet to a warming of about 5-15°C in 2075. By contrast, in Mintzer's Slow Buildup
scenario, a warming associated with the doubled CO2 equivalent atmosphere is postponed beyond
the end of the simulation period in 2075. In the Slow Buildup scenario this level of risk reduction
is achieved by aggressively pursuing policies to increase energy efficiency, limit tropical deforestation,
reduce the use of the most dangerous CFCs, and shift the fuel mix from carbon-intensive fuels like
coal to hydrogen-intensive fuels like natural gas, and ultimately, to energy sources that emit no CO2.
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More recently, Rotmans et al. (1988) have used a framework similar to the Model of Warming
Commitment to develop scenarios of greenhouse warming based on alternative policy assumptions.
Also, Rotmans and Eggink (1988) have analyzed the role of methane in greenhouse wanning.
Major Uncertainties
Major uncertainties underlie many aspects of our understanding of the climate change problem.
These uncertainties encompass our understanding of the geophysical processes underlying the
sensitivity of the climate to perturbations, that is, the processes that control how fast greenhouse
gases flow into and out of the atmosphere and biosphere, including feedback processes that may
affect future concentrations of greenhouse gases, and socio-economic uncertainties that are inherent
in any energy/economic model used to forecast long-term emissions. The physical uncertainties
include uptake of heat and CO2 by the ocean and any other sinks, geophysical and biogeochemical
feedback mechanisms, and natural rates of emission of the greenhouse gases. The social and
economic uncertainties include population growth, GNP growth, structural changes in economic
systems, rates of technological change, future reliance on fossil fuels, and future compliance with the
Montreal Protocol. Future rates of greenhouse gas emissions cannot be predicted with certainty.
Future emissions rates will be determined by the emerging pattern of human industrial and
agricultural activities as well as by the effects of feedback processes in the Earth's biogeophysical
system whose details are not well understood at the present time.
All existing climate models encompass large uncertainties that limit the accuracy of the models
and the level of geographic detail that can be considered. Even the best General Circulation Models
(GCMs) are limited by the assumptions necessarily made about the influence of clouds, vegetation,
ice and snow, soil moisture, and terrain, all of which affect the energy balance of the Earth's surface.
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Two of the largest uncertainties involve our limited understanding of the role that clouds, and ocean
uptake and transport of beat, play in the climate system.
Conclusions From Previous Studies
Despite the significant uncertainties that underlie our understanding of climate change, several
important conclusions emerge from the existing literature. First, emissions of a number of other
trace gases will amplify the future warming effect of any further buildup in the atmospheric
concentration of CO2. Second, it is too late to prevent all future global warming. Trace gases
released over the last century have already committed the planet to an ultimate warming (of up to
2°C) that may be greater than any other in the period of written human history. Finally, policy
choices and investment decisions made during the next decade that are designed to increase the
efficiency of energy use and shift the fuel mix away from fossil fuels could slow the rate of buildup
sufficiently to avoid the most catastrophic potential impacts of rapid climate change. Alternatively,
decisions to rapidly expand the use of coal, extend the use of the most dangerous CFCs, and rapidly
destroy the remaining tropical forests could "push up the calendar," accelerating the onset of a
dangerous global warming.
The rate at which climate may change must be of particular concern to policy makers. The
temperature increases resulting from doubling the concentration of CO2 that are predicted by most
GCMs are comparable to the increase that has occurred since the last ice age. The difference is that
the period of tune within which this increase could happen is much shorter. Atmospheric scientists
predict that within approximately 100 years we could experience temperature increases equivalent to
those that have "occurred over the past 18,000 years (about 5°C; see Chapter III). It is not clear that
our ecosystems and economic systems will be able to adjust to such a rapid change in global mean
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temperatures. Increases in world population, coupled with limited environmental and agricultural
resources, increase the vulnerability of social systems to climatic change.
The potential impacts of climatic change are highly uncertain and are beyond the scope of this
report. They are addressed hi the companion volume, Tlie Potential Effects of Global Climate Change
on the United States (Smith and Tirpak, 1989). The findings of this study collectively suggest that the
climatic changes associated with a global warming of roughly 2-4°C would result in "a world that is
different from the world that exists today. Global climate change will have significant implications
for natural ecosystems; for when, where, and how we farm; for the availability of water to drink and
water to run our factories; for how we live in our cities; for the wetlands that spawn our fish; for the
beaches we use for recreation; and for all levels of government and industry." Although sensitivities
were identified in this report, detailed regional predictions of climate change cannot be made at this
time. Thus potential responses to the greenhouse gas buildup must be viewed hi the context of risk
management, or buying insurance.
A second major concern is that the greenhouse gases have very long lifetimes once they are
introduced into the atmosphere. Although there is a substantial lag between the time when a
greenhouse gas is introduced into the atmosphere and when its full impact on climate is realized,
once the gases are in the atmosphere they will remain there for a long time. The longer the delay
before mitigating action is taken, the larger will be the commitment to further global warming.
Policy makers must determine how best to minimize the costs of global warming to the peoples
of the world and the damage to ecosystems. But global warming is a complex problem for which
there is no single, simple solution. No single policy initiative will completely mitigate man-made
climate change. The sources, sectors, and countries contributing to the emissions of greenhouse gases
are numerous (Chapter IV).
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Compounding the difficulty of identifying solutions to the greenhouse problem is that the
greenhouse gases do not all have the same forcing effect on global temperatures. In fact, CO2 is the
least effective absorbent of infrared radiation of all of the greenhouse gases per additional molecule
added to the atmosphere. Because the combined effect of the other greenhouse gases is comparable
to the effect of CO2, mitigatory policies cannot be directed solely at reducing CO2 emissions. The
sources of methane, CFCs, nitrous oxide, and other gases must therefore be carefully considered.
As we explore the options for limiting greenhouse gas emissions in this report, it is important
to remember two salient points: (1) Global warming is an international problem whose solution will
require extensive cooperation between both industrialized and developing countries; and (2) No single
economic sector can be held entirely responsible for the greenhouse effect. In focusing on strategies
to stabilize climate in this Report, we recognize that the optimal mix of adaptation and prevention
is uncertain. The Earth is already committed to some degree of climatic change, so adaptation to
some level of change is essential. On the other hand, the highest rates of potential change may be
considered unacceptable, requiring some degree of prevention. Stabilizing strategies would require
global cooperation of an unprecedented nature and could be costly for some countries. The activities
responsible for greenhouse gas emissions are economically valuable, the distribution of emissions is
large, and the responsible countries reflect diverse economies and a variety of interests. Adaptation
strategies, on the other hand, can be adopted unilaterally. They may also be less burdensome
because the costs will be spread out into the future when countries may be better able to afford
them. Imposing climatic change on our grandchildren, however, raises serious concerns regarding
intergenerational equity.
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CURRENT NATIONAL AND INTERNATIONAL ACTIVITIES
Subsequent to the Congressional request to produce this report and the companion document on
potential effects of climate change there have been a wide variety of new domestic and international
initiatives related to climatic change.
National Research and Policy Activities
The Global Climate Protection Act of 1987 requires that:
The President, through the Environmental Protection Agency, shall be
responsible for developing and proposing to Congress a coordinated national
policy on global climate change.
This Act is a very broad mandate that will require close cooperation between EPA and other
agencies (including NASA, NOAA, the Corps of Engineers, and the Departments of Energy,
Agriculture, and the Interior, the National Climate Program Office, and the Domestic Policy
Council).
The Global Climate Protection Act also requires that the Secretary of State and the EPA
Administrator jointly submit, by the end of 1989, a report analyzing current international scientific
understanding of the greenhouse effect, assessing U.S. efforts to gain international cooperation in
limiting global climate change, and describing the U.S. strategy for seeking further international
cooperation to limit global climate change. This report, along with those being developed by other
Federal agencies, will provide a foundation upon which a national policy can be formulated.
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International Activities
The greenhouse gas problem is an international issue. In order to respond effectively to this
problem, the nations of the world must act in concert. Several international organizations have
recognized the need for multilateral cooperation and have become involved with the global climate
change issue. The United Nations Environment Programme (UNEP) is responsible for conducting
climate impact assessments. The World Meteorological Organization (WMO) is supporting research
on and monitoring of atmospheric and physical sciences. The International Council of Scientific
Unions (ICSU) is developing an international geosphere-biosphere program.
The U.S. Government is supporting the Intergovernmental Panel on Climate Change (IPCC)
established under the auspices of UNEP and WMO. The IPCC, which held its first meeting in
November 1988, will help ensure an orderly international effort in responding to the threat of global
climate change. At its first meeting the IPCC established three working groups: the first, to assess
the state of scientific knowledge on the issue, will be chaired by the United Kingdom; the second,
to assess the potential social and economic effects from a warming, will be chaired by the Soviet
Union; and the third, to examine possible response strategies, including options for limiting emissions
and adapting to change, will be chaired by the United States.
In addition, several countries have held or plan to hold international conferences on global
climate change. These include Canada, The Federal Republic of Germany, Italy, Japan, India, Egypt,
and the Netherlands. The Netherlands and the Federal Republic of Germany (through the Enquete
Commission) are undertaking analyses of policy options.
The efforts of all of these organizations may be hindered if some countries perceive themselves
as winners instead of losers, as a result of climate change. For example, both Canada and the Soviet
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Union, which have vast land areas that are currently largely unusable because of their severe
climates, could benefit from increased agricultural productivity as those areas become warmer. But
the notion that a global warming might be beneficial to some may prove fallacious. Two particular
problems might limit anticipated benefits. First, if the shift in climatological zones happens too
quickly, ecosystems may not be able to keep up and may be severely disrupted. Although the
Canadian climate may become more conducive to certain types of forests, if the forests can't migrate
fast enough and therefore die back as the climatological zones shift northward, benefits to the
Canadians will be reduced. Second, although shifts towards more favorable climatic conditions may
be a necessary condition for increased agricultural productivity, a warmer climate in itself may not
be sufficient. For example, the northern areas of Canada might not have the proper soil composition
for high agricultural yields. The conclusion must therefore be drawn that it is difficult to predict
what the net costs and benefits of climate change will be for any one country.
The global warming issue is an international concern. In order to develop a responsible
program, the U.S. government must consider the feasibility of achieving both domestic and
international acceptance and implementation of policy initiatives. Otherwise, the effectiveness of
programs instituted by any one country could be compromised by the lack of participation by other
countries. International collaboration must be pursued.
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Komhyr, W.D., R.H. Gammon, T.B. Harris, L.S. Waterman, TJ. Conway, W.R. Taylor, and K.W.
Thoning. 1985. Global atmospheric CO2 distribution and variations from 1968-1982 NOAA/GMCC
CO2 flask sample data. Journal of Geophysical Research 90:5567-55%.
Lacis, A., J. Hansen, P. Lee, T. Mitchell, and S. Lebedeff. 1981. Greenhouse effect of trace gases,
1970-1980. Geophysical Research Letters 8:1035-1038.
Lashof, D. 1988. The Dynamic Greenhouse: Feedback Processes That May Influence Future
Concentrations of Atmospheric Trace Gases. U.S. Environmental Protection Agency, Washington, D.C.
Legasov, VA., I.I. Kuzmin, and A.I. Chernoplyokov. 1984. The influence of energetics on climate.
Fizika Atmospheri i Okeana 11:1089-1103. USSR Academy of Sciences.
MacCracken, M. C, and F. M. Luther, eds. 1985a. Projecting the Climatic Effects of Increasing Carbon
Dioxide. U.S. Department of Energy, Washington, D.C.
MacCraken, M.C. and F.M. Luther, eds. 1985b. Detecting the Effects of Increasing Carbon Dioxide.
U.S. Department of Energy, Washington, D.C.
Mintzer, I.M. 1987. ,4 Matter of Degrees: The Potential for Controlling the Greenhouse Effect. World
Resources Institute, Washington, D.C.
NAS (National Academy of Sciences). 1979. Carbon Dioxide and Climate: A Scientific Assessment.
National Academy Press, Washington, D.C.
NAS (National Academy of Sciences). 1983. Changing Climate. National Academy Press, Washington,
D.C.
NAS (National Academy of Sciences). 1987. Current Issues in Atmospheric Change. National Academy
Press, Washington, D.C.
NOAA (National Oceanographic and Atmospheric Administration). 1987. Geophysical Monitoring for
Climatic Oiange No. IS, Summary Report 1986. Schnell, R.C., ed. U.S. Department of Commerce,
NOAA Environmental Research Laboratories, Boulder. 155 pp.
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Nordhaus, W.D., and G. Yohe. 1983. Future paths of energy and carbon dioxide emissions. In
Changing Climate. National Academy Press, Washington, D.C.
Ramanathan, V., R. J. Cicerone, H. B. Singh, and J. T. Kiehl. 1985. Trace gas trends and their
potential role in climate change. Journal of Geophysical Research 90:5557-5566.
Reilly, J., J. Edmonds, R. Gardner, and A. Brenkeri. 1987. Uncertainty analysis of the IEA/ORAU
CO2 emissions model. Tlie Energy Journal 8:1-29.
Rose, DJ., M.M. Miller, and C. Agnew. 1983. Global Energy Futures and CO2-Induced Climate
Change. MITEL 83-015. Prepared for National Science Foundation. Massachusetts Institute of
Technology, Cambridge.
Rotmans, J., H. de Boois, and RJ. Swart. 1988. An integrated model for the assessment of the
greenhouse effect: The Dutch approach. Working paper. National Institute of Public Health and
Environmental Protection, Bilthoven, The Netherlands.
Rotmans, J., and E. Eggink. 1988. Methane as a greenhouse gas: A simulation model of the
atmospheric chemistry of the CH4 - CO - OH cycle. Working paper. National Institute of Public
Health and Environmental protection, Bilthoven, The Netherlands.
Rotty, R.M. 1987. A look at 1983 CO2 emissions from fossil fuels (with preliminary data for 1984).
Tellus 396:203-208.
Seidel, S., and D. Keyes. 1983. Can We Delay a Greenhouse Warming? Office of Policy and
Resources Management, U.S. Environmental Protection Agency, Washington, D.C.
Siegenthaler, U. 1983. Uptake of excess CO2 by an outcrop-diffusion model of the ocean. Journal
of Geophysical Research 88:3599-3608.
Smith, J. and D. Tirpak, eds. 1989. Tlie Potential Effects of Global Climate Change on the United
States. U.S. Environmental Protection Agency, Washington, D.C.
Strain, B.R. and J.D. Cure, eds. 1985. Direct Effects of Increasing Carbon Dioxide on Vegetation. U.S.
Department of Energy, Washington, D.C.
Trabalka, J.R., ed. 1985. Atmospheric Carbon Dioxide and the Global Carbon Cycle. U.S. Department
of Energy, Washington, D.C.
WMO (World Meteorological Organization). 1985. Atmospheric Ozone 1985: Assessment of Our
Understanding of the Processes Controlling its Present Distribution and Change. Volume 1. WMO,
Geneva. 392+ pp.
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CHAPTER II
GREENHOUSE GAS TRENDS
FINDINGS 11-2
INTRODUCTION II-5
CARBON DIOXIDE II-7
Concentration History and Geographic Distribution II-7
Mauna Loa II-8
Ice-core Data II-9
GMCC Network 11-10
Sources and Sinks 11-14
Fossil Carbon Dioxide H-14
Biospheric Cycle 11-16
Ocean Uptake 11-17
Chemical and Radiative Properties/Interactions 11-18
METHANE 11-22
Concentration History and Geographic Distribution 11-22
Sources and Sinks 11-24
Chemical and Radiative Properties/Interactions 11-29
NITROUS OXIDE 11-30
Concentration History and Geographic Distribution 11-30
Sources and Sinks II-32
Chemical and Radiative Properties/Interactions 11-35
CHLOROFLUOROCARBONS (CFCs) II-36
Concentration History and Geographic Distribution 11-36
Sources and Sinks 11-37
Chemical and Radiative Properties/Interactions 11-39
OZONE 11-40
Concentration History and Geographic Distribution 11-40
Sources and Sinks 11-43
Chemical and Radiative Properties/Interactions 11-44
OTHER FACTORS AFFECTING COMPOSITION 11-45
Global Tropospheric Chemistry 11-46
Carbon Monoxide 11-47
Nitrogen Oxides 11-48
Stratospheric Ozone and Circulation 11-49
CONCLUSION 11-50
REFERENCES 11-59
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FINDINGS
The composition of the atmosphere is changing as the result of human activities. Increases in
the concentration of carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons (CFCs) are
well documented. In addition, tropospheric (lower atmospheric) and stratospheric (upper
atmospheric) chemistry are being modified due to the addition of these gases as well as emissions
of carbon monoxide, nitrogen oxides, and other compounds. Specifically, we find that:
• The concentration of carbon dioxide in the atmosphere has increased by 25% since the
industrial revolution. Detailed measurements since 1958 show an increase of about 35
parts per million by volume. Both land clearing and fossil fuel combustion have
contributed to this rise, but the fossil fuel source has dominated in recent years. Carbon
dioxide is increasing at a rate of about 0.4% per year and is responsible for about half
of the current increases in the greenhouse effect. Carbon cycle models indicate that the
oceans are responsible for the uptake of most of the fossil fuel CO2 that does not remain
in the atmosphere. The total net uptake of CO2 by the oceans and the net
uptake/release of CO2 by the terrestrial biosphere cannot be precisely determined at this
time.
• The concentration of methane has more than doubled during the last three centuries.
There is considerable uncertainty about the total emissions from specific sources of
methane, but the observed increase is probably due to increases in a number of sources
as well as changes in tropospheric chemistry. Agricultural sources, particularly rice
cultivation and animal husbandry, have probably been the most significant contributor
to historical increases in concentrations. But there is the potential for rapid growth in
emissions from landfills, coal seams, permafrost, natural gas exploration and pipeline
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leakage, and biomass burning associated with forest clearings in the future. Methane is
increasing at a rate of 1% per year and is responsible for about 20% of the current
increases in the greenhouse effect.
• The concentration of nitrous oxide has increased by 5-10% since preindustrial times.
The cause of this increase is highly uncertain, but it appears that the use of nitrogenous
fertilizer, land clearing, biomass burning, and fossil fuel combustion have all contributed.
Nitrous oxide is over 200 times more powerful, on a per molecule basis, than carbon
dioxide as a greenhouse gas, and can also contribute to stratospheric ozone depletion.
Nitrous oxide is currently increasing at a rate of about 0.25% per year, which represents
an imbalance between sources and sinks of about 30%. Nitrous oxide is responsible for
about 6% of the current increases in the greenhouse effect.
• CFCs were introduced into the atmosphere for the first time during this century; the
most common species are CFC-12 and CFC-11 which had atmospheric concentrations
in 1985 of 380 and 220 parts per trillion by volume, respectively. While these
concentrations are tiny compared with that of carbon dioxide, these compounds are
about 30,000 times more powerful, on a per molecule basis, than carbon dioxide as a
greenhouse gas and are increasing very rapidly ~ 5% per year from 1978 to 1983. Of
major concern because of their potential to deplete stratospheric ozone, the CFCs also
represent about 15% of the current increases in the greenhouse effect.
• The chemistry of the atmosphere is changing due to emissions of carbon monoxide,
nitrogen oxides, and volatile organic compounds, among other species, in addition to the
changes in the greenhouse gases just described. This alters the amount and distribution
of ozone and the oxidizing power of the atmosphere, which changes the lifetimes of
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methane and other greenhouse gases. Changes in global ozone are quite uncertain, and
may have contributed to an increase or decrease in the warming commitment during the
last decade.
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INTRODUCTION
The composition of the Earth's atmosphere is changing. Detailed background atmospheric
concentration measurements combined with analyses of ancient air trapped in Antarctic and
Greenland ice now give a compelling picture, not only of recent trends, but also of major changes
that have occurred since preindustrial times. Mounting evidence that the atmosphere is changing has
increased the urgency of understanding the processes that control atmospheric composition and the
significance of the changes that are taking place. In this chapter we examine what is known and not
known about the gases expected to be most important in altering climate during the coming decades.
For each gas, we present data regarding its concentration history and geographic distribution, its
sources and sinks, and its chemical and radiative interactions in the atmosphere. This information
is summarized at the end of the Chapter in Table 2-2.
The concentrations of a number of greenhouse gases have already increased substantially over
preindustrial levels. The estimated relative radiative forcing from the major gases (excluding water
vapor and clouds) is illustrated in Figure 2-1 for the period 1880-1980 and for the expected
concentration changes during the 1980s. Carbon dioxide accounted for about two-thirds of the total
forcing over the last century, but its relative importance has declined to about half the total in recent
years due to more rapid growth in other gases during the last few decades (see Chapter IV).
Particularly important has been the recent growth in chlorofluorocarbon (CFC) concentrations.
Methane (CH4) has remained the second most important greenhouse gas, responsible for 15-20% of
the forcing. With the recent signing of the Montreal Protocol on Substances that Deplete the Ozone
Layer, growth in CFC concentrations is likely to be substantially restrained compared with what has
been assumed until recently (e.g., Ramanathan et al., 1985; see Chapters IV and V). The relative
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Chapter II
FIGURE 2-1
GREENHOUSE GAS CONTRIBUTIONS TO GLOBAL WARMING
Other (8%)
CO2(66%)
Other (13V.)
CO2 (49%)
CFC-11 &-12
(14%)
CH4
(15%)
CH4(18%)
1860-1980
1980s
Figure 2-1. Based on estimates of the increase in concentration of each gas during the specified
period. The "Other" category includes other halons, tropospheric ozone, and stratospheric water
vapor. The contribution to warming of the "Other" category is highly uncertain. (Sources: 1880-
1980: Ramanathan et al.( 1985; 1980s: Hansen et al., 1988.)
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importance of CO2 is therefore likely to increase again in the future unless these emissions are also
restricted (Chapter V).
The radiative impact of greenhouse gases is characterized here hi terms of the effect of
concentration changes on surface temperatures in the absence of climate feedbacks. Climate
feedbacks are defined and discussed in Chapter III, where the climatic effects of changes in
greenhouse gases are put into the broader context of other factors that influence climate. The
human activities that are apparently responsible for the concentration trends documented in this
chapter are described in Chapter IV.
CARBON DIOXIDE
Concentration History and Geographic Distribution
Carbon dioxide (CO^ is the most abundant and single most important greenhouse gas (other
than water vapor) in the atmosphere. Its role in the radiative balance, and its potential for altering
the climate of the Earth have been recognized for over a hundred years. Chemical measurements
of atmospheric CO2 were made in the 19th Century at a few locations (Fraser et al., 1986a; From
and Keeling, 1986). However, the modern high-precision record of CO2 in the atmosphere did not
begin until 1958, the International Geophysical Year (IGY), when C.D. Keeling of Scripps Institution
of Oceanography pioneered measurements of CO2 using an infrared gas analyzer at Mauna Loa
Observatory (MLO) in Hawaii and at the South Pole. Since 1974, background measurements of
atmospheric CO2 have been made continuously at four stations (Pt. Barrow, Alaska; Mauna Loa,
Hawaii; American Samoa; and the South Pole) as part of the Geophysical Monitoring for Climatic
Change (GMCC) program of the National Oceanic and Atmospheric Administration (NOAA) of the
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U.S. Department of Commerce. In addition to the continuous monitoring stations, NOAA/GMCC
also operates a cooperative sampling network. Flask samples of air are collected weekly from these
sites and are shipped to the GMCC facility in Boulder, Colorado, for analysis. The sampling network
began before 1970 at a few initial sites, expanded to a network of 15 stations in 1979, and, as of
1986, consisted of -26 stations (Komhyr et al., 1985; Conway et al., 1988). In addition to the U.S.
programs, surface measurements of atmospheric CO2 around the globe are made by many countries
including Australia, Canada, France, Italy, Japan, New Zealand, West Germany, and Switzerland.
Mauna Loa
The MLO CO2 record is shown in Figure 1-2 in Chapter I. CO2 steadily increased from 315
parts per million by volume (ppm) in 1958 to 346 ppm in 1986. This corresponds to an increase
at the rate of 0.4% per year, or a mean increase of 1.5 ± 0.2 ppm per year. From 1958 to 1986,
CO2 at Mauna Loa increased by 31 ppm; over the same period, fossil fuel combustion (shown also
in Figure 1-2) was a source of 117 petagrams (Pg)1 of carbon (C) as CO2 to the atmosphere, which
is equivalent to 56 ppm of CO2. The apparent fraction of the fossil fuel sources of CO2 that
remained in the atmosphere during this period is thus 55%. As other net sources of CO&
particularly deforestation (see below), may have been important during this period, the actual fraction
of anthropogenic carbon emissions remaining in the atmosphere is uncertain. Superimposed on the
increasing secular trend of atmospheric CO2 are regular seasonal oscillations: the concentration
peaks in May/June, decreases steadily through the summer, and reaches a minimum in
September/October. The seasonal peak-to-trough amplitude is ~5.8 ppm. The seasonal cycle of
CO2 at Mauna Loa and at other northern hemispheric locations is caused primarily by the natural
dynamics of the terrestrial biosphere: There is net removal of CO2 from the atmosphere via
1 peta = 1015' giga = 109, 1 ton = 106 grams. Thus, 1 petagram (Pg) = 1 gigaton (Gt).
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photosynthesis during the growing season, and net return of CO2 to the atmosphere via respiration
and decomposition processes during the rest of the year.
Despite its regular appearance, there are interannual variations in the CO2 concentration
measured at MLO. Annual mean concentration changes do not remain uniform throughout the
duration of the record, but have large fluctuations around the mean (Keeling, 1983). These
excursions of atmospheric CO2 from the mean generally occur during El Nino-Southern Oscillation
events, where the large-scale perturbations of atmospheric temperature, precipitation, and other
circulation statistics also alter the biological, chemical, and physical aspects of carbon cycling between
the atmosphere, land, and ocean reservoirs. These El Nino excursions highlight the possibility of
climatic feedbacks in the carbon cycle. They do not mask the increasing secular trend, which mainly
tracks the trend in fossil fuel combustion.
The seasonal amplitude also does not remain constant and has a ±10% variation about the
mean. Recent analysis reveals a statistically significant positive trend in the seasonal amplitude since
1976 (Bacastow et al, 1985; Enting, 1987). The causes of this amplitude trend have not been
unambiguously identified; hypotheses involve shifts in the seasonality of photosynthesis and respiration,
faster cycling of carbon as a result of climatic warming, and the direct effects of CO2 on plants (also
referred to as the CO2 fertilization effect).
Ice-core Data
Bubbles in natural ice contain samples of ancient air. Analysis by gas chromatography and laser
infrared spectroscopy of gases occluded in gas bubbles in polar ice has provided a unique
reconstruction of atmospheric CO2 history prior to the modern high-precision instrumental record
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(Oeschger and Stauffer, 1986). Deep ice cores have been drilled from many locations in both
Greenland and Antarctica.
From the ice-core data, it is deduced that in pre-industrial times, ~ 1800, the CO2 concentration
was 285 ± 10 ppm and has increased at an accelerating rate since the industrial era (Neftel et al.,
1985; Raynaud and Barnola, 1985; Pearman et al., 1986) (Figure 2-2). The ice-core data reveal the
possible existence of natural fluctuations of the order of ± 10 ppm occurring at decadal time scales
during the last few thousand years (Delmas et al., 1980; Neftel et al., 1982; Stauffer et al., 1985;
Raynaud and Barnola, 1985; Oeschger and Stauffer, 1986).
Recent analysis of the 2083-meter-deep ice core from Vostok, East Antarctica, provides for the
first tune information on CO2 variations in the last 160,000 years (Barnola et al., 1987; Figure 3-3
in Chapter III). Large CO2 changes were associated with the transitions between glacial and
interglacial conditions. CO2 concentrations were low (~200 ppm) during the two glaciations and high
(~285 ppm) during the two major warm periods. The Vostok ice-core data also emphasize that
current levels of atmospheric CO2 are higher than they have ever been in the past 160,000 years.
The CO2 increase since 1958 is larger than the natural CO2 fluctuations seen in the Greenland and
Antarctic ice-core record.
GMCC Network
The CO2 concentrations from the -26 globally distributed sites in the NOAA/GMCC cooperative
flask sampling network have been reviewed in Komhyr et al. (1985) and Conway et al. (1988). The
distribution for 1981-1985 is shown in Figure 2-3. There are large-scale, coherent, temporal and
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FIGURE 2-2
CARBON DIOXIDE CONCENTRATION
(Parts Per Million)
o-
\D
o-
CO
00
o-
111
Q.
H-
£C
2 g'
o
O
f--
CM
1720
1760
1800
1640
I860
i r^ i i
1920 1960
ZOOO
YEAR
Figure 2-2. The history of atmospheric CO2 presented here is based on ice core measurements
(open spaces, closed triangles) and atmospheric measurements (crosses). The data show that CO2
began to increase in the 1800s with the conversion of forests to agricultural land. The rapid rise
since the 1950s, due primarily to fossil fuel combustion, is at a rate unprecedented in the ice core
record, (Sources: Neftel et al, 1985; Friedli et al., 1986; Keeling, pers. communication; as cited in:
Siegenthaler and Oeschger, 1987).
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FIGURE 2-3
C02 ATMOSPHERIC CONCENTRATIONS BY LATITUDE
Figure 2-3. The distribution of CO2 by latitude from 1981-1985 shows that CO2 is increasing
globally. Superimposed on the increasing trend are coherent seasonal oscillations reflective of
seasonal dynamics of terrestrial vegetation. The seasonal cycle is strongest at high Northern latitudes,
and is weak and of opposite phase in the Southern Hemisphere, reflecting the distribution of
terrestrial vegetation. The data are from the NOAA/GMCC flask sampling network. (Sources:
Komhyr et al., 1985; NOAA, 1987; Conway et al., 1988.).
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spatial variations of CO2 in the atmosphere. Concentrations of CO2 at all the stations are increasing
at the rate of -1.5 ppm per year (ppm/yr), similar to the rate of increase at Mauna Loa.
Annually averaged CO2 concentrations are higher in the Northern Hemisphere than in the
Southern Hemisphere. The interhemispheric difference was ~1 ppm in the 1960s and is ~3.2 ppm
now, reflecting the Northern Hemisphere mid-latitude source (about 90%) of fossil fuel CO2. This
gradient has remained approximately constant in the past decade. Also evident in the north-south
distribution of atmospheric CO2 is the relative maximum of ~ 1 ppm in the equatorial regions, caused
mainly by the outgassing of CO2 from the super-saturated surface waters of the equatorial oceans.
Although tropical deforestation may also contribute to the equatorial maximum in atmospheric CO2)
models of the global carbon cycle suggest that the observations are inconsistent with a net
deforestation source greater than approximately 1.5 Pg C/yr (Pearman et al., 1983; Keeling and
Heimann, 1986; Tans et al., 1989).
There is a coherent seasonal cycle at all the observing stations: the Northern Hemisphere cycles
resemble that at Mauna Loa. The seasonal amplitude is largest, -16 ppm, at Pt. Barrow, Alaska,
and decreases toward the equator to ~6 ppm at Mauna Loa (Figure 2-3). The CO2 concentration
is flat through the year in the equatorial region and is of opposite seasonality in the Southern
Hemisphere, The seasonal cycle in the Northern Hemisphere is caused primarily by seasonal
exchanges with the terrestrial biosphere (Fung et al., 1987; Pearman and Hyson, 1986), while in the
Southern Hemisphere, oceanic and terrestrial exchanges are equally important in determining the
seasonal oscillations in the atmosphere (Pearman and Hyson, 1986'). The CO2 seasonal cycle shows
a consistent amplitude increase with time for some sites (Cleveland et al., 1983; Thompson et al.,
1986).
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The geographical variations of CO2 growth rates at the GMCC sites show more clearly the El
Nino perturbations, as noted already in the Manna Loa data. For example, the El Nino-caused
cessation of upwelling that resulted in the devastation of the fishing industry and marine wildlife in
the eastern equatorial Pacific is also evidenced by reduced outgassing of CO2 to the atmosphere
(Feely et al., 1987) and a concomitant decrease in the global CO2 growth rate (Conway et al., 1988).
These variations in the growth rate contain information about the response of the carbon system to
climatic perturbations, some of which are under investigation currently.
Sources and Sinks
The atmosphere exchanges CO2 with the terrestrial biosphere and with the oceans. Averaged
over decades, sources must approximately equal sinks if the system is to remain in quasi-steady state;
however, the individual flux in each direction may be large (50-100 Pg C/yr). The fluxes of carbon
to the atmosphere associated with anthropogenic activities are roughly ten times smaller than the
natural fluxes of carbon. However, the anthropogenic fluxes are unidirectional and are thus net
sources of carbon to the atmosphere (Figure 2-4).
Fossil Carbon Dioxide
•
The combustion of fossil fuels, in liquid, solid, or gas forms, is the major anthropogenic source
of CO2 to the atmosphere. A recent documentation and summary of the fossil fuel source of CO2
is given by Rotty (1987a, 1987b). In 1985, about 5.2 Pg C were released in the form of CO2 as a
result of fossil fuel combustion. Of this, the USA, USSR, and China contributed 23%, 19%, and
10%, respectively (Rotty, pers. communication). The emissions for 1987 were 5.5 Pg C. The history
and mix of activities and fuels giving rise to these emissions are discussed in detail in Chapter IV.
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Chapter II
FIGURE 2-4
(a)
THE CARBON CYCLE
ATMOSPHERE: 770
SEDIMENTS
orgonic C: 12,000,000
limestone: 50,000,000
FOSSIL FUEL:5000
BIOSPHERE:
living plants: 800
young soils: 1500
old soils: 1500
(b)
60
100
Figure 2-4. (a) Major reservoirs of the global carbon cycle. Reservoirs (or stocks) are in units of
10T5 grams of carbon (Pg C).
(b) Fluxes of carbon, in 1015 grams of carbon per year (Pg C/yr).
Source: Adapted from Keeling, 1983.
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Biospheric Cycle
The terrestrial biosphere absorbs CO2 from the atmosphere via photosynthesis on the order of
60 Pg C/yr. Approximately the same amount is returned to the atmosphere annually via
heterotrophic respiration and decomposition processes. While the net exchange of the unperturbed
biosphere is close to zero over a period of one year, the seasonal asynchronicity of the exchange
gives rise to the regular oscillations seen in the atmospheric CO2 records.
In general, land-use modification is a net source of CO2 to the atmosphere. CO2 is released
as a result of burning and decay of dead plant matter and oxidation of soil organic matter. The
amount of this release exceeds the amount of CO2 absorbed as a result of regrowth of live vegetation
and accumulation of soil organic matter. Recently, Houghton et al. (1987) and Detwiler and Hall
(1988) estimated a net source of 0.4-2.6 Pg C/yr to the atmosphere from land-use changes.
Deforestation in the tropics accounted for nearly all the flux: The release of carbon from temperate
and boreal regions was only 0.1 Pg C/yr. The regional and temporal patterns and causes of
deforestation are taken up in Chapter IV.
Natural changes in terrestrial biospheric dynamics may result from climate warming and/or from
increased CO2 concentrations in the atmosphere. The possibility of such natural changes is suggested
by the increasing amplitude of CO2 oscillations in the atmosphere (Bacastow et al., 1985; Cleveland
et al., 1983; Thompson et al. 1986; Enting, 1987). The amplitude change may signal a tendency
towards a biospheric sink of CO2, as photosynthesis responds to increasing temperatures and CO2
concentrations (Pearman and Hyson, 1981; D'Arrigo et al., 1987; Kohlmaier et al., 1987). The
amplitude change can also mean increased sources via respiration and decay, which are strongly
temperature-dependent processes (Houghton, 1987). Because growth and decay cycles are intimately
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linked, it is difficult to tell whether atmosphere-biosphere interactions will act as a positive or a
negative feedback without further theoretical and field studies (see the discussion of biogeochemical
feedbacks in Chapter III).
Ocean Uptake
The exchange of CO2 across the air-sea interface depends on the degree of CO2 supersaturation
in the surface waters of the oceans and the rate at which CO2 is transferred across the interface
itself. Because of the very nature of shipboard measurements, data on oceanic CO2 partial pressure
(pCO2) are sparse, both spatially and temporally. Most of the data have come from oceanographic
research programs, mainly Scripps Institution of Oceanography in the 1960s (Keeling, 1968),
Geochemical Sections (GEOSECS) in the 1970's (Takahashi et aL, 1980,1981), and Transient Tracers
in the Oceans (TTO) in the early 1980s (Brewer et al., 1986) and more recently from NOAA survey
cruises and from ships of opportunity.
Depending on the regional interplay between temperature, carbon supply from upwelling, and
carbon consumption by biological activities, the seasonal cycle of CO2 in surface water may peak at
different times of the year in different oceanic regions (Peng et al., 1987; Takahashi et al., 1986,
1988). This makes it extremely difficult to interpret the sparse oceanic carbon data in the context
of the global carbon cycle. The interpretation is aided by data from carbon-14 and other transient
tracers in the ocean.
Based on the available data and an understanding of carbon dynamics in the ocean, it is
estimated that, on an annual basis, about 90 Pg C/yr is exchanged between the atmosphere and the
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ocean. This exchange results in a net outgassing of approximately 1 Pg C/yr from the equatorial
oceans and a net absorption of about the same amount by the middle to high latitude oceans.
Superimposed on this exchange of 90 Pg C/yr in either direction is the penetration of fossil fuel
CO2 into the oceans, estimated to be ~3 Pg C/yr currently. This oceanic uptake of fossil fuel CO2
is corroborated by the observations of anthropogenic tracers penetrating gradually into the oceanic
thermocline. These tracers include tritium and carbon-14, by-products of nuclear testing in the 1960s,
and the chlorofluorocarbons, a recent man-made compound. The magnitude of the oceanic uptake
of fossil fuel CO2 is estimated using numerical models calibrated by tracers. Because of the
variability of the oceanic carbon system and the precision of ocean carbon measurements, the oceanic
signature of fossil fuel CO2 has not been demonstrated unambiguously. Takahashi et al. have
demonstrated that in the Atlantic, the oceanic pCO2 increased by 8 ± 8 microatmospheres (fiatm)
from 1958 to the mid-1970s, an increase that is consistent with estimates from numerical models.
Chemical and Radiative Properties/Interactions
Carbon dioxide is chemically inert in the atmosphere, but it has a very important impact on the
Earth's radiation budget and hence on climate and the chemistry of the atmosphere. After water
vapor, CO2 is the most abundant and most significant infrared (IR) absorbing gas in the atmosphere.
As discussed in Chapter III, the Earth's climate is determined by the point at which incoming solar
(short-wave) radiation is balanced by IR (long-wave) emissions to space from the warm surface and
atmosphere. Increasing the concentration of CO2 and other greenhouse gases in the atmosphere
elevates the average surface temperature required to achieve this balance. Doubling the atmospheric
CO2 concentration from 315 to 630 ppm would produce a radiative forcing (the equilibrium surface
temperature increase in the absence of climate feedbacks) of 1.2-1.3°C. At current concentrations
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THE RADIATIVE EFFECTS OF GREENHOUSE GASES
The radiative effects of greenhouse gases have received a great deal of attention over
the last decade. Recent reviews are given by Dickinson and Cicerone (1986) and
Ramanathan et al. (1987). In the absence of an atmosphere the Earth would radiate energy
to space as a black body with a temperature of about 250° K (-23°C). Figure 2-5 shows
the actual emissions, indicating the absorption bands of the major greenhouse gases. Not
shown is water vapor, which has continuous absorption throughout this spectral range and
dominates all other gases at wavelengths < 8 micrometers Qim) and > 18 /im (Dickinson
and Cicerone, 1986). The 15 jUm band of CO2 dominates absorption in the spectral range
from 12 to 18 Mm> and its absorption in the other parts of the spectrum amounts to 15% or
less of its impact in this region.
The shaded region in Figure 2-5, between about 7 and 13 Jim, is called the atmospheric
window because it is relatively transparent to outgoing radiation: 70-90% of the radiation
emitted by the surface and clouds in these wavelengths escapes to space (Ramanathan et al.,
1987), Many trace gases happen to have absorption bands in this window region and are
therefore very effective greenhouse absorbers. For example, CFCs are as much as 20,000
tunes more effective than CO2 per incremental increase in concentration (see Table 2-1).
CO2 already absorbs most of the radiation emitted from the Earth's surface in the wavelengths where
it is active. As a result, each additional molecule of CO2 added to the atmosphere has a smaller
effect than the previous one. Hence, radiative forcing scales logarithmically, rather than Linearly, with
increases in the concentration of atmospheric CO2. For example, a 50 ppm increase in CO2 from
350 to 400 ppm yields a radiative forcing of 0.23°C, while the same increment from 550 to 600 ppm
yields a radiative forcing of only 0.16°C. Despite the reduced greenhouse effectiveness of each
molecule of CO2 as concentrations increase, CO2 will remain the dominant greenhouse gas in the
future, responsible for 50% or more of the increased greenhouse effect during the next century for
plausible scenarios of future trace gas emissions (Hansen et al., 1988; Chapter V).
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Chapter II
FIGURE 2-5
emission
wavelength 5
15 17.5/20
Figure 2-5. Infrared (long-wave) emissions to space from the Earth. Many of the absorption bands
of the greenhouse gases fall within the atmospheric window -- a region of the spectrum, between 7
and 13 Jim, in which there is little else to prevent radiation from the Earth escaping directly into
space. (Source: UNEP, 1987.)
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Chapter II
TABLE 2-1
Radiative Forcing for a Uniform
Increase in Trace Gases From Current Levels
Compound
CO2
CH<
N2O
CFC-11
CFC-12
CFC-13
Halon 1301
F-116
CC14
CHC13
F-14
HCFC-22
CH2C12
CH3CC13
C2H2
SO2
Radiative Forcing
(No Feedbacks)
(°C/pPb)
.000004
.0001
.001
.07
.08
.10
.10
.08
.05
.04
.04
.03
.02
.01
.01
.01
Source: Adapted from Ramanathan et al., 1985.
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METHANE
Concentration History and Geographic Distribution
High-precision atmospheric measurements of methane (CH4) have been made in the past decade
at many different locations. The data show clearly that the concentration of methane, 1675 parts per
billion by volume (ppb) in 1987, has been increasing at the rate of about 16 ppb per year (Blake and
Rowland, 1986, 1988) (Figure 2-6).
Since 1982, air samples from ~25 globally distributed sites of the NOAA/GMCC cooperative
network have been analyzed for CH4 (Steele et al., 1987). In addition to flask sampling, continuous
measurements of atmospheric CH4 are now made at Cape Meares, Oregon (Khalil and Rasmussen,
1983); Pt. Barrow, Alaska; and Mauna Loa, Hawaii (NOAA, 1987).
The data show that CH4, like COj, exhibits very coherent spatial and temporal variations. CH4
is approximately uniform from mid- to high latitudes in the Southern Hemisphere, and increases
northward. The Northern Hemisphere concentration is approximately 140 ppb higher than that in
the Southern Hemisphere. The seasonal cycle in the Southern Hemisphere shows a minimum in the
summer, consistent with higher summer abundances of the hydroxyl radical (OH) and
temperature-dependent destruction rates. In the Northern Hemisphere, the seasonal cycle is more
complex, showing the interaction mainly between chemical destruction and emissions from
high-latitude peat bogs.
Analysis of air bubbles in ice cores shows that in pre-industrial years, CH4 was relatively constant
at ~700 ppb, from 100,000 years before present (100 kyBP) until the mid-19th century, and exhibited
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Chapter II
FIGURE 2-6
METHANE CONCENTRATION
(a)
Atmospheric Data
(Parts P«r Million)
1.7
I"1
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Policy Options for Stabilizing Global Climate — Review Draft Chapter II
a factor of 2.5 increase to its present value in only the last 300 years (Stauffer et al., 1985; Pearman
et al., 1986) (Figure 2-6). The 2083-meter ice core recovered by the Soviet Antarctic Expedition at
Vostok, Antarctica, shows that the CH4 concentration was as low as 340 ppb during the penultimate
ice age (-155 kyBP) and nearly doubled to 610 ppb in the following interglacial (130 kyBP). The
trend in CH4 closely followed the trend in air temperature deduced from deuterium, confirming the
role of CH4 as an important greenhouse gas (Raynaud et al., 1988). These measurements show that
current concentrations of CH4, like that of CO^ are higher than they have been in the past 160,000
years.
Sources and Sinks
Methane is produced via anaerobic decomposition in biological systems. It is also a major
component of natural gas and of coal gas. While the major sources of CH4 have been identified,
their individual contributions to the global budget are highly uncertain. A recent review of the
sources and sinks of CH4 is given by Cicerone and Oremland (1988) (see Figure 2-7).
The major sink of CH4 is reaction with OH radicals in the atmosphere.2 Based on chemical
considerations, it is estimated that the global sink of methane is ~510 teragrams (Tg) CH4/yr.3 By
inference, the annual global source is the sink plus the annual increase, i.e., about 550 Tg CH4/yr.
Cicerone and Oremland (1988) estimate a range of 400 to 640 Tg/yr for the annual global source.
2 A radical is an atom or group of atoms with at least one unpaired electron, making it
highly reactive.
1 Tg = 1 teragram = 1012 grams
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Chapter II
FIGURE 2-7
CURRENT EMISSIONS OF METHANE BY SOURCE
(Teragrams)
Fossil Fuel Production
50-95 TG
Domestic Animals
65-100 TG
Biomass Burning
50-100 TG
Rice Production
60-170 TG
Landfills
30-70 TG
Natural Sources
116-346 TG
Rice Production
1. India
2. China
3. Bangladesh
TOP THREE PRODUCERS
Domestic Animals
I.India
Z. USSR
3. Brazil
Fossil Fuel Production
1. United States
2, USSR
3. China
Figure 2-7. Human activities in the agricultural sector (domestic animals, rice production and
biomass burning) and the energy sector (fossil fuel production) are the major sources of atmospheric
CH4. Natural sources, from wetlands, oceans, and lakes, may contribute less than 25% of total
emissions. (Sources: Cicerone and Oremland, 1988; Crutzen et al., 1986; Lerner et al., 1988; United
Nations, 1987; IRRI, 1986.)
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Estimates of methane emissions from natural wetlands have ranged from 11-150 Tg/yr (e.g.,
Seiler, 1984; Khalil and Rasmussen, 1983). A recent study by Matthews and Fung (1987) estimated
that there are 530 million hectares of natural wetlands that account for a global emission of ~110
Tg CH4/yr. Of this, about 50% of the CH4 is emitted from productive peat bogs at high latitudes
in the Northern Hemisphere, a regional emission that is likely to increase with greenhouse wanning.
While this study has employed more extensive field data than earlier estimates (e.g., Sebacher et al.,
1986; Harriss et al., 1985), uncertainties in the global estimate remain due to the variability of
natural wetlands and then- CH4 fluxes.
Rice paddies are environments very similar to natural wetlands in terms of CH4 production and
emission to the atmosphere. In 1984, there were 148 million hectares of rice harvest area globally,
with ~50% in India and China. Methane emission studies have been performed in controlled
mid-latitude environments (Cicerone et al., 1983; Holzapfel-Pschorn and Seiler, 1986). These studies
have identified factors affecting methane fluxes to the atmosphere: inter alia, temperature, soil
properties, fertilizer, and irrigation practices. These factors make global extrapolation of methane
emissions very difficult. Cicerone and Oremland (1988) estimate a global emission of 60-170 Tg
CH4/yr.
Methane is also produced by enteric fermentation in animals, especially ruminants. The amount
of CH4 produced is dependent on enteric ecology, the composition and quantity of feed, and the
energy expenditure of the individual animal. Estimates of emission rates range from 94 kg
CH4/animal/year from West German dairy cattle, to -35 kg CH4/animal/year from Indian cattle
fed on kitchen refuse, to 5-8 kg CH4/animal/year from sheep. Using these emission coefficients and
population statistics of animals in the world, Crutzen et al. (1986) obtained a global emission of 78
Tg CH4 for 1983. This emission includes ~5 Tg CH4 from wild animals and <1 Tg CH4 from
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humans. About 75% of the emissions are from cattle and dairy cows. India, the USSR, Brazil, the
USA, and China are the five major countries in terms of CH4 emission from domestic animals
(Lerner et al., 1988).
Other natural sources of CH4 include termites, and exchange with oceans and lakes. The source
from termites is highly uncertain and controversial. Estimates of global emissions range from close
to zero (Seiler et al., 1984a) to 20 Tg (Fraser et al., 1986b), and as high as 200 Tg CH4 (Zimmerman
et al., 1982, 1984), on the order of half the global emission. The oceanic source is small, estimated
to be 5-20 Tg CH4/yr (Cicerone and Oremland, 1988).
There are several anthropogenic sources of methane. Methane is produced by incomplete
combustion during biomass burning. The amount of CH4 produced depends on the material burned
and the degree of combustion. Estimates range from 50-100 Tg CH4/yr (see Cicerone and
Oremland, 1988). While a few studies have attempted to understand and measure CH4 emission
during biomass burning (Crutzen et al., 1979, 1985), extrapolation to a global estimate is difficult
because of the lack of global data on area burned, fire frequency, and characteristics of fires. The
feasibility of monitoring fires from space (Matson and Holben, 1987; Matson et al., 1987) will improve
this estimate significantly.
Methane is also produced in large municipal and industrial landfills, where biodegradable carbon
in the refuse is decomposed to form a mixture of CO2 and CH4. As in the case of many other CH4
sources, the fraction of gas produced that escapes to the atmosphere is debated. Recently, Bingemer
and Crutzen (1987) estimated that this source produces 45-70 Tg CH4/yr. These estimates assume
that a large fraction of all organic carbon deposited in landfills eventually is subject to methanogenesis
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and subsequent emission to the atmosphere. Cicerone and Oremland (1988) adopt a range of 30-
70 Tg CJVvr.
Methane is the major component (-90%) of natural gas, and so the leakage of natural gas
from pipelines and the venting of natural gas from oil and gas wells represent sources of CH4 to the
atmosphere. Although natural gas production and consumption statistics are available globally, the
nature of this CH4 source makes it difficult to estimate how much this source contributes to the
atmospheric abundance. From U.S. and Canadian natural gas statistics it is estimated that
approximately 2-2.5% of the marketable gas is unaccounted for. Assuming that all of the
unaccounted for gas is lost to the atmosphere, 25-30 Tg CH4/yr from line loss is obtained globally
(Cicerone and Oremland, 1988). An additional 15 Tg CH4/yr is estimated from natural gas sources,
assuming that -20% of the gas that is vented and flared at oil and gas wells is not combusted,
escaping to the atmosphere as CH4 (Darmstadter et al., 1987). Together these estimates suggest a
source of up to 50 Tg CH4/yr from natural gas production and consumption. Much of the
unaccounted for gas, however, may represent meter discrepancies, and venting of natural gas has been
declining in recent years (Darmstadter et al., 1987). Thus a reasonable range for these sources may
be 20-50 Tg CH4/yr.
Methane is also the major component of gas trapped in coal. The percentage of the CH4
component increases with the age and depth of the coal and is released to the atmosphere during
mining and processing/crushing of coal. Globally, the amount of CH4 in coal is -0.5% of the mass
of coal extracted. This source is estimated to be 15-45 Tg CH4/yr in 1980 (Darmstadter et al.,
1987; Cicerone and Oremland, 1988).
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A highly uncertain but potentially large source of CH4 is methane hydrates in sediments under
permafrost and on continental margins (Kvenvolden, 1988). The magnitude of the current CH4
release from this source is unknown. Climate warming presents the potential for destabilization of
the hydrates and subsequent release of CH4 to the atmosphere (Chapter III).
Chemical and Radiative Properties/Interactions
Methane is the most abundant trace gas in the atmosphere that is active both radiatively and
chemically. Although the abundance of methane is 1/200 that of CO2, CH4 is a more efficient
absorber of thermal radiation than is CO2: Donner and Ramanathan (1980) and Lacis et al. (1981)
estimate that at present levels, an additional molecule of CH4 will contribute a radiative forcing that
is equivalent to that contributed by approximately 25 molecules of CO2. These radiative transfer
calculations suggest that a doubling of atmospheric CH4 (1.6-3.2 ppm) will contribute a radiative
forcing of 0.16°C (Hansen et al., 1988).
The destruction rate of CH4 is dependent on the amount of OH (and hence water vapor) in the
atmosphere as well as on temperature. The lifetime (atmospheric abundance divided by destruction
rate) of CH4 is approximately 10 years, the lifetime being shorter in the tropics. Using estimates of
the average concentration of atmospheric hydroxyl radicals derived from measurements of
methylchloroform, Prinn et al. (1987) have deduced the average atmospheric lifetime of methane to
be 9.6 (+2.2, -1.5) years. The reaction between CH4 and OH eventually produces CO; CO itself
reacts with OH, producing CO2 (Thompson and Cicerone, 1986). Thus, an increase in the
background levels of either CH4 or CO can reduce OH and the oxidizing power of the entire
atmosphere. It is estimated that increases in CO alone from 1960 to 1985 would have lowered OH
concentrations in the atmosphere, increased the methane lifetime, and resulted in a 15-20% increase
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in CH4 concentrations (Khalil and Rasmussen, 1985; Levine et al., 1985; Thompson and Cicerone,
1986).
Because of the interactions between CO, CH4, and OH in the atmosphere, it is difficult to
predict the effects of climate change on OH destruction of CH4, as increasing atmospheric water
vapor and increased precipitation (and removal of OH reservoirs like nitric acid [HNO3] and
hydrogen peroxide [H2OJ) have opposite effects on OH concentrations. Changes in nitrogen oxides
(NOJ and tropospheric ozone (O3) also strongly affect atmospheric OH (see below).
NITROUS OXIDE
Concentration History and Geographic Distribution
Nitrous oxide (N2O) is present in minute amounts in the atmosphere but it is nonetheless of
great importance. Its concentration is three orders of magnitude less than that of CO2, but its
radiative forcing per molecule is 230 times greater. The first high-precision measurements of
atmospheric N2O in the late 1970s showed an unambiguous increasing trend in its concentration
(Weiss, 1981). Continuous measurements at four Atmospheric Lifetime Experiment/Global
Atmospheric Gases Experiment (ALE/GAGE) sites have been made since 1979 (Figure 2-8). Flask
samples of air from the globally distributed cooperative network of NOAA-GMCC are also being
analyzed for N2O (Thompson et al., 1985; Komhyr et al., 1988).
The ALE/GAGE data show that the mid-1980s concentration of atmospheric N2O is 310 ppb
and that its annual growth rate is -0.8 ppb per year, or 0.2-0.3% per year. The concentrations at
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Chapter II
FIGURE 2-8
z
o
CO
cc
at
o.
a:
<
O.
CO
ec 300
Ui
a.
cc
<
a.
3 SO -
NITROUS OXIDE CONCENTRATIONS
(Parts P«r Billion)
Atmospheric Data
3 1 0
N -, O
308 I-
i
306 r
304 r
H ,
302 t-
300
1979
1983
Ice Core Data
1986
ieoo
i;oo
ieoo
itoo
YEAR
Figure 2-8. Concentration of atmospheric N2O has been increasing at the rate of 0.25%/yr in the
last decade (upper panel). The ice core record shows that N2O was relatively constant from the
1600's to the beginning of the 20th century, and began increasing rapidly in the last SO years.
(Sources: Khalil and Rasmussen, 1987; Pearman et al., 1986.)
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the two Northern Hemisphere sites are 0.8 ppb higher than those at the Southern Hemisphere sites,
suggesting the dominance of a northern source.
Ice-core data show that the preindustrial concentration of N2O was 285 ± 10 ppb averaged
between 1600-1800 (Pearman et al., 1986; Khalil and Rasmussen, 1987). Unlike CO2, whose
concentration began to increase significantly in the 1800s, N2O remained fairly constant until the
1900s, and then began increasing more rapidly in the 1940s (Pearman et al., 1986; Khalil and
Rasmussen, 1987). (See the ice-core data in Figure 2-8).
Sources and Sinks
While a lot of progress has been made during the last five years in quantifying the sources and
sinks of N2O in the atmosphere, there remain considerable uncertainties in the global budget and
in the contributions of individual source terms. The uncertainties arise not just because of the
scarcity of measurements of N2O fluxes, but also because of the complexity of the biogeochemical
interactions and heterogeneous landscape where N2O is produced.
Nitrous oxide is simultaneously produced and consumed in soils via the metabolic pathways of
denitrification, nitrification, nitrate dissimilation, and nitrate assimilation. These processes are affected
by various environmental parameters such as temperature, moisture, the presence of plants, and the
characteristics and composition of the soils (e.g., Seiler and Conrad, 1987; Sahrawat and Keeney,
1986). The flux of N2O to the atmosphere also depends on the location of the N2O-producing and
N2O consuming microorganisms and their relative activity within the soil column (Conrad and Seiler,
1985). Because of the complexity of the N2O production and destruction processes, and the inherent
heterogeneity of soils, it is difficult to estimate the contribution of natural soils to the global N2O
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budget. Slemr et al. (1984) calculated N2O emissions from natural temperate and subtropical soils
to be 4.5 Tg N/yr. Recent measurements (Livingston et al., 1988; Matson and Vitousek, 1987) show
that N2O emission rates from tropical soils are higher than those from temperate soils and that a
relationship exists between the N2O flux and the rate of nutrient cycling in the tropical forest soils.
However, no budget of N2O emissions from tropical soils has been attempted. Seller and Conrad
(1987) give a very tentative estimate of 6 ± 3 Tg N/yr from natural soils globally.
Measurements of supersaturation of N2O in the oceans indicate that the oceans contribute
additional N2O to the atmosphere, though in smaller quantities (Seiler and Conrad, 1981; Weiss,
1981). Seiler and Conrad (1987) estimated the oceanic contribution to be 2 ± 1 Tg N/yr.
Little is known about N2O emissions from terrestrial freshwater systems. Extrapolating from
measurements in the Netherlands and in Israel of elevated N2O levels in aquifers contaminated by
the disposal of human or animal waste, cultivation, and fertilization, Ronen et al. (1988) estimated
a global source 0.8-1.7 Tg N/yr from contaminated aquifers.
Nitrous oxide is also produced during combustion, but the importance of this source is unclear
at this time. A recent study of this N2O source was reported by Hao et al. (1987) who found that
the amount of N2O in flue gases was correlated with the nitrogen content of fuels. N2O emission
rates were highest during coal combustion, lower when oil was used as the fuel, and lowest when the
fuel was natural gas. They found that hi conventional single-stage boilers, on average, 14% of fuel
nitrogen was converted to N2O during combustion. Conversion of fuel nitrogen to N2O was much
less efficient (2-4%) in a two-stage experimental combuster and in wood fires. They also found
consumption of N2O in fuel-rich flames with low air-fuel ratios, reducing significantly the emission
factor. Using statistics on solid- and liquid-fuel production, they estimated an emission of 3.2 Tg
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N2O-N in 1982. Very recent studies, however, suggest that many of the N2O measurements reported
in the literature have been affected by a sampling artifact. A reaction between water, sulfur dioxide
(SO2), and nitrogen oxides (NOJ generates N2O in sample cylinders over a period of hours,
sometimes increasing N2O concentrations by more than an order of magnitude unless the samples
are carefully dried or N2O is measured immediately (Muzio and Kramlich, 1988). Reanalysis of
measurements made in the U.S., excluding those that were apparently affected by this reaction,
found no significant difference between N2O emissions from gas and coal-fired boilers (Piccot, pers.
communication). Recent measurements conducted by EPA with an on-line analyzer confirm this
finding: In both utility and small experimental boilers N2O concentrations in the exhaust gases were
always less than 5 ppm and generally less than 2 ppm (Hall, pers. communication). This suggests
that the relationship between N2O and fuel-nitrogen found by Hao et al. (1987) may have actually
been due to differences in SO2 and NOX emissions. Emissions of N2O do appear to vary with
combustion technology. Preliminary measurements suggest that fluidized-bed combusters and catalyst-
equipped automobiles may have substantially elevated N2O emissions (De Soot, pers. communication).
Total N2O emissions from fossil-fuel combustion cannot be estimated with any confidence at this
time, but may be closer to 1 Tg N/yr than to 3 Tg N/yr (Chapter V).
The addition of nitrogenous fertilizers to soils enhances the emission of N2O and other nitrogen
gases to the atmosphere. This emission depends on temperature, soil moisture, rainfall, fertilizer
type, fertilizer amount, and the way the fertilizer is applied. It also depends on the properties of the
soils and the crops grown. The fraction of fertilizer nitrogen lost to the atmosphere as nitrous oxide
ranges from -0.001-0.05% for nitrate, -0.01-0.1% for ammonium fertilizers, to -0.5->5% for
anhydrous ammonia. With a global consumption of approximately 705 million tons nitrogen as
nitrogenous fertilizers in 1984, an N2O contribution of 0.14-2.4 Tg N/yr is estimated. Although the
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estimated N2O emissions associated with the use of nitrogenous fertilizers are small compared with
those from natural sources, they are, nonetheless, a source subject to rapid growth.
Land-use modification in the tropics may also contribute N2O to the atmosphere. N2O is
produced during biomass burning, but because direct estimates of total N2O emissions are difficult,
N2O emissions are estimated by ratios with emissions of C02 or other nitrogen gases. Crutzen
(1983) estimated this source to be 1-2 Tg N/yr, although the accuracy of this estimate is highly
uncertain.
Recently, Bowden and Bormann (1986) found enhanced N2O fluxes to the atmosphere from
cleared areas in a temperate forest and elevated N2O concentration in ground water adjacent to the
cut watershed. Similarly, tenfold increases in N2O fluxes are found in pastures and forest clearings
in the Amazon (Matson, pers. communication). Robertson and Tiedje (1988) postulate, on the basis
of observations in Central America, that the loss of primary tropical rain forest may decrease the
emissions of N2O to the atmosphere. These studies suggest that rapid deforestation in the tropics
may significantly alter the N2O budget, although an estimate of its contribution to the global budget
has not been attempted.
Chemical and Radiative Properties/Interactions
N2O has a low concentration in the atmosphere, and its rate of increase is much smaller than
that of the other trace gases. Yet it still plays an important role in the radiative and chemical
budgets of the atmosphere. The seemingly small growth rate, ~0.25%/year, is the result of a large
unbalance (-30%) between the sources and sinks. The extremely long lifetime of N2O, -160 years,
means that the system has a very long memory of its emission history.
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Nitrous oxide is an effective greenhouse gas. The radiative forcing of one molecule of N2O is
equivalent to that of -230 molecules of C02; a doubling of N2O and a doubling of CH4 yield
approximately the same radiative forcing, even though the N2O concentration is less, by a factor of
5, than that of CH<. A 25% increase in the current burden of N2O in the atmosphere will yield a
radiative forcing of 0.07°C.
Nitrous oxide is not chemically reactive in the troposphere. However, its destruction in the
stratosphere, by photolysis and by reaction with atomic oxygen in the excited state [OC'D)], makes
N2O the dominant precursor of odd nitrogen in the stratosphere. Thus, an increase in N2O would
lead to an increase in stratospheric NO^ which would significantly influence stratospheric ozone
chemistry.
CHLOROFLUOROCARBONS (CFCs)
Concentration History and Geographic Distribution
High-precision measurements of CFC-11 (CC13F) and CFC-12 (CC12F2) began in 1971 with the
development of electron capture detector/gas chromatograph techniques (Lovelock, 1971). Like
CO2 and CH4, surface measurements have consisted of high-frequency observations at a few dedicated
sites as well as flask samples of air collected from a global network of stations or from irregular
global transects.
High-frequency in situ measurements of surface concentrations have been or are currently being
made at the five coastal/island stations of the Atmospheric Lifetime Experiment/Global Atmospheric
Gases Experiment (ALE/GAGE) (Cunnold et al., 1986; Prinn et al., 1983; Rasmussen and Khalil,
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1986; Simmonds et al., 1987). In addition, analysis of CFC concentrations in the flask samples of
air collected at the NOAA-GMCC globally distributed network of sites have begun at the GMCC
facility hi Boulder (Thompson et al., 1985; NOAA, 1987).
CFC-12 is the most abundant halocarbon in the atmosphere. Its average tropospheric
concentration in 1986 was 392 parts per trillion by volume (ppt), corresponding to a total burden of
about 7.6 Tg. Its concentration is increasing at 4%/yr.
With a total burden of about 5.0 Tg, CFC-11 is the second most abundant halocarbon in the
atmosphere. Its average concentration in 1986 was 226 ppt, and is also increasing at 4%/yr.
The other important halocarbons include methyl chloroform (CH3CC13), 125 ppt in 1986,
increasing at ~6%/yr; carbon tetrachloride (CC14), 121 ppt in 1982, increasing at 1.3%/yr; HCFC-
22 (formerly denoted CFC-22; CHC1F2), -100 ppt in 1986, increasing at 7%/yr; and Halon-1211
(CF2ClBr), -2 ppt in 1986 and increasing at >10%/yr (Prinn, 1988).
Sources and Sinks
CFCs are the product solely of the chemical industry. CFC-11 is used in blowing plastic foams
and in aerosol cans. CFC-12 is used primarily in refrigeration and aerosol cans. Comprehensive
data on production of CFC-11 and CFC-12 are published by the Fluorocarbon Program Panel (FPP)
of the Chemical Manufacturers Association (CMA). The peak year for CFC production was 1974,
in which a total of 812.5 gigagrams (Gg) of CFC-11 plus CFC-12 was produced.4 The annual total
CFC production decreased for a number of years following a ban on non-essential aerosol uses in
4 1 Gg = 109 grams = 106 Kg
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the United States, Canada, and Sweden. Non-aerosol uses have continued to increase, however, and
the total has risen rapidly in recent years, reaching 703.2 Gg in 1985. Of this total about 70% was
consumed by the U.S., the European Economic Community, and Japan (see Figure 4-9 in Chapter
IV).
The CMA data do not cover the USSR. The FPP has estimated Soviet production; however,
these estimates are considered unreliable. Data for China and the countries of Eastern Europe are
lacking entirely, rendering large uncertainties in the magnitude of world emissions. Cunnold et al.
(1986) and Fraser et al. (1983) have found the measured trend of CFC-11 and CFC-12 concentrations
is relatively consistent with the CMA estimates of CFC-11 release but not CFC-12 release, which
suggests that the USSR and Eastern Europe contribute a substantial amount to CFC-12 emissions.
Methyl chloroform (CH3CC13) is widely used in the manufacturing industry as a solvent for
degreasing, CFC-113 is used in the electronics industry, mainly for circuit board cleaning, and HCFC-
22 is used mainly in refrigeration. The sources of these gases have been estimated in various studies,
but a survey of sources-equivalent to that conducted for CFC-11 and CFC-12 ~ has not been done.
Fully halogenated CFCs (those that contain no hydrogen) are destroyed almost solely by
photolysis in the stratosphere. The atmospheric lifetimes of CFCs estimated from the ALE/GAGE
analyses are 111+22 years for CFC-12, 73+31 years for CFC-11, 6.3+li2 years for methyl chloroform,
-44 -17 -OS
and approximately 40 years for carbon tetrachloride. Compounds containing hydrogen (HCFCs)
react with OH in the troposphere and have lifetimes on the order of 20 years or less.
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Chemical and Radiative Properties/Interactions
CFCs absorb infrared radiation in the window region of the atmospheric spectrum (Figure 2-5).
Although CFCs are present in minute amounts in the atmosphere, together they are one of the
dominant greenhouse gases. They have the highest annual fractional increase (~5%/yr) of all the
greenhouse gases. Furthermore, the radiative forcing due to each additional molecule of CFC is
equivalent to that due to about 30,000 molecules of CO2, and at present levels increases linearly
with the concentration (Ramanathan et al., 1987). A 2 ppb increase in both CFC-11 and CFC-12
will contribute a radiative forcing of 0.3°C, equivalent to that from a 65 ppm increase in CO2. In
the 1980s, CFC-11 and CFC-12 together will contribute about 15% of the increase in global
greenhouse forcing.
Chlorinated and brominated compounds contribute chemically active halogens into the
atmosphere, where they are broken down by solar ultraviolet radiation or by reaction with OH. In
addition to CFC-11 and CFC-12, these species include methyl chloroform (CH3CC13), carbon
tetrachloride (CCIJ, and methyl halides (CH3C1, CH3Br, and CH3I). Also important are two rapidly
increasing chlorofluorocarbons, CFC-113 (C2C13F3) and HCFC-22 (CHCIF^. Halocarbons with at
least one hydrogen, such as methyl chloroform and HCFC-22 are destroyed primarily by reaction with
OH radicals in the troposphere. Long-lived species survive the 1-3 year transport time from the
surface up to stratospheric levels to play a critical role in ozone photochemistry. All of these species
can contribute to the stratospheric burden of chlorine, but the longer-lived CFCs can accumulate,
reaching higher concentrations before a steady state balance is achieved.
The dissociation products of halocarbons are the dominant sources of chlorine (Cl) and fluorine
(F) for the stratosphere (WMO, 1985), which are major components in the catalytic cycles that
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control ozone abundance. Ground-based data and aircraft data of trends for major halocarbon
reservoirs (HC1, HF, CIO) are highly uncertain. Within the limits of the uncertainties, the estimated
trends in these species were not in disagreement with trends in the source gases themselves.
OZONE
Concentration History and Geographic Distribution
Ozone (O3) is both produced and destroyed in situ in the atmosphere. Unlike the other trace
gases, the vertical distribution of O3 in the atmosphere is of prime importance in determining its
radiative and chemical effects (Figure 2-9).
Ozone sondes from a diverse and globally distributed network provide our only record of possible
trends in the vertical distribution of tropospheric O3. A review of ozone sonde and surface data have
been given by Logan (1985), Tiao et al. (1986), and more recently by Crutzen (1988). Since the
1970's, surface O3 concentrations are measured routinely at the four continuous monitoring
stations operated by NOAA-GMCC: Pt. Barrow, Alaska; Mauna Loa, Hawaii; American Samoa;
and the South Pole (see e.g. NOAA, 1987). NOAA-GMCC also participates in international
cooperative ozone sonde profiling activities.
The O3 data taken near populated and industrial regions in the 1930's to the 1950's generally
showed an annually averaged concentration of 10-20 ppb at the surface, with a seasonal cycle that
peaked in summer. The data showed a generally increasing trend, especially in the summer, in
surface concentrations of O3 at sites in western Europe, the USA and northern Japan. For example,
a factor of two increase, from -30 ppb in 1933 to ~60 ppb in the 1980s, is found in the summer
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Chapter II
FIGURE 2-9
OZONE CONCENTRATION (cm'3)
10
140
TEMPERATURE PROFILE
AND OZONE
DISTRIBUTION IN
THE ATMOSPHERE
120 -
100
200 300
TEMPERATURE IK)
Figure 2-9. On the left, temperature profile and ozone distribution in the atmosphere. On the right,
sensitivity of global surface temperature to changes in vertical ozone distribution. Ozone increases
in Region I (below -30 km) and ozone decreases in Region II (above ~30 km) warm the surface
temperature. The results are from a 1-D radiative transfer model in which 10 Dobson unit ozone
increments are added to each layer. The heavy solid line is a least square fit to step-wise
calculations. (Sources: Watson et al., 1986; Lacis et al., no date)
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concentrations in south Germany and Switzerland. Similarly, summertime concentrations of O3 at
the surface in rural areas in the Eastern U.S. have increased by 20%-100% since the 1940s (Logan,
1985). The surface O3 trend is 1%/yr or more at those sites in close proximity to population and
industrial centers. At Pt. Barrow and at Mauna Loa, geographically removed from but still under
the influence of urban centers, surface O3 was about 25 ppb in 1986 with summer values of 35-40
ppb. A small positive trend (0.7±0.5%/yr) is detected at these two sites from 1973-1986.
Analysis of the ozone sonde data at these populated sites show a small but significant positive
trend in mid-tropospheric ozone. In general, the mid-tropospheric trends are smaller than those at
the surface of the same O3 profile, and upper-tropospheric and lower-stratospheric trends are
negative, —0.5%/yr.
At remote locations, surface O3 exhibits a behavior very different from that near populated and
industrial regions. At remote sites in the Canadian Arctic and in Tasmania, Australia, for example,
the seasonal cycle of surface O3 has a minimum, rather than a maximum, in summer or autumn.
Surface O3 at the South Pole was 20 ppb in 1986, similar to that measured in Western Europe in
the 1930s. Also, unlike populated sites in the Northern Hemisphere, O3 at remote sites in the
Northern Hemisphere exhibit no significant trends near the surface, but significant positive trends at
700 millibars (mb) and 500 mb. Mid-tropospheric O3 at Resolute, Canada (75°N), for example, is
found to be increasing at 1%/yr, while there is a negative trend in the lower stratosphere. In the
Southern Hemisphere, however, there appear to be no significant trends in surface or
mid-tropospheric O3, although O3 in the lower stratosphere has clearly decreased and the seasonal
cycle at the South Pole has doubled in amplitude.
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The recent record of O3 concentrations in the upper atmosphere has been reviewed by a NASA
panel of experts (Watson et al., 1988). They report a general decreasing trend in the total O3
concentration in the entire atmospheric column from 1969 to 1986, the period of examination.
Ground-based Dobson instruments, located mainly between 30 and 64 degrees in the Northern
Hemisphere, show a total decrease of 1.7-3.0% in 17 years in the annually averaged O3 concentration
of the entire atmospheric column. The decrease was more rapid in the winter months, from
December to March. Satellite data, calibrated by coincident Dobson measurements, show a decrease
of about 2-3% from October 1978 to October 1985 in the column O3 concentrations between 53°S
and 53°N. The observations of stratospheric O3 in the Northern Hemisphere indicate that O3
abundances have declined over the past 20 years. The rate of decrease in the summer is consistent
with the predicted change, due to CFCs. However, the measured ozone loss poleward of 40°N in
winter is greater than that predicted by theory (Rowland, 1989). This depletion of O3 in the north
is not associated with the unusual chemistry of the Antarctic ozone hole and may be the beginning
of a truly global decline.
Sources and Sinks
Ozone is not directly emitted by human activity, but its concentration in the troposphere and
stratosphere is strongly governed by anthropogenic emissions of NOW hydrocarbons, and CFCs, among
others. Because of the short lifetimes of NOX and many of the other chemical species important in
tropospheric O3 chemistry, O3 concentrations exhibit large variability horizontally, vertically and
temporally. Its annual concentration, seasonal cycle, as well its trend have different behaviors in
different parts of the globe so that the observations from a few regions cannot be viewed as globally
representative. Global trends in tropospheric O3 cannot be unambiguously extracted from trends in
column O3 either. Stratospheric O3 dominates the column abundance and its decreasing trend may
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obscure positive or negative trends in tropospheric ozone. The difficulty in determining the globally
representative trend in tropospheric O3 translates into uncertainties in the O3 contributions to the
greenhouse warming.
Chemical and Radiative Properties/Interactions
Radiative forcing of O3 is more complex than that of the other greenhouse gases because (1) O3
is the major source of atmospheric heating due to ultraviolet and visible absorption bands, in addition
to being a greenhouse gas, and (2) O3 trends are not uniform in the atmosphere ~ anthropogenic
effects are expected to include upper stratospheric losses, lower tropospheric increases, and latitude
dependent changes in the lower stratosphere and upper troposphere.
Radiative transfer calculations reveal that the climate forcing due to an O3 perturbation changes
sign at 25-30 km altitude (see Figure 2-9). Ozone increases below this level lead to surface warming
because its greenhouse effect dominates its impact on solar radiation, while O3 added to the
stratosphere above ~30 km increases stratospheric absorption of solar energy at the expense of solar
energy that would otherwise have been absorbed at lower altitudes. On a per molecule basis, the
O3 changes with by far the largest net effect on surface temperatures are those near the tropopause
where the temperature contrast between absorbed and emitted thermal radiation is greatest. O3
changes near the surface produce little greenhouse forcing since the outgoing thermal radiation
absorbed is of nearly the same temperature as the radiation emitted by surface ozone.
While the radiative effects of O3 are understood theoretically, quantifying surface temperature
changes due to O3 changes is difficult because of the uneven data and the lack of global coverage
in the observations. Available ozone trend data are limited to northern mid-latitudes. Based on the
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observed decreases in O3 in the upper troposphere and lower stratosphere, which outweighed the
warming caused by decreases in the upper stratosphere and by increases in O3 in the lower
troposphere, Lacis et al. (no date) find that during the 1970s, surface cooling resulted from these
O3 changes. This cooling was equal in magnitude to about half of the warming contributed by CO2
increases during the same time period. These results differ from previous assessments (e.g.,
Ramanathan et al., 1985; Wang et al., 1988) that were based on one-dimensional photochemical
model results which predict ozone increases in the lower stratosphere and upper troposphere, and
thus produce surface warming. Predictions of two-dimensional photochemical models for increases
in CFCs suggest that ozone should decrease in the lower stratosphere at middle and high latitudes,
but increase in the tropics (Ko et al., 1984; WMO, 1985). This implies a strongly latitude-dependent
climate forcing for O3 distributional changes with surface cooling in the middle and high latitudes and
warming in the tropics.
The global nature of O3 changes in the upper troposphere and lower stratosphere cannot be
deduced, at this point, from current observations. This makes highly uncertain the evaluation of O3
contributions to the global greenhouse warming. Lacis et al. (no date) note that even the best
sampled O3 data from mid-latitudes in the northern hemisphere are of uneven quality, and that the
associated trends have large uncertainties and may not be globally representative.
OTHER FACTORS AFFECTING COMPOSITION
In addition to those greenhouse gases cited above that have a direct impact on the radiative
balance of the Earth, we must consider those forces that control the chemical balance of the
atmosphere, in turn controlling the abundance of greenhouse gases. With the exception of ozone,
the greenhouse gases are generally not very reactive in the atmosphere; they have long chemical
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lifetimes, on the order of 10 to 200 years; and they accumulate in the atmosphere until their rate of
chemical destruction balances their emissions. The chemistry of the stratosphere and troposphere
provides the oxidizing power to destroy the majority of trace pollutants in the Earth's atmosphere (a
major exception is CO^ see above). We outline below those primary and secondary components of
the Earth's atmosphere that affect the chemically reactive gases and note changes that may have
occurred in the recent past and those possible in the future.
Global Tropospheric Chemistry
In the troposphere, many species are removed in a chain of reactions beginning with the hydroxyl
radical, OH, and ending with the deposition or rainout of a soluble compound, or with the complete
oxidation of the original compound (i.e., net: CH4 + 2O2 = = > CO2 + 2H2O). For CH4, most
hydrocarbons, and halocarbons containing a hydrogen atom (e.g., anthropogenic HCFCs such as
CHClFj), the chemical lifetime will vary inversely with the suitably averaged global OH concentration.
The OH radicals in the troposphere are short-lived (<1 sec) and are produced by sunlight in the
presence of O3 and H2O; they are consumed rapidly by reaction with CO, CH4, and other
hydrocarbons. Moderate levels of nitrogen oxides (NOX: NO and NOj) can play an important role
in recycling the odd-hydrogen (HOJ from HO2 to OH, thus building up the concentrations of OH;
but high levels of NOX can reduce both OH and O3. The short lifetime of OH means that, when
we integrate the loss of even a well-mixed gas like CH4 against consumption by reaction with OH,
we are integrating over the myriad of conditions of the troposphere in terms of sunlight, O3, H2O,
i
CO, CH4, NO,, and others. These tropospheric conditions vary over scales that range from smooth
in latitude and height, to irregular in plumes downwind from metropolitan areas. At present we do
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not have adequate models for OH that can describe these varied conditions and accurately integrate
the global loss of a greenhouse gas such as CH4.
In spite of these problems in modeling the chemically complex, heterogenous conditions of the
global troposphere, we do understand tropospheric chemistry sufficiently to make some simple
generalizations:
• Most loss of CH4 occurs in marine environments, particularly in the tropics and
subtropics, remote from the influence of urban areas and the continental boundary layer,
• Increasing concentrations of CO and CH, will reduce global or hemispheric levels of OH,
• Large scale perturbations to tropospheric O3 and H2O (from climate change) may have
equal impact on OH concentrations,
• Changes in anthropogenic emissions of NOX are expected to have only a moderate direct
impact on globally integrated OH, but may lead to more significant increases in Northern
Hemispheric O3.
Carbon Monoxide
Carbon monoxide (CO) has a lifetime of about one month in the tropics that becomes
indefinitely long in winter at high latitudes. The globally averaged destruction of CO corresponds
to an estimated lifetime of 3 months. Carbon monoxide is lost almost exclusively through
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tropospheric reactions with OH (and in this non-linear system, CO is also a major sink for OH).
There are some estimates of plant/soil uptake of CO, but these are not of major significance.
Detailed observations of CO concentrations are available over the past decade (Dianov-Klokov
and Yurganov, 1981; Khalil and Rasmussen, 1988) and there are sporadic measurements since 1950
(Rinsland and Levine, 1985). These data indicate that CO concentrations have grown modestly, but
consistently (1-2%/yr) in the northern mid-latitudes over the last few decades. There is no
convincing evidence for growth in the Southern Hemisphere (Seiler et al., 1984b). This pattern is
consistent with a growing anthropogenic source, since the short lifetime precludes significant
interhemispheric transport. Since CH< concentrations have also increased similarly (about 1%, as
noted above), we would expect a similar change of opposite sign in tropospheric OH.
Nitrogen Oxides
One form of odd-nitrogen denoted as NOX is defined as the sum of two species, NO + NO2.
NOX is created in lightning, in natural fires, in fossil fuel combustion, and in the stratosphere from
N2O. The NOX levels over the continental boundary layer and in the aircraft flight lanes of the
Northern Hemisphere are likely to have increased over the last several decades. Nevertheless, the
levels of NO, in the clean marine environment are so low that they might be accounted for entirely
by natural sources (i.e., lightning, fires, stratospheric HNO3).
The anticipated changes in NO, levels over limited regions of the Northern Hemisphere are
expected to have only a small direct effect on the globally integrated OH concentration. A more
important impact of NO, emissions is likely for tropospheric O3, where a substantial fraction of the
global tropospheric ozone production is predicted to take place in small regions with elevated levels
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of NOX and hydrocarbons (Liu et al., 1987). These issues are unresolved and are currently the focus
of photochemical studies with multi-dimensional tracer models.
Stratospheric Ozone and Circulation
Some species such as N2O and CFCs do not react with OH, and these gases are destroyed only
in the stratosphere by short-wavelength ultraviolet light and by reactions with the energetic state of
atomic oxygen, O(1D). For CFCs and N2O the abundances will be perturbed by changes in the rate
of stratosphere-troposphere circulation and changes in the stratospheric O3 that shields the solar
ultraviolet radiation. Major perturbations to stratospheric O3 and circulation may also alter the
concentrations of tropospheric O3, since the stratosphere represents a major source for this gas.
Predictions have been made over the past decade that stratospheric O3 will change due to
increasing levels of CFCs, and that the circulation of the stratosphere may be altered in response to
changes in climate induced by greenhouse gases. Recent detection of the Antarctic ozone hole has
dramatized the ability of the atmosphere to change rapidly in response to perturbations. There are
currently underway many theoretical studies of the impact of the ozone hole on stratospheric
circulation, O3 fluxes, and the mean chemistry of the stratosphere (e.g., N2O losses). As discussed
above, there are also indications of a declining trend in Northern Hemisphere O3 not associated with
the Antarctic hole. In summary, we expect stratospheric O3 to change in the next few decades, which
will lead to alterations in the lifetimes of the long-lived greenhouse gases and will also perturb
tropospheric chemistry through the supply of O3 and through the increase in solar ultraviolet light
available to generate OH.
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CONCLUSION
Anthropogenic emissions of both long-lived greenhouse gases and short-lived highly reactive
species are altering the composition of the atmosphere. The concentrations of CO2 and CH4 have
increased dramatically since the preindustrial era, and CFCs have been introduced into the
atmosphere for the first time. As a result of the rapid pace of human-induced change, neither
atmospheric composition nor climate is currently in equilibrium. Thus, significant global change can
be anticipated over the coming decades, no matter what course is taken in the future. The rate and
magnitude of change, however, are subject to human control, which serves as the motivation for this
report.
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TABLE 2-2
Trace Gas Data
CO2 CARBON DIOXIDE 1012 kg C
Atmospheric Burden 720
348.4 ±0.2 ppm in 1987
Not photochemically active
Annual Trend 2.2
1.15 ppm/yr (0.3%/yr) since 1984
Annual Sources 6-7
1. Fossil fuel combustion 5.4
4.5%/yr since 1984
2. Land use Modification 0.4 - 2.6
3. Biosphere -- climate feedback ?
Enhanced aerobic decomposition
of detrital material due to
more favorable climate
Annual Sinks ~ 2.5
1. Ocean ~ 2.5
Ocean's capacity to absorb CO2
will be altered by changes in
temperature, salinity as wells a
biological activity of ocean.
2. Biosphere ?
Enhanced photosynthetic uptake of
CO2 due to more favorable climate
and/or due to CO2 fertilization
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TABLE 2-2 (Continued)
CH, METHANE 109 kg CH4
Atmospheric^ Burden
1675 ppb in 1987 4600 - 4800
Lifetime: 5-10 years
Annual Trend
14 - 16 ppb/yr (0.8-1%/yr) 40 - 46
Annual Sources 500 ± 100
1. Fossil fuel
Coal mining 15 - 45
Natural gas drilling, venting, and transmission loss 25 - 50
2. Biomass burning 50 - 100
3. Natural wetlands 100 - 200
4. Rice Paddies 60 - 170
5. Animals - mainly ruminants 65 - 100
6. Termites 10 - 100
Population unknown
7. Oceans and freshwater lakes 5-45
8. Landfills 30 - 70
9. Methane hydrate destabilization 0 - 100 (future)
Annual Sinks 495 ± 145
1. OH destruction 495 ± 145
2. Dry soils ?
absorption by methane -
oxidizing bacteria in dry soils
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Chapter II
TABLE 2-2 (Continued)
N,O
NITROUS OXIDE
109 kg N
Atmospheric Burden
340 ppb in 1986
Lifetime: 100 - 175 years
Annual Trend
0.7 ± 0.1 ppb/yr
0.2%/yr
Annual Sources
1. Combustion of coal and oil
2. Land use modification
Biomass burning
Forest clearings
3. Fertilized agricultural lands
4. Contaminated aquifers
5. Tropical and subtropical forests and woodlands
6. Boreal and temperate forests
7. Grasslands
8. Oceans
Annual Sinks
Stratospheric photolysis and reaction with O(:D)
1500
3.5 ± 0.5
14 ± 3 (inferred)
1 - 3
1 - 2
?
0.2 - 2.4
0.8 - 1.7
6 ± 3
0.1 - 0.5
<0.1
2 ± 1
10.5 ± 3
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TABLE 2-2 (Continued)
NOX NITROGEN OXIDES 109 kg N
NOX = NO + NO2
nitric nitrogen
oxide dioxide
NOy = NOX + HNO2 + HNO3 + HOzNOa + NOs + 2N2Os + PAN + Participate Nitrate
Atmospheric Burden ~ large variability, lifetime 1-2 days in summer ?
Marine air 4 ppt (NO)
Continental air
non-urban sites 2-12 ppb
U.S. & European cities 70 - 150 ppb
(100 ppt = 240 x 10' kg N)
Annual Trend ?
Annual Sources ~ Spatially and temporally concentrated sources 25 - 99
1. Combustion of coal, oil and gas 14 - 28
2. Biomass burning 4-24
3. Lightning 2 - 20
4. Oxidation of ammonia 1-10
5. Emission from soils (mostly NO) 4-16
6. Input from stratosphere (by reaction of O(JD) with N2O) ~ 0.5
Sinks
1. Wet deposition (precipitation scavenging) 24-64
ocean 4-12
continents 8-30
2. Dry deposition 12 - 22
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Chapter II
TABLE 2-2 (Continued)
CO CARBON MONOXIDE
109 kg CO
Atmospheric Burden 42Q
~ 90 ppb
(150 - 200 ppb Northern Hemisphere, 75 ppb Southern Hemisphere)
Lifetime: 0.4 year
Annual Trend
1 - 2%/yr Northern Hemisphere
0 - 1%/yr Southern Hemisphere
Annual Sources 1500 - 4000
1. Fossil fuel combustion 400 - 1000
50% for automobiles
residential use of coal
industrial activities, e.g., steel production
2. Oxidation of anthropogenic hydrocarbons 0 - 180
45% emissions from automobiles
40% evaporation of fuels and solvents
3. Biomass burning
Wood used as fuel 25 - 150
Forest wildfires 10 - 50
Forest clearing . 200-800
Savanna burning 100 - 400
4. CH< oxidation 400 - 1000
5. Oxidation of natural hydrocarbons (\soprenes and terpenes) 280 - 1200
6. Emission by plants 50 - 200
7. Ocean 20 - 80
Annual Sinks 3420
1. Soil uptake 250
2. Photochemistry 3170
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Geophysical Research 86:7185-7195.
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WMO (World Meteorological Organization). 1985. Atmospheric Ozone 1985: Assessment of our
Understanding of the Processes Controlling its Present Distribution and Change. Volume 1. WMO,
Geneva, 392+pp.
Zimmerman, P.R., J.P. Greenberg, and J.P.E.C. Darlington. 1984. Response to: Termites and
atmospheric gas production (Technical Comment by N.M. Collins, and T.G. Wood). Science 224:84-86.
Zimmerman, P.R., J.P. Greenberg, S.O. Wandiga, and PJ. Crutzen. 1982. Termites: A potentially
large source of atmospheric methane, carbon dioxide and molecular hydrogen. Science 218:563-565.
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CHAPTER III
CLIMATIC CHANGE
FINDINGS • III-2
INTRODUCTION III-4
CLIMATIC CHANGE IN CONTEXT III-6
CLIMATE FORCINGS III-8
Solar Luminosity 111-12
Orbital Parameters 111-13
Volcanoes 111-13
Surface Properties 111-14
The Role of Greenhouse Gases 111-14
Internal Variations Ill-15
PHYSICAL CLIMATE FEEDBACKS 111-15
Water Vapor - Greenhouse Ill-17
Snow and Ice 111-17
Clouds 111-19
BIOGEOCHEMICAL CLIMATE FEEDBACKS 111-20
Release of Methane Hydrates 111-20
Oceanic Change 111-22
Ocean Chemistry 111-23
Ocean Mixing 111-23
Ocean Biology and Circulation 111-24
Changes in Terrestrial Biota 111-25
Vegetation Albedo 111-25
Carbon Storage 111-26
Other Terrestrial Biotic Emissions 111-26
Summary 111-27
EQUILIBRIUM CLIMATE SENSITIVITY 111-28
THE RATE OF CLIMATIC CHANGE 111-31
CONCLUSION 111-35
REFERENCES 111-37
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FINDINGS
• Climate exhibits natural variability on all time scales, from years to millions of years.
This variability is caused by a combination of changes in external factors, such as solar
output, and internal dynamics and feedbacks, such as the redistribution of heat between
the atmosphere and the oceans.
• The ultimate warming that can be expected for a given increase in greenhouse gas
concentrations is uncertain due to our inadequate understanding of the feedback
processes of the climate system. For the benchmark case of doubling carbon dioxide
concentrations, the National Academy of Sciences has estimated that the equilibrium
•>
increase in global average temperature would most likely be in the range of 1.5-4.5°C;
recent analyses suggest that the warming could be as much as 5.5°C; a reasonable
central uncertainty range is 2-4°C.
• A variety of geophysical and biogenic feedbacks exist that have generally been ignored
in quantifying the temperature change that could occur for any given initial increase in
greenhouse gases. In particular, the potential of future global warming to increase
emissions of carbon from northern latitude reservoirs in the form of both methane and
carbon dioxide, to alter uptake of CO2 by oceans, and a variety of other temperature
dependent phenomena indicate that the true sensitivity of the Earth's climate system to
increased greenhouse gases could exceed 5.5°C for an initial doubling of CO2. While
there are biogenic and geochemical feedbacks that could decrease greenhouse gas
concentrations - enhanced photosynthesis due to higher CO2, for example ~ it appears
that the risk of all biogenic and geochemical feedbacks raising the Earth's true
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temperature sensitivity to anthropogenic emissions of greenhouse gases is greater than
the possibility of such feedbacks decreasing this sensitivity.
• Uncertainties about ocean circulation and heat uptake, and about future internal climate
oscillations and volcanic eruptions, make it difficult to predict the time-dependent
response of climate to changes in greenhouse gas concentrations. Because the oceans
delay the full global warming that would be associated with any increase in greenhouse
gases, significant climatic change could continue for decades after the composition of the
atmosphere were stabilized. The Earth already is committed to a total warming of about
0.7-1.5°C (assuming that the climate sensitivity to doubling CO2 is 2-4°C). The Earth
has warmed by 0.3-0.7°C during the last century, which is consistent with expectations
given the uncertain delay caused by ocean heat uptake.
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INTRODUCTION
The increasing concentrations of greenhouse gases documented in the last chapter are expected
to alter significantly the Earth's climate in the coming decades. The magnitude and timing of actual
climatic change will be determined by future emissions (Chapter V), by changes in other climatic
forcings, and by the sensitivity of the climate system to perturbations. Weather and climate (the
time-average of weather) are determined by complex interactions between the atmosphere, land
surface, snow, sea ice, and oceans, involving radiative and convective exchange of energy within and
among these components. As is readily apparent, this system exhibits considerable variability from
day to day, month to month, and year to year.
Systematic diurnal (day-night) and seasonal variations are driven by changes in the distribution
and amount of solar energy reaching the top of the Earth's atmosphere as the Earth rotates on its
axis and orbits around the sun. Changes in the amount of energy emitted by the sun, changes in the
atmospheric composition (due to volcanic eruptions and human input of aerosols and greenhouse
gases), and changes in the earth's surface (such as deforestation) can also affect the earth's energy
balance. Such factors are considered "external forcings" because they do not depend on the state of
the climate system itself.
In contrast, much of the day to day and year to year variation results from the internal dynamics
of the climate system. For example, the polar front may be unusually far south in North America
during a given year, producing colder than normal weather in the northern Great Plains, but there
can be warmer than average weather somewhere else, leaving the global average more or less
unchanged. Similarly, upwelh'ng of cold water off the Pacific Coast of South America may fail for
several years. This irregularly recurring event, referred to as El Nino, leads to various regional
weather anomalies, impacts like the collapse of the Peruvian anchovy fishery, and warmer global
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temperatures. In this case there is a temporary net release of heat from the ocean to the
atmosphere, which is usually followed by a reversal, sometimes referred to as La Nina (Kerr, 1988).
Such random variations of the atmospheric and oceanic circulation can produce anomalous
redistributions of energy in the climate system resulting in random climate variations, with amplitudes
and time scales which may be comparable to climate changes expected from past increases in
greenhouse gases (Lorenz, 1968; Hasselmann, 1976; Robock, 1978; Hansen et al., 1988).
In order to determine precisely the potential effects of the input of greenhouse gases on future
climate, it would be necessary not only to be able to understand all the physics of the climate system
and the effects of each potential cause of climate change, but also to be able to predict the future
changes of these forcings. If we could do this, we could explain past climate change and separate
the effects of greenhouse gases from the other factors that have acted during the past 100 years for
which we have instrumental temperature records. We could also use theoretical climate models to
calculate the future size and timing of climate changes due to greenhouse gases. Since our
measurements of past climate are incomplete, our understanding of the climate system is incomplete,
and some (not well known) portion of climate change is random and unpredictable, we can only
estimate the impact of greenhouse gas buildup within a broad range of uncertainty.
In this chapter we discuss in brief the magnitude and rate of past changes in climate and examine
the various factors influencing climate in order to place the potential warming due to increasing
greenhouse gas concentrations in context. Feedback mechanisms that can amplify or lessen imposed
climate changes are discussed next. The overall sensitivity of climate to changes in forcing is then
considered, followed by a discussion of the time-dependent response of the Earth system. The focus
is on global temperature as an indicator for the magnitude of climatic change. Regional climate and
the potential impacts of climatic change are not discussed here, but are considered in the companion
report Potential Effects of Global Climate Change on the United States (Smith and Tirpak, 1989).
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CLIMATIC CHANGE IN CONTEXT
The most detailed information on climate is, of course, from the modern instrumental record,
but even this data set is quite sparse in the Southern Hemisphere and over the oceans. Wigley et
al. (1986) reviewed a number of recent analyses, noting that independent groups (including Hansen
et al., 1981 and Vinnikov et al., 1980; more recent publications are Hansen and Lebedeff, 1988 and
Vinnikov et al., 1987), necessarily relying on the same basic data sources, but using different data
selection and averaging approaches, have obtained very similar results. Given the various
uncertainties due to factors such as poor spatial coverage hi some regions, changes in the number
and location of stations, growth of urban heat islands, and changes in instrumentation, Wigley et al.
conclude that the warming since 1900 has been in the range of 0.3-0.7°C. The most complete and
up-to-date global surface air temperature record available (Jones et al., 1988) is displayed in Figure
3-la, which shows a global warming of about 0.3°C from 1900 to 1940, a cooling of about 0.1°C
from 1940 to 1975 and a warming of about 0.2°C from 1975 to 1987. The four warmest years in
the record occurred during the 1980s: 1980, 1981, 1983, and 1987. The overall warming is similar in
the land air temperature record of the Northern and Southern Hemispheres (Figures 3-lb,c), though
the long-term trend is steadier in the Southern Hemisphere where the 1940-1975 cooling is less
evident. While the gradual warming seen in Figure 3-1 during the past century is not inconsistent
with the increasing greenhouse gases during this period (Chapter II), the large interannual variations
and the relatively flat curve from 1940 to 1975 show that there are also other important causes of
climate change. The differences between the two hemispheres also show that there are regional
differences in the climate response to a global forcing (greenhouse gases), that important other
forcings (such as large volcanic eruptions) are not global in their effects, or that internal climate
variations produce regional differences. Because of past and potential future emissions of greenhouse
gases (see below and Chapter V), climate changes during the next century may be larger than the
variations shown for the past 100 years.
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FIGURE 3-1
(a)
(b)
(c)
0.5
SURFACE AIR TEMPERATURE
(Degrees Celsius)
Global
-0.5
'900
1920
'940
(960
1980 2000
Northern Hemisphere
l»0 1870 1890 1910 1930 1350 1970 1990
Southern Hemisphere
1950 1870 1890 1910 1330 1950 1970 1990
Figure 3-1. (a) Global surface air temperature, 1901-1987. This record incorporates measurements
made both over land and from ships. The smooth curve shows 10-year Gaussian filtered values. The
gradual warming during this period is not inconsistent with the increasing greenhouse gases during
this period, but the large interannual variations and the relatively flat curve from 1940 to 1975 show
that there are also other important causes of climate change. (Source: Jones et al., 1988.)
(b,c) Land surface air temperatures, 1851-1987 for the Northern Hemisphere (NH) and 1857-1987
for the Southern Hemisphere (SH). Note the larger interannual variability before 1900, when data
coverage was much more sparse. (Source: Jones, 1988.)
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Recent climate variations are put in a longer-term perspective in Figures 3-2 through 3-4. The
amplitude of climatic change over the last millennium (Figure 3-2) is similar to what has been seen
during the last century. The Medieval Warm Epoch (800-1200 AD) may have been restricted to the
North Atlantic Basin (Wigley et al., 1986), and in any case appears to have been about as warm as
the present. The Little Ice Age (1430-1850; Robock, 1979) appears to have been as cool as the early
20th Century in parts of Europe. The peak of the most recent glaciation is generally given as 18
thousand years before the present (kyBP) (Figures 3-3, 3-4) with globally averaged temperatures
about 5°C cooler than today (Hansen et al., 1984) between 15 and 20 kyBP. Even over the 700,000
year period illustrated in Figure 3-4 the maximum global temperature swing appears to have been
no greater than about 5°C, with the periods of greatest warmth being the present and the interglacial
peaks which occurred approximately every 100,000 years for the past million years (Figure 3-4). The
temperature change shown in Figure 3-3 is for Antarctica and is substantially greater than what is
believed to represent the globe as a whole. (Such high-latitude amplification of temperature increases
can be expected for greenhouse-induced warming in the future). The CO2 variations are, in general,
in step with the temperature variations deduced from deuterium variations in the same ice core
(Jouzel et al., 1987), demonstrating the importance of CO2 in amplifying the relatively weak orbital
forcings during past climate variations (Genthon et al., 1987; see Orbital Parameters). While it is
difficult to assign a cause for these past changes, it is reasonable to conclude that, given current
greenhouse gas concentrations, global temperatures will soon equal or exceed the maximum
temperatures of the past million years.
CLIMATE FORCINGS
The patterns of climate variations discussed in the last section are the result of a combination
of external forcings, internal feedbacks, and unforced internal fluctuations. The strictly external
forcings are changes in solar output and variations in the Earth's orbital parameters, while changes
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Chapter III
FIGURE 3-2
X J
03
"o
' » I > ' v / V r- ' h /
\r \ / w - \ / v V
w w
400 r,oj HT) 100Q 1?00 MOO 1600 1HOO
AD
Figure 3-2. Oxygen isotope (6 O) variations from ice cores in Greenland. This is an index of
Northern Hemisphere temperature, with the maximum range equal to about 1°C. (Source: record
of Dansgaard as given by Lamb, 1977.)
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Chapter III
FIGURE 3-3
u
o
2.5
0
-2.5
-5.0
- 7.5
-10.0
Figure 3-3. Carbon dioxide levels and temperatures over the last 160,000 years from Vostok 5 Ice
Core in Antarctica. The temperature scale is for Antarctica; the corresponding amplitude of global
temperatures swings is thought to be about 5°C. (Source: Barnola et al., 1987.)
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Chapter HI
FIGURE 3-4
STAGES
100
200 300 400
AGE IN THOUSANDS OF YEAUS
500
600
Figure 3-4. Composite 618O record of Emiliani (1978) as given by Berger (1982). This comes from
deep sea sediment cores and is an index of global temperature, with the temperature range from
stage 1 (present) to stage 2 (18,000 years ago) equal to about 5°C.
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in aerosols and greenhouse gas concentrations may be viewed as external forcings or internal
feedbacks, depending on the time scale and processes considered. The sensitivity of the climate
system is determined by the feedbacks that modify the extent to which climate must change to
restore the overall energy balance of the Earth as external forcings change.
Solar Luminosity
The solar luminosity (or total energy output from the sun) has an obvious and direct influence
on climate by determining the total energy reaching the top of the Earth's atmosphere. Theories of
stellar evolution suggest that solar output was 25% lower early in Earth history, but geologic evidence
and the fact that life was able to evolve on Earth shows that the Earth was not an inhospitable ice-
covered planet. An important part of the explanation for this "faint young sun paradox" now appears
to be that the CO2 content of the atmosphere was many times higher than it is at present. The
enhanced greenhouse effect from CO2 was probably the main factor in counteracting the lower solar
luminosity (see below). Geochemical models suggest that over millions of years CO2 has acted as
an internal feedback that has kept the Earth's climate in a habitable range (Walker et. al., 1981;
Berner and Lasaga, 1988; Figure 3-2b).
Solar luminosity also varies by small but significant amounts over shorter time periods. Various
attempts have been made to explain past climate variations by assuming a link between solar
luminosity and observed parameters, such as sunspot activity, solar diameter, and the umbral-
penumbral ratio (Wigley et al., 1986). Unfortunately, measurements with sufficient precision to
detect solar luminosity changes have only been available sbce 1979 - too short a time period to be
able to definitively confirm or refute the proposed relationships. These measurements show a decline
in solar luminosity between 1980 and 1986; whereas the most recent data show a reversal of this
trend (Willson and Hudson 1988; Willson et al., 1986). The luminosity data are positively correlated
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with sunspot number and suggest an 11-year cycle with an amplitude of 0.04% or 0.1 W/m2 at the
top of the atmosphere (Willson and Hudson, 1988).
Orbital Parameters
Cyclic changes in Earth orbital characteristics are now widely accepted as the dominant trigger
behind the glacial/interglacial variations evident in Figure 3-3 and extending back to at least 1.7
million years before present (e.g., Wigley et al., 1986; COHMAP, 1988). While causing only small
changes in the total radiation received by the Earth, the orbital changes (known as the Milankovitch
cycles) significantly alter the latitudbal and seasonal distribution of insolation. For example, Northern
Hemisphere summer insolation was about 8% greater 9 kyBP than it is now, but winter insolation
was 8% lower. Changes of this type, in combination with internal feedbacks as discussed below, are
presumed to have determined the pattern of glaciations and deglaciations revealed in the geologic
record. Attempts have been made to compare model predictions with paleoclimatic data. There has
been reasonably good agreement between the two, given specified ice sheet extent and sea surface
temperatures (COHMAP, 1988; Hansen et al., 1984). To the extent that the Milankovitch
explanation of ice ages is correct, one would expect the Earth to be heading toward a new ice age
over the next 5000 years, but the very gradual changes in orbital forcings expected in this period will
be overwhelmed if current trends in greenhouse gas concentrations continue (Wigley et al., 1986).
Volcanoes
Large volcanoes can significantly increase the stratospheric aerosol concentration, increasing the
planetary albedo and reducing surface temperatures by several tenths of degree for several years
(Hansen et al., 1978, 1988; Robock, 1978, 1979, 1981, 1984). Because of the thermal inertia of the
climate system, discussed below, volcanoes can even be responsible for climate changes over decades,
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and in fact the warming shown in Figure 3-1 from 1920 to 1940 can be attributed to a period with
very few volcanic eruptions (Robock, 1979). Since large eruptions occur fairly frequently and cannot
now be predicted, this component of climate change will have to be considered when searching past
climate for a greenhouse signal and when projecting future climate change.
Surface Properties
The Earth's radiative balance can also be changed by variations of surface properties. While
interactions with the ocean, which covers 70% of the Earth's surface, are considered internal to the
climate system and are discussed below, land surfaces also exert a strong influence on the climate.
Human activities, such as deforestation, not only provide a source of CO2 and CH4 to the
atmosphere, but also change the surface albedo and moisture flux into the atmosphere. Detailed
land surface models, incorporating the effects of plants, are now being developed and incorporated
into GCM studies of climate change (Dickinson, 1984; Sellers et al., 1986).
The Role of Greenhouse Gases
The greenhouse effect does not increase the total energy received by the Earth, but it does alter
the distribution of energy in the climate system by increasing the absorption of infrared (IR) radiation
by the atmosphere. If the Earth had no atmosphere, its surface temperature would be strictly
determined by the balance between solar radiation absorbed at the surface and emitted IR. The
• amount of IR emitted by any body is proportional to the fourth power of its absolute temperature,
so that an increase in absorbed solar radiation (due to increased solar luminosity or decreased
albedo, for example) would be balanced by a small increase in the surface temperature, increasing
IR emissions until they are again equal to the absorbed solar radiation. The role of greenhouse
gases can be understood by thinking about the atmosphere as a thin layer that absorbs some fraction
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of the IR emitted by the surface (analogous to the glass in a greenhouse). The energy absorbed by
the atmosphere is then remitted in all directions, and the downward half of this energy flux warms
the surface (Figure 3-5). Higher concentrations of greenhouse gases increase the IR absorption in
the atmosphere, raising surface temperatures.
Changes in the atmosphere's radiative properties can result from external perturbations (such
as anthropogenic emissions of CO2) or from internal adjustments to climatic change. The amount
of water vapor, the dominant greenhouse gas, is directly determined by climate and contributes the
largest positive feedback to climatic change (Hansen et al., 1984; Dickinson, 1986). Similarly, clouds
are an internal part of the climate system that strongly influence the Earth's radiative balance
(Ramanathan et al., 1989). Changes in the concentrations of other greenhouse gases may be imposed
by human activity or may result from changes in their sources and sinks induced by climatic change.
Such feedbacks are discussed below.
Internal Variations
As discussed in the introduction, even with no changes in external forcings, climate still exhibits
variations due to internal rearrangements of energy within the atmosphere and between the
atmosphere and the ocean. The total amplitude and time scales of these internal stochastic climate
variations are not well known; these variations therefore pose an additional difficulty in interpreting
the past record and projecting the level of future climate change.
PHYSICAL CLIMATE FEEDBACKS
Any imposed imbalance in the Earth's radiative budget, such as discussed above, will be
translated into a changed climate through feedback mechanisms which can act to amplify or decrease
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Chapter III
FIGURE 3-5
GLOBAL ENERGY BALANCE
(Watts/Square Meter)
Solar
340
100
90
150
86
154
Atmosphere
300
308
2XC02
\
240
390
160
394
I
154
Earth's Surface
(b)
SPACE
INCOMING
SOLAR
RADIATION
340
OUTGOING RAC'IATION
Short wdve Longwave
27 66 20 31
136
68
ATMOSPHERE
Absorbed by
Water Vapor.
Dusi, 0,
Net Emission
by
Water Vapor.
co2.o3
Absorption
by Clouds
Water Vapor,
^35)C°2'°3
Emission
by Clouds
LONGWAVE RADIATION
Ldtent
Heat Flux
Sensible
Heat Flux
OCEAN, LAND
166
380
340 24
62
Figure 3-5. (a) Highly simplified schematic of the global energy balance illustrating the mechanism
by which increased greenhouse gas concentrations warm the Earth's surface. The atmosphere is
treated as a thin layer that does not absorb solar radiation; the role of convective and latent heat
transfer is also neglected. Doubling the concentration of CO2 increases the absorption and emission
of infrared radiation by the atmosphere, increasing the total energy absorbed at the surface. In the
equilibrium depicted, total emissions to space remain unchanged.
(b) A more realistic schematic of the global energy balance for current conditions. (Source: adapted
from MacCracken, 1985.)
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the initial imposed forcing. In this section, several of these mechanisms which are internal to the
physical climate system are discussed. In the next section, several recently quantified mechanisms
involving the planet's biology and chemistry are described.
By no means do we understand or even know about all the mechanisms involved in climate
feedbacks. Figure 3-6 shows some of the physical climate feedbacks involved in changing surface
temperature. Current state-of-the-art climate models attempt to incorporate most of the physical
feedbacks that have been identified, but are forced, for example, to provide a very crude treatment
for one of the most important — changes in clouds — because of inadequate understanding of cloud
physics and because of the small spatial scale on which clouds form compared to the resolution of
climate models.
Water Vapor - Greenhouse
When the climate warms, the atmosphere can hold more water vapor. This enhances the
warming because it increases the greenhouse effect from water vapor, producing still more
evaporation from the warmed surface. This positive feedback acts to approximately double imposed
forcings.
Snow and Ice
When climate warms, snow and ice cover are reduced, exposing land or ocean with a lower
albedo than the snow or ice. In addition, the albedo of the remaining snow and ice is reduced due
to meltwater puddles and debris on the surface. This acts to absorb more energy at the surface,
further enhancing the warming. This albedo feedback was originally thought to be the dominant
positive feedback effect of snow and ice, but we now understand that the thermal inertia feedback
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Chapter III
FIGURE 3-6
EQUILIBRIUM TEMPERATURE CHANGES FROM DOUBLED C02
(Degrees Celsius; Based on 1.5-5.5 Degree Sensitivity)
5.5
5.0
4.5
4.0
CO
2 3.5
CO
UJ
o
v> 3.0
UJ
UJ
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Policy Options for Stabilizing Global Climate - Review Draft Chapter III
of sea ice plays a much more important role (Manabe and Stouffer, 1980; Robock, 1983). The
albedo feedback requires that the sun be shining, and since the maximum ice and snow extent is in
the winter, it plays a small role in influencing the albedo except in the spring, when the snow and
ice are present along with high insolation.
The thermal inertia feedback acts to increase the thermal inertia of the oceans when climate
warms by melting sea ice, reducing its insulating effect and increasing the transfer of heat from the
ocean to the atmosphere at high latitudes. This acts to reduce the seasonal cycle of surface
temperature and is the prime reason for the enhancement of imposed climate change in the polar
regions in the winter (Robock, 1983). If sea ice retained its current seasonal cycle, there would be
no preferential latitude or time of year for climate change.
Clouds
Clouds respond directly and immediately to changes in climate and may represent the most
important uncertainty in determining the sensitivity of the climate system to the buildup of
greenhouse gases. Fractional cloud cover, cloud altitude and cloud optical depth can all change when
climate changes (Schlesinger, 1985). It has not been possible to calculate the net effect of cloud
feedbacks because all these properties of clouds can change simultaneously, because clouds affect
long-wave radiation, short-wave radiation, and precipitation (which affects soil moisture and hence
albedo, thermal inertia, and moisture flux of land), and because the net effect depends on the
location of the cloud (in 3 dimensions), the underlying surface albedo, and the time of day and year
of the changes. The current net effect of clouds has only recently been measured (Ramanathan et
al., 1989). Current climate models include crude calculations of clouds and have difficulty even
reproducing the current cloud distribution.
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BIOGEOCHEMICAL CLIMATE FEEDBACKS
In addition to the climatic processes discussed above, a number of biogeochemical feedback
processes will influence future concentrations of greenhouse gases and climatic change. Increased
greenhouse gas concentrations will alter not only the climate, but also biogeochemical processes that
affect sources and sinks of radiatively important gases. Climatically important surface properties,
such as albedo and evapotranspiration, will also be modified by vegetation changes. The major
biogeochemical feedback links, illustrated in Figure 3-7, can be categorized as follows: physical
effects of climatic change, changes in marine biology, and changes in terrestrial biology. Potential
physical effects of climatic change include release of methane hydrates and changes in ocean
chemistry, circulation, and mixing. Changes in marine biology may alter the pumping of carbon
dioxide from the ocean surface to deeper waters and the abundance of biogenic cloud condensation
nuclei. Potential biological responses on land include changes in surface albedo, increased flux of
CO2 and CH4 from soil organic matter to the atmosphere due to higher rates of microbial activity,
increased sequestering of CO2 by the biosphere due to CO2 fertilization, and changes in moisture
flux to the atmosphere.
Release of Methane Hydrates
Potentially the most important biogeochemical feedback is the release of CH4 from near-shore
ocean sediments. Methane hydrates are formed when a methane molecule is included within a
lattice of water molecules; the ratio can be as small as 1:6, that is, one methane molecule for every
six water molecules (Bell, 1982). The hydrate structure is stable under temperature and pressure
conditions that are typically found under a water column of a few hundred meters or more in the
Arctic and closer to a thousand meters in warmer waters; the region where hydrates are found can
start at the sea floor and extend up to a few hundred meters into the sediment, depending on the
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Chapter HI
FIGURE 3-7
tn
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CD
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DRAFT - DO NOT QUOTE OR CITE
111-21
February 16, 1989
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Policy Options for Stabilizing Global Climate -- Review Draft Chapter HI
geothermal temperature gradient (Kvenvolden and Barnard, 1984). Estimates of the total quantity
of CH4 contained in hydrates range from 2x10* to 5xl06 Pg (Kvenvolden, 1988). Given the climatic
change associated with a doubling of CO2) Bell (1982; as corrected by Revelle, 1983) estimated that
there could be a release of -120 Tg CH4/yr from Arctic Ocean sediments, and Revelle (1983)
calculated global emissions of ~640 Tg CH4/yr from continental slope hydrates. These estimates
are, however, highly uncertain both because the total quantity of hydrates potentially subject to
destabilization is not known and because bottom water may be insulated from surface temperature
increases throughout much of the ocean (Kvenvolden, 1988). Nonetheless, a very strong positive
feedback from this source cannot be excluded at this time.
Oceanic Change
The oceans are the dominant factor in the Earth's thermal inertia to climate change as well as
the dominant sink for anthropogenic CO2 emissions. The mixed layer (approximately the top 75 m)
alone contains about as much carbon (in the form of H2CO3, HCO3", and CO3") as does the
atmosphere (see Chapter II). Furthermore, the ocean biota play an important role in carrying
carbon (as organic debris) from the mixed layer to deeper portions of the ocean (see, for example,
Sarmiento and Toggweiler, 1984). Thus, changes in ocean chemistry, biology, mixing, and large-scale
circulation have the potential to substantially alter the rate of CO2 accumulation in the atmosphere
and the rate of global warming.
Because the oceans are such an integral part of the climate system, significant changes in the
oceans are likely to accompany a change in climate. For example, the oceans are responsible for
about 50% of heat transport from the equator toward the poles (Dickinson, 1986), surface mixing is
driven by winds, and deep circulation is driven by thermal and salinity gradients. The feedbacks
involving the ocean can be divided into three categories: the direct effect of temperature on
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carbonate chemistry, reduced mixing due to increased stability of the thermocline, and the possibility
of large-scale reorganization of ocean circulation and biological activity1.
Ocean Chemistry
The most straightforward feedback is on ocean carbonate chemistry. As the ocean warms, the
solubility of CO2 decreases and the carbonate equilibrium shifts toward carbonic acid; these effects
combine to increase the partial pressure of CO2 (pCO2) in the ocean by 4-5%/°C for a fixed
alkalinity and total carbon content. Because the total carbon content would only have to decrease
by about one-tenth this amount to restore pCO2 to its previous level, the impact of this feedback is
to increase atmospheric CO2 by about 1%/°C for a typical scenario (Lashof, 1989; Chapter VI).
Ocean Mixing
As heat penetrates from the mixed layer of the ocean into the thermocline the stratification of
the ocean will increase and mixing can be expected to decrease, resulting in slower uptake of both
CO2 and heat. This feedback raises the surface temperature that can be expected in any given year
for two reasons: First, the atmospheric CO2 concentration will be higher because the oceans will
take up less CO2. Second, the realized temperature will be closer to the equilibrium temperature
due to reduced heat transport into the deep ocean (see the discussion of the transient response
below).
1 The thermocline starts at the base of the mixed layer and extends to a depth of about 1000m.
It is characterized by a rapid decrease in temperature with increasing depth, which inhibits mixing
in the water column because the colder deeper water is denser than the warmer overlying water.
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Ocean Biology and Circulation
A more speculative, but potentially more significant, feedback involves the possibility of large-
scale changes in the circulation of the atmosphere-ocean system as suggested by Broecker (1987).
This possibility is illustrated by the apparently very rapid changes in the CO2 content of the
atmosphere during glacial-interglacial transitions as revealed by ice-core measurements (e.g., Jouzel
et al., 1987; Figure 3-2a). Only shifts in carbon cycling in the ocean are thought to be capable of
producing such large, rapid, and sustained changes in atmospheric CO2. A number of papers have
attempted to model the changes in ocean circulation and/or biological productivity required to
account for the change in pCO2, emphasizing the importance of high-latitude processes (Kerr, 1988;
Sarmiento and Toggweiler, 1984; Siegenthaler and Wenk, 1984; Knox and McElroy, 1984). Given
that continuation of current trends could lead to a climate change during the next century of the
same magnitude as that which occurred between glacial and interglacial periods, one must take
seriously the possibility of sudden changes hi ocean circulation. Should this happen, the oceans could
even become a CO2 source rather than a sink - significantly accelerating climatic change. Such
changes in circulation could also cause abrupt changes in climate, a scenario that conflicts with the
general assumption that the warming will be gradual (Broecker, 1987).
A different feedback involving ocean biology has been proposed by Charlson et al. (1987). It
is also uncertain, but potentially significant. Dimethyl sulfide (DMS) emitted by marine
phytoplankton may act as cloud condensation nuclei in remote marine environments, affecting cloud
reflectivity and therefore climate (Charlson et al., 1987; Bates et al., 1987). Climate presumably
affects biogenic DMS production but the relationship is complex and poorly understood at this tune
(Charlson et al., 1987). While this mechanism was originally proposed as a potential negative
feedback consistent with the Gaia Hypothesis (Lovelock, 1988; Lovelock and Margulis, 1973), ice-
core data indicate that aerosol levels were higher during the last glacial maximum, suggesting that
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biogenic DMS production may act instead as a positive feedback (Legrand et al., 1988). This is only
one possible cloud optical property feedback (discussed above), and the net effect cannot be
determined because other cloud properties (amount, elevation) would also change in a complex way.
Changes in Terrestrial Biota
The terrestrial biota interact with climate in a wide variety of important ways (Figure 3-7). The
most significant effects on climate may result from large-scale reorganization of terrestrial ecosystems
as well as the direct effects of temperature and CO2 increases on carbon storage.
Vegetation Albedo
Probably the most significant global feedback produced by the terrestrial biota, on a decades-
to-centuries time scale is due to changes in surface albedo (reflectivity) as a result of changes in the
distribution of terrestrial ecosystems. Changes in moisture flux patterns are probably also important.
Cess (1978) argued that vegetation albedo feedback could have played a major role in explaining the
glacial-interglacial temperature change. Dickinson and Hanson (1984) reanalyzed this problem and
found a much smaller, but still significant effect, i.e., that the planetary albedo was 0.0022 higher at
the glacial maximum due to differences in mean annual vegetation albedo. A similar result was
obtained by Hansen et al. (1984) using a prescriptive scheme to relate vegetation type to climate in
GCM simulations for current and glacial times. (The much larger effect found by Cess was due to
differences in the albedo assigned to similar vegetation types for 18 kyBP versus the present
[Dickinson and Hanson, 1984].) This feedback may be less important in the future than it was during
the last deglaciation because of direct human effects on the surface, such as deforestation, and
because the pattern of vegetation change will be different.
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Carbon Storage
Other significant feedbacks are related to the role of the terrestrial biosphere as a source and
sink for CO2 and CH4. The carbon stored in live biomass and soils is roughly twice the amount in
atmospheric CO2, and global net primary production (NPP) by terrestrial plants absorbs about 10%
of the carbon held in the atmosphere each year. On average this is nearly balanced by decay of
organic matter, about 0.5-1% of which is anaerobic and thus produces CH4 rather than CO2. Small
shifts in the balance between NPP and respiration, and/or changes in the fraction of NPP routed to
CH4 rather than CO2, could therefore have a substantial impact on the overall greenhouse forcing,
because CH4 has a much larger greenhouse effect than CO2 per molecule. Both NPP and respiration
rates are largely determined by climate and NPP is directly affected by the CO2 partial pressure of
the atmosphere. Thus the potential for a substantial feedback exists.
Other Terrestrial Biotic Emissions
The biosphere plays an important role in the frequency and quantity of emissions of various
other atmospheric trace gases, which are also likely to be influenced by climatic change. For
example, as much as half of nitrous oxide (N2O) emissions are attributed to microbial processes in
natural soils (Bolle et al., 1986). Emissions of N2O tend to be episodic, depending strongly on the
pattern of precipitation events in addition to temperature and soil properties (Sahrawat and Keeney,
1986). Thus, climatic change could be accompanied by significant changes in N2O emissions, although
there is not sufficient understanding of the microbiology to predict these changes at present. The
biosphere is also a key source of atmospheric non-methane hydrocarbons (NMHCs), which play an
important role in global tropospheric chemistry; the oxidation of NMHCs generates a substantial
share of global carbon monoxide and therefore influences the concentration of OH and the lifetime
of CH4 (Mooney et al., 1987; Thompson and Cicerone, 1986). As much as 0.5-1% of photosynthate
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is lost as isoprene and terpene (Mooney et al., 1987). Lamb et al. (1987) found that the volume of
biogenic NMHC emissions in the United States is greater than anthropogenic emissions by about a
factor of two. The ratio for the globe is probably greater. Emissions, at least for isoprene and tt-
pinene are exponentially related to temperature (Lamb et al., 1987; Mooney et al., 1987). The first-
order impact of climatic change, then, would be to increase NMHC emissions, producing a positive
feedback through the CO-OH-CH4 link. The actual impact when changes in ecosystem distribution
are considered is uncertain, however, as different species have very different emissions (Lamb et al.,
1987).
Summary
Of the feedbacks that will come into play during the next century, the physical climate feedbacks
discussed earlier (water vapor, clouds, ice cover, and ice and snow albedo) will almost certainly have
the greatest impact. In comparison, the potential individual impacts of the biogeochemical feedbacks
discussed here are rather modest. If the physical climate feedbacks approach the strongly positive
end of their ranges, then the overall sensitivity of the climate system would be substantially increased
by even small additional feedbacks. However, since both the internal and biogeochemical feedbacks
are presently so poorly understood, and since other feedbacks may be discovered, the overall
equilibrium response of the climate system (discussed below) can only be specified with a fairly wide
range.
The perturbations to global biogeochemical cycles reflected in the feedback processes discussed
here are of great importance in their own right in addition to whatever warming they may produce.
The vegetation albedo feedback, for example, contributed only 03°C out of the 3.6°C global cooling
in the ice-age analysis of Hansen et al. (1984), but this represented a massive change in terrestrial
ecosystems. A better assessment of both the impact of climatic change on biogeochemical cycles and
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the associated feedbacks is needed. Several aspects of the impact of climatic change on
biogeochemical processes are discussed in the companion report Potential Effects of Global Climate
Change on the United States (Smith and Tirpak, 1989). A quantitative estimate of the impact of some
of the feedbacks discussed here is presented in Chapter VI based on incorporating them in the
Atmospheric Stabilization Framework developed for this study.
EQUILIBRIUM CLIMATE SENSITIVITY
When any forcing, such as an increase in the concentration of greenhouse gases, is applied to
the climate system, the climate will start to change. Since both the imposed forcings and the climatic
response are time-dependent, and since the climate system has inertia due to the response times of
the ocean, the exact relationship between the timing of the forcings and the timing of the response
is complex. In an attempt to simplify the problem of understanding the sensitivity of the climate
system to forcings, it has become a standard experiment to ask the question, "What would be the
change in global average surface air temperature if the CO2 concentration in the atmosphere were
doubled from the preindustrial level, all other climate forcings were held constant, and the climate
became completely adjusted to the new radiative forcing?" This quantity is called the "equilibrium
climate sensitivity to doubled CO2" and is indicated as AT^ (see Box 3-1).
The actual path that the climate system would take to approach the equilibrium climate would
be determined by the time scales of the forcings and the various elements of the climate system.
This is called the "transient response" and is discussed in the next section. Because the climate
system response always lags the forcing, there will always be a built-in unrealized warming that will
occur in the future, even if no more forcing occurs. Thus, there is certain to be some future climate
response to greenhouse gases that were put into the atmosphere in the past, even if no more are put
in starting today. Another way of saying this is that societal responses to the greenhouse problem
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BOX 3-1. SIMPLIFIED MODELING FRAMEWORK
The concepts discussed in this chapter can be summarized b a simple zero-
dimensional or one-box model of climate as discussed by Dickinson (1986):
(1) C(dAT/dt) + AAT = AQ
where AQ is the climate forcing and could be due to changes in solar output, volcanoes,
surface properties, stochastic processes or greenhouse gases (as discussed under Climate
Forcings and Feedbacks); AT is the change in tropospheric/mixed-layer temperature from
the preindustrial equilibrium climate; the factor A, called the "feedback parameter" by
Dickinson, gives the change in upward energy flux resulting from a change in surface
temperature, AT, and is the net result of all the climate feedbacks (as discussed in the
section on Climate Sensitivity); t is time; and C is the effective heat capacity of the
Earth, which is determined by the rate of heat uptake by the ocean (C must be a
function of time to account for the gradual penetration of heat into an increasing volume
of the deep ocean and changing sea ice cover). In equilibrium the first term in (1) is
zero, so the equilibrium climate sensitivity is simply given by
(2) AT = AQ/X
For a doubling of CO2, AQ is about 43 W/m2, so the range 1.5-5.5°C of A-T^ discussed
above corresponds to a range of 2.9-0.8 W m"2 "C1 in X. This conceptual model, with
AQ calculated from changes in greenhouse gases and C replaced by a diffusive model
of the ocean, is incorporated into the Integrating Framework used in the modeling
exercises for this report (see Chapter V and Appendix A).
that are undertaken now will be felt for decades in the future, and lack of action now will similarly
bequeath climate change to future generations.
Analysis of past climate change, and model calculations of future climate change can both be
used to determine AT^. Unfortunately, our knowledge of both past climate change and the
responsible forcings are too poor to reliably determine AT^ from past data. Wigley and Raper
(1987) estimate that if all of the warming of the past 100 years was due to greenhouse gases, then
ATjx would be approximately 2°C. If however, one allows for other possible forcings, natural
variability, uncertainties hi ocean heat uptake and the transient response, and for uncertainties hi
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preindustrial greenhouse gas concentrations (see below, Hansen et al., 1985; Wigley and Schlesinger,
1985; Wigley et al., 1986), then the climate record of the last 100 years is consistent with any ATjx
between 0 and 6°C (Wigley, personal communication).
Due to the various problems with direct empirical approaches, mathematical models of the
climate system are the primary tool for estimating climate sensitivity. While they have inherent
errors, they can isolate the greenhouse forcing, and many theoretical calculations can be made to test
the importance of various assumptions and various proposed feedback mechanisms. The simplest
climate model is the zero-dimensional global average model described in the box below. Models that
are one-dimensional in the vertical, often called "radiative-convective" models, and that are one-
dimensional in the horizontal, often called "energy-balance" models, are very useful for quickly and
inexpensively testing various components of the climate system. In order to calculate the location
of future climate change, however, and in order to incorporate ah1 the important physical interactions,
especially with atmospheric circulation, fully three-dimensional general circulation models (GCMs) are
necessary. These sophisticated models solve simultaneous equations for the conservation of energy,
momentum, mass, and the equation of state on grids with horizontal resolution ranging from 3 to 8
degrees of latitude by 3 to 10 degrees of longitude and with varying vertical resolution. The radiation
schemes attempt to account for the radiatively significant gases, aerosols and clouds. They generally
use different schemes for computing cloud height, cover and optical properties. The models also
differ in their treatment of ground hydrology, sea ice, surface albedo, and diurnal and seasonal cycles
(Schlesinger and Mitchell, 1988). Perhaps the most important differences lie in the treatment of
oceans, ranging from prescribed sea surface temperatures, to "swamp" oceans with mixed layer
thermal capacity but no heat transport, to mixed layers with specified heat transport, to full oceanic
GCMs.
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A series of reviews by the National Academy of Sciences (NAS, 1979, 1983, 1987) as well as
the "State-of-the-Art" report of the Department of Energy (MacCracken and Luther, 1985) have
concluded that the equilibrium sensitivity of climate to a 2xCO2 forcing (ATjx) is probably in the
range of 1.5 to 4.5°C. An independent review by Dickinson (1986) attempts to quantitatively combine
the uncertainties indicated by the range of recent GCM results and concludes that the range should
be broadened to 1.5-5.5°C. The GCM result of Wilson and Mitchell (1987) giving AT^ = 5.2°C
was published after all of the reviews cited here. Dickinson's estimates of the contributions of the
individual factors to climate sensitivity are shown in Figure 3-6. The largest positive feedback is from
changes in the amount and distribution of water vapor. Substantial positive feedback may also be
contributed by changes in sea ice and surface albedo and clouds, although the uncertainty range
includes the possibility that clouds contribute significant negative feedback. The differences in the
strength of these feedbacks between models is the result of different parameterizations of the relevant
processes as well as differences in the control (lxCO2) simulation (Cess and Potter, 1988). Even
though the exact value of AT^ is not known, we can study the potential impact of climatic warming
caused by greenhouse gases by choosing scenarios that span the range of theoretical calculations.
Thus, we adopt 2-4°C as a putative one standard deviation (1(7) confidence interval about the center
of the range proposed by the National Academy of Sciences, and the range proposed by Dickinson
(1.5 to 5.5°C) as 20 bounds for subsequent modeling (Chapter V). When the biogeochemical
feedbacks discussed above are also considered, a &TK as great as 8-10°C cannot be ruled out
(Lashof, 1989).
THE RATE OF CLIMATIC CHANGE
The Earth's surface does not immediately come to an equilibrium following an increase in
radiative forcing. Excess radiation captured by the Earth heats the land surface, the ocean, and the
atmosphere. The effective heat capacity of the oceanic part of the climate system, in particular, is
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enormous. The result is that the warming realized in any given year may be substantially less than
the warming that would occur in equilibrium if greenhouse gas concentrations were fixed at their
levels in that year. Hundreds of years would be required for the entire ocean to equilibrate with the
atmosphere, but only the surface layer (about 100 m) is well mixed by winds and therefore tightly
linked to climate in the short term. The heat capacity of the surface layer is about l/40th that of
the entire ocean and this layer by itself would equilibrate with a response time (the time required
to reach 1 - 1/e, or 63%, of the equilibrium response) of 2-15 years, depending on the climate
sensitivity and assumed mixed layer depth. The equilibration tune is longer if the climate sensitivity
is greater because the feedback processes that increase climate sensitivity respond to the realized
changes in climate, not to the initial change in forcing (Hansen et al., 1985). When the transfer of
heat from the mixed layer into the deep ocean is considered, it is impossible to characterize the
oceanic response with a single time constant (Harvey and Schneider, 1985; Wigley and Schlesinger,
1985).
While the main features of ocean circulation and mixing, and therefore the rate of heat and
carbon uptake, have been identified, they are not well defined or modeled on a global scale. The
theory and modeling of ocean circulation are currently limited by the inadequacy of the database
(Woods, 1985). The development of Ocean General Circulation Models (OGCMs) lags significantly
behind their atmospheric counterparts, mainly because it is difficult and expensive to obtain the
necessary data with sufficient temporal and spatial coverage, because fewer scientists have addressed
this problem, and because a large amount of computer power is needed to resolve the necessary time
and space scales. Due to these problems it may be a decade or more before OGCMs reach the state
of development achieved by current atmospheric GCMs.
Lacking well-tested OGCMs, the main tools used so far to investigate ocean heat uptake have
been highly parameterized models, very similar to those used for carbon (see Chapter II). These
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models are calibrated with data on the penetration of tracers such as tritium and 14C produced by
atmospheric nuclear weapons tests during the 1950s and early '60s and/or with the steady-state
profiles of various ocean parameters, such as natural WC and temperature. The simplest models
that yield a plausible time-dependence for heat (and carbon) uptake lump the entire ocean into two
compartments: A well-mixed surface layer and a deep ocean compartment in which mixing is
parameterized as a diffusive process (Box-Diffusion or BD model). This approach was introduced
by Oeschger et al. (1975) for modeling carbon uptake, and has been applied to ocean heat absorption
by Hansen et al. (1985) and Wigley and Schlesinger (1985), among others. A more elaborate version
of this model which includes a representation of upwelling implicitly balanced by high-latitude bottom-
water formation (UpweUing-Diffusion or UD model), has been used by Hoffert et al. (1980), Harvey
and Schneider (1985), and Wigley and Raper (1987). The addition of an upwelling term allows the
observed mean thermal structure of the ocean to be approximated (Hoffert et al., 1980), but given
the highly parameterized nature of both of these models, there is no convincing reason to favor one
approach over the other for modeling small perturbations to heat flows.
The response tune, T, of Box-Diffusion models is proportional to ^(ATjJ2, where K is the
diffusion constant used to characterize deep ocean mixing (Hansen et al., 1985; Wigley and
Schlesinger, 1985). Data on the penetration of tracers into the ocean suggests that K = 1-2 cm2/s
(Hansen et al., 1985). Hoffert and Hannery (1985) have argued that mixing rates derived from
tracers may be too high for heat because mixing rates are highest along constant density surfaces,
which are nearly parallel to ocean isotherms. On the other hand, in a preliminary coupled GCM-
OGCM run, Bryan and Manabe (1985) found that heat was taken up more rapidly than with a
passive tracer because of reduced upward heat convection. Using a range of 0.5-2 cm2/s for K and
the la range for AT& given above (2-4°C) in the equation derived by Wigley and Schlesinger (with
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their recommended values for other parameters) yields 7 = 6-95 years.2 Correspondingly, the
warming expected by now, based on past increases in greenhouse gases and assuming no other
climate forcings, is roughly 40-80% of the equilibrium warming (Wigley and Schlesinger, 1985). In
other words, even if greenhouse gas concentrations could be fixed at today's level, the Earth would
still be subject to significant climatic change that has yet to materialize. The large uncertainty
surrounding ocean heat uptake, combined with uncertainty about potential climate forcings other than
those from greenhouse gases, also implies that it is not possible to obtain a useful constraint on AT^
from the observed temperature record as discussed above (see also Hansen et al, 1985; Wigley and
Schlesinger, 1985).
Experiments with Upwelling-Diffusion models demonstrate the importance of the bottom water
formation process for the rate of ocean heat uptake. The impact of using an Upwelling-Diffusion
ocean model rather than a Box-Diffusion ocean model is that the heat that diffuses into the
thermocline is pushed back toward the mixed layer, which decreases the effective heat capacity of the
ocean and the time constant for tropospheric temperature adjustment, assuming that the upwelling rate
and the temperature at which bottom water is formed do not change. If the initial temperature of the
downwelling water is assumed to warm as much as the mixed layer, however, then a UD model
actually takes up more heat in the ocean than a BD model, leading to a larger disequilibrium
between a given radiative forcing scenario and the expected realized warming. While there are
reasons to think that the temperature of Antarctic Bottom Water will not increase as climate changes,
the temperature of North Atlantic Deep Water could increase or decrease (Harvey and Schneider,
1985). Furthermore, there is no reason to assume that the rate of bottom-water formation will
remain constant as climate changes. The tropospheric temperature could even overshoot equilibrium
2 It is important to note that the actual response does not correspond to exponential decay with
a single time constant, so that while T gives tie time required for one e-folding and is a useful
measure, it would not apply to subsequent e-foldings (the time constant would be substantially longer)
and must be interpreted with care.
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if the average bottom-water temperature cools as the surface temperature warms or if the upwelling
rate increases with warming (Harvey and Schneider, 1985). One must also recognize the potential
for sudden reorganizations of the ocean-atmosphere circulation system as suggested by Broecker
(1987), which could lead to discontinuous, and perhaps unpredictable, changes in climate that cannot
be included in the models used in this report.
Another major limitation of the BD and UD models generally used to analyze the climate
transient problem is that they have limited or no spatial resolution (at best hemispheric, land-sea)
and thus cannot consider spatial heterogeneity in either the magnitude or rate of climatic change.
Work at the Goddard Institute for Space Studies (Hansen et al., 1988) has produced one of the few
three-dimensional time-dependent analyses of climatic change that have been published to date. This
study employed three simple, but reasonably realistic, scenarios of future greenhouse gas
concentrations and volcanic eruptions. The results suggest that the areas where warming is initially
most prominent relative to interannual variability are not necessarily those where the equilibrium
warming is greatest. For example, low-latitude ocean regions warm quickly because ocean heat
uptake is limited by strong stratification in these regions. Warming is also prominent in high-latitude
ocean areas where a large equilibrium warming is expected due to increased thermal inertia as sea
ice melts. Global average temperatures are used in this report as an indicator of the rate and
magnitude of global change but, as these results emphasize, it must be recognized that major
variations among regions are a certainty.
CONCLUSION
The changing composition of the atmosphere will in turn drive significant changes in the Earth's
climate. These changes may have already begun, but because of the uncertainties in temperature
data sets and the complexity of the interaction between climate sensitivity and the transient response,
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definitive predictions are subject to a good deal of controversy at this time. Whether next year is
warmer or cooler than this year, however, has no direct bearing on how the greenhouse effect should
be viewed. Internal fluctuations or countervailing forcings may temporarily mask the warming due
to increasing concentrations of greenhouse gases or make the climate warmer than expected solely
from greenhouse warming. Therefore, to derive our estimates of the magnitude and rate of change
that can be expected during the next century we must continue to rely on model calculations, which
indicate that by early in the next century the Earth could be warmer than at any time during the last
million years or more, and that the rate of change could be unprecedented in Earth history.
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REFERENCES
Barnola, J.M., D. Raynaud, Y.S. Korotkevich, and C. Lorius. 1987. Vostok ice core provides
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CHAPTER IV
HUMAN ACTIVITIES AFFECTING TRACE GASES
AND CLIMATE
FINDINGS IV-2
INTRODUCTION IV-5
HISTORICAL OVERVIEW OF POPULATION TRENDS IV-5
Global Population Trends IV-7
Population Trends by Region IV-7
Industrialized Countries IV-10
Developing Countries IV-10
ENERGY CONSUMPTION IV-12
History of Fossil-Fuel Use IV-13
Current Energy Use Patterns and Greenhouse Gas Emissions IV-18
Emissions by Sector IV-20
Fuel Production and Conversion IV-25
Future Trends IV-27
The Fossil-Fuel Supply IV-29
Future Energy Demand IV-29
INDUSTRIAL PROCESSES IV-31
Chlorofluorocarbons, Halons, and Chlorocarbons IV-33
Historical Development and Uses IV-33
The Montreal Protocol IV-37
Landfill Waste Disposal IV-40
Cement Manufacture IV-43
LAND USE CHANGE IV-45
Deforestation IV-47
Biomass Burning IV-50
Wetland Loss IV-51
AGRICULTURAL ACTIVITIES IV-55
Enteric Fermentation In Domestic Animals IV-55
Rice Cultivation IV-56
Use of Nitrogenous Fertilizer IV-61
IMPACT OF CLIMATIC CHANGE ON ANTHROPOGENIC EMISSIONS IV-63
REFERENCES IV-67
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FINDINGS
Various human activities affect the Earth's climate by altering the level of trace gases in the
atmosphere. These activities include energy consumption, particularly fossil-fuel consumption;
industrial processes; land use change, particularly deforestation; and agricultural practices such as
waste burning, fertilizer usage, rice production, and animal husbandry. Economic development and
population growth are key factors affecting the level of each activity.
• Population levels and growth rates have increased tremendously over the last 200 years.
Between 1650 and 1980, the global population doubling time shrunk from 200 to 35 years.
At the beginning of this century, global population was about 1.6 billion; in 1987, it reached
5 billion. By the early part of the next century total population is likely to reach 8 billion.
The rate of population increase is most acute in the developing regions, particularly Africa
and Asia where annual rates of growth exceed 2%.
• Fossil fuel combustion emits carbon dioxide and other radiatively important gases and is the
primary cause of atmospheric warming. Energy consumption currently accounts for more than
five of the six to eight billion tons of carbon dioxide emitted to the atmosphere annually
from anthropogenic sources. Between 1950 and 1986, annual global fossil fuel consumption
grew 3.6-fold and annual carbon dioxide emissions grew 3.4-fold.
• Emissions of other trace gases due to fossil fuel consumption are more uncertain.
Approximately 0 to 2 million tons nitrogen as nitrous oxide, 20 million tons nitrogen as
nitrogen oxides, and 180 million tons carbon as carbon monoxide are emitted annually from
fossil fuel combustion. Leaking and venting of natural gas contributes approximately 20 to
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50 million tons methane annually to the atmosphere, and coal mining contributes
approximately 25 to 45 million tons methane.
• Three significant non-energy sources of greenhouse gases are associated with industrial
activity. Production and use of chlorofluorocarbons (CFCs), halons, and chlorocarbons; waste
disposal in landfills; and cement manufacture. Production of CFC-11 and CFC-12 grew 4.7-
fold between 1960 and 1985. Consumption of major CFCs and halons reached nearly one
million tons in 1985. An international agreement (the Montreal Protocol), however, came
into force on January 1, 1989 to reduce future production of certain CFCs and halons.
Anaerobic decay of organic wastes in landfills currently contributes approximately 30-70
million tons of methane to the atmosphere annually. Cement production, which has
increased sevenfold since the 1950s, contributed approximately 134 million tons carbon as CO2
to the atmosphere in 1985.
• Land use change has resulted in substantial emissions of greenhouse gases to the atmosphere.
Since 1850, approximately 15% of the world's forests have been converted to agricultural and
other land uses. Currently, deforestation contributes between one-tenth and one-third of the
total anthropogenic carbon dioxide emissions to the atmosphere, i.e., between 0.4 to 2.6
billion tons of carbon. Between one-quarter and one-half of the world's swamps and marshes
also have been destroyed by man. Wetlands currently contribute approximately one-fifth of
the total methane emissions to the atmosphere; continued changes to wetlands could
significantly alter the global methane budget. Biomass burning, in addition to contributing
to the atmospheric concentrations of carbon dioxide, contributes approximately 10 to 20%
of total annual methane emissions, 5 to 15% of the nitrous oxide emissions, 10 to 35% of
the nitrogen oxide emissions, and 20 to 40% of the carbon monoxide emissions.
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• Three agricultural activities directly result in major contributions to atmospheric emissions
of greenhouse gases: animal husbandry, rice cultivation, and nitrogenous fertilizer use.
Domestic animals, which produce methane as a by-product of enteric fermentation, currently
contribute approximately 65 to 85 million tons of methane annually. Over the past several
decades, domestic animal populations have grown by up to 2% annually. Methane is also
produced by anaerobic decomposition in rice paddies. Currently, about one-fifth of annual
methane emissions, or between 60 and 170 million tons, comes from rice cultivation. Rice
production has grown rapidly since the mid-1900s due to bcreases in crop acreage, double
cropping, and higher yields. Between 1950 and 1984 rice production increased nearly
threefold, and harvested area grew by about 70%. Use of nitrogenous fertilizers results in
nitrous oxide emissions, either directly from the soil, or indirectly from groundwater. Global
use of organic and inorganic fertilizers has risen markedly, and nitrogen-based fertilizers
increased their market share of total inorganic fertilizer consumption from 28% in 1950 to
64% in 1981. Nitrogenous fertilizer use may contribute between 0.14 and 2.4 million tons
nitrogen as nitrous oxide per year to the atmosphere.
• In addition to the human activities that directly affect trace gas emissions, future
concentrations of greenhouse gases will be influenced by feedback processes resulting from
humans living in a world that has undergone climatic change. Two potential feedbacks of
increased temperatures, which may counteract each other to some extent, are increased
energy demand for air conditioning in the summer, and decreased energy demand for heating
in the winter.
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INTRODUCTION
As discussed in Chapter III, the Earth's climate has been in a constant state of change
throughout geologic time due to natural perturbations in the global geobiosphere. However, various
human activities have the potential to cause future global warming over a relatively short amount of
time. These activities, which affect the Earth's climate by altering the concentrations of trace gases
in the atmosphere, include energy consumption, particularly fossil-fuel consumption; industrial
processes (production and use of chlorofluorocarbons, halons, and chlorocarbons, landfilling of wastes,
and cement manufacture); changes in land use patterns, particularly deforestation and biomass
burning; and agricultural practices (waste burning, fertilizer usage, rice production, and animal
husbandry). Population growth is an important underlying factor affecting the level of growth in each
activity.
This chapter describes how the human activities listed above contribute to atmospheric change,
the current pattern of each activity, and how levels of each activity have changed since the early part
of this century. Figure 4-1 illustrates the current contributions to the greenhouse gas buildup by
region. Almost 50% of the warming is attributable to activities in the United States, the USSR, and
the European Economic Community. As background to the discussion of trace-gas producing
activities, we first provide an overview of population trends. This historical perspective is meant to
serve as a framework for the discussion of possible future scenarios of trace-gas emissions in Chapter
V.
HISTORICAL OVERVIEW OF POPULATION TRENDS
One of the major factors affecting trends in greenhouse gas emissions is the increase in human
population. As population levels rise, increasing pressures are placed on the environment as the
larger population strives to feed and clothe itself and achieve a higher standard of living. Without
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Chapter IV
FIGURE 4-1
REGIONAL CONTRIBUTION TO GREENHOUSE WARMING
1980s
(Percent)
Rest of the World (36%)
United States (21%)
USSR(14%)
China (7%)
EEC(14%)
Figure 4-1. Estimated regional contribution to greenhouse wanning for the 1980s, based upon
regional shares of current levels of human activities that contribute to greenhouse gas emissions.
(Sources: U.S. EPA, 1988a; United Nations, 1987; U.S. BOM, 1985; IRRI, 1986; FAO, 1986a, 1987;
Bolle et al., 1986; Rotty, 1987; Lerner et al., 1988; Seiler, 1984; WMO, 1985; Hansen et al., 1988;
Houghton et al., 1987; Matthews and Fung, 1987.)
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changes in the methods used to meet people's needs, higher population levels invariably lead to
increased emissions of greenhouse gases.
Global Population Trends
Not only has global population grown rapidly over the past few centuries, but the rate of growth
has also increased (see Figure 4-2). World population hi the year 1 A.D., approximately 0.25 billion,
doubled by 1650 (Wagner, 1971). By 1850 (i.e., 200 years later), global population had roughly
doubled again to 1.1 billion. The global population doubling time has continued to decline — 80
years later, in 1930, world population was 2 billion. By 1975 global population had reached 4 billion,
and according to some estimates the population will double once again within 35 years (world
population reached 5 billion in 1987). Moreover, despite recent declines in the world's annual
population growth rate (IIED and WRI, 1987), world population is expected to continue to grow
rapidly. Several studies estimate that world population will exceed 8 billion by 2025 (Zachariah and
Vu, 1988; U.S. Bureau of the Census, 1987). Such rapid population growth can be expected to result
in increasing pressure on the global environment, particularly as the burgeoning human population
strives to improve its living standards through economic growth.
Population Trends by Region
The rapid population growth in recent decades has not occurred uniformly around the world (see
Figure 4-2). Between 1950 and 1985, population in developed countries increased by 41%, compared
to 117% in developing countries (IIED and WRI, 1987). Recent trends indicate these differences will
continue: Annual growth rates in the developed countries are generally less than 1%, while many
developing countries continue to experience rates of growth between 2 and 3% (see Table 4-1).
These higher growth rates in the 20th century in developing countries have been due primarily to the
combined effects of declining death rates and continued high birth rates.
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Chapter IV
FIGURE 4-2
REGIONAL POPULATION GROWTH
1750-1985
(Billions)
z
o
CD
North America
& Oceania
Latin America
Africa
Europe & USSR
Asia
1750
1985
Figure 4-2. Since about 1850, global population has grown at increasingly rapid rates. In 1850, the
population doubling time was approximately 200 years; by 1975, the doubling time had declined to
approximately 45 years. Most of the growth has occurred in the developing world, particularly Asia.
(Sources: Matras, 1973; Hoffman, 1987.)
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TABLE 4-1
Regional Demographic Indicators
Region
North America
Europe
East Asia
Oceania
Caribbean
Southeastern Asia
Latin America
Southern Asia
Western Asia
Africa
•
Total
Fertility4
1980-85
1.83
1.88
2.34
2.65
3.34
4.11
4.17
4.72
5.22
6.34
Infant
Mortality"
1980-85
11
15
36
31
65
73
61
115
81
112
Annual
Population
Growth Rate
1980-85
(percent)
0.90
0.30
1.22
1.51
1.53
2.05
2.34
2.14
2.79 .
2.92
World Average
3.52
60
1.67
a The total fertility rate is the average number of children that a woman bears in a
lifetime.
The infant mortality rate is the average number of infant deaths (deaths before the first
birthday) per 1,000 live births.
Source: Adapted from IIED and WRI, 1987.
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Industrialized Countries
Population growth rates in the industrialized countries are substantially lower compared with
growth rates in the developing world. For example, while most developing countries contend with
growth rates that will double their populations within 20-40 years, current growth rates in North
America and Europe will lead to a doubling within about 100 years and 250 years, respectively (IIED
and WRI, 1987). This trend toward lower growth rates is due to many complex economic and social
factors, including the changing role of women in the labor force, the higher economic costs of child
rearing, and the reduced need for children as a labor pool.
Developing Countries
The highest rates of population growth are in the developing countries: From 1950 to 1985
developing countries increased their share of the world's population from 66.8% to 75.6% (IIED and
WRI, 1987). During this time Asia's population grew from 1.3 to 2.7 billion, Africa's from 224 to
555 million, and Latin America from 165 to 405 million. Key trends are summarized below.
Africa. Africa currently has the highest fertility rates and population growth rates in the world.
Its growth rate has increased recently: Between 1955 and 1985, Africa's average annual growth rate
increased from 2.3% to 2.9%. The total fertility rate (i.e., average number of children that a woman
bears in a lifetime) is six or higher in 38 African countries, most of which have experienced declining
infant mortality rates (infant deaths per thousand live births) over the past 20 years (IIED and WRI,
1987). For example, in Kenya, where the total fertility rate is 7.8, the infant mortality rate fell from
112 to 91 between 1965 and 1985. Between 1965 and 1985, the crude birth rate (births per thousand
population) for Kenya grew by 4.7%, while the crude death rate fell by 37.7%. The average annual
growth rate reached 4.1% in the 1980s (World Bank, 1987). The United Nations expects the African
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population to continue to grow rapidly, with the average annual growth rate increasing to 3% in
1990 (United Nations, 1986).
Asia.. From 1850 to 1950, Asia experienced the largest increase in population in the world
(Ehrlich and Ehrlich, 1972). Rates of growth have continued at high levels ~ annual growth rates
since 1960 have exceeded 2%. These rates are likely to remain high in several Asian countries in
future years (United Nations, 1986). For example, China currently is the most populous country in
the world, with 22% of the world's total (Ignatius, 1988). Although its strong population policy of
one child per family helped to halve the 2% annual growth rates of the 1960s, growth rates have
recently turned upward, approaching 1.5% annually. This trend of growth could lead to population
levels in China in excess of 1.7 billion by 2025.
India's population has also been rapidly expanding. It is the second most populous country hi
the world (United Nations, 1986), with 765 million people as of 1985. India's rate of growth has
been relatively high this century, although it has declined in recent years; in 1960 its annual rate of
growth was 2.3%, but has since dropped to 1.7% (IIED and WRI, 1987). Despite this recent decline,
its population is expected to grow for many years; e.g., the United Nations estimates that India's
population will be over 1.2 billion by 2025 (United Nations, 1986).
Latin America. Latin America currently has one of the highest population growth rates hi the
world: From 1980 to 1985 the annual rate of growth averaged 2.3% for the region (IIED and WRI,
1987), although these rates of growth varied substantially between countries. Argentina, Chile, and
Uruguay have the lowest growth within Latin America, while countries such as Bolivia, Ecuador, El
Salvador, Guatemala, Honduras, Nicaragua, Paraguay, and Venezuela have annual population growth
rates that exceed 2.5%. Fertility rates have been declining throughout the region due to
industrialization, urbanization, rising incomes, and official population policies, although one source
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estimates that Latin America's share of world population will nonetheless increase from 8.4 to 9.5%
between 1985 and 2025 (IIED and WRI, 1987). The two population projection sources used in this
report (U.S. Bureau of the Census, 1987, and Zachariah and Vu, 1988; see Chapter V) project that
by 2025, Latin America's share of world population will grow to 9.1 and 8.7%, respectively.
ENERGY CONSUMPTION
The major human activity affecting trace-gas emissions is the consumption of energy, particularly
energy from carbon-based fossil fuels. As discussed in Chapter II, global carbon dioxide (COj)
emissions from anthropogenic sources currently range from 6 to 8 petagrams (Pg) of carbon (C)
annually, with commercial energy consumption accounting for approximately 65-85% of this total.1
Non-commercial (biomass) energy consumption accounts for approximately 7%. Energy consumption
and production also produce substantial amounts of other greenhouse gases, including carbon
monoxide (CO), methane (CH4), nitrogen oxides (NOX, or NO and NOj), and nitrous oxide (N2O).2
This section explores the role of energy consumption in climate change. We first discuss the
world's increasing reliance on fossil fuels, the roles that fossil-fuel production (e.g., coal mining and
oil drilling) and fossil-fuel combustion play in the emission of trace gases to the atmosphere, and the
implications of the continuation of current energy consumption patterns on future global warming.
1 Anthropogenic sources of trace gases are those resulting from human activities, e.g.,
combustion of fossil fuels. These sources are distinguished from natural sources, since emissions
from anthropogenic sources result in unbalanced trace-gas budgets and accumulation of gases in
the atmosphere.
1 Throughout the report these gases are often referred to as greenhouse gases, although
strictly speaking, CO and NO, are not greenhouse gases since they do not directly affect radiative
forcing (see Chapter II). These two gases indirectly affect global wanning due to their chemical
interactions with other gases in the troposphere. As a result, for simplicity, we shall refer to
them as greenhouse gases.
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History of Fossil-Fuel Use
Prior to the discovery and development of fossil fuels (coal, oil, and natural gas), people relied
on readily-available energy resources such as wood and other forms of biomass (i.e., living matter),
as well as water and wind, to satisfy their basic energy needs. Since the beginning of the 19th
century, fossil fuels have played an increasingly important role in the world economy, particularly for
developed countries, by providing the energy required for industrial development, residential and
commercial heating, cooling, and lighting, and transportation services. Fossil fuels now provide about
85% of the world's total energy requirements. This dependence on fossil fuels is greatest in
industrialized countries, where over 95% of all energy needs are provided by fossil fuels, compared
with about 55% in developing countries (Hall et al., 1982) .3
Global consumption of fossil fuels has increased rapidly over the past century as human
populations and their economic activities have grown, resulting in the development of additional
fossil-fuel resources. Since 1950, global primary energy consumption has increased nearly fourfold
(Figure 4-3), with energy consumption per capita approximately doubling. In 1985, 42% of global
energy demand was supplied by liquid fossil fuels (primarily petroleum); solid fuels (coal) supplied
31%, natural gas, 22%, and other fuels combined accounted for 5% of the market share.4 These
relative proportions have changed considerably since 1950, when coal supplied 59% of total
commercial energy requirements, liquids, 30%, natural gas, 9%, and other fuels, 2%.
The increase hi fossil fuel consumption over the last century has caused a substantial increase
in the amount of CO2 emitted to the atmosphere. Carbon dioxide emissions from fossil fuels grew
3 In some developing countries, the dependence on biomass can approach 95 percent of total
energy requirements.
4 Non-commercial biomass estimates are not included in these figures.
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Chapter IV
FIGURE 4-3
GLOBAL ENERGY DEMAND BY TYPE *
1950 - 1985
(Exajoules)
300
250 -
200 -
V)
Ul
_1
o
<
X
OJ
150 -
100 -
50
Other
Natural Gas
Liquid Fuels
Solid Fuels
1950 1955 1960 1965 1970 1975 1980 1985
YEAR
* Data is for commercial energy only; biomass Is not included
Figure 4-3. Global demand for fossil fuels has more than tripled since 1950. Today, about 85% of
the world's energy needs are met by fossil fuels. (Sources: United Nations, 1976, 1982, 1983, 1987.)
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from less than 0.1 Pg C annually in the mid-nineteenth century, to about 5.4 Pg C in 1986 (see
Figure 4-4).5 This rate of increase is about 3.6% per year and is the major reason why atmospheric
CO2 concentrations increased from about 290 ppm in 1860 to about 348 ppm as of 1987 (Farman et
al, 1985). Currently fossil fuel combustion also contributes approximately 0-2 Tg N as N2O, 20 Tg
N as NOX and 180 Tg C as CO annually to the atmosphere.
In recent decades there has also been a significant shift hi global energy use patterns. In 1950,
countries belonging to the Organization for Economic Cooperation and Development (OECD)
consumed about three fourths of all commercial energy supplies, the centrally-planned economies of
Europe and Asia, 19%, and developing countries, 6% (United Nations, 1976, 1983).6 By 1985 OECD
countries consumed just over one-half of all commercial energy globally, while the European and
Asian centrally-planned economies and the developing countries had increased their relative shares
to 32% and 15%, respectively (see Figure 4-5). Between 1950 and 1985, commercial energy use per
capita in the OECD grew from 93 to 189 gigajoules per capita (GJ/cap) (103%), in centrally-planned
economies from 16 to 59 GJ/cap (269%), and in the developing countries from 3 to 18 GJ/cap
(500%).7 The proportion of energy consumed by the OECD is expected to decline further as the
developing world continues to experience population growth and economic development and, thus,
significantly expands their energy requirements (Chapter V).
5 In 1986 CO2 emissions from fossil fuels were approximately 5370 million metric tons C, or
5.37 Pg C. 1 billion metric tons = 1 gigaton = 1 Pg = 1015 grams.
6 The OECD countries include the U.S., Canada, Japan, Australia, New Zealand, the United
Kingdom, France, Spain, Portugal, the German Federal Republic, Belgium, the Netherlands,
Sweden, Norway, Finland, Italy, Ireland, Iceland, Denmark, Austria and Switzerland.
7 1 GJ = 1 gigajoule = 109 joules. 1055 joules = 1 Btu.
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Chapter IV
FIGURE 4-4
C02 EMISSIONS DUE TO FOSSIL FUEL CONSUMPTION
1860-1985
(Petagrams Carbon)
6
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Chapter IV
FIGURE 4-5
GLOBAL COMMERCIAL ENERGY DEMAND BY REGION
(Exajoules)
350
CO
111
O
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Current Energy Use' Patterns and Greenhouse Gas Emissions
The allocation of energy consumption among sectors varies considerably from one region to the
next. Figure 4-6 summarizes 1985 end-use energy demand (for both commercial and non-commercial,
or biomass, fuels) by sector for the OECD countries, the centrally-planned economies of Asia and
Europe (including China and the USSR), and the developing countries. Whereas the OECD split
is approximately one-third industrial, one-third transportation, and one-third residential/commercial,
centrally-planned economies of Asia and Europe consume more than 50% of their energy in the
industrial sector.
These energy consumption patterns partly reflect the basic differences in the structure of
economic activity at the current stage of each region's economic development. The centrally-planned
economies and the developing countries devote a greater share of their energy requirements to the
industrial sector because they are at a stage of economic development where energy-intensive basic
industries account for a large share of total output, while infrastructure in the transportation and
commercial sectors has not been extensively developed. In the OECD, transportation consumes a
larger share of total energy compared with other regions, primarily because of the large number of
automobiles in the OECD. For example, in the U.S. there are 550 cars and light trucks/1000 people,
compared with 60 cars and light trucks/1000 people in the USSR, and 6 cars and light trucks/1000
people in China. Also, biomass is very important to the residential energy requirements of the
developing economies compared with those of industrialized countries; the industrial sector is the
major consumer of fossil fuels in most developing countries.
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Chapter IV
FIGURE 4-6
1985 SECTORAL ENERGY DEMAND BY REGION
COMMERCIAL AND NON-COMMERCIAL FUELS
(Exajoules)
42.6,
33.4
29.3
39.7
19.0
27.0
OECD
8.4
CENTRALLY PLANNED
Residential/Commercial
Industrial
Transportation
10.0
DEVELOPING
Figure 4-6. End-use energy demand by sector for three global regions. While energy demand in the
OECD countries is split almost equally between the three sectors, over 50% of the energy b the
centrally planned countries is consumed by the industrial sector, and almost 50% of the energy in the
developing countries is consumed by the residential/commercial sector. (Sources: Sathaye et al.,
1988; Mintzer, 1988.)
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Emissions by Sector
The differences among regions in terms of the share of energy consumed by each sector and the
types of applications for which the energy within each sector is used can have a major impact on the
amount and types of greenhouse gases emitted. This section discusses how emissions of greenhouse
gases vary as a result of differences in type of fossil energy consumed and combustion technology
used.
Electric Utility Sector. Energy is increasingly desired in the form of electricity. The amount of
greenhouse gases produced from electricity generation is a function of the type of primary energy
used to produce the electricity and the production technology. For example, nuclear, hydroelectric,
or solar primary energy sources emit little or no greenhouse gases, while fossil fuels generate
substantial quantities of CO2, as well as other gases (see Table 4-2). The amount of greenhouse gas
emissions varies according to the type of fossil fuel used because of inherent differences in the
chemical structure of the fuels. Additionally, the level of emissions varies as a function of production
efficiency. For example -
• Coal-fired power plants produce about two to three times as much CO2 as natural
gas-fired units per unit of electricity generated (330 kg CO2/GJ for a pulverized coal
wall-fired unit compared with 120 kg CO2/GJ for a combined cycle gas-fired unit).
Oil-fired units produce more CO2 than natural gas units produce, but less CO2 than
coal-fired units produce. Within fuel types the emission levels may vary. For ex-
ample, when natural gas is used as the fuel, combined cycle or ISTIG units produce
about 40% lower CO2 emissions than simple cycle units (see Chapter VII) because
of the greater generating efficiency obtained through the use of these technologies.
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Chapter IV
TABLE 4-2
Emission Rate Differences by Sector
(grams per gigajoule)'
Source
Electric Utility (g/GJ delivered
Gas Turbine Comb. Cycle
Gas Turbine Simp. Cycle
Residual Oil Boilers
Coal - F. Bed Comb. Cycle
Coal - PC Wall Fired
Coal - PC Cyclone
Coal - Integrated Gas
Efficiency
(%)
electricity)
42.0
26.4
32.4
35.0
31.3
313
27.3
C02
120,300
191,400
230,000
290,000
330,000
330,000
253,600
Industrial (g/GJ delivered steam for boilers; energy output for
Boilers
Coal-Fired
Residual Oil-Fired
Natural Gas-Fired
Kilns - Coal
Dryer - Natural Gas
Dryer - Oil
Dryer - Coal
Residential /Commercial (g/GJ
Wood Stoves
Coal Stoves
Distillate Oil Furnaces
Gas Heaters
Wood Boilers
Gas Boilers
Residual Oil Boilers
Coal Boilers
80
85
85
65-75
30-65
30-65
30-65
energy output)
50
50
75
70
67.5
80.9
84.9
75.9
130,000
88,000
57,000
300-350,000
75-170,000
100-240,000
155-340,000
[150,000]
198,000
111,000
101,000
[138,0001
61,800
86,000
135,000
CO
70
110
43
NA
42
42
222
others)
110
17
18
75
10
15
170
17,600
3,400
17
13
280
10.6
19
244
CH4
13
20
2.2
1.8
2.0
2.0
NA
2.9
3.3
1.5
1
1
1
1
70
NA
7
1
21
1.4
1.8
13
N2O
20
30
44
40
45
45
51
18
16
35
2
NA
NA
NA
NA
NA
NA
NA
6
2.7
14
16
NOX
400
640
590
690
1,400
2,600
760
390
180
71
500
52
160
215
190
170
65
61
47
53
183
295
Total
Carbon
32,850
52,060
62,750
79,090
90,020
90,020
69,260
35,510
24,010
15,550
81-95,490
20-46,370
27-65,460
42-92,800
48,500
55,460
30,290
27,550
37,770
16,860
23,460
36,930
Transportation (g/GJ energy input)
Rail
Jet Aircraft
Ships
Light Duty Gasoline Vehicle
Light Duty Diesel Vehicle
Light Duty Compressed
N. Gas Vehicle
NA
NA
NA
NA
NA
NA
69,900
72,800
70,000
54,900
73,750
50,200
570
120
320
10,400
340
4
13
2
20
36
2
120
NA
NA
NA
0.5
20
7
1,640
290
830
400
300
140
19,320
19,910
19,240
19,460
20,260
13,780
a All emission rates, except for the total carbon estimates in the last column, are based on total molecular weight.
Total carbon estimates refer only to the total amount of carbon emitted.
NA - Not Available
[ ] = No Net CO2 if based on sustainable yield
Source: Radian, 1988; except N2O data, which is based on unpublished EPA data. N2O emission coefficients are highly
uncertain and currently undergoing further testing and review.
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Similarly, coal-fired fluidized bed units produce less NOX emissions than do coal-
fired cyclone units because the higher operating temperatures typical of cyclone-
units are more conducive to NOX formation.
Industrial Sector. The industrial sector includes mining, construction, and manufacturing, which
are some of the most energy-intensive economic activities. Energy consumption in this sector can
be subdivided into four categories:
• Boilers — Boilers produce steam for many different purposes, including mach-
ine drive, on-site electricity production, high-pressure cleaning, and process
requirements. Virtually any fuel can be consumed to produce steam (e.g., fossil
fuels, biomass, hazardous wastes, by-product wastes, etc.). In the U.S., boilers
consume about 30% of all industrial energy.
• Process Heat — Many industrial processes that do not use steam require the use
of some form of heat during production. Examples of process heat applications
include ovens, furnaces, dryers, melters, and kilns. The degree of flexibility in
fuel choice a consumer may have depends on the process heat application —
some applications may use technologies or production processes that require a
particular fuel.8 Process heat applications consume about 40% of the energy in
the U.S. industrial sector.
8 For example, some food production processes use natural gas because its relatively clean-
burning characteristic allows it to be used when product contamination may be an issue.
Similarly, melters in the glass industry are often designed to burn natural gas because of the
flame characteristics of this fuel. Use of other fuels would tend to produce an inferior product
and likely require the redesign of equipment.
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• Feedstocks - Fuels may be used as a raw material for the production process.
Examples of such applications include the conversion of metallurgical coal to
coke for use in the manufacture of steel, natural gas for fertilizer production,
and petroleum for asphalt. It is usually very difficult to switch to alternative
fuels with these applications. In the U.S., feedstocks consume about 15% of
industrial energy.
• Other - This category consists primarily of industrial activities requiring
electricity, e.g., lighting, motor drive, etc. These applications account for 15%
of all energy consumed by U.S. industry.
The amount of greenhouse gas emissions generated from industrial energy consumption is a
function of fuel type and the process in which it is consumed (see Table 4-2 for emissions from
selected industrial applications).
Residential and Commercial Sectors. In the residential and commercial sectors the main end-
use applications for energy are heating, cooling, cooking, and lighting. The form and amount of
energy used to meet these needs varies, as summarized in Table 4-3 for the U.S. and
South/Southeast Asia. In developing countries, most of the energy in these two sectors is consumed
for cooking purposes, with consumers relying on biomass or kerosene for fuel. In industrialized
countries, however, space heating and water heating consume the most energy, which is supplied
primarily by fossil fuels and, to some extent, electricity; gas and electricity are the primary energy,
forms for cooking in industrialized countries. Because of the wide variety of end-use applications,
types of energy consumed, and conversion efficiencies in the residential and commercial sectors it is
difficult to generalize about emission trends in these sectors; for illustrative purposes, emission
coefficients for several major applications in industrialized countries are listed in Table 4-2.
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Chapter IV
TABLE 4-3
End-Use Energy Consumption Patterns for
the Residential/Commercial Sectors
(% of Total Energy)
End-Use
Type of Energy
Biomass
Fossil Fuels
Electricity
Total
South /Southeast Asia
Heating
Cooling
Cooking
Lighting
TOTAL
United States
Heating
Cooling
Cooking
Lighting
Other
TOTAL
0
0
75
o
75%
<1
0
0
0
-°-
<1%
16
0
3
1
20%
59
0
7
0
-Q-
66%
NA
NA
NA
NA
5%
8
6
3
7
IQ
34%
NA
NA
NA
NA
100%
67
6
10
7
^Q
100%
Sources: Sathaye et al., 1988; Mintzer, 1988; U.S. DOE, 1987; Leon Schipper, pers. communication.
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Transportation Sector. As consumers become wealthier, the absolute quantity and share of
energy used in the transportation sector increases. For example, as discussed earlier, in many
developing countries, such as China, the transportation sector consumes a much smaller portion of
the country's energy requirements than the portion consumed by this sector in industrialized countries,
such as the United States. Energy requirements in the transportation sector are typically met with
fossil fuels, particularly petroleum-based products such as gasoline, diesel, or jet fuel. For example,
in 1985 countries belonging to the OECD met 91% of their transportation energy requirements with
oil-derived products, 8% with electricity, and the remaining 1% with natural gas and coal (OECD
1987). As countries become wealthier, increased use of petroleum to meet transportation needs can
significantly increase greenhouse gas emissions to the atmosphere (see Table 4-2).
The amount and type of greenhouse gases emitted can also be affected by the transportation
technology. For example, gasoline vehicles produce about 25% less CO2 on an energy input basis
than do diesel vehicles, while producing substantially more CO. However, the CO is eventually
oxidized to CO2, so the CO2 emissions attributable to gasoline vehicles are comparable to those of
diesel vehicles. Also, the efficiencies of diesel engines are generally greater than those of gasoline
engines for a similar vehicle, implying that diesel vehicles would actually have lower effective CO2
emissions per mile travelled. Similarly, vehicles powered with compressed natural gas would emit
CO2 and CO at lower levels than would either gasoline or diesel vehicles, although CH4 emissions
might be higher.
Fuel Production and Conversion
Significant quantities of greenhouse gases are emitted during the production of energy and its
conversion to end-use energy forms. Several major components of these fuel production and
conversion processes are discussed below.
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Natural Gas Flaring. Venting, and Leaking. During the production of oil and natural gas, some
portion of natural gas, which is mostly methane, is typically vented to the atmosphere (as CH4) or
flared (thereby producing CO2) rather than produced for commercial use. Venting typically occurs
during natural gas drilling and well maintenance operations to avoid pressure buildup, to test well
drawdown, and during required maintenance at existing production wells. Flaring is most common
in conjunction with oil production when no market can be found for the natural gas associated with
oil reservoirs. In some circumstances, the gas may be vented rather than flared. On average, the
amount of natural gas flared and vented amounts to about 2-3% of global natural gas production,
although in some regions virtually all of the natural gas may be vented or flared, while in other
regions (like the U. S.) the total amount flared or vented is less than 0.5% of total production (U.S.
DOE, 1986).' Currently, approximately 50 teragrams (Tg) of CO2 are released to the atmosphere
from flaring of natural gas (Rotty, 1987).10
Leaks of natural gas also occur during the refining, transmission, and distribution of the gas.
These leaks may occur at the refinery as the gas is cleaned for market, from the pipeline system
during transportation to the end-user, or during liquefaction and regasification if liquified natural gas
(LNG) is produced. About 20-50 Tg of CH4 are released to the atmosphere each year from leaking
and venting of natural gas (Crutzen, 1987; Cicerone and Oremland, 1989).
Coal Mining. As coal forms, CH4 produced by the decomposition of organic material, becomes
trapped in the coal seam. This CH4 is released to the atmosphere during coal extraction operations.
The amount of CH4 released by coal mining varies depending on factors such as depth of the coal
seam, quality of the coal, and characteristics of the geologic strata surrounding the seam. The
9 U.S. regulations strictly govern the flaring and venting of natural gas. In other parts of the
world, however, insufficient data exists to determine whether the natural gas is flared or vented,
although safety precautions would strongly encourage flaring rather than venting.
10 1 Tg = 1 teragram = million metric tons = 1012 grams.
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amount of CH4 emitted as a result of coal mining is highly uncertain, with estimates ranging from
25 to 45 Tg per year (Cicerone and Oremland, 1989). If coal mining operations intensify, the
quantity of methane released as an indirect result of mining is expected to increase at a comparable
rate.
Synthetic Fuel Production. As conventional petroleum resources are depleted, some of the
demand for liquid (oil and natural gas liquids) and gaseous (natural gas) fuels may be met by
synthetic fuels. Although there is currently little synthetic fuel produced in the world, processes have
been developed to convert relatively abundant solid energy resources such as coal, oil shale, and tar
sands to liquid or gaseous products that could be consumed in the same end-use applications as
conventional oil and gas.
Significant amounts of energy are typically required to produce synthetic fuels. The conversion
process produces greenhouse gas emissions, particularly CO2, so that the net emissions per unit of
energy for synthetic fuels are greater than those for conventional fossil fuels. For example, the CO2
emissions from production and consumption of synthetic liquid fuels from coal are about 1.8 times
the amount from conventional liquid fuels from crude oil (Marland, 1982). Table 4-4 lists emission
rates for both conventional fossil fuels and synthetic fuels produced from coal and shale oil.
Future Trends
As shown in Figure 4-4, the quantity of CO2 emitted to the atmosphere as a result of the
combustion of fossil fuels has increased dramatically in the last century. This increase in fossil-fuel-
produced CO2 emissions is the main factor that has led to an increase in atmospheric CO2
concentrations - from about 280 ppm in preindustrial periods to about 345 ppm today. As discussed
in Chapter II, future CO2 concentrations will depend on many factors, but most important will be
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Chapter IV
TABLE 4-4
Carbon Dioxide Emission Rates for Conventional
and Synthetic Fuels
Fuel
CO2 Emission Rate
(g C/109J)
Notes
Conventional Fossil Fuels (rates for consumption)
Natural Gas 13.5-14.2
Liquid Fuels from Crude Oil 18.2-20.6
Bituminous Coal 23.7-23.9
Synthetic Fuels (rates for production and consumption)
Shale Oil
Liquids from Coal
High-Btu gas from coal
104.3
66.4
47.6
28.4
51.3
41.8
39.9
38.6
37.2
31.9
30.5
40.7
40.1
36.2
32.7
Differences are partly attributable to product
mix, i.e., gasoline versus fuel oil and gasoline.
High temperature, 10 gal/ton shale
High temperature, 25 gal/ton shale
Modified in situ, 28 gal/ton shale
Low temperature retorting
Gasoline from methanol using Mobil MTG
process
Sasol-type technology, Eastern coal
FHP process
Exxon-Donor Solvent, Eastern coal
H-coal
Generic 75% thermal efficiency
SRC-II, liquid and gas products
Lurgi
Hygas
Generic 66% thermal efficiency
Via synthesis gas with by-product credits
Source: Marland, 1982.
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the rate of growth in energy demand and the type of energy that is consumed hi order to satisfy this
demand.
The Fossil-Fuel Supply
Higher levels of energy demand will produce higher levels of greenhouse gas emissions if the
demand is satisfied with fossil fuels. As indicated above, fossil fuels currently supply a majority of
the world's energy needs, and it seems likely that fossil fuels will continue to play a key role in the
world's energy supply picture for decades to come. However, supplies of fossil fuels are not
unlimited. Resource and reserve estimates for coal, oil, and gas are listed in Table 4-5. A resource
is any presently or potentially extractable mineral supply, a reserve is a presently extractable supply.
A resource that is not presently economic to extract, may become economic in the future and then
be called a reserve. The estimates of the lifetimes of fossil-fuel reserves are based on current (1985)
rates of production. The lifetime estimates of fossil-fuel resources are based on linear and
exponential extrapolations of current energy demand (described below). Despite uncertainties about
the size of the resource base and the rate at which the resource base may be depleted, it is clear
from a technical standpoint that the consumption of fossil fuels could continue for a very longtime.
As will be discussed in Chapter V, if the world continues to rely on fossil fuels to meet the majority
of its energy needs, the amount of carbon emitted to the atmosphere may be many times greater than
current levels.
Future Energy Demand
The future rate of energy demand depends on many variables, including the rate of population
growth, the rate of economic growth, energy prices, the types of energy services demanded by
consumers, the type and efficiency of technology used, and the type and amount of energy supplies
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Chapter IV
TABLE 4-5
Estimates of Global Fossil-Fuel Resources"
(Exajoules)
Coal
Oil
Gas
Geological
Resources
(exajoules)
315,800
12,800
10,100
Reserves/ Reserves Resource Lifetime (Years)
Reserves . Resources Lifetime11 Linear
(exajoules) (%) (Years) Extrapolation
20,400 6 229 524
4,300 34 41 69
3,700C 37 61 86
Exponential
Extrapolation
103
39
44
1 Resources estimates, as of 1985, are from the World Energy Conference (1980), adjusted for global
production from 1979-85. Reserve estimates are from DOE/EIA (1986): oil and gas estimates as of
January 1, 1986; coal estimates as of 1981.
b Based on 1985 rates of production.
e Includes estimates for the Middle East and USSR.
Sources: World Energy Conference, 1980; United Nations, 1983, 1987; U.S. DOE, 1986.
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available (Chapter V). Two hypothetical cases based on crude extrapolations illustrate potential
upper and lower bounds on future energy demand (see Figure 4-7) and the lifetime of fossil fuel
resources (Table 4-5). For example, from 1950 to 1973, the average annual growth rate in energy
demand was 5.2%. If this rate of growth were exponentially extrapolated to 2050, global energy
demand would be about 254 TW (or equivalently about 8,000 EJ), almost 30 times the 1985 level.11
This amount of energy demand could lead to an increase in annual CO2 emissions from the current
5.2 Pg C to about 140 Pg C in 2050, assuming that this demand is met by consumption of fossil fuels.
Cumulative energy demand for 1985 through 2050 based on this extrapolation represents over five
times the amount of fossil fuels in proven reserves and about 45% of the resource estimate. On the
other hand, the average annual growth rate in energy demand from 1973 to 1985 was much lower:
about 2.2%. If this rate were linearly extrapolated to 2050, global energy demand would be about
23 TW (720 EJ) - almost 150% greater than the demand in 1985 - which could increase annual CO2
emissions from fossil fuels to nearly 13 Pg C. Cumulative energy demand for 1985 through 2050
based on the linear extrapolation represents about 115% of proven fossil fuel reserves, or nearly 10%
of estimated resources.
INDUSTRIAL PROCESSES
There are three significant non-energy sources of greenhouse gases associated with industrial
activity: the use of chlorofluorocarbons (CFCs), halons, and chlorocarbons (collectively, halocarbons);
cement manufacture; and waste disposal in landfills. The use of CFCs, halons, and chlorocarbons,
which are man-made chemicals with a variety of applications, results in their release to the
atmosphere. Certain uses, such as aerosol propellants and solvents, result in instantaneous release
(when the product is used), while others, such as foam-blowing agents and refrigerants, result in a
11 TW = Terawatt-years per year = 1012 watt-years per year; 1 TW = 31.53 EJ; 1 EJ = 1
Exajoule = 1018 joules; 1055 joules = 1 Btu.
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Chapter IV
FIGURE 4-7
POTENTIAL FUTURE ENERGY DEMAND
(Exajoules)
8000
7000 -
6000 -
5000 -
oo
LU 4000
O
-3
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Policy Options for Stabilizing Global Climate - Review Draft Chapter IV
delayed release. Cement manufacture results in CO2 emissions, and waste disposal in landfills results
in CO2 and CH4 emissions, although only the CH4 emissions are significant in terms of the total
global source.
Chlorofluorocarbons, Halons, and Chlorocarbons
Historical Development and Uses
Chlorofluorocarbons are man-made chemicals containing chlorine, fluorine, and carbon, hence the
name CFCs (HCFCs contain hydrogen as well). Table 4-6 lists the major CFCs with their chemical
formulae. CFCs were developed in the late 1920s in the United States as a substitute for the toxic,
flammable, refrigerator coolants in use at that time. The chemicals, which are noncorrosive, nontoxic,
nonflammable, and highly stable in the lower atmosphere, provided the refrigerator industry with a
safe, efficient coolant that soon proved to have numerous other uses as well. Commercial
development of CFCs began in 1931. During World War II, CFCs were used as propellants in
pesticides against malaria-carrying mosquitos. Since then, CFCs have been used as aerosol
propellants hi a wide range of substances, from hairsprays to spray paints. In the 1950s, industries
began using CFCs as blowing agents for plastic foam and foam insulation products. Chillers, used
for cooling large commercial and industrial buildings, as well as cold storage units for produce and
other perishable goods, became feasible at this tune with the use of CFCs. Mobile air conditioners
(in automobiles, trucks, and buses) currently constitute the largest single use of CFCs in the United
States. CFCs are also used in gas sterilization of medical equipment and instruments, solvent
cleaning of manufactured parts, especially electronic components and metal parts, and miscellaneous
other processes and products such as liquid food freezing.
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Chapter IV
TABLE 4-6
Major Halocarbons:
Statistics and Uses
Chemical
Chlorofluorocarbons
CFC-11 (CFC13)
CFC-12 (CFjClO
HCFC-22 (CHClFj)
CFC-113 (QClsFj)
1986
Atmospheric
Concentration
(pptv)
226
392
-100
30-70
Atmospheric
Lifetime
(Years)
+32
75
-17
289
111
-46.
20
90
Current Annual
Atmospheric
Concentration
Growth Rates Major
(%/yr) Uses
4 Aerosols,
Foams
4 Aerosols,
Refrigeration
7 Refrigeration
11 Solvents
Halons (Bromofluorocarbons)
Halon-1211 (CBrClFj)
Halon-1301 (CBrF3)
Chlorocarbons
Carbon tetrachloride
Methylchloroform
(CHjCdj)
-2
-2
75-100
125
25
110
-50
5.5-10
> 10 Fire
extinguisher
> 10 Fire
extinguisher
1 Production of
CFC-11 and
CFC-12
7 Solvents
Sources: U.S. EPA, 1988a; Hammitt et al., 1987; Wuebbles, 1983; WMO, 1985.
NA = No data available.
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Halons, or bromofluorocarbons, are man-made chemicals containing carbon, fluorine, and chlorine
and/or bromine (see Table 4-6 for the chemical formulae of the major halons in use today). These
chemicals were developed in the 1970s, and are used primarily as fire extinguishants. Halon-1211 is
used almost exclusively for portable (i.e., wheeled or handheld) fire extinguishers, particularly for
situations where human exposure to the chemical is possible, such as in airplanes. Halon-1301 is used
exclusively for total flooding fire extinguishing systems such as those used to protect computer centers,
document rooms, libraries, and military installations. A summary of the 1985 end-use applications
for the major CFC and halon compounds is shown in Table 4-6.
Chlorocarbons, man-made chemicals containing chlorine and carbon (see Table 4-6), are used
primarily as solvents and chemical intermediates. The primary chlorocarbons are carbon tetrachloride
and methylchloroform. In the United States, carbon tetrachloride was once used extensively as a
solvent and grain fumigant, but because of its toxicity, only small amounts of it are used in such
applications today. Its primary use in the United States is in the manufacture of CFC-11 and CFC-
12, a process which consumes or destroys almost all of the carbon tetrachloride, resulting in very
small emissions. However, carbon tetrachloride is believed to be used as a solvent in developing
countries, resulting in considerable emissions. Methylchloroform is used worldwide as a cleaning
solvent in two applications: 1) vapor degreasing (the solvent is heated and the item to be cleaned
is suspended in the vapor); and 2) cold cleaning (the part to be cleaned is submerged in a tank of
solvent). Small amounts are also used in adhesives, aerosols, and coatings.
Production of CFCs, halons, and chlorocarbons has grown steadily as new uses have developed.
Production of the two largest CFC compounds, CFC-11 and CFC-12, increased rapidly in the 1960s
and early 1970s (see Figure 4-8). Production peaked in 1974 at 812.5 gigagrams (Gg) and then
declined due to a ban on most aerosol use in the United States, Canada, and Sweden in the late
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Chapter IV
FIGURE 4-8
HISTORICAL PRODUCTION OF CFC-11 AND CFC- 12
(Glgagrams)
900
800
700
600
oo
5 500
«t
oc
o
<
o
3 400
300
200
100
Dashed line indicates estimates
I i
Total
Non-Aerosol
Aerosol
1960 1965 1970 1975 1980 1985
YEAR
Figure 4-8. While non-aerosol production of CFC-11 and CFC-12 has grown fairly steadily since
1960, aerosol production declined in the 1970s and then leveled off in the 1980s due to a ban on
most aerosol use of CFCs in the United States, Canada, and Sweden. (Source: U.S. EPA, 1987.)
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1970s.12 However, non-aerosol use has continued to grow, with 1985 production of 703.2 Gg.
Globally, major CFC and halon consumption reached nearly one Tg in 1985 (see Table 4-7). Global
production of carbon tetrachloride and methylchloroform in 1985 was estimated at nearly 1,029 Gg
and 545 Gg, respectively (Hammitt et al., 1987).
Most CFC and halon consumption occurs in the United States and other industrialized nations.
Of the 7032 Gg of CFC-11 and CFC-12 produced in 1985, about 70% was consumed by the U.S.,
the EEC, and Japan (see Figure 4-9). Although CFC use is concentrated in the industrialized world,
consumption has also increased recently in developing countries.
The Montreal Protocol
Concern over the effect on the Earth's atmosphere of CFCs and related anthropogenically-
produced compounds containing chlorine, bromine, and nitrogen began in the 1970s. Because of their
stability (i.e., their long lifetimes, see Table 4-6), CFCs are transported to the stratosphere where
they contribute to the destruction of ozone. Since the early 1970s, improved understanding of this
process, accumulation of data indicating growing atmospheric concentrations of CFCs, and observed
depletion of stratospheric ozone, particularly in the Antarctic, have fueled international concern over
this issue.
International negotiations to protect the stratosphere began in 1981 under the auspices of the
United Nations Environment Programme (UNEP). These negotiations culminated in September 1987
in Montreal, Canada, where a Diplomatic Conference was held, resulting in an international
agreement (The Montreal Protocol on Substances That Deplete the Ozone Layer," or the Montreal
Protocol) to begin reducing the use of CFCs and halons (chlorocarbons were not included). The
12 1 Gg = 109 grams = 1 million kilograms.
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TABLE 4-7
Estimated 1985 World Use of Potential
Ozone-Depleting Substances
(gigagrams)
Other
United Reporting Communist
Chemical World States Countries Countries
CFC-11 341.5 75.0 225.0 41.5
CFC-12 443.7 135.0 230.0 78.7
CFC-113 163.2 73.2 85.0 5.0
Halon 1301 10.8 5.4 5.4 0.0
Halon 1211 10.8 2.7 8.1 0.0
Carbon tetrachloride 1029.0 280.0 590.0 159.0
Methylchloroform 544.6 270.0 186.7 87.0
Source: Hammit et al., 1986.
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Chapter IV
FIGURE 4-9
CFC-11 AND CFC-12 PRODUCTION/USE
FOR VARIOUS COUNTRIES
(Glgagrams)
240
210 -
180 -
150 -
2
2 120
o
<3
90 -
60 -
30 -
Thailand
Figure 4-9. The EEC, the United States, and Japan accounted for almost 70% of the 1985 global
production of CFC-11 and CFC-12. (Source: U.S. EPA, 1988a.)
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Montreal Protocol came into force on January 1, 1989 and has been ratified by 31 countries,
representing over 90% of current world production of these chemicals (as of January 11, 1989). As
a result of this historic agreement, the very high growth rates in atmospheric CFC concentrations
projected in earlier studies (e.g., Ramanathan et al., 1985) are not likely to occur. Nevertheless,
because of the long atmospheric lifetimes of CFCs, their concentrations could continue to increase
for several decades (see Chapter V).
Landfill Waste Disposal
Humans have generated solid wastes since they first appeared on Earth, although disposal of
these wastes did not become a major problem until the rise of synthetic materials (e.g., plastics) and
densely-populated urban areas. The environment can usually assimilate the smaller amounts of wastes
produced by rural, sparsely-settled communities. However, because urban populations produce such
high volumes of waste, due to both the sheer concentration of individuals contributing to the waste
stream and the high use of heavily-packaged products, urban waste disposal has become a formidable
task.
Approximately 80% of the municipal solid wastes collected in urban areas around the world is
deposited in landfills or open dumps (Bingemer and Crutzen, 1987). Sanitary landfilling (compaction
of wastes, followed by daily capping with a layer of clean earth), which became common in the
United States after World War II, is used primarily in urban centers in industrialized countries.
Open pit dumping is the most common "managed" disposal method in developing countries (30-50%
of the solid wastes generated in cities of developing countries is uncollected [Cointreau, 1982]). Most
landfills and many open dumps develop anaerobic conditions, resulting in decay of organic carbon to
CH4 and CO2. The amount of CH4 resulting from anaerobic decay of organic municipal and
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industrial wastes in landfills is currently about 30-70 Tg per year (Bingemer and Crutzen, 1987),
approximately 10% of the total annual CH4 source.13
The primary variable affecting gas generation in landfills is the composition of the refuse. Wastes
high in organic material (e.g., food wastes, agricultural wastes, paper products) decompose readily,
while inorganics are relatively unaffected by the decomposition process. While agriculture is the
largest single source of solid wastes in the U.S. (Berry and Horton, 1974), most of these wastes are
not landfilled. Increasing urbanization and demand for "convenience" items, which encourages
marketing of single-serving and heavily-packaged products, has resulted in increasingly greater
proportions of plastics, glass, metals, and paper products, in the waste stream. Other factors
influencing gas generation include inclusion of sewage sludge (which enhances gas generation), oxygen
concentration, moisture content, pH, and available nutrients.
Disposal of municipal solid waste in industrial nations increased by 5% per year during the 1960s,
and by 2% per year in the 1970s (CEQ, 1982). Currently, per capita waste production in
industrialized countries is considerably larger than in developing countries (see Table 4-8), and the
largest contribution of landfill CH4 conies from the industrialized world (Bingemer and Crutzen,
1987). Although current rates of waste disposal in landfills have begun to level off in many
industrialized countries, associated CH4 emissions are probably still growing because the total quantity
of waste in place is still increasing. In the developing world, with its high population growth rates
and increasing urbanization, municipal solid waste disposal is projected to double by the year 2000
(Kresse and Ringeltaube, 1982), so CH4 production from waste dumps and/or sanitary landfills can
be expected to increase rapidly in developing countries.
13 This estimate does not include methane from anaerobic decomposition of agricultural
wastes, which could be a significant quantity. The total amount of carbon in agricultural wastes in
the United States alone is already 2.5 times larger than the 113 million metric tons of waste
carbon that are generated and dumped in landfills worldwide (Bingemer and Crutzen, 1987).
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TABLE 4-8
Refuse Generation Rates in Selected Cities
Per Capita
Waste
Generation Rate
City (kg Per Day)
Industrial Cities
New York, United States 1.80
Singapore 0.87
Hong Kong 0.85
Hamburg, West Germany 0.85
Rome, Italy 0.69
Developing Cities
Jakarta, Indonesia 0.60
Lahore, Pakistan 0.60
Tunis, Tunisia 0.56
Bandung, Indonesia 0.55
Medellin, Colombia 0.54
Surabaya, Indonesia 0.52
Calcutta, India 0.51
Cairo, Egypt 0.50
Karachi, Pakistan 0.50
Manila, Philippines 0.50
Kanpur, India 0.50
Kano, Nigeria 0.46
Source: Cointreau, 1982.
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Cement Manufacture
Cement manufacture produces CO2, as well as numerous other exhaust gases. As demand for
cement has grown over the last century, CO2 emissions associated with this industry have increased
from 18 to 134 Tg C between 1950 and 1985 (see Figure 4-10). In recent years CO2 emissions from
cement production have grown at a faster rate than those from fossil fuel combustion: In the early
1950s CO2 emitted as a result of cement manufacture was approximately 1% of the amount emitted
from the consumption of fossil fuels; by the early 1980s this fraction had increased to 2.5% (Rotty,
1987).
The CO2 emissions resulting from cement manufacture occur during the production of clinker,
a material produced midway through the process. After the raw materials (cement rock, limestone,
clay, and shale) are quarried and crushed, they are ground and blended to a mixture that is
approximately 80% limestone by weight. The mixture is then fed into a kiln for firing, where it is
exposed to progressively higher temperatures that cause heating, then drying, calcining, and sintering.
Finally, the feed is heated to the point of fusion (approximately 1595°C), and clinker (round, marble-
sized particles) is produced. It is during the calcination process, which occurs at approximately 900
to 1000°C, that the limestone (CaCO3) is converted to lime (CaO) and CO2, and the CO2 is released.
For every million tons of cement produced, approximately 0.137 Tg C as CO2 is emitted from
calcining (Rotty, 1987).14 An additional 0.165 Tg C is emitted per million ton of cement produced
from fossil fuel used for kiln firing and electricity. This CO2 is accounted for as part of industrial
energy use emissions.
World cement production has increased at an average annual rate of approximately 6% since the
1950s, from 133 million tons in 1950 to 972 million tons in 1985 (U.S. BOM, 1949-1986). Cement
14 1 ton = 1 metric ton = 1000 kg.
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Chapter IV
FIGURE 4-10
C02 EMISSIONS FROM CEMENT PRODUCTION
1950-1985
(Teragrams Carbon)
140
120
100
o
co 80
M
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Policy Options for Stabilizing Global Climate •- Review Draft Chapter IV
production growth rates in individual countries have varied during this period (see Figure 4-11) due
to economic fluctuations in cement's primary market, i.e., the construction industry, and competitive
shifts internationally among the primary cement-producing countries. For example, in 1951 the
United States produced approximately 28% of the global total, while by 1985 its share had shrunk
to 7%.15 During the same time, the production shares for the USSR grew from 8% to 13%, for
China, from less than 1% to 15%, and for Japan, from 4% to 8%. Although many national markets,
except the United States', experienced low levels of demand during the 1980s, global cement
production is expected to continue to grow faster than the GNP for some time.
LAND USE CHANGE
Over the past few centuries, man has significantly changed the surface of the Earth. Forests have
been cleared, wetlands have been drained, and agricultural lands have been expanded. All of these
activities have resulted in considerable changes in trace gas emissions to the atmosphere.
Deforestation results in a net release of carbon from both the biota and the soils (unless the land
is reforested) as these organic carbon pools burn or are decomposed. Biomass burning, due to
shifting agriculture, burning of savanna, use of industrial wood and fuelwood, and burning of
agricultural wastes, is a source of CO2, as well as CH4, N2O, and NOX. 16 Destruction of wetlands,
from either filling or dredging, can alter the atmospheric CH4 budget, since anaerobic decomposition
in wetlands produces CH4.
15 The U.S. is currently a net importer of cement; the volume of its imports has grown,
representing only a few percent of consumption in the early 1980s but as much as 18 percent hi
1986 (International Trade Administration, 1987).
16 Shifting agriculture is the practice of clearing and planting a new area, farming it until
productivity declines, and then moving on to a new plot to start the cycle over again. If the land
is allowed to reforest, there are no net CO2 emissions.
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Chapter IV
FIGURE 4-11
CEMENT PRODUCTION IN SELECTED COUNTRIES
1951-1985
(Thousand Metric Tons)
200
150
v>
z
o
o
B
o
o
100
50
United States
^
s >• •
/
I
J_
I
1951 1955 1960 1965 1970 1975 1980 1985
YEAR
Figure 4-11. World cement production grew at an average annual rate of about 6% between 1950
and 1985. Growth has been particularly rapid in China, the U.S.S.R., and Japan. (Source: U.S.
BOM, selected years.)
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Deforestation
Estimates of net emissions of CO2 to the atmosphere due to changes in land use (deforestation,
reforestation, logging, and changes in agricultural area) in 1980 range from 0.4 to 2.6 Pg C
(Houghton et al., 1987; Detwiler and Hall, 1988), which accounts for approximately 10-30% of annual
anthropogenic CO2 emissions to the atmosphere. Deforestation in the tropics accounted for almost
all of the flux; the carbon budget of temperate and boreal regions of the world has been
approximately hi balance in recent years. Of the net release of carbon from tropical deforestation,
55% was produced by only six countries in 1980: Brazil, Indonesia, Columbia, the Ivory Coast,
Thailand, and Laos (see Figure 4-12).
The world's forest and woodland areas have been reduced 15% since 1850, primarily to
accommodate the expansion of cultivated lands (IIED and WRI, 1987). The largest changes in forest
area during this period have occurred in Africa, Asia, and Latin America. Europe is the only region
that has experienced a net increase in forest area over this time interval. Forest area began to
increase in Europe in the 1950s and in North America in the 1960s (see Table 4-9). However, recent
data from the Food and Agriculture Organization of the United Nations (FAO) and the U.S. Forest
Service indicates that net deforestation may be occurring in the United States — although there are
discrepancies between the two data sets. The FAO data indicates that between 1980 and 1985 the
area of U.S. forest and woodlands decreased by approximately 3.8 million hectares (million ha) per
year, or 1.4% per year (FAO, 1986b).17 The U.S. Forest Service (Alig, 1988) estimates that between
1977 and 1987 the area of U.S. forests decreased by approximately 0.41 million ha per year, or 0.14%
per year.
17 1 ha = 1 hectare = 2.471 acres.
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Chapter IV
FIGURE 4-12
NET RELEASE OF CARBON FROM
TROPICAL DEFORESTATION
1980
(Teragrams Carbon)
Rest of World (516)
Peru(45)
Burma (51)
Philippines (57)
Nigeria (60)
Laos(85)
Thailand (95)
Brazil (336)
Indonesia (192)
Ivory Coast (101)
Colombia (123)
Figure 4-12. Tropical deforestation accounts for approximately 10-30% of the annual anthropogenic
CO2 emissions to the atmosphere. Over half of the 1980 CO2 emissions from deforestation was
produced by six countries: Brazil, Indonesia, Columbia, the Ivory Coast, Thailand, and Laos.
(Source: Houghton et al., 1987.)
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TABLE 4-9
Land Use: 1850-1980
Area (million hectares')
Percentage
Change
1850 to
TEN REGIONS
Forests and Woodlands
Grassland and Pasture
Croplands
Tropical Africa
Forests and Woodlands
Grassland and Pasture
Croplands
North Africa and Middle East
Forests and Woodlands
Grassland and Pasture
Croplands
North America
Forests and Woodlands
Grassland and Pasture
Croplands
Latin America
Forests and Woodlands
Grassland and Pasture
Croplands
China
Forests and Woodlands
Grassland and Pasture
Croplands
South Asia
Forests and Woodlands
Grassland and Pasture
Croplands
Southeast Asia
Forests and Woodlands
Grassland and Pasture
Croplands
Europe
Forests and Woodlands
Grassland and Pasture
Croplands
USSR
Forests and Woodlands
Grassland and Pasture
Croplands
Pacific Developed Countries
Forests and Woodlands
Grassland and Pasture
Croplands
1850
5,919
6,350
538
1,336
1,061
57
34
1,119
27
971
571
50
1,420
621
18
96
799
75
317
189
71
252
123
7
169
150
132
1,067
1,078
94
267
638
6
1860
5,898
6,340
569
1,333
1,062
58
34
1,119
28
968
559
65
1,417
623
19
93
799
78
315
189
73
252
123
7
158
147
136
1,060
1,081
98
267
638
6
1870
5,869
6,329
608
1^29
1,064
61
33
1,118
30
965
547
80
1,414
625
21
91
798
81
311
189
77
251
122
8
157
145
140
1,052
1,083
103
266
638
7
1880
5,833
6,315
659
1,323
1,067
64
32
1,117
32
962
535
95
1,408
627
24
89
796
84
307
189
81
251
121
10
157
144
142
1,040
1,081
118
265
637
9
1890
5,793
6301
712
1,315
1,070
68
31
1,116
35
959
522
110
1,401
630
28
86
797
86
303
189
85
250
119
12
156
143
143
1,027
1,079
132
264
635
12
1900
5,749
6,284
773
1396
1,075
73
30
1,115
37
954
504
133
1,394
634
33
84
797
89
299
189
89
249
118
15
156
142
145
1,014
1,078
147
263
634
14
1910
5,696
6,269
842
1,293
1,081
80
28
1,113
40
949
486
156
1383
638
39
82
797
91
294
190
93
248
116
18
155
141
146
1,001
1,076
162
262
632
17
1920
5,634
6,260
913
1,275
1,091
88
27
1,112
43
944
468
179
1,369
646
45
79
796
95
289
190
98
247
114
21
155
, 139
147
987
1,074
178
261
630
19
1930
5353
6,255
999
1,251
1,101
101
24
1,108
49
941
454
1%
1,348
655
57
76
796
96
279
190
108
246
111
25
155
138
149
973
1,072
194
260
629
22
1940
5,455
6,266
1,085
1,222
1,114
118
21
1,103
57
940
450
201
1,316
673
72
73
794
103
265
190
122
244
108
30
154
137
150
961
1,070
208
259
627
24
1950
5,345
6,293
1,169
1,188
1,130
136
18
1,097
66
939
446
206
1,273
700
87
69
793
108
251
190
136
242
105
35
154
136
152
952
1,070
216
258
625
28
1960
5,219
6,310
1,278
1,146
1,147
161
17
1,085
79
939
446
205
1,225
730
104
64
789
117
235
190
153
240
102
40
156
136
151
945
1,069
225
252
617
42
1970
5,103
6,308
1396
1,106
1,157
190
15
1,073
93
941
447
204
1,186
751
123
59
784
127
210
189
178
238
97
47
161
137
145
940
1,065
233
247
609
56
1980
5,007
6,299
1401
1,074
1,158
222
14
1,060
107
942
447
203
1,151
767
142
58
778
134
180
187
210
235
92
55
167
138
137
941
1,065
233
246
608
58
1980
-15
-1
179
-20
9
288
-60
-5
294
-3
-22
309
-19
23
677
-39
-3
79
-43
-1
196
-7
-25
670
4
-8
4
-12
-1
147
-8
-5
841
Source: IIED and WRI, 1987.
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Currently, it is estimated that approximately 11.3 million ha of tropical forests are lost each year,
while only 1.1 million ha are reforested per year (FAO, 1985). Most of the tropical deforestation
is due to transfer of forest land to agricultural use, through shifting agriculture and conversion to
pasture. FAO has estimated a demand for an additional 113-150 million ha of cultivated land for
the 20-year period between 1980 and 2000 to meet food production needs (FAO, 1981). Most of this
land will have to come from areas that were once forested, however there is a large potential to use
land currently under shifting cultivation by adapting low-input agricultural techniques (Chapter VII).
Fuelwood use also contributes to deforestation, particularly in Africa where fuelwood is a major
source of residential energy. Sixty-three percent of the total energy consumption of developing
African countries, 17% in the Asian countries, and 16% in the Latin American countries, is provided
by fuelwood. In the Sudan, Senegal, and Niger, fuelwood provides 94, 95, and 99%, respectively, of
household energy consumption (Anderson and Fishwick, 1984). Rapidly-increasing populations,
particularly in developing nations, will result in increasing demands on forest lands to meet growing
agricultural and energy needs.
Biomass Burning
Biomass burning, in addition to contributing to the atmospheric CO2 budget, contributes
approximately 10-20% of total annual CH4 emissions, 5-15% of the N2O emissions, 10-35% of the
NOX emissions, and 20-40% of the CO emissions (Crutzen et al., 1979; WMO, 1985; Logan, 1983;
Stevens and Engelkemeir, 1988; and Andreae et al., 1988). These estimates are for instantaneous
emissions from combustion. Recent research has shown that biomass burning also results in longer-
term (at least up to 6 months after the burn) emissions of NO and N2O due to enhancement of
biogenic soil emissions (Anderson et al., 1988). Estimates of emissions of trace gases due to biomass
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burning are very uncertain for two reasons: 1) data on amounts and types of biomass burned are
scarce, and 2) emissions per unit of biomass burned are highly variable.
Activities associated with biomass burning include agriculture, colonization, wildfires and
prescribed fires, and burning of industrial wood and fuelwood. Currently, agricultural burning, due
to shifting agriculture and burning of agricultural wastes is estimated to account for over 50% of the
biomass burned annually (Table 4-10). Biomass burning is a particularly important source of trace-
gas emissions in the tropics, where forest exploitation is unsurpassed. Continued rapid population
growth and exploitation of forests may substantially increase emissions from biomass burning hi the
future.
Wetland Loss
Annual global emissions of CH4 from freshwater wetlands are estimated to be 110 Tg,
approximately 25% of the total annual source of 400 to 600 Tg (Matthews and Fung, 1987). Of the
approximately 530 million ha producing this CH4, 39% is forested bog, 17% is nonforested bog, 21%
is forested swamp, 19% nonforested swamp, and 4% alluvial formations.18 The bulk of the bog
acreage is located between 40°N and 70°N, while swamps predominate between 10°N and 30°S.
Alluvial formations are concentrated between 10°N and 40°S (see Figure 4-13). Coastal saltwater
and brackish water environments produce minor amounts of CH4 in comparison, probably due to the
inhibitory effects of dissolved sulfate (SO4) in the interstitial water of salt-marsh sediments (DeLaune
et al., 1983; Bartlett et al., 1985).
18 Bogs are peat- or organic-rich systems, usually associated with waterlogging and seasonal
freeze-thaw cycles; swamps are low-organic formations occurring most commonly in the tropics,
and alluvial formations are low-organic riverine formations.
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Chapter IV
TABLE 4-10
Summary Data on Area and Biomass Burned
Activity
Burned and/or
Cleared Area
(million ha)
Burned Biomass
(100 Tg dry matter)
Burning due to shifting agriculture
Deforestation due to population
increase and colonization
Burning of savanna and brushland
Wildfires in temperate and
boreal forests
Prescribed fires in temperate forests
Burning of industrial wood and fuelwood
Burning of agricultural wastes
TOTAL
21-62 (41)
8.8-15.1 (12.0)
(600)
4.0-6.5 (5.4)
2.0-3.0 (2.5)
630-690 (660)
9-25 (17)
5.5-8.8 (7.2)
4.8-19 (11.9)
1.9-3.2 (2.6)
0.1-0.2 (0.2)
10-11 (10.5)
17-21 (19)
48-88 (68)
Data in parentheses represent average values.
Source: Crutzen et al., 1979.
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FIGURE 4-13
WETLAND AREA AND ASSOCIATED METHANE EMISSIONS
<
o
tc.
(3
K
UJ
SON 70 60 50 40 30 20 10
10 20 30 40 60S
80N 70 60 SO 40 30 20 10 0
LATITUDE
Alluvial [//; Forested swamp
jxyj Nonf orested bog
10 20 30 40 SOS
Forested bog
O\j Nonfcrested swamp
Figure 4-13. Estimated latitudinal distribution of wetland area (top) and associated methane
emissions (bottom). Forested and non-forested bogs located between 40° and 70°N account for
approximately 50% of the current CH4 emissions from wetlands. (Source: Matthews and Fung,
1987.)
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The latitudinal distribution of wetland CH4 emissions is estimated to be very similar to the
latitudinal distribution of freshwater wetland area. About 50% of the emissions originate between
50°N and 70°N, and about 25% between 20°N and 30°S. The source of the high-latitude emissions
is organic-rich bogs, while most of the low-latitude emissions come from swamps (see Figure 4-13).
Between 25 and 50% of the world's original swamps and marshes have been eliminated by human
activities (IIED and WRI, 1987). For centuries people have drained and filled marshes and swamps
to create dry land for agricultural and urban development. Wetland areas have been converted to
open water by dredging and installation of flood-control levees, and have been used as disposal sites
for dredge materials and solid wastes. Peat mining and pollution from agricultural and industrial
runoff have also contributed to the destruction of wetlands. By 1970, more than half of the original
wetland acreage in the United States had been destroyed (IIED and WRI, 1987). Between the mid-
1950s and mid-1970s, there was a net loss of wetlands in the United States of approximately 4.6
million ha, 97% of which occurred hi inland freshwater areas (U.S. OTA, 1984). Agricultural
conversions were responsible for 80% of this freshwater wetland loss.19 Wetland loss has also been
extensive in Europe and the Asia-Pacific region. For example, approximately 40% of the coastal
wetlands of Brittany, France, have been lost in the last 20 years, and 8100 ha of wetlands on the east
coast of England have been converted to agricultural use since the 1950s. Large-scale wetland losses
have not been as prevalent in the developing world, but rising populations will result in increasing
demands for agricultural expansion. There is already pressure to develope two large wetland systems
in Africa, the Okavango Swamps of Botswana and the Sudd Swamps of southern Sudan, for
agricultural use (IIED and WRI, 1987).
19 For example, drainage of prairie potholes hi Iowa to provide new farmland has resulted in
the reduction of Iowa's original wetlands by over 98%, from 930,000 ha when settlement began, to
10,715 ha today.
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AGRICULTURAL ACTIVITIES
Three agricultural activities contribute directly to atmospheric emissions of greenhouse gases:
enteric fermentation in domestic animals, rice cultivation, and use of nitrogenous fertilizer. Global
demand for food and agricultural products has more than doubled since 1950, fueled by rising
populations and incomes. Agricultural advancements during the post-war years, such as the "Green
Revolution," brought improvements in soil management and disease control, new high-yielding varieties
of crops, increased application of commercial fertilizers, and increased use of machinery. Between
1950 and 1986, world grain production increased from 624 to 1,661 million tons and average yield
more than doubled, from 1.1 to 2.3 tons per ha (Wolf, 1987). Over this same time interval, growth
of various domestic animal populations ranged from 20 to 150% (Crutzen et al., 1986) and fertilizer
consumption grew approximately 750% (Herdt and Stangel, 1984). According to projections by the
Food and Agriculture Organization of the United Nations, by the year 2000, a world population of
about 6 billion will require an agricultural output approximately 50 to 60% greater than that required
in 1980 (FAO, 1981).
Enteric Fermentation In Domestic Animals
Methane is produced as a by-product of enteric fermentation in herbivores, a digestive process
by which carbohydrates are broken down by microorganisms into simple molecules for absorption into
the bloodstream. Both ruminant animals (e.g., cattle, dairy cows, sheep, buffalo, and goats) and some
non-ruminant animals (e.g., pigs and horses) produce CH4. The highest CH4 losses are reported for
ruminants (approximately 4-9% of total energy intake), which are able to digest cellulose due to the
presence of specific microorganisms in their digestive tracts. The amount of CH4 that is released
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from both ruminant and non-ruminant animals depends on the type, age, and weight of the animal,
the quality and quantity of feed, and the energy expenditure of the animal.
Of the annual global source of 400-600 Tg CH4, domestic animals contribute approximately 65-
85 Tg (Crutzen et al., 1986; Lerner et al., 1988). Domestic animals that produce the bulk of the CH4
are (in decreasing order of amount produced) cattle, dairy cows, buffalo, goats, sheep, camels, pigs,
and horses. Currently, approximately 57% comes from cattle, and 19% from dairy cows. Domestic
animals in six countries, India, the USSR, Brazil, the U.S., China, and Argentina, produce over 50%
of the methane by enteric fermentation (Lerner et al., 1988).
The domestic animal population has increased considerably during the last century. Between the
early 1940s and 1960s, increases in global bovine and sheep populations averaged 2% per year. Since
the 1960s, the rates of increase have slowed somewhat, to 1.2% and 0.6% per year, respectively (see
Figure 4-14). The annual increases in global populations of pigs, buffalo, goats, and camels since the
1960s have been comparable: 1.4%, 1%, 1.2%, and 0.5%, respectively. The horse population
declined about 0.25% per year. For comparison, the average annual increase in global human
population since the 1960s has been about 1.8%.
Rice Cultivation
Anaerobic decomposition in flooded rice fields produces methane, which escapes to the
atmosphere by ebullition (bubbling) up through the water column, diffusion across the water/air
interface, and transport through the rice plants. Research suggests that the amount of CH4 released
to the atmosphere is a function of rice species, number and duration of harvests, temperature,
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FIGURE 4-14
TRENDS IN DOMESTIC ANIMAL POPULATIONS
1890-1985
(Millions)
o
1400
1200
1000
800
600
400
200
1890
1925 1945 1960
YEAR
1985
Cattle
Sheep
Pigs
Goats
Buffalos
Horses
Camels
Figure 4-14. Global domestic animal populations have grown by about 0.5 to 2.0% per year during
the last century. Currently, domestic animals account for about 15% of the annual anthropogenic
CH4 emissions. Note: The cattle population figures include dairy cows. (Sources: Crutzen et al.,
1986; FAO, 1971, 1982, 1986a.)
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irrigation practices, and fertilizer use (Holzapfel-Pschorn and Seiler, 1986; Seiler et al., 1984; Cicerone
et al., 1983).
Rice cultivation has grown tremendously since the mid-1900s, due both to increases in crop
acreage and yields.20 Between 1950 and 1984, rough rice production grew from 163 to 470 million
tons, nearly a 200% increase.21 During the same time, harvested rice paddy area increased
approximately 40%, from 103 to 148 million ha, and average global yields doubled, from 1.6 to 3.2
tons per ha (IRRI, 1985).n Average yields higher than 5 tons per ha have already been obtained
in parts of the developed world (FAO, 1986a). The increase in rice production has been due both
to the "Green Revolution" of the 1960s, which resulted in the development and dissemination of high-
yield varieties of rice and an increase in fertih'zer use, and to a significant expansion of land area
under cultivation. Methane emissions are probably primarily a function of area under cultivation,
rather than yield, although yield could influence emissions, particularly if more organic matter is
incorporated into the paddy soil.
Over 90% of global rice acreage and production occurs in Asia. Five Asian countries, China,
India, Indonesia, Bangladesh, and Thailand, account for 75% of global production and 73% of the
harvested area (IRRI, 1986; see Figures 4-15 and 4-16). Rice fields contribute 60-170 Tg of methane
20 Rice statistics are for rice grown in flooded fields, i.e., they do not include upland rice,
since methane emissions result only from flooded rice fields.
21 Rough rice, also called paddy rice, is rice with the hull, or husk, attached. The hull
contributes about 20% of the weight of rough rice. The kernel remaining after the hull is
removed is brown rice. Milling of brown rice, which removes the bran, followed by polishing,
results in white rice.
22 Harvested area is the area under cultivation multiplied by the number of crops per year.
For example, 1 ha that is triple-cropped is counted as 3 ha of harvested area.
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Chapter IV
FIGURE 4-15
ROUGH RICE PRODUCTION
1984
(Million Tons)
Rest of World
(75.1)
Vietnam
(15.4)
Japan
(14.8)
Burma
(14.5)
Thailand
(19.2)
Bangladesh
(21.5)
Indonesia
(37.5)
Figure 4-15. Distribution of the total rough rice production of 470 million tons. Five Asian
countries, China, India, Indonesia, Bangladesh, and Thailand, accounted for approximately 75% of
the 1984 global rice production. (Source: IRRI, 1986.)
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Chapter IV
FIGURE 4-16
RICE AREA HARVESTED
1984
(Million Hectacres)
Rest of World
(27.9)
Burma
(4.7)
Vietnam
(5.6)
Thailand
(9.7)
Indonesia
(9.7)
Bangladesh
(10.5)
India
(42.8)
China
(34.3)
Figure 4-16. Distribution of the total harvested rice paddy area of 148 million ha. Five Asian
countries, India, China, Bangladesh, Indonesia and Thailand accounted for 73% of the 1984 rice
acreage harvested. (Source: IRRI, 1986.)
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per year to the atmosphere, or approximately, 20% of the global flux (Cicerone and Oremland, 1989).
This estimate is highly uncertain because there have been no comprehensive rice-paddy flux
measurements in the major rice-producing countries in Asia.
Use of Nitrogenous Fertilizer
Nitrous oxide is released through microbial processes in soils, both through denitrification and
nitrification. Nitrogenous fertilizer application enhances N2O flux rates, since some of the applied
fixed N is converted to N2O and released to the atmosphere. The amount of N2O released depends
on rainfall, temperature, the type of fertilizer applied, mode of application, and soil conditions.
Nitrogen is currently the most abundant commercial fertilizer nutrient consumed worldwide. Its
dominance in the fertilizer markets has increased steadily over the last few decades, from 28% of
total nutrients (nitrogen, phosphorus, and potassium) in 1950 to 64% in 1981 (Herdt and Stangel,
1984). Approximately 70.5 million tons N was consumed worldwide in 1984/1985 in the form of
nitrogenous fertilizers (FAO, 1987). A preliminary estimate suggests that this produced N2O
emissions of 0.14-2.4 Tg N of the global source of approximately 8-22 Tg N per year (Fung et al.,
1988) although this estimate is highly uncertain. Experiments to determine the fraction of fertilizer
nitrogen lost to the atmosphere as nitrous oxide have shown a wide range of results (see Table 4-
11, and Chapter II). Anhydrous ammonia, which requires sophisticated equipment for application (it
is injected under pressure into the soil), is used exclusively in the United States. It comprises about
38% of the U.S. nitrogenous fertilizer consumption. Urea, which is usually broadcast as pellets by
hand, comprises about 69% and 58% of nitrogenous fertilizer consumption in Asia and South
America, respectively.
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TABLE 4-11
Nitrous Oxide Emissions by Fertilizer Type
Fertilizer Type Percent of Nitrogenous Fertilizer Evolved as N2O
Anhydrous Ammonia 0.5 to 6.84
Ammonium Nitrate 0.04 to 1.71
Ammonium Type 0.025 to 0.1
Urea 0.067 to 0.5
Nitrate 0.001 to 0.50
Source: Eichner, 1988; GalbaUy, 1985.
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Asia, Western Europe, Eastern Europe, and North America consume the major share of the
world's nitrogenous fertilizers (collectively, about 85%). China, the Soviet Union, and the United
States together account for approximately one-half of the world's fertilizer consumption. The twelve
largest nitrogen fertilizer consumers, all of which consume more than one million tons N annually,
are (in decreasing order): China, the United States, the Soviet Union, India, France, the United
Kingdom, West Germany, Canada, Indonesia, Poland, Mexico, and Italy (see Figure 4-17). Together,
these twelve countries account for approximately 74% of the annual nitrogenous fertilizer
consumption.
Although developed nations will probably increase their consumption of commercial fertilizer over
the next few decades, most of the increased demand will occur in developing nations. The World
Bank estimates that over 90 million tons N will be consumed in 1997/98, a 30% increase over
consumption in 1986/87. Almost 50% of the growth between 1986/87 and 1997/98 is expected to
occur in the developing nations (World Bank, 1988).
IMPACT OF CLIMATIC CHANGE ON ANTHROPOGENIC EMISSIONS
Climate change will affect human activity in a myriad of ways, and thus influence anthropogenic
emissions of greenhouse gases (see Chapter III for a discussion of the biogeochemical feedbacks of
climate change). The impact of climatic change on land-use patterns and agricultural practices could
be particularly significant in influencing the trace gas emissions from these sources. The magnitude
(or even the direction) of such changes have not been examined to date. More information is
available regarding the impact of climatic change on electric utilities (Under et al., 1987). A brief
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Chapter IV
FIGURE 4-17
NITROGEN FERTILIZER CONSUMPTION
1984/1985
(Million Metric Tons Nitrogen)
Poland (1.2)
Indonesia (1.3) » Mexico (1.2)
Canada (1.3)
West Germany (1.
United Kingdom (1.6)
Rest of World (18.9)
France (2.4)
India (5.7)
Soviet Union (10.9)
China (13.7)
United States (9.5)
Figure 4-17. Distribution of the total nitrogenous fertilizer consumption of 70.5 million tons N.
China, the United States, and the Soviet Union together accounted for just over 50% of the
1984/1985 global fertilizer consumption. Currently, 5-35% of the total anthropogenic N?O emissions
are attributed to nitrogenous fertilizer consumption. (Source: FAO, 1987.)
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discussion of this subject is presented here as an illustration of some ways in which climatic change
can, in turn, influence trace gas emissions.
Linder and Inglis (1988) estimate that annual electricity consumption increases by 0.5 to 2.7%/°C
for utilities in the United States, depending on the local climate and the fraction of buildings with
electrical heating and air-conditioning equipment. If climate change leads to increases in ownership
levels, then substantially greater sensitivities are possible (Linder et al., 1987). Currently, 37% of total
CO2 emissions from fossil fuels are produced by electric utilities and this share is expected to increase
in the future (see Chapter V). Applying the U.S. average sensitivity of 1.0%/°C obtained by Linder
and Inglis (1988) to the rest of the world implies a feedback on CO2 emissions of 0.4%/°C. This
feedback would be offset to an extent that has not been estimated by lower fuel use for heating, but
as the penetration of air conditioning rises in developing countries this feedback could increase.
Climate change may affect the electricity industry from the supply side as well. When steam is
produced to generate electricity in a power plant, either water (usually from a nearby reservoir or
river) or air is used as a coolant to condense the steam back into water and start the process over
again. Higher atmospheric temperatures will result in warming of these coolants, and reduction in
the efficiency of the power plants. This effect is not likely to be as significant as others, however,
since seasonal temperature changes are already much greater than the warming predicted for the next
century (Linder et al., 1987).
More immediate and acute effects of climate change on electric utilities are likely to occur due
to reduced availability of water. The drought of the summer of 1988 resulted in such low river levels
in the U.S. Midwest, that some electric plants were forced to reduce generation due to lack of
cooling water. More frequent and severe droughts would also result in reduced hydropower for
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generation of electricity. (This change would also affect barge shipping, since many rivers would
become unnavigable, and result in increased trace gas emissions from truck and rail transport.)
Sea-level rise and lowered stream flows resulting from climate change would also have adverse
effects on electric utilities. Salinities in rivers and estuaries would increase, and stream chemistry
could change, so that the water may become too corrosive to be used as a coolant. A few power
plants in the United States use saltwater for cooling purposes, so the technology exists to adapt to
more saline coolants, although the conversion process is costly.
These feedback mechanisms are likely to have a smaller influence on future warming than the
biogeochemical feedbacks discussed in Chapter III. The impact of climatic change on anthropogenic
trace gas emissions may nevertheless prove to be important and should be investigated further.
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REFERENCES
Alig, R.J. 1988. Projecting land cover and use changes, (review draft). In Bones, J.T., ed. An
Analysis of the Land Situation in the United States: 1989-2040. General Technical Report WO-OO.
USDA Forest Service, Washington, D.C. hi press.
Anderson, D., and R. Fishwick. 1984. Fuelwood Consumption and Deforestation in African Countries.
World Bank Staff Working Papers, Number 704. The World Bank, Washington, D.C. 52 pp.
Anderson, I.C., J.S. Levine, MA. Poth, and PJ. Riggan. 1988. Enhanced biogenic emissions of nitric
oxide and nitrous oxide following surface biomass burning. Journal of Geophysical Research 93:3393-
3898.
Andreae, M.O., E.V. Browell, M. Garstang, G.L. Gregory, R.C. Harriss, G.F. Hill, DJ. Jacob, M.C.
Pereira, G.W. Sachse, A.W. Setzer, P.L. Silva Dias, R.W. Talbot, A.L. Torres, and S.C. Wofsy. 1988.
Biomass-burning emissions and associated haze layers over Amazonia. Journal of Geophysical
Research 93:1509-1527.
Bartlett, K.B., R.C. Harriss, and D.I. Sebacher. 1985. Methane flux from coastal salt marshes.
Journal of Geophysical Research 90:5710-5720.
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World Bank. 1988. World Bank, FAO, UNIDO Fertilizer Working Group Nitrogen Supply, Demand
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University Press, Baltimore. 439 pp.
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CHAPTER V
THINKING ABOUT THE FUTURE
FINDINGS V-2
INTRODUCTION V-4
APPROACH TO ANALYZING FUTURE EMISSIONS V-5
Production V-7
Consumption V-ll
SCENARIOS FOR POLICY ANALYSIS V-13
Scenarios with Unimpeded Emissions Growth V-17
Scenarios with Stabilizing Policies V-21
ANALYTICAL FRAMEWORK V-22
Energy Module V-25
Industry Module V-26
Agriculture Module V-26
Land Use and Natural Source Module V-27
Ocean Module V-27
Atmospheric Composition and Temperature Module V-28
Assumptions V-29
Population Growth Rates V-29
Economic Growth Rates V-29
Oil Prices V-30
Limitations V-30
SCENARIO RESULTS V-33
Energy Sector V-33
End-use Consumption V-33
Primary Energy Supply V-39
Greenhouse Gas Emissions From Energy Production and Use V-44
Comparison to Previous Studies V-45
Industrial Processes V-56
Halocarbon Emissions V-56
Emissions From Landfills and Cement V-59
Changes in Land Use V-60
Agricultural Activities V-63
Total Emissions V-64
Atmospheric Concentrations V-71
Global Temperature Increases V-76
Comparison with General Circulation Model Results V-81
Relative Effectiveness of Selected Strategies V-82
CONCLUSIONS V-82
REFERENCES V-87
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FINDINGS
• Decisions made in the next few decades, about how electricity is produced, homes are
constructed, and cities are laid out, for example, will have an impact on the climate in 2100
and beyond. While it is not possible to predict the level of greenhouse gas emissions over this
time period, it is possible to construct scenarios of economic and technological development,
and a reasonable range for resulting greenhouse gas emissions, atmospheric concentrations, and
global temperature changes. Global temperature change estimates provide an indicator for the
rate and magnitude of climatic change.
• Carbon dioxide emissions are likely to grow by a factor of 2 to 5 during the next century if
stabilizing policies are not adopted, primarily due to expansion of global coal consumption.
Options are available, however, that could stabilize or reduce carbon dioxide emissions.
Despite the Montreal Protocol to control CFCs, global emissions of these compounds could
remain constant or even increase significantly unless the agreement is strengthened. Methane
emissions could increase by 60-100% during the next century unless measures to control these
emissions are taken.
• Although per capita emissions of greenhouse gases are currently very low in developing
countries, their share of global emissions will rise significantly in the future.
• The relative contribution of carbon dioxide to greenhouse warming is likely to increase
significantly in the future. Carbon dioxide accounts for more than 70% of the increased
commitment to global warming between 2000 and 2100 in all of the scenarios analyzed in this
report. This represents a significantly higher estimate of the role of CO2 compared to its
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roughly 50% contribution to global warming in the last few decades, but is similar to the
estimated contribution of CO2 to increases in the greenhouse effect over the last century.
• If there is no policy response to the risk of climatic change carbon dioxide concentrations are
likely to reach twice preindustrial levels sometime in the latter half of the 21st Century, but
total greenhouse gas concentrations equivalent to this level may occur by 2030 or even earlier,
and are likely to occur before 2050.
• Even with modest economic growth and optimistic assumptions regarding technical progress,
the world could be committed to an equilibrium warming of 1-2°C by 2000, 2-4°C by 2050 and
3-6°C by 2100 (assuming the climate sensitivity to doubling CO2 is 2.0-4.0°C). Realized
warming would be about 2°C by 2050 and 3-4°C by 2100.
• With rapid, but not unprecedented rates of economic growth, the world could be committed
to an equilibrium warming of 1-2°C by 2000, 3-5°C by 2050 and 5-10°C by 2100 (assuming that
the climate sensitivity to doubling CO2 is 2.0-4.0°C). Realized warming would be 2-3°C by
2050 and 4-6°C by 2100. Estimated warming commitments greater than 5°C may not be fully
realized because the strength of some positive feedback mechanisms may decline as the Earth
warms.
• The adoption of policies to limit emissions on a global basis, such as simultaneous pursuit of
energy efficiency, non-fossil energy sources, reforestation, the elimination of CFCs, and other
measures, could reduce the rate of warming during the 21st century by 60% or more. Even
under these assumptions, the Earth could ultimately warm by 1-3°C or more relative to
preindustrial times. Extremely aggressive policies to reduce emissions would be necessary to
ensure that total warming is less than 2°C.
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INTRODUCTION
Although technological advances in industry and agriculture have provided extraordinary wealth
to a portion of the global population of over 5 billion people, these technologies have the potential
to dramatically alter the Earth's climate by causing changes in the composition of the atmosphere as
discussed in Chapters II through IV. Global increases in the atmospheric concentrations of carbon
dioxide (CO2), nitrous oxide (N2O), methane (CH4), and chlorofluorocarbons (CFCs) are now well
documented (Chapter II), perhaps already committing the Earth to significant climatic change.
Myriad human activities are contributing to this situation, and continued population and economic
growth raises the prospect of accelerated greenhouse gas buildup in the future (Chapter IV).
If current trends hi trace-gas concentrations continue, climatic change could be noticeable to
the "man-in-the-street" during the 1990s, and the average surface temperature of the Earth could be
warmer than at any time in recorded human history by the second decade of the 21st century
(Hansen et al., 1988). If the composition of the atmosphere were stabilized by 2000, on the other
hand, detectable climatic change is still possible, but its magnitude would be limited and the rate of
change might be similar to natural fluctuations recorded in the geologic record (Hansen et al., 1988).
What will happen in the future cannot be predicted. The future evolution of the atmosphere
will depend largely on the paths of economic development and technological change, as well as on
the physical, chemical, and biological processes of the Earth-atmosphere system. While we have no
control over this system once gases enter the atmosphere, economic and technological change will be
influenced by policy choices made at local, national, and international levels. This chapter explores
some of the paths the world might follow in the decades ahead and provides an indication of the
relative climatic consequences under these alternatives. After a discussion of the economic and social
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factors that determine emissions, four scenarios of economic and technological development are
presented. These scenarios cannot capture all the possibilities, of course; rather, they have been
developed in order to explore the probable climatic effects under significantly different, but plausible,
economic and technological conditions. The climatic implications of these scenarios are analyzed
using an integrated framework described briefly in this chapter and in greater detail in Appendix A.
The chapter concludes with the results of this analysis and a comparison of these results with other
studies.
APPROACH TO ANALYZING FUTURE EMISSIONS
The scope of this analysis must be global, and because of the long lags built into both the
economic and climatic systems, this study must consider a time horizon of more than a century~we
chose 2100 as the ending year for the analysis. While this is an eternity for most economists and
planners, it is but a moment for geologists. And indeed, decisions made in the next few decades,
about how electricity is produced, homes are constructed, and cities are laid out, for example, will
have an impact on the climate in 2100 and beyond. Decisions about what kinds of automobiles and
other industrial products to produce and how to produce them will also have a profound impact.
These choices, which will affect the amount and type of fuel we use to travel, to heat and light our
homes and offices, and to run our factories, will influence the magnitude of greenhouse gas emissions
for many years.
The vast difference between the energy demand projections of the early 1970s and what has
actually occurred illustrates the danger inherent in simple trend extrapolations and, indeed, even in
making predictions based on results obtained from more complex models. Our approach then is not
to attempt to predict the future, but to construct what we believe are logically coherent scenarios of
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possible paths of economic and technological development. An analytical framework is used to keep
track of the assumptions, data, and relationships needed to define the scenarios. Our intent is to
define the probable climatic effects under the various economic/social/technological alternatives and,
in so doing, increase the likelihood that these consequences will be taken into account when policy
decisions are made. If we believe that under a wide variety of assumptions about long-term
economic growth and technological change the world will face severe climate problems in the absence
of political or economic forces arising from concerns over the greenhouse problem, then it will be
necessary to seriously examine the options available for reducing greenhouse gas emissions.
The difficulty we face is that projections of greenhouse gas emissions are very uncertain,
because of uncertainties in world economic growth, future fuel prices (which demonstrably affect both
the intensities of their use and the substitution amongst alternative energy sources), future rates of
land clearing, and rates of technological change, among other factors. For example, both the vagaries
of the world oil market in the medium term, as well as true uncertainties regarding the long-term
relationship between the cost of producing fossil fuels and the cost of using those fuels in ways that
are relatively benign to the local environment, mean that at best we can only guess at future fossil
fuel use.
Another avenue of analysis, however, yields information that can guide policy makers faced with
these uncertainties. If we can construct scenarios of future energy demand, land-clearing rates, CFC
production, etc., that are driven by reasonable assumptions about population, economic growth,
technologies, and energy prices, then we can develop a plausible range of future greenhouse gas
emissions. To accomplish this task we must consider the structural factors that determine the
quantities and patterns of emissions of radiatively-important gases (i.e., those gases whose presence
in the atmosphere contribute to a greenhouse warming).
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It is conceptually useful to distinguish between production activities and consumption activities.
Production emissions arise largely from the processing of bulk materials—steel from ore, plastics from
petroleum, cement and glass from limestone and silicate rock—which requires large amounts of
energy per unit of industrial value added (i.e., the difference in value between an industry's products
and its inputs) and may also be associated with direct emissions of greenhouse gases. For example,
during cement making, CaCO3 is reformed to CaO + CO2, which is released to the atmosphere, and
during the making of plastic foams, CFCs are released. Much lower emissions per unit of value
added are generally associated with fabrication and finishing. Food production leads to emissions of
methane and nitrous oxide as discussed in Chapter IV, as well as to emissions of CO2 and other
gases as a result of the energy used on and, even more, off the farm. The large amount of energy
required to move freight is also attributable to production activities. Consumption leads to
greenhouse gas emissions as individuals use energy, primarily in pursuit of comfort (heating and air
conditioning) and mobility (automobile and air travel). Other major end-uses for energy include
refrigeration, lighting, water heating, and cooking.
Production
As societies develop over time, both the quantity and the structure of activities that influence
emissions change radically. For example, energy use per unit of Gross National Product (GNP) has
declined steadily and dramatically in industrialized countries, even in periods of declining real energy
prices (Figure 5-1). This decline is due to a combination of two factors. First, improvements in
production processes, which often save capital and labor as well as energy, reduce the energy intensity
per ton of physical output. For example, in steel production modern energy recovery and process
technology make it possible to produce a ton of steel using only 13xl09 joules (13 GJ) of final
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Chapter V
FIGURE 5-1
TOTAL U.S. ENERGY CONSUMPTION PER GNP DOLLAR
1900-1985
50
45
40
35
cc
0
5 30
03
O)
T—
J2 25
0
S 20
15
10
5
0
19
(Megajoules/1982 Dollar)
~Ji
-r KM
i \ A
\J\
—
—
—
—
i i i i
Vs^-~/~\
V
\
I ! I
00 1910 1920 1930 1940 1950 1960 1970 1980
YEAR
Sources: U.S. DOE, 1987a; U.S. Bureau of the Census, 1975.
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February 16, 1989
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energy, less than half of the current U.S. average.1 New processes under development in Sweden
(Hired and Plasma-Smelt) that integrate a number of operations have even lower energy requirements
and reduced overall costs (Goldemberg et al., 1988). Second, the bulk of value added by industry
tends to shift from basic materials processing to fabrication and finishing as a country's infrastructure
matures. Strout (1985) suggests that consumption of steel, cement, and other raw materials begins
to decline after income surpasses about $5000 (1985$) per capita (Figure 5-2). These shifts in
technology and the mix of products generally increase the share of energy consumed as electricity,
but do not significantly increase absolute electricity intensity because efficiency in electric end-uses
improves as well (Kahane, 1986). Rapid economic growth over the long term can be expected to
accelerate the reduction of industrial energy intensity in wealthier countries by promoting the
replacement of old plant and equipment with more efficient technology, as well as by accelerating the
shift toward a less energy-intensive product mix.
In industrialized societies services such as public and private administration, health care, and
education, are likely to grow faster than GNP, both because much of industry is being redefined as
services and because much of our new wealth is being created by the development and transfer of
information. Heating, air conditioning, and lighting, which dominate energy and electricity use in
buildings today, will become less energy-intensive (even as indoor environmental quality continues to
rise) as more efficient technology is adopted. At the same time, information technology is exerting
upward pressure on electricity use per square foot in office buildings and schools. The Business
Services sector depends more on electricity than does any other sector in the economy, although it
still uses less electricity per unit of output than industry. If there is a large increase in electricity use
in industrialized countries, it will come from a massive expansion of the service sector.
1 1 GJ = 0.948 million British Thermal Units (BTU).
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Chapter V
FIGURE 5-2
a.
U
a-
u
I
CONSUMPTION OF BASIC MATERIALS
Consumption Per Dollar of GNP
3000
4000
G.N.P. Per Capita (In 1983 Dollars)
5000 8000
70.
60-
50-
40-
30-
20-
10-
11000
I
14000
x'\
.6
Q.
5 §
.4
_3
_1
i i
Consumption Per Capita
TOO.
600.
500-
400.
300-
200-
100-
-60
-50
.40
70
T I I I i r i i i
1890 1900 1910 1920 1930 1940 1950 1960 1970 1980
Year
Source: Williams et al., 1987
s
Is
U
,30 g"
2
.20
10
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The greatest potential for large increases in production-related emissions lies in developing
countries. As these countries expand their industrial infrastructure the demand for basic materials
could skyrocket. But developing countries have the opportunity to take advantage of new processes
and materials that sharply reduce the energy required to produce a given level of amenity. As a
result, it is unlikely that materials and energy intensity per capita in developing countries would reach
the levels of industrialized countries today, even as similar levels of per capita income are achieved.
The extent to which developing countries seize these kinds of opportunities will strongly influence
future greenhouse gas emissions.
Consumption
The factors influencing emissions arising from consumption are quite different from those that
affect production. In developing countries energy use in consumer products can be expected to
increase rapidly as the number of households that can afford to acquire fans, televisions, refrigerators,
and automobiles grows. Part of the reason that developing country energy demand has historically
increased faster than it did in OECD countries is that developing-country households can afford to
purchase these products at lower income levels than was the case for industrialized-country
households. The declining price-to-income ratios for many of the energy-intensive consumer goods
make this possible, with the consequence that developing country energy consumption tends to grow
more rapidly than the experience of the industrialized countries might indicate At the same time,
the efficiency of many of these products is increasing, so that per capita energy consumption in
developing countries may not reach the levels of industrialized countries today, even if these income
levels are surpassed.
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As a society becomes wealthier, the penetration of energy-intensive equipment saturates (there
are, for example, 600 cars for every 1000 people in the U.S. compared to 6 in Asian countries), and
changes in the efficiency of the stock and how the stock is used becomes more important than
changes in the levels of ownership alone. For example, as automobile ownership shifts from
corporate to private hands the number of vehicles increases dramatically, but the miles driven per
vehicle declines. Consumers rarely consider energy use in making major purchases, and many key
decisions that determine energy requirements are made by developers rather than the consumers who
pay the energy bills (Ruderman et al., 1987). Increased amenity levels can often be achieved while
simultaneously reducing energy use and emissions (better insulated houses are more comfortable
because they are less drafty and more efficient air conditioners are usually quieter), but more affluent
consumers are likely to choose powerful cars and spacious dwellings, paying less attention to the
associated operating costs. Further, because a very wide range of efficiency can be achieved with a
small impact on total costs over the life-cycle of the product (see Chapter VIII; von Hippie and Levi,
1983; Ruderman et al., 1987), consumers who are concerned about initial cost are unlikely to choose
a product whose level of efficiency is optimal from a social perspective.
The level and pattern of mobility may be the most significant uncertainty in future energy use.
Will we spend our free time in our air-conditioned homes watching rented movies on the VCR/TV,
or are we more likely to drive to the countryside to go for a hike? Not surprisingly, the pattern
of automobile use at present (roughly 1/3 of all passenger-kilometers driven in the U.S. are to/from
work, 1/3 are for family business, and 1/3 are hi pursuit of leisure activities; OTA, 1988) is a
function both of distances among where we live, work, and relax, and of how often we choose to
move about. Similarly, airline travel, already dominated in the U.S. by personal rather than business
travel, is more and more determined by how and where people want to spend their free time.
Meanwhile in cities like Hong Kong and Sao Paulo, but also in New York and Los Angeles,
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congestion is increasingly constraining automobile use. The level of fuel economy and emissions
achieved by a particular automobile in practice is very sensitive to average speed, which is down to
about 15-20 miles per hour in LA. and under 10 miles per hour in New York City (Walsh, personal
communication). How and whether cities solve these congestion problems-with roads, car pools,
buses, light rail, or all of the above-will have a large impact on both urban and global environmental
quality.
SCENARIOS FOR POLICY ANALYSIS
In order to explore some of the implications of the relationships discussed briefly above, we have
constructed four scenarios of future patterns of economic and technological development starting with
alternative assumptions about the rate of economic growth and the adoption of policies that influence
climatic change (Table 5-1). These four scenarios cannot capture all the possibilities of course;
rather, they allow us to explore likely climatic outcomes and the impact of strategies for stabilizing
the atmosphere. The sensitivity of the results to a wide range of specific assumptions has been tested
and is discussed in Chapter VT.
Two scenarios explore alternative pictures of how the world may evolve in the future assuming
that policy choices allow unimpeded growth in emissions of greenhouse gases (these are referred to
as the "No Response" scenarios). One of these scenarios, called a Rapidly Changing World (RCW),
assumes rapid economic growth and technical change; the other assumes more gradual change and
is called the Slowly Changing World (SCW). That is, we have invented a future with relatively high
and robust economic growth, and one representing a more pessimistic view of the evolution of the
world's economies. The first world would likely illustrate the upper half of the potential range of
future greenhouse gas emissions, because in general higher economic activity means a higher total
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Chapter V"
TABLE 5-1
Overview of Scenario Assumptions
Slowly Changing World
Rapidly Changing World
Slow GNP Growth
Continued Rapid Population Growth
Minimal Energy Price Increases
Slow Technological Change
Carbon-Intensive Fuel Mix
Increasing Deforestation
Montreal Protocol/Low Participation
Rapid GNP Growth
Moderated Poulation Growth
Modest Energy Price Increases
Rapid Technological Improvements
Very Carbon-Intensive Fuel Mix
Moderate Deforestation
Montreal Protocol/High Participation
Slowly Changing World
with Stabilizing Policies
Rapidly Changing World
with Stabilizing Policies
Slow GNP Growth
Continued Rapid Population Growth
Minimal Energy Price Increases/Taxes
Rapid Efficiency Improvements
Moderate Solar/Biomass Penetration
Rapid Reforestation
CFC Phase-Out
Rapid GNP Growth
Moderated Population Growth
Modest Energy Price Increases/Taxes
Very Rapid Efficiency Improvements
Rapid Solar/Biomass Penetration
Rapid Reforestation
CFC Phase-Out
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energy use and emissions; conversely, the second world could serve as a useful guide to the lower
half of the range. In either case, our scenarios are first constructed as if there were no interventions
motivated by global climate problems.
In constructing these two worlds/scenarios, we have borne two important ideas in mind. First,
evidence is clear that with more rapid economic growth, energy efficiency improves more rapidly than
with slower growth (Schurr, 1983). This occurs because innovation proceeds more rapidly and
because older, less efficient systems are more rapidly replaced with new technology. History shows,
for example, that for almost every country, energy efficiency in industry increases with increasing
incomes, as sophistication and scale win over brute force. At the same time, higher incomes allow
people to spend more money on two key energy-intensive uses, space conditioning (heating and air
conditioning), and automobiles. Thus not all of the technological benefits of rapid economic growth
put the brakes on overall energy use. But more rapid economic growth does allow society to put
resources aside to improve the efficiency of both space comfort and personal transportation. Similar
patterns can be expected in other emissions sectors.
Conversely, slower economic growth retards innovation, in part because both consumers and
producers do not see bright economic times that make innovation and expansion into new
technologies useful. Comfort and mobility still manage to increase as important drivers of personal
energy demand, but at a slower rate. When these two paths are compared, the effect of more rapid
efficiency increases hi the higher growth world is to narrow the difference in greenhouse gas
emissions; that is, the likely difference between emissions in the Rapidly and Slowly Changing Worlds
is less than the differences hi Gross National Product. This result makes our scenarios somewhat
more robust than one might otherwise think.
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The second idea concerns energy prices. In a world of high and robust economic growth, which
we have assumed in the Rapidly Changing scenario, energy demand will likely increase, and in the
medium term, so will energy prices. Yet if energy efficiency increases, then energy costs can increase
more rapidly than the rate of economic growth and still not consume an increasing share of national
wealth and income. In other words, energy prices can rise without putting the brakes on economic
growth, as long as the price increases are gradual (CONAES, 1979). But in a world of sluggish
economic growth, energy demand rises more slowly, so that energy prices would rise very little. This
idea is an additional reason why we believe that energy efficiency increases more rapidly in the high
growth scenario (RCW) than in the low growth scenario (SCW).
With these ideas in mind, we can build scenarios of world energy demand by end use and region
as well as levels of other activities that emit greenhouse gases. The scenarios are not exact
predictions, but serve as guides to the level of emissions associated with each important purpose or
end use in the worlds we constructed.
One benefit of using this approach is that we can compare the utilization efficiencies that we
assume for the No Response scenarios with those we believe achievable if more than just market
forces were acting. Two additional scenarios (referred to as the "Stabilizing Policy" scenarios) start
with the same economic and demographic assumptions, but examine the effect that policies could
have on global warming. These scenarios are called the Slowly Changing World with Stabilizing
Policies (SCWP) and the Rapidly Changing World with Stabilizing Policies (RCWP).
Using our best information about technologies that could become available, or technologies that
are already available but not taken up by the market because of market failures or other reasons,
we can reconstruct activity patterns that are still consistent with our overriding economic assumptions,
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but produce much lower levels of greenhouse gas emissions. Key changes are assumed in energy
efficiency, the energy supply mix, land-clearing rates, and other factors that might be changed by
government policies or other means.
In other words, we keep the basic scenarios but, for example, manipulate important energy use
patterns within these scenarios. These manipulations can only be carried out if greenhouse gas
emissions in each scenario are constructed from the bottom up, i.e., by specifying the level of each
major-emitting activity, as well as the emissions per unit of activity (e.g., total harvested rice paddy
area and methane emissions per square meter of paddy).
Thus the scenarios we constructed are a necessary step towards illustrating both ranges of
greenhouse gas emissions under two quite different assumptions about economic growth, and where
there is scope for reducing emissions through a variety of strategies. In the final analysis, our work
can be turned around: we can consider the levels of emissions that under the best and worst
assumptions about how emissions are coupled to climatic change leave the world's climate tolerable.
Scenarios with Unimpeded Emissions Growth
In a "Slowly Changing World" (SCW) we consider the possibility that the recent experience of
modest economic growth will continue indefinitely, with no concerted policy response to the risk of
climatic change. In this scenario we assume that the aggregate level of economic activity (as
measured by GNP) increases relatively slowly on a global basis (Table 5-2). Per capita income is
stagnant for some time in Africa and the Middle East as rapid population growth continues. Modest
increases in per capita income occur elsewhere, and per capita growth rates increase slightly over
time in all developing countries as population growth rates slowly decline (Figure 5-3). The share
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Chapter V
TABLE 5-2
Economic Growth Assumptions
(percent per year)
Slowly Changing World
US & OECD
USSR&
Eastern Europe
Centrally
Planned Asia
Other Developing
Countries
WorM
1965-1975
3.9
6.2
7.0
5.6
4.4
1975-1985
2.8
NA
7.8
3.2
2.91
1985-2025
1.7
2.2
3.2
2.7
2.0
2025-2100
1.0
1.6
2.5
2.1
1.5
Rapidly Changing World
1985-2025
2.7
4.3
5.1
4.5
3.4
2025-2100
IS
2.6
4.0
3.3
2.6
a Excludes USSR and Eastern Europe.
Source: IMF, 1988.
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Chapter V
14000
12000 —
FIGURE 5-3
POPULATION BY REGION
(Millions)
Slowly Changing World
1985 2000
2025 2050
YEAR
2075
Other Developing
S & SE Asia
China & CP Asia
USSR & E.Europe
Rest of OECD
United States
2100
Rapidly Changing World
1985 2000 2025 2050
YEAR
Source: U.S. Bureau of the Census, 1987
2075
Other Developing
S & SE Asia
China & CP Asia
USSR & E,Europe
Rest of OECD
United States
2100
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of global income going to the developing world does increase with time, but not dramatically. The
population engaged in traditional agriculture and shifting cultivation continues to increase, as do
demand for fuelwood and speculative land clearing. These factors lead to accelerated deforestation
until tropical forests are virtually eliminated toward the middle of the next century.
In industrialized countries economic growth is sluggish, although per capita income reaches about
$40,000 by 2100 in the OECD. Because of slack demand, real energy prices increase slowly.
Correspondingly, existing capital stocks turn over slowly and production efficiency in agriculture and
industry improve at only a moderate rate. The energy efficiency of buildings, vehicles, and consumer
products also improve at a slow rate.
In a "Rapidly Changing World" (RCW) we assume that rapid economic growth and structural
change occurs and that little attention is given to the global environment. Per capita income rises
rapidly in most regions and consumer demand for energy increases, putting upward pressure on
energy prices. On the other hand, there is a high rate of innovation in industry, and capital stocks
turn over rapidly, which leads to an accelerated reduction in energy required per unit of industrial
output. An increasing share of energy is consumed in the form of electricity, produced mostly from
coal. The fraction of global economic output produced in the developing world increases dramatically
as post-industrial structural change continues in the industrialized world. As educational and income
levels rise, population growth declines more rapidly than in the SCW scenario (Figure 5-3).2
Deforestation continues at about current rates, spurred by land speculation and commercial logging,
despite reduced rates of population growth. Energy efficiency is not much of a factor in consumer
2 The sole exception is China, where aggressive policies are assumed in both cases. Slightly
higher population growth is shown in the Rapidly Changing World scenario based on the sources
of the alternative estimates (see Appendix B). This could be attributed to a relaxation of the one-
child-per-family policy hi response to greater economic growth.
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decisions, as incomes increase faster than real energy prices. Private vehicle ownership increases
rapidly hi developing countries while air travel increases rapidly in wealthier ones. Nonetheless,
significant reductions in energy intensity occur with technological innovation and structural change.
Scenarios with Stabilizing Policies
Two variants of the above scenarios explore the impact of policy choices aimed at reducing the
risk of global warming. These scenarios, labelled "Slowly Changing World with Stabilizing Policies"
(SCWP) and "Rapidly Changing World with Stabilizing Policies" (RCWP), start with the same
economic and demographic assumptions used in the SCW and RCW scenarios, respectively, but
assume that government leadership is provided to ensure that limiting greenhouse gas emissions
becomes a consideration in investment decisions beginning in the 1990s. We assume that policies to
promote energy efficiency in ail sectors succeed in substantially reducing energy demand relative to
the No Response scenarios and that efforts to expand the use of natural gas increase its share of
primary energy supply relative to other fossil fuels in the near term. Research and development
into non-fossil energy supply options such as photovoltaics (solar cells) and biomass-derived fuels
(fuels made from plant material) assure that these options are available and begin to become
competitive after 2000. As a result, non-fossil energy sources meet a substantial fraction of total
demand in later periods. The existing protocol to reduce CFC emissions is assumed to be
strengthened, leading to a phase-out of fully-halogenated compounds and a freeze on methyl
chloroform. A global effort to reverse deforestation transforms the biosphere from a source to a sink
for carbon, and technological innovation and controls reduce agricultural, industrial, and transportation
emissions.
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While these general assumptions apply to both the SCWP and RCWP cases, the degree and
speed of improvement are higher in the Rapidly Changing variant because technological innovation
and capital stock replacement are greater hi this case. The policies considered do not require
changes hi basic life styles. For example, energy use in buildings is greatly reduced hi the Stabilizing
Policy scenarios relative to the No Response scenarios, but the floor space available per person and
the amenity levels provided are assumed to be the same. The technological strategies and policy
options available to achieve the Stabilizing Policy scenarios are discussed in detail in Chapters VII,
VIII, and IX.
ANALYTICAL FRAMEWORK
To make it possible to assess the implications of the kinds of scenarios just described, we have
developed an integrated analytical framework to organize the data and assumptions required to
calculate emissions of radiatively and chemically active gases, concentrations of greenhouse gases, and
the rate of climatic change. This framework is described very briefly here, and hi more detail hi
Appendix A.
The analytical framework consists of four emissions modules and two concentration modules as
shown in Figure 5-4. The four emissions modules use input data, including scenario specifications
for population growth, GNP, energy efficiency, etc., to estimate emissions of greenhouse gases for
nine regions of the globe (Figure 5-5). Emissions are calculated every 5 years from 1985 to 2025 and
then every 25 years through 2100. Emissions of the greenhouse gases CO2, CH4, N2O, and a number
of CFCs are explicitly calculated within the framework. Emissions of CO and NOX, which are not
themselves greenhouse gases, are also explicitly calculated, as these gases can significantly alter the
chemistry of the atmosphere and thus affect the concentrations of the greenhouse gases.
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Chapter V
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Policy Options for Stabilizing Global Climate - Review Draft
Chapter V
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Policy Options for Stabilizing Global Climate - Review Draft Chapter V
The concentrations of other greenhouse gases, such as water vapor and ozone, are calculated
implicitly or explicitly as a function of the other gases. The atmospheric composition and ocean
modules together estimate global concentrations of the greenhouse gases resulting from the projected
emissions, and increases in global temperatures resulting from the calculated concentrations. The
atmospheric trace gas concentrations and temperatures affect the emissions and concentration modules
in the next time period.
Energy Module
The energy module consists of a Global Energy Supply Model (SUPPLY), which is based on the
energy-CO2 model of Edmofids and Reilly (1983a, 1984), and was developed by ICF Inc. for this
study; a global energy end-use analysis (DEMAND), conducted by the World Resources Institute and
Lawrence Berkeley Laboratories; and combustion emission coefficients developed by Radian (1987).
DEMAND estimates energy consumption based on specific assumptions about the level of energy
using activities and technical efficiency by region and sector (industry, transportation, buildings).
Although this analysis provided more detail than most previous global studies, this level of aggregation
obscures many important variations, particularly for developing countries. For example, per capita
incomes vary from $150 for Bangladesh to $7000 for Singapore within the South and East Asia
region. The share of energy used by the manufacturing sector, vehicle ownership levels, and types
of fuels used (particularly the importance of biofuels), all vary from one economy to another. In
conducting the analysis we capture some of this diversity by examining energy use by regions and by
income groups within regions. Detailed analysis was performed for 2025 to anchor the demand
estimates calculated for other years using SUPPLY.
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SUPPLY includes estimates of energy resources and costs by region and can balance supply and
demand using a highly-aggregated estimate of demand as a function of price and income. Supply-
demand equilibration takes place within SUPPLY, which projects fuel mix and final prices. Trace-
gas emissions are calculated by allocating the final fuel consumption among the individual combustion
technologies for which emission coefficients are available. Additional emissions associated with fuel
production are also estimated.
Industry Module
The industry module consists of a CFC model and a model for other non-combustion trace-gas
sources. The CFC model was developed by EPA for use in assessing stratospheric ozone depletion
(U.S. EPA, 1987). It projects production and emissions of the following compounds: CFC-11, CFC-
12, HCFC-22, CFC-113, CC14, CH3CCl3, CH3C1, CH3Br, CF4, Halon 1211, and Halon 1301. Other
industrial sources of trace gases include landfilling and cement production. Emissions from these
activities are estimated as a simple function of population and per capita income.
Agriculture Module
The agricultural module uses the IIASA/IOWA Basic Linked System, or BLS, (Frohberg, 1988)
to forecast fertilizer use and agricultural production. These estimates are used with emission
coefficients derived from the literature to calculate emissions of N2O from fertilizer use, CH4 from
rice production, CH4 from enteric fermentation in domestic animals, and emissions of CH4, N2O,
NOX, and CO from burning agricultural wastes.
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Land Use and Natural Source Module
This module consists of components dealing with a number of land surface processes and other
natural sources of trace gases. The most important of these is carbon dioxide released from land
use change, particularly deforestation, which is projected with the Marine Biological
Laboratory/Terrestrial Carbon Model, or MBL/TCM, (Houghton et al., 1983) based on assumptions
about future rates of land clearing. Other anthropogenic emissions related to land clearing, such as
a portion of CO emissions from biomass burning and N2O emissions from land disturbance, are
scaled based on the CO2 emissions calculated by the MBL/TCM. Natural emissions of CO, CH4,
N2O, and NOX from sources such as forest fires, wetlands, soils, oceans, and fresh water are based
on values from the literature, and generally are held constant throughout the projection period
(biogeochemical feedbacks can be assumed to alter these emissions, see Chapter VI).
Ocean Module
Ocean uptake of heat and CO2 are modeled using the Box-Diffusion approach introduced by
Oeschger et al. (1975) as implemented for the GISS GCM (Hansen et al., 1984). The ocean mixing
parameter for heat uptake is chosen to reproduce, as closely as possible, the timescales obtained in
the time-dependent calculations with the GISS GCM (Hansen et al., 1988). Alternative values for
this parameter can be used to approximate the timescales of other approaches to estimating ocean
heat uptake (Chapter VI). Alternative ocean model formulations for CO2, such as the Advective-
Diffusive Model (Bjorkstrom, 1979) and the Outcrop-Diffusion Model (Siegenthaler, 1983), are
included in the integrating framework and can be used for alternative estimates of CO2 uptake. Total
carbon uptake is calibrated using estimates of historical emissions of CO2 from fossil fuels (Rotty,
1987a,b) and deforestation (Houghton, 1988). The atmospheric CO2 concentration is assumed to be
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285 ppm in 1800 and is forced to be equal to the values obtained at Mauna Loa for the period of
record (1960-1985). The excess flux required to meet these conditions is calculated and held constant
in the future at the average value for 1975-1985. Alternative assumptions are considered in Chapter
VI.
Atmospheric Composition and Temperature Module
The atmospheric composition model was developed for this study (Prather, 1988). It estimates
changes in the concentration of key atmospheric constituents and the global radiation balance based
on the emissions/uptake projected by the other modules. Perturbations to atmospheric chemistry
are incorporated based on first-order (and occasionally second-order) relationships derived from more
process-based chemical models and observations. The model is essentially zero-dimensional, but it
does distinguish between the northern hemisphere, southern hemisphere, troposphere, and
stratosphere. Global surface temperature change is calculated based on the radiative forcing of the
greenhouse gases derived from Lacis et al. (1981) and Ramanathan et al. (1985) coupled to heat
uptake by the ocean model using a specified climate sensitivity parameter. This sensitivity parameter
is set to yield a global equilibrium temperature increase of 2 or 4°C when the CO2 concentration
is doubled, reflecting a central estimate of the range of uncertainty; a broader range of possibilities
is examined in Chapter VI (see discussion in Chapter III).
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Assumptions
Population Growth Rates
The population estimates for the Rapidly Changing World scenario were developed from
Zachariah and Vu (1988) of the World Bank; for the Slowly Changing World scenario estimates were
taken from U.S. Bureau of the Census (1987) of the U.S. Bureau of the Census. These two sources
agree quite closely on the size of the world's population through 2000, then diverge thereafter due
to different assumptions on the rate at which the global population will stabilize. Zachariah and Vu
(1988) assumes that population growth rates in developing countries will begin to decline markedly
after 2000, achieving a net reproduction rate of unity hi every country by 2040. (A net reproduction
rate of unity indicates that people of child-bearing age have children at a replacement rate; it
eventually leads to a stable population level.) U.S. Bureau of the Census (1987) assume that global
population stability will occur at a later date, with developing countries experiencing rapid population
growth rates until the middle of the next century.
Economic Growth Rates
The primary source for the economic growth rate estimates was the World Bank (1987). In their
report, Gross National Product (GNP) forecasts were provided for the 1986-1995 period for several
different types of country groups. Most countries could be classified into one of these three general
categories-low income, middle income, or industrialized. In addition, the World Bank defined several
other more select groups for which separate growth rates were estimated, including oil exporters,
exporters of manufactures, highly-indebted countries, and sub-Saharan Africa. The low growth case
was used as a starting point for this analysis because these estimates were more consistent with recent
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historical trends and other forecasts. For the RCW (SCW) scenario these initial values were
generally increased (decreased) by one percentage point for developing and East Bloc countries and
one-half percentage point for OECD countries to reflect the greater uncertainty regarding future
growth in developing and centrally-planned economies. The growth rates were applied for the period
1985-2000, and were generally reduced by one-half percentage point each 25-year period, beginning
in 2000, to reflect structural change and the decline hi population growth rates over time.
Nonetheless, GNP per capita continues to increase throughout the projection period, although the
rate of growth is substantially lower in the Slowly Changing World scenario.
Oil Prices
The oil prices used in this analysis were taken from U.S. DOE (1988), which supplied a range
of oil price forecasts. The Middle Price forecast from DOE was used for the Rapidly Changing
World scenario (by 2000 the world oil price is about $31/barrel in 1987 dollars), while the Low Price
forecast was used for the Slowly Changing World scenario (oil prices by 2000 were about $25/barrel
in 1987 dollars). Since the DOE price forecasts did not extend beyond 2000, oil prices were derived
from the SUPPLY model; in each scenario prices escalated about 0.8% annually from 2000-2100.
Limitations
This analytical framework attempts to incorporate some representation of the major processes
that will influence the rate and magnitude of climatic change during the next century within a
structure that is reasonably transparent and easy to manipulate. In so doing we recognize a number
of major limitations:
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Policy Options for Stabilizing Global Climate -- Review Draft Chapter V
• Economic growth rates are difficult to forecast. Our alternative assumptions may not
adequately reflect the plausible range of possibilities. In particular, we have assumed that
aggregate economic growth rates will generally decline over time from the levels assumed
for 1985-2000; this may not be the case.
• Economic linkages are not fully captured. The economic analysis uses a partial-
equilibrium framework, making it impossible to ensure that the activity levels assumed
in each sector are completely consistent with the aggregate economic assumptions. In
addition, capital markets are not explicitly considered. This is particularly significant in
examining developing countries as it is unclear if they will be able to obtain the capital
investments needed to develop the energy supplies assumed in some of the scenarios.
• Technological changes are difficult to forecast. Substantial improvements in the efficiency
of energy using and producing technologies are assumed to occur even in the absence
of substantial energy price increases or policy measures. If this assumption proves to
be untrue, then greenhouse gas emissions may be substantially underestimated in the
No Response scenarios. Similarly, aggressive research and development is assumed to
substantially reduce the cost of renewable technologies in the Stabilizing Policy scenarios.
The impact of policies may be overestimated if such improvements fail to materialize or
if they would have materialized as rapidly even without increased government support.
• Detailed cost analyses have not been conducted. Technological strategies have been
screened based on judgments about their potential cost-effectiveness, but no attempt has
been made to rank the cost-effectiveness of each strategy or to estimate the government
expenditures or total costs associated with the stabilizing strategies.
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• The modules of the framework are not fully integrated. Existing models of individual
processes that affect greenhouse gas emissions were assembled within the analytical
framework and were used with consistent assumptions. However, it was not possible to
ensure complete consistency of results. For example, while the biomass energy supplies
arrived at in the Energy module do not appear to be inconsistent with the land use
patterns calculated in the Agriculture and Land Use and Natural Source modules, there
is no explicit coupling among these results.
• The ocean models employed are highly simplified. The ocean plays an important role
in taking up both CO2 and heat. The one-dimensional models used to represent this
process may not adequately reflect the underlying physical processes, particularly as
climate changes.
• Changes in atmospheric chemistry are calculated in a highly-simplified fashion. Chemical
interactions are analyzed based on parameters derived from detailed chemical models.
These parameters may not adequately reflect the underlying chemistry, particularly as the
atmospheric composition changes significantly from current conditions. Also, it is not
possible to explicitly model the heterogeneous conditions that control, for example,
tropospheric ozone concentrations. In our analysis we also assume that non-methane
hydrocarbon emissions remain constant, which may cause future methane and ozone
changes to be underestimated.
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Policy Options for Stabilizing Global Climate -- Review Draft Chapter V
SCENARIO RESULTS
We have estimated the implications of the four scenarios described above for emissions of
radiatively-important gases arising from energy production and use, industrial processes, changes in
land use, and agricultural activities using the integrated analytical framework developed for this study.
The resulting changes in atmospheric composition and global climate are also estimated.
Energy Sector
The single most important determinant of greenhouse gas emissions is the level of energy demand
and the combination of sources that are used to supply that energy.
End-use Consumption
Government policies that affect demand for energy are likely to be the most important
determinant of greenhouse gas emissions in the near term. Figures 5-6 and 5-7 illustrate global
end-use energy consumption by region for fuel and electricity, respectively. Total end-use energy
consumption increases from 220xl018 joules (220 EJ) in 1985 to 320 EJ in 2025 hi the SCW versus
420 EJ in the RCW.3 Greater improvements in energy efficiency in the SCWP and RCWP cases
reduce end-use demand in 2025 by 13% and 15%, respectively, relative to the No Response scenarios.
Extrapolating these trends to 2100 yields 430 EJ in the SCW and 780 EJ in the RCW scenarios, while
in the Stabilizing Policy cases there is 20% and 35% lower demand, respectively.
1 EJ = 0.948 quadrillion BTU (Quad).
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Chapter V
FIGURE 5-6
END-USE FUEL DEMAND BY REGION
(Exajoules)
SCW RCW
Other Developing
China & CP Am
USSR * CP Europe
Other OECO
United State!
1985 2000 202S 2060 2075 2100
1986 2000 2025 2050 2075 2100
SCWP
RCWP
Reduction From
No Response Scenario
Other Developing
China » CP Asia
USSR » CP Europe
Other OECD
United States
1985 2000
2026 2050
VEAR
2075 2100
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Policy Options for Stabilizing Global Climate -- Review Draft
Chapter V
FIGURE 5-7
END-USE ELECTRICITY DEMAND BY REGION
(Exajoules)
SCW RCW
2025 2050 2075 2100
SCWP
Other Developing
Chin* & CP Asia
USSR 9i CP Europe
Other OECD
United States
1985 2000 2025 20SO 2075
RCWP
Reduction From
No R«spons« Scenario
1986 2000 2025 2060 2075 2100
VEAR
1S85 2000 202E 2050 2075 2100
VEAR
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In each scenario the growth in end-use demand is driven almost entirely by countries outside the
OECD (USSR, Eastern Europe, China, and other developing countries) as a result of higher rates
of economic and population growth in these regions and from more rapid efficiency improvements
and the saturation of energy-intensive technologies in the OECD (e.g., steel production, automobile
transportation, and central heating). Fuel use, in particular, is not expected to grow significantly in
the U.S. and other OECD countries as efficiency gains compensate for increases in floor space,
mobility, and production. Electricity use is projected to grow much more rapidly than fuel use in all
cases, and significant increases in OECD electricity demand are reflected in the RCW.
It is important to note that both the SCW and RCW scenarios assume substantial efficiency gains
due to technological innovation and market forces. For example, fuel use per square meter of
residential and commercial floor space is assumed to fall by 45-55% in the United States and
Western Europe by 2025. Similarly, fleet average fuel efficiency of U.S. cars and light trucks reaches
7.8 and 6.9 liters per 100 kilometers (liters/100 km), or 30 and 34 miles per gallon (mpg), in the
SCW and RCW scenarios, respectively. In the SCW, industrial energy use per unit of GNP falls by
1.5-2%/yr hi the industrialized countries, in accordance with recent trends. This rate accelerates to
2-3.5%/yr in the RCW, the highest rate of improvement being for the East Bloc countries as they
have the highest initial industrial energy intensities. Less optimistic assumptions about efficiency gains
in the No Response scenarios would imply higher rates of associated climatic change and greater
relative improvement ha the Stabilizing Policy scenarios.
In developing countries the use of biofuels for cooking is strongly influenced by urbanization
and the efficiency with which these fuels are used. Urban populations have better access to modern
fuels and thus a smaller share of urban households will use traditional fuels. There is substantial
scope for improvement of the efficiency of biomass use. Laboratory experiments in Asia with
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Policy Options for Stabilizing Global Climate -- Review Draft Chapter V
improved cookstoves suggest that it is possible to achieve efficiencies of up to 33% (compared with
current averages of 8%). However, experience from the last decade of improved cookstove
dissemination projects suggests that efficiencies are unlikely to exceed 20% in the field. We assume
the dissemination of efficient cookstoves to almost all users of biomass only in the Stabilizing Policy
cases. Thus, the average efficiency of biomass use is assumed to improve to 15-17% in each region
in these scenarios. As a result of these efficiency improvements and because an increasingly larger
share of the population moves to urban areas, where there is better access to modern fuels, the
amount of biofuels consumption declines hi the household sector for each scenario.
Important structural shifts underlie the aggregate trends in these scenarios. Electricity's share
of end-use consumption more than doubles in the RCW, from 16% in 1985 to 19% in 2025 and 34%
in 2100, while it grows less dramatically hi the SCW, reaching 24% in 2100. These trends are
accentuated hi the policy scenarios as there appears to be even greater room for reductions in fuel
use than in electricity use, partly because electricity is substituted for fuel hi some highly-efficient
applications. In particular, electricity accounts for 40% of end-use consumption by 2100 in the
RCWP scenario because of dramatic increases in electricity use in developing countries. The
distribution of energy use among the industrial, transportation, and residential and commercial sectors
also shifts significantly, as shown hi Figure 5-8. In the Rapidly Changing World the share of end-
use energy going to the residential and commercial sectors declines continuously, while the share
going to industry increases until the middle of the 21st Century and then declines. This pattern
reflects the increasing importance of developing countries, which generally have low heating demands
and a greater percentage of modern energy devoted to the Industrial sector. This occurs despite a
decline hi the share of commercial energy use, particularly electricity, going to the industrial sector
within developing country regions. As the most intense phase of industrialization is completed the
transportation sector begins to take off, its share rising steadily after 2025. In the SCW scenario the
share of end-use energy consumed hi the industrial sector grows less dramatically and does not peak
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Policy Options for Stabilizing Global Climate •- Review Draft
Chapter V
FIGURE 5-8
SHARE OF END-USE ENERGY DEMAND BY SECTOR
(Percent)
SCW
1985 2000 202S 2050 2075 2100
SCWP
RCW
Residential ft
Commercial
!• Industrial
1985 2000 2025 2050 207S 2100
RCWP
19S5 2000
202E 20EO
YEAH
2075 2100
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until 2075 as industrialization in developing countries is stretched out over a longer time-span and
dramatic increases in mobility are delayed. In the Stabilizing Policy scenarios the growth hi
transportation energy use is suppressed by much higher fuel efficiency and the share of end-use
energy going to the residential and commercial sectors increases toward the end of the next century.
Primary Energy Supply
While policies affecting demand will have the largest impact on near-term greenhouse gas
emissions, changes in the supply mix will also be very important over the long term. Global primary
energy supply is shown by source for the four scenarios in Figures 5-9 and 5-10. Growth in primary
energy production is substantially higher than growth in end-use energy consumption because of
increased requirements for electricity and synthetic fuel production. This is most dramatic in the
RCW, where primary energy production increases from 290 EJ in 1985 to 580 EJ in 2025 and 1410
EJ in 2100; a 100% and 380% increase, respectively, compared with 90% and 260% increases in end-
use consumption.
The use of synthetic fuels to supplement conventional oil and gas production becomes particularly
important after 2025, influencing both total requirements and the mix of sources (Figure 5-11). In
the RCW conventional oil and gas production increases through 2050, then begins to decline due to
resource depletion (the share of primary energy supplied by oil and gas declines throughout the
projection period). As a result, synthetic fuels are increasingly relied on to supply liquid and gaseous
fuel requirements. By 2050 19% of primary energy is used in synthetic fuels production, and this
value increases to 40% by 2100. In the SCW heavy dependence on synthetic fuels begins later
because conventional oil and gas resources are depleted more gradually. Coal is the dominant
feedstock for synfuel production in both of these scenarios.
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Chapter V
FIGURE 5-9
PRIMARY ENERGY SUPPLY BY TYPE
(Exajoules)
SCW RCW
H8S 2000 202S 2050 207S 2100
SCWP
Reduction From
No A*spons«
Sc«n«no
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Policy Options for Stabilizing Global Climate •- Review Draft
Chapter V
FIGURE 5-10
SHARE OF PRIMARY ENERGY SUPPLY BY TYPE
(Percent)
SCW RCW
1985 2000 2025 2050 2075 2100
SCWP
1985 2000 2025 2050 2075 2100
RCWP
1385 2000
202S 2060
VCAK
2075 2100
2000 2025 20EO 2075 2100
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Policy Options for Stabilizing Global Climate - Review Draft
Chapter V
FIGURE 5-11
ENERGY DEMAND FOR SYNTHETIC FUEL PRODUCTION
(Exajoules)
SCW RCW
2050
SCWP
2060
YEAR
20SO 2100
RCWP
20SO
VEAR
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The mix of primary energy resources used to generate electricity is also crucial in determining
future greenhouse gas emissions. While non-fossil energy sources (nuclear, solar, and hydro) increase
their absolute contribution to primary energy supply in all scenarios, in the absence of policies to limit
greenhouse gas emissions, it is likely that future electricity production will be dominated by coal-
based technologies over the long term (in the near term, current gas prices make gas-based
combustion turbine technology very attractive in many regions). Thus in the RCW, demand for
electricity and synfuel production pushes global coal consumption up by more than a factor of 10
between 1985 and 2100. Correspondingly, the share of primary energy supplied by coal increases
from 27% in 1985 to 40% in 2025 and 63% in 2100 (Figure 5-10). The same forces are at work
in the SCW, but the results are less dramatic: Coal production increases by less than a factor of 5,
and its share of primary energy reaches just over 50% by 2100.
In the Stabilizing Policy scenarios natural gas is relied on more heavily in the near term while
accelerated research and development and other incentives are assumed to make several non-fossil
electricity supply technologies strongly competitive over the long term. In particular, photovoltaics,
biomass-based combustion turbines, and advanced nuclear reactors appear to be strong candidates
to make a large contribution to future electricity production (these and other options are discussed
in some detail in Chapter VII). In the policy scenarios these technologies begin to supply energy
after 2000 and become strongly competitive by 2025. By 2050 they supply 60% and 70% of global
electricity in the SCWP and RCWP scenarios, respectively.4 It is also assumed that research priorities
and other policies promote the use of biomass-derived fuels rather than coal-based synfuels. In fact,
in 2025 and 2050 total synfuel production is higher in the policy scenarios because biomass production
and conversion is assumed to become competitive with imported oil and gas in many developing
4 This value includes all of the electricity generated from gas, reflecting the assumption that
a little over half of the synthetic gas generated from biomass is actually both produced and consumed
in integrated gasifier-combustion turbine units.
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Policy Options for Stabilizing Global Climate •- Review Draft Chapter V
regions starting around 2010 (Walter, 1988). The particular mix among the non-fossil supply
technologies shown in Figure 5-10 is rather arbitrary, but the type of non-fossil technologies is of little
consequence to total greenhouse gas emissions.
Greenhouse Gas Emissions From Energy Production and Use
The heavy reliance on coal in both the SCW and RCW scenarios leads to large increases in both
CO2 and CH4 emissions (see Figures 5-14 and 5-16 later in the chapter). In the SCW energy-related
emissions of CO2 increase from 5.1 petagrams of carbon (Pg C) in 1985 to 7.2 Pg C in 2025 and 11.1
Pg C in 2100.5 Emissions reach more than twice this level in the RCW scenario: 10.3 and 24.4 Pg
C in 2025 and 2100, respectively. This growth in emissions of 0.5 Pg C per decade in the SCW and
1.3 Pg C per decade in the RCW between 1985 and 2025 compares with average growth of 1.1 Pg
C per decade between 1950 and 1980. Emissions of CH4 from fuel production, predominantly coal
mining, grow even more dramatically. The estimated emissions from fuel production in 1985 are 60
teragrams of CH4 (Tg CH4) or just over 10% of the total.6 In the SCW this source increases to
86 Tg CH4 in 2025 and 160 Tg CH4 in 2100. The corresponding values for the RCW are 130 Tg
CH4 in 2025 and 360 Tg CH4 in 2100, about 20% and 30% of the CH4 total, respectively.
The combination of higher efficiency and greater reliance on non-fossil fuels assumed in the
Stabilizing Policy scenarios serves to substantially curtail CO2 and CH4 emissions. In both the SCWP
and RCWP cases CO2 emissions from energy use reach only 5.5 Pg C in 2025, after which time they
decrease, reaching 3.2 and 4.3 Pg C by 2100 in the two cases, respectively. Similarly, CH4 emissions
from fuel production remain relatively constant in both of these scenarios.
5 1 petagram = 1015 grams.
6 1 teragram = 1012 grams.
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Energy-related emissions, other than of CO2 and CH4, are strongly affected by the type of control
technology employed in addition to the total amount and type of energy used. Emissions of CO and
NOX associated with energy use can be expected to increase almost as rapidly as primary energy
consumption in the absence of new policies. On the other hand, in the Stabilizing Policy scenarios
NO., emissions are roughly constant and CO emissions are cut by more than half. This assumes that
the rest of the world gradually adopts control technology similar to that required of new mobile and
stationary sources in the United States today, and that industrialized countries adopt standards
consistent with the use of Selective Catalytic Reduction technology in utility and industrial applications
after 2000, with developing countries following after 2025.
Comparison to Previous Studies
Despite the large range of outcomes illustrated by the four scenarios developed here, none of
the global rates of change are unprecedented (Table 5-3). Global reductions in aggregate energy
intensity generally fall within the range of 1-2% per year; the lower value is consistent with long-
term trends and the higher value is consistent with recent experience. Reductions in the amount of
carbon emitted per unit of energy consumed (carbon intensity) varies from 0.0-1.3% per year with
significant declines only apparent hi the Stabilizing Policy cases. These values are not unprecedented
as carbon intensity declined by an average of 1.5% per year between 1925 and 1985 due to increased
reliance on oil and gas over coal.
While we know of no previous attempts to develop long-term scenarios for emissions of the full
set of gases discussed above based on explicit economic and technological assumptions, there have
been a number of previous studies that relate to many of the components examined here. Over the
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Policy Options for Stabilizing Global Climate ~ Review Draft
Chapter V
TABLE 5-3
Key Global Indicators
Parameter
*
GNP/capita
(1000 1988 $)
Primary Energy
(EJ)b
Fossil Fuel CO2
GNP/capita
(%/y)
Energy/GNP
(%/yr)
Fossil Fuel
COj/Energy
(%/yr)
Scenario"
SCW, SCWP
RCW, RCWP
SCW
RCW
SCWP
RCWP
SCW
RCW
SCWP
RCWP
SCW, SCWP
RCW, RCWP
SCW
RCW
SCWP
RCWP
SCW
RCW
SCWP
RCWP
1985
3.0
290
5.1
1985-2025
0.5
2.0
-1.1
-1.6
-1.3
-1.9
-0.1
0.0
-0.5
-1.3
Year
2025
3.7
6.7
430
580
380
520
7.2
10.3
5.5
5.5
2025-2100
0.9
2.3
-0.8
-1.4
-1.0
-1.8
-0.0
-0.0
-1.2
-1.1
2100
7.1
35.6
680
1410
550
940
11.1
24.4
3.2
4.3
SCW = Slowly Changing World; SCWP = Slowly Changing World with Stabilizing Policies;
RCW = Rapidly Changing World; RCWP = Rapidly Changing World with Stabilizing Policies.
EJ = exajoule = 0.948 quadrillion BTUs
Pg C = petagrams of carbon; 1 petagram = 1015 grams.
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last decade there have been many studies of U.S. energy futures that can be compared to our U.S.
results. In addition there have been several recent studies of long-term global energy use and CO2
emissions (Chapter I). One recent study has developed "conventional wisdom reference scenarios"
for CH4, CO, NOX, and N2O emissions related to major energy sources (Darmstadter et al, 1987).
This section compares the scenarios presented here to those developed in previous work.
•
Since the OPEC oil embargo focused the world's attention on energy in 1973 a number of
studies have examined the future of energy supply and demand in the United States. Those analyses
contain much more detail, particularly in the short term, than is possible in this study, as our focus
is necessarily global and long term. Nonetheless, it is useful to compare the results of this study for
the U.S. with selected previous work. The National Energy Policy Plan (NEPP) prepared by the
Department of Energy (U.S. DOE, 1987b) and Energy for A Sustainable World (ESW), an
international study supported by the World Resources Institute (Goldemberg et al., 1985, 1987, 1988)
are examples of two important recent studies.
The results of these studies for the United States are summarized and compared with our
scenarios in Tables 5-4 and 5-5. A key point is that both of the No Response scenarios developed
here incorporate much lower growth in energy use and CO2 emissions than is projected in the NEPP
reference and NEPP high-efficiency cases. The largest discrepancies are in demand for electricity and
consumption of coal, although all energy sources other than gas and all sectors show higher
consumption in the NEPP projections. The NEPP Reference Case projects an increase of almost
40% in U.S. CO2 emissions between 1985 and 2010, while the High Efficiency case produces about
a 20% increase. By contrast, the RCW scenario, which has GNP assumptions similar to those used
in NEPP, estimates about a 10% increase in CO2 emissions, while the SCW scenario predicts
essentially flat emissions. Had the NEPP reference case been adopted as one of our No Response
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Chapter V
TABLE 5-4
Comparison of No Response Scenarios and NEPP
End-Use Energy Demand
(exaioules)
Sector
Residential/Commercial
Transport
Industry
Total
Fuel
Elec
Fuel
Elec
Fuel
Elec
Fuel
Elec
1985
11
6
21
0
18
3
50
9
SCW"
10
6
21
0
20
3
51
9
Primary
Estimated
RCW"
11
7
19
0
21
4
51
10
for 2010
NEPP-RC0
13
9
23
0
26
7
58
15
NEPP-HEd
19'
22
0
28e
51
13
Energy Consumption
(exaioules)
Estimated for 2010
Primary Energy
Coal
Oil
Gas
Other*
Total
1985
19
33
19
7
77
sew
18
32
19
8
77
RCW
22
29
22
8
82
NEPP-RC
38
35
19
18
110
NEPP-HE
31
33
17
16
97
Carbon Dioxide Emissions
(oetaarams of carbon)
CO2
1985
1.3
sew
1.3
Estimated
RCW
1.4
for 2010
NEPP-RC
1.8
NEPP-HE
1.6
1 Slowly Changing World scenario.
b Rapidly Changing World scenario.
c National Energy Policy Plan (NEPP) Reference Case (DOE, 1987b)
d National Energy Policy Plan (NEPP) High Efficiency Case (DOE, 1987b)
* Fuel + Electricity. Separate values not given.
' Excludes dispersed wood.
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Policy Options for Stabilizing Global Climate - Review Draft
Chapter V
TABLE 5-5
Comparison of Stabilizing Policy Scenarios and ESW
End-Use Energy Demand
(exaioulesl
Sector
Residential/
Commercial
Transportation
Industry
Total
Primary Energy
Coal
Oil
Gas
Other
Total
Fuel
Elec
Fuel
Elec
Fuel
Elec
Fuel
Elect
1985
11
6
21
0
18
3
50
9
1985
19
33
19
7
77
SCWP"
8
'5
15
0
18
3
41
8
Primary
SCWP
11
24
18
12
65
Estimated
RCWP"
5
5
12
0
21
4
38
9
Energy Consumption
fexaioulesl
Estimated
RCWP
9
18
22
18
64
for 2020
ESW-S0
5
4
12
0
14
5
31
9
for 2020
ESW-S
11
13'
13e
14
52
ESW-Rd
5
4
14
0
15
5
34
9
ESW-R
13 •
14e
14e
14
56
Carbon Dioxide Emissions
Cpetaerams of carbon")
CO2
1985
1.3
SCWP
1.0
Estimated
RCWP
0.8
for 2020
ESW-S
0.7
ESW-R
0.8
* Slowly Changing World with Stabilizing Policies.
b Rapidly Changing World with Stabilizing Policies.
c Energy for a Sustainable World, Goldemberg et al., 1987, 1988. Assumes a 50% increase in per capita GNP from 1980 to
2020. Note that the SCWP case assumes a 50% increase from 1985 to 2020.
d Energy for a Sustainable World, Goldemberg et al., 1987, 1988. Assumes a 100% increase in per capita GNP from 1980 to
2020. Note that the RCWP case assumes a 120% increase from 1985 to 2020.
* Given as Oil + Gas. A 50% split is assumed following the global supply scenario given by Goldemberg et al., 1987, 1988.
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scenarios, the U.S. contribution to global emissions would have been substantially higher than what
we have estimated in the SCW and RCW cases, and the difference between the No Response and
Stabilizing Policy cases would have been significantly greater.
Comparing low emissions scenarios, U.S. energy use is considerably higher in the Stabilizing
Policy cases than in those given in Energy for a Sustainable World. The largest differences in
consumption are in the industrial sector, with significant differences also in the residential and
commercial sectors in the slow-growth cases. We assume that slower turnover of the housing stock
leads to higher residential and commercial demand in the slow growth variant, whereas Goldemberg
et al. assume that income does not effect demand in this sector. Despite higher energy consumption
in our scenario, the two rapid-growth cases have similar CO2 emissions due to lower consumption
of coal and heavier reliance on gas and non-fossil energy sources in the RCWP scenario compared
with the ESW cases.
The global energy use and CO2 emissions calculated for 2050 in the four scenarios developed
here are compared to the bounding extrapolations discussed in Chapter IV and the results of selected
previous studies in Table 5-6. The total energy use derived in our scenarios falls within the lower
end of the range given by trend extrapolation and previous analyses. In those studies that included
a "Base Case" that did not assume the implementation of policies to reduce CO2 emissions, the
estimated primary energy demand for the year 2050 ranges from 21 to 52 terawatts (TW)7. This
level of energy demand is approximately 2.2 to 5.5 times 1985 consumption levels of 9.4 TW. The
Rapidly Changing World scenario has total energy demand that is quite similar to the Base Case
given by a number of previous studies, including Edmonds and Reilly (1984), Seidel and Keyes (1983),
and World Energy Conference (1983). The Slowly Changing World scenario, with almost 50% less
7 1 terawatt = 1012 watts = 31.54 EJ per year.
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' fi- c~-_ cs -4 '
>-* (S t-i «S
ON
l(S
00 Tf VO 00
1— I
-------
Policy Options for Stabilizing Global Climate ~ Review Draft Chapter V
total energy use in 2050, lies between the median and 25th percentile non-zero correlation scenario
of Edmonds et al. (1986) and Reilly et al. (1987). The estimated uncertainty bounds in the systematic
uncertainty analysis conducted by Edmonds et al. are not symmetric; their median scenario has
significantly lower energy use and CO2 emissions than both the mean of their results and the result
of using the median values for all model parameters. The implication is that very high energy use
scenarios may be much less probable than is suggested by simply considering the range given by many
studies.
Compared with the energy use estimates there is substantially less, though still considerable,
variation in the CO2 emissions estimates for 2050. None of the studies cited in Table 5-6 approach
within a factor of four the result of exponentially extrapolating the pre-1973 rates of energy demand
growth, assuming no change in the mix of sources. This reflects the constraint due to the finite size
of the fossil fuel resource base (Chapter IV), which implies that very high growth in energy
consumption would have to be accompanied by a significant shift away from fossil fuels (but not
before atmospheric CO2 concentrations reached extraordinarily high levels). Considering the full
range of values for both energy use and CO2 emissions represented in Tables 5-4 and 5-5, it does
appear that, as intended, the Slowly Changing and Rapidly Changing World scenarios represent very
different but not extreme possibilities.
While the general agreement found between this study and previous studies at the aggregate
level may be comforting, substantial disagreements are possible when the results are examined more
closely. For example, the global increase in energy demand obtained in the Rapidly Changing World
scenario is the result of almost level demand in OECD countries coupled with very vigorous demand
growth in developing countries. Other scenarios with nearly identical global demand in 2050 may not
distinguish among regions (e.g., Nordhaus and Yohe, 1983) or may have a more even pattern of
DRAFT - DO NOT QUOTE OR CITE V-52 February 16, 1989
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Policy Options for Stabilizing Global Climate -- Review Draft Chapter V
energy demand growth (e.g., Edmonds and Reilly, 1984). Similarly, the GNP growth rate assumed
in the RCW scenario is higher than what was assumed by Seidel and Keyes (1983), but because
higher rates of technical efficiency improvements were assumed in the RCW case, energy demand
and CO2 emissions are almost identical in 2050.
The results obtained in the policy scenarios developed here are most appropriately compared
with the results of Lovins et al. (1981), Rose et al. (1983), and Goldemberg et al. (1985, 1987, 1988).
These studies all emphasize the possibility that increased efficiency of energy use could limit energy
demand and CO2 emissions while allowing for sustainable economic growth. They conclude that
energy demand in 2050 could be held to between 5 and 16 TW by supplying energy services with
advanced cost-effective technology that is either available or nearly commercial today. In these
scenarios efficiency improvements combined with shifts in energy supply allow CO2 emissions to be
held at or below today's level, and Lovins et al. (1981) argue that it is technically feasible to reduce
fossil fuel CO2 emissions by about 80% over 50 years. The SCWP and RCWP scenarios have energy
consumption of 15 and 24 TW respectively—similar to, but somewhat higher than, what previous
studies suggested was feasible. Part of this difference may be explained by the high rate of economic
growth assumed in the RCWP case and, particularly in comparison to Lovins et al., our assumption
that efficiency measures are not adopted up to their technical potential. The CO2 emissions in the
policy scenarios are 10-20% below current levels, again consistent with some previous analyses. This
result is obtained in different ways, however. For example, the lowest CO2 scenario given by Rose
et al. assumes substantially more contribution from non-fossil energy sources than do the policy
scenarios developed here, while the Goldemberg et al. high demand scenario has somewhat more oil
and gas and less coal than does the RCWP case in 2020.
DRAFT - DO NOT QUOTE OR CITE V-53 February 16, 1989
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Policy Options for Stabilizing Global Climate - Review Draft Chapter V
The estimates of energy-related (fossil fuel and wood use) emissions of CH4, N2O, NOX, and
CO developed here are compared in Table 5-7 with results of a study by Darmstadter et al. (1987).
While the main purpose of their study was to develop an historical database, reference values for
future emissions are presented assuming either constant emission coefficients or coefficients declining
by 1% per year. The emissions calculated with constant coefficients by Darmstadter et al. increase
much more rapidly than those obtained in any of our scenarios. These differences are not too
surprising given our explicit assumptions regarding technological change, including increasing
penetration of emission control technologies. The largest discrepancy is for N2O, reflecting not only
our assumptions regarding technical change, but also the much higher initial emission coefficient
adopted by Darmstadter et al. based on Hao et al., 1987 (see Chapter II). The initial estimate of
CO emissions given by Darmstadter et al. is a factor of two lower than ours, probably due primarily
to their extrapolation of the U.S. emission coefficient for gasoline to the rest of the world; we have
attempted to account for variations hi automobile emission control technology by region, with most
regions having higher average CO emissions than the U.S. The closest agreement is for CH4,
probably because these emissions are directly proportional to the total quantity of coal and gas
produced, and are not assumed to depend on production technology in our No Response scenarios.
When Darmstadter et al. assume that all emission coefficients decline by 1% per year, they
obtain estimates of NOX emissions that are similar to those occurring in the RCW case and CH4
emissions estimates closer to those obtained in the RCWP case; their CO emissions estimate falls
between these two cases. Overall, the RCWP case has significantly lower emissions than are obtained
by Darmstadter et al. even when they decrease their emission coefficients by 1%/y for a full century.
This is a result not only of the assumptions regarding emission control technology, but also because
our policy scenarios have substantially lower total energy demand and a very different fuel mix.
DRAFT - DO NOT QUOTE OR CITE V-54 February 16, 1989
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Policy Options for Stabilizing Global Climate - Review Draft
Chapter V
TABLE 5-7
Comparison of Energy-Related Trace-Gas Emissions Scenarios
Trace Gas
CH4
(Tg CH4)
N2O
(TgN)
NOX
(TgN)
CO
(TgC)
Scenario
RCW
RCWP
* Darmstadter et al.
** Darmstadter et al.
RCW
RCWP
* Darmstadter et al.
** Darmstadter et al.
RCW
RCWP
* Darmstadter et al.
** Darmstadter et al.
RCW
RCWP
* Darmstadter et al.
** Darmstadter et al.
1985/1980
68
63
1.1
4.3
25
20
202
108
Emissions of Trace
(teragrams')
2025/2030
141
73
192
117
2.1
1.2
16
9.5
43
27
62
37
318
122
292
177
Gases
2075/2080
301
74
432
131
3.8
1.4
57
21
77
21
184
68
651
82
614
226
* Constant Emission Coefficients.
** Emission Coefficients Decline 1% Per Year.
DRAFT - DO NOT QUOTE OR CITE
V-55
February 16, 1989
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Policy Options for Stabilizing Global Climate - Review Draft Chapter V
Industrial Processes
Halocarbon Emissions
The most important industrial source of greenhouse gases not directly associated with energy
use is the production and release of CFCs and halons. In both the Slowly Changing World and
Rapidly Changing World scenarios, the Montreal Protocol as currently formulated is assumed to come
into force and apply throughout the projection period. This agreement (described in Chapter IV,
VIII, and IX) calls on developed countries to reduce their emissions of certain CFCs 50% from 1986
levels by 1998, and to freeze the use of halons. Developing countries with low per capita
consumption, however, are allowed to increase the use of these compounds for up to 10 years~as a
result, emissions of the controlled compounds could actually increase substantially, depending on the
number of countries that participate in the Protocol and the rate at which use increases in developing
and non-participating nations (Hoffman and Gibbs, 1988). For the Slowly Changing World scenario
we adopt the assumptions of the Protocol scenario developed for the Regulatory Impact Assessment
of rules to implement the Montreal Protocol in the United States (U.S. EPA, 1988). Namely that,
in addition to the U.S., 94% (in terms of current CFC consumption) of developed countries and 65%
of developing countries participate in the agreement. In this scenario the global average annual
growth rate in demand for products and services that would use CFCs, if they were available, is
approximately 4.0% from 1986 to 2000 and 2.5% from 2000 to 2050 (constant production is assumed
after 2050). Growth in demand is much higher in certain developing countries, particularly India and
China. These growth rates are not applied directly to CFC use in non-participating and developing
countries, however, because it is assumed that shifts in technology development away from CFCs in
DRAFT - DO NOT QUOTE OR CITE V-56 February 16, 1989
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Policy Options for Stabilizing Global Climate - Review Draft Chapter V
the United States and other participating countries "rechannels" demand in other countries as well.8
The Stabilizing Policy scenarios assume that the Montreal Protocol is strengthened to produce a
complete phase-out of CFCs in participating countries by 2003.9
Figure 5-12 shows the estimates for emissions of CFC-11, -12, and -13, and HCFC-22 under
the four scenarios. Emissions change more slowly than production because a significant portion of
each year's production is "banked" in air conditioners, refrigeration systems, and closed-cell foams.
The model keeps track of the size of this bank and estimates the gradual release of these CFCs.
In the Slowly Changing World scenario emissions are relatively constant despite the Protocol's
requirement of a 50% reduction in participating industrialized countries. After declining to 12%
below 1985 levels between 1990 and 2020, emissions of CFC-11 begin to rise again, reaching 1985
levels by the end of the projection period. CFC-113 emissions also fall significantly for a few decades
but rise again toward 1985 levels. CFC-12 emissions never decline to 1985 levels: they decline by
11% between 1990 and 2015, reaching a few percent above 1985 values, then they rise slowly, almost
reaching the 1990 peak levels towards the end of the 21st Century. Emissions of HCFC-22 grow
rapidly as a substitute for the fully-halogenated species that have the highest ozone-depletion potential.
Although HCFC-22 has a shorter lifetime and weaker radiative forcing than the fully-halogenated
compounds, it could make a significant contribution to global warming during the next century
because it is not controlled by the Montreal Protocol.
8 In the Slowly Changing World scenario this rechanneling effect is assumed to decrease growth
in CFC demand by 63% hi developed countries and by 50% in developing countries. In the Rapidly
Changing World scenario the baseline growth rates are increased by a factor of 1.7 to reflect the
higher rate of economic growth, but participation is assumed to be 100% in developed countries and
75% in developing countries, and rechanneling reduces the baseline growth rates by 63% in
developing countries (rechanneling does not affect developed countries in this scenario as 100%
participation in the Protocol is assumed).
' Participation is assumed to be 100% in industrialized countries and 85% in developing
countries; rechanneling reduces the baseline growth rates of non-participants by 75%.
DRAFT - DO NOT QUOTE OR CITE V-57 February 16, 1989
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Policy Options for Stabilizing Global Climate - Review Draft
Chapter V
FIGURE 5-12
EMISSIONS OF MAJOR CFCs
(Gigagrams)
CFC-11
CFC-113
400
300
O
200
100
0
19
500
400
300
i
o
0
200
100
0
19
-
RCW
•^"^'•-. .•'•'
•"V"0^ "
- \\
\\
\\
\\
J RCWP
SCWP
96 2000 2025 2050 2076 21
CFC-12
....
// ""-"'
•°x/'«""^"""''"«— — ^*"'***""~^ scw
''~\\
- \\
\\
\\
\\
\\
\\
\\ RCWP
1
SCWP
tC 2000 2025 2060 2075 2
400
300
200
100
0
00 19
3000
2500
2000
1500
1000
500
0
00 "
-
-
RCW
*••''-.
y ,_\> SCW
' \ \
" V
*" s RCWP
SCWP \ \
— — ^™"
85 2000 2025 2050 207E 21
HCFC-22
RCW
,' RCWP
1
•
.
1
1
SCW
/ „ - .. -T
* s*' — "" scwp
1 1 1 1
96 2000 2025 2050 2076 21
YEAR VEAR
DRAFT - DO NOT QUOTE OR CITE V-58
February 16, 1989
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Policy Options for Stabilizing Global Climate - Review Draft Chapter V
In the RCW, higher growth rates in developing countries more than compensate for higher
participation and rechanneling rates. CFC-11 emissions decline by no more than 6% below 1985
levels, while CFC-12 and -113 each increase by more than 25%. Emissions' of HCFC-22 grow
dramatically in this scenario. In the SCWP case emissions of the fully-halogenated compounds fall
by more than 80% from 1985 levels by 2025, which is sufficient to reverse the trend in concentrations
(see Figure 5-18). Emission reductions in the RCWP case are not quite as large, but still lead to
declining concentrations after 2025. HCFC-22 emissions are assumed to be the same in the No
Response and Stabilizing Policy cases; however, these emissions could rise as a result of a CFC
phase-out if chemical substitution is the primary approach to eliminating CFCs, or fall, if product
substitution and process redesign are the major approaches (see Chapter VI).
Emissions From Landfills and Cement
Other important activities bcluded in the industrial category are CO2 emissions from cement
making and CH4 emissions from landfills. The growth of these activities in developing countries is
assumed to be related to per capita income in a simple fashion, although growth is curtailed as
current per capita levels in industrialized countries are approached. The result is a three- to four-
fold increase in CO2 emissions from cement in the SCW and RCW scenarios, respectively, though
emissions remain less than 0.5 Pg C/yr hi all cases. Landfill CH4 emissions increase by more than
five-fold hi the RCW, reaching 15% of the total by 2100. In the policy scenarios, advanced materials
are assumed to reduce the demand for cement (relative to the No Response scenarios), while gas
recovery systems and waste reduction policies are assumed to limit emissions from landfills. The
result is that emissions from cement making still increase by a factor of two to three, but CH4
emissions from landfills are held essentially constant.
DRAFT - DO NOT QUOTE OR CITE V-59 February 16, 1989
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Policy Options for Stabilizing Global Climate - Review Draft Chapter V
Changes in Land Use
Deforestation has been a significant source of CO2 to the atmosphere over the last two
centuries, as clearly shown by the measurements of CO2 concentrations in Greenland and Antarctic
ice (Chapter II). If current trends continue, tropical forests could be completely eliminated during
the next century, adding significantly to the CO2 emissions from fossil fuels. On the other hand,
efforts are underway to reverse deforestation; if these efforts succeed, reforestation could become a
net sink for atmospheric CO2. The total amount of carbon that can move in either direction between
the atmosphere and terrestrial ecosystems is ultimately constrained by the area of forests available
for deforestation or by the area of land available to support new forests. The timing and magnitudes
of these fluxes of carbon are determined by the timing and extent of changes in land use as
influenced by local, national, and international policies.
The causes of deforestation are complex and vary from country to country. This makes it
difficult to directly tie assumptions about deforestation rates to the economic and demographic
assumptions of the general scenarios. Qualitatively, we assume that in a Slowly Changing World
poverty, unsustainable agricultural practices, and rapid population growth lead to continuously
increasing pressure on remaining forests. The rate of deforestation is assumed to increase from
current levels at the rate of population growth and tropical deforestation increases from 11 million
hectares per year (Mha/yr) in 1980 to 34 Mha/yr in 2047, when ah1 the unprotected forests in Asia
are exhausted. In a Rapidly Changing World improved agricultural practices and the substitution of
modern fuels for traditional uses of wood could ease the pressure on forests. Nonetheless, clearing
of forest lands for agriculture, pasture, logging, and speculation could continue apace, even if small
areas are set aside as biological preserves. In this scenario tropical deforestation is assumed to
DRAFT - DO NOT QUOTE OR CITE V-60 February 16, 1989
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Policy Options for Stabilizing Global Climate -- Review Draft Chapter V
increase very gradually, reaching 15 Mha/yr in 2097, before the unprotected forest areas of Latin
America are exhausted.
In the Stabilizing Policy scenarios it is assumed that a combination of policies succeed in
stopping deforestation by 2025, while more than 1000 Mha is reforested by 2100. Only land that once
supported forests and is not intensively cultivated is assumed to be available for reforestation. These
lands include 85% of the area currently involved in shifting cultivation (370 Mha) under the
assumption that this practice is replaced by sustainable low input agriculture (Sanchez and Benites,
1987). In addition, fallow agricultural land in the temperate zone (250 Mha), planted pasture in Latin
America (100 Mha), and degraded land in Africa and Asia (400 Mha) is assumed to be reforested.
Of the reforested land, about 380 Mha is assumed to be in plantations (sufficient to produce the
biomass energy requirements of the RCWP case with productivity increases expected by the
Department of Energy; Walter, 1988), the rest absorbs carbon at a much lower rate but reaches a
higher level of average biomass.
The carbon fluxes associated with these deforestation/reforestation scenarios based on
Houghton's (1988) low estimates of average biomass are shown in Figure 5-13. In the SCW CO2
emissions from deforestation increase rapidly from 0.7 Pg C/yr to more than 2 Pg C/yr in 2047
before the Asian forests are exhausted. Latin American and African forests are exhausted by 2075,
reducing emissions drastically. Total deforestation emissions are almost the same in the RCW but
they are spread out over a longer period. Emissions are close to 1 Pg C/yr from 2000 to 2100. In
the Stabilizing Policy scenarios the biosphere becomes a sink for carbon by 2000 and reaches its peak
Plantation products decay at various rates at the end of each rotation; no attempt to protect
this carbon from oxidation is assumed.
DRAFT - DO NOT QUOTE OR CITE V-61 February 16, 1989
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Policy Options for Stabilizing Global Climate - Review Draft
Chapter V
FIGURE 5-13
C02 EMISSIONS FROM DEFORESTATION
C02 From Deforestation
(Petagrams Carbon)
o
CD
CC.
<
O
-------
Policy Options for Stabilizing Global Climate - Review Draft Chapter V
absorption of 0.7 Pg C/yr before 2025. The size of this sink declines gradually, after 2025 as forests
reach their maximum size and extent.
Agricultural Activities
The demand for agricultural products is a direct function of population, but is not strongly
dependent on income levels. Thus, there are only small differences between the scenarios as the
much higher incomes largely offset the somewhat lower populations in the RCW compared with the
SCW. The land area used for rice production, and thus the methane emissions from this source,
bcreases by only about 50% by 2100 in both the SCW and RCW scenarios (production per hectare
increases by 80-100%). Meat production increases more, about 125%, as demand rises with income
to some extent. Satisfying the demands of increasing populations with a finite amount of land
requires more intensive cultivation, and fertilizer use increases by 160% as a result.
In the Stabilizing Policy scenarios we do not assume changes in the demand for agricultural
commodities, but rather changes in technology and production methods that could reduce greenhouse
gas emissions per unit of product. Although the impact of specific technologies cannot be estimated
at present, a number of techniques have been identified for reducing methane emissions associated
with rice and meat production and nitrous oxide emissions related to the use of fertilizer (Chapter
VII). For simplicity, we have assumed that CH4 emissions per unit of rice, meat, and milk
production decrease by 0.5% per year (emissions from animals not used in commercial meat or milk
production are assumed to be constant). Emissions of N2O per unit of nitrogenous fertilizer applied
are also assumed to decrease by 0.5% per year for each fertilizer type. In addition, fertilizer use is
assumed to shift away from those types with the highest emissions after 2000. Based on these
assumptions, CH4 emissions from rice production remain roughly constant until 2075, after which time
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Policy Options for Stabilizing Global Climate - Review Draft Chapter V
they fall by about 20% as the global population stabilizes. Methane emissions from enteric
fermentation increase by 40-50% by the middle of the 21st Century, before falling to within about
30% of 1985 levels. Similarly, N2O emissions from fertilizer increase from 0.3 to 0.5 Tg N/yr
between 1985 and 2025 and then remain roughly constant.
Total Emissions
Total emissions of the key radiatively-important gases, the aggregate of estimates of the
emissions from each activity discussed above and of natural emissions, are shown in Table 5-8.
Overall, emissions increase gradually in the Slowly Changing World scenario and more dramatically
in the Rapidly Changing World, while in the policy scenarios emissions are reasonably stable or
declining.
In the No Response scenarios CO2 emissions are projected to increase to a much greater
extent than emissions of the other gases. This is because all net CO2 emissions are assumed to be
anthropogenic in origin and because CO2 is a fundamental product of all fossil-fuel combustion. In
the SCW increased deforestation contributes significantly to near-term growth in CO2 emissions, and
total emissions are relatively constant between 2025 and 2075 as forests are exhausted (see Figure
5-14). In the RCW CO2 emissions are dominated by the growth in fossil-fuel combustion and total
emissions increase by a factor of three by 2050. In the Stabilizing Policy scenarios increased end-
use efficiency and reforestation contribute significantly to producing decreasing emissions in the near-
term, while decreased reliance on fossil fuels in conjunction with continued improvements in efficiency
allow for further decreases later on.
DRAFT - DO NOT QUOTE OR CITE V-64 February 16, 1989
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Policy Options for Stabilizing Global Climate - Review Draft
Chapter V
TABLE 5-8
Trace Gas Emissions
C02 (Pg C)
sew
RCW
SCWP
RCWP
N20 (Tg N)
sew
RCW
SCWP
RCWP
CH4 (Tg CH4)
sew
RCW
SCWP
RCWP
NOX (Tg N)
sew
RCW
SCWP
RCWP
CO (Tg C)
sew
RCW
SCWP
RCWP
CFC-11 (Gg)
sew
RCW
SCWP
RCWP
CFC-12 (Gg)
sew
RCW
SCWP
RCWP
CFC-22 (Gg)
sew
RCW
SCWP
RCWP
CFC-113 (Gg)
sew
RCW
SCWP
RCWP
1985
5.9
5.9
5.9
5.9
11.3
11.3
11.3
11.3
514.4
510.5
514.4
510.5
53.3
53.2
53.3
53.2
502.3
502.0
502.3
502.0
278.3
278.3
278.3
278.3
363.8
363.8
363.8
363.8
73.8
73.8
73.8
73.8
150.5
150.5
150.5
150.5
2000
7.2
7.6
5.3
5.5
12.3
12.0
11.0
11.0
569.4
576.7
519.0
527.3
59.3
60.0
50.9
51.5
616.1
571.1
3823
382.3
292.0
314.2
261.8
293.1
401.1
471.8
354.2
433.3
192.1
247.4
192.1
247.4
119.3
171.8
93.2
148.7
2025
9.2
11.5
5.1
5.2
13.5
13.0
10.7
10.8
676.4
712.4
545.0
558.7
68.8
72.9
44.8
50.7
842.0
699.1
286.1
290.8
248.1
267.3
47.3
60.2
379.7
4373
54.9
85.9
385.0
829.1
385.0
829.1
124.2
167.4
9.0
19.7
2050
9.8
16.6
4.2
5.1
13.7
14.0
10.6
10.8
739.9
879.6
534.0
567.2
70.8
91.7
40.9
46.1
858.7
894.6
2453
241.7
275.6
300.3
47.8
52.4
404.8
483.6
62.3
83.6
6863
2,194.2
6863
2,194.2
140.4
190.3
13.8
25.4
2075
9.7
22.2
3.7
4.8
12.2
15.0
10.6
10.8
773.3
1,025.4
522.2
542.6
65.2
108.2
41.0
44.8
594.7
1,049.9
250.6
242.0
280.4
306.3
50.3
54.2
410.3
492.3
65.7
86.4
785.7
2,744.9
785.7
2,744.9
140.4
190.3
13.8
25.4
2100
11.4
253
3.1
43
12.1
15.0
10.7
10.9
815.9
1,089.0
484.7
508.0
70.1
118.2
42.6
43.8
6033
1,207.1
250.9
244.9
280.8
306.8
50.6
54.3
410.8
493.1
66.0
86.6
794.8
2,795.6
794.8
2,795.6
140.4
190.3
13.8
25.4
DRAFT - DO NOT QUOTE OR CITE
V-65
February 16, 1989
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Policy Options for Stabilizing Global Climate -- Review Draft
Chapter V
FIGURE 5-14
C02 EMISSIONS BY TYPE
(Petagrams Carbon)
SCW
RCW
19*6 2000 2025 2060 2076 2100
198S 2000 2026 2050 2075 2100
SCWP
I
5 10
1MB 2000 202S 20CO 2071 2100
VEAR
RCWP
Commercial
En.rgy
2026 2060 2076 2100
YEAR
DRAFT - DO NOT QUOTE OR CITE V-66
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Policy Options for Stabilizing Global Climate — Review Draft Chapter V
The regional allocation of CO2 emissions shows a rapid increase in the share attributed to
developing countries in all scenarios (Figure 5-15). This share increases from about 34% currently
to 57% by 2025 and levels off at about 60% after 2050 in the RCW. The developing countries
account for a little over 50% of CO2 emissions in the SCW after 2025, with the share from
developing countries other than China decreasing after 2050 as deforestation emissions decline.
China's share of emissions grows most dramatically in the Stabilizing Policy scenarios as deforestation
is eliminated in other developing countries and China becomes by far the world's largest coal
consumer. About 70% of global CO2 emissions are from China and other developing countries by
2100 in the RCWP scenario, but only 42% in the SCWP.
The projected increases in CH4 emissions in the No Response scenarios are a result of growth
in a variety of sources (see Figure 5-16). In the SCW almost 60% of the increase between 1985 and
2050 is due to enteric fermentation and rice cultivation, whereas in the RCW these sources account
for less than 40% of the increase and the growth in emissions from fuel production accounts for
another 40%. In both of these scenarios emissions from landfills increase steadily, becoming quite
significant by the end of the period. Reduced growth in each component is responsible for relatively
stable CH4 emissions in the policy scenarios. The total increases gradually until 2025 and declines
after 2050, falling below 1985 levels by the end of the period in both the SCWP and RCWP cases.
The regional contributions to CH4 emissions do not shift as dramatically as for CO2 (Figure
5-17). The share of CH4 emissions from industrialized countries increases in the RCW scenario due
to rapid growth in coal production, but this share declines somewhat in the other three scenarios.
Total N2O emissions do not increase dramatically in any of the scenarios, although we note
again that current, and therefore future, emissions of N2O are highly uncertain. These uncertainties,
DRAFT - DO NOT QUOTE OR CITE V-67 February 16, 1989
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Policy Options for Stabilizing Global Climate - Review Draft
Chapter V
FIGURE 5-15
SHARE OF C02 EMISSIONS BY REGION
(Percent)
SCW
1?li 2000 2025 20(0 2075 2100
SCWP
RCW
Other Developing
20 p
Chin* a> CP An.
USSR & CP Europe
Reit of OECD
United St«t«<
1*11 2000 2021 20CO 2075 2100
RCWP
Other Developing
U«5 2000
202S 20SO
YEAR
2075 2100
Chin* k CP Afl.
USSR k CP Europe
1MS 2000
2025 20CO
VEAB
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Chapter V
sew
FIGURE 5-16
CH4 EMISSIONS BY TYPE
(Teragrams)
RCW
D*tor«jt*tion
iiom*« Burning
1986 2000 2025 2060 2075 2100
1986 2000 2026 2060 2076 2100
SCWP
RCWP
1000 -
Blomass Burning
Fuel Production
Ric« Production
2075 2100
2025 2050
VEAR
2075 2100
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Chapter V
FIGURE 5-17
SHARE OF CH4 EMISSIONS BY REGION
(Petagrams)
SCW RCW
1111 tooo ioai toio aori a 100
100
mi tooo ion ioio
RCWP
1071
Other Developing
China * CP Alia
USSR k CP Europe
Other Developing
China k CP Ana
USSR k CP Europe
Rest of OECD
United States
Oceans
2026 2050
YEAR
2076 2100
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however, do not appear to have a large impact on the overall rate or magnitude of climatic change
in these scenarios (see Chapter VI). In the SCW, emissions related to deforestation and land-
clearing, as well as fertilizer-induced emissions, increase significantly through 2025, and total emissions
decline after 2050. In the RCW, emissions growth is driven mainly by a four-fold increase in fossil-
fuel combustion emissions between 1985 and 2100. In the policy cases, total emissions decline slightly
due to the assumed decreases in emissions per unit of fertilizer use and fossil-fuel combustion, and
because deforestation is halted.
In the No Response scenarios, emissions of both NOX and CO increase significantly through
2050. After 2050 declining emissions related to deforestation in the SCW compensate for continued
increases in energy-related emissions. The deforestation assumptions have a particularly large impact
on CO emissions as deforestation accounts for 40-50% of the total between 2000 and 2050 in this
case. In the RCW, deforestation emissions are relatively uniform, and both CO and NOX emissions
continue to increase through 2100. In the Stabilizing Policy cases emission controls produce relatively
stable NOX emissions and declining CO emissions from fossil-fuel combustion sources, while
deforestation emissions are eliminated. The result is moderate decline in NO, emissions and more
than a 50% cut in CO emissions by 2050.
Atmospheric Concentrations
Figure 5-18 shows concentrations of greenhouse gases that result from the pattern of emissions
discussed above. Because CO2, N2O, and CFCs are long-lived in the atmosphere, their concentrations
respond gradually to changes in emissions. CH4 has an intermediate lifetime (about 10 years), which
is itself affected by changes in emissions of CO, NOX, CH4, and other trace gases, so its atmospheric
concentration responds rapidly to changes hi emissions.
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Chapter V
FIGURE 5-18
800
ATMOSPHERIC CONCENTRATIONS
(3.0 Degree Celsius Climate Sensitivity)
CARBON DIOXIDE . METHANE
(Parti Par Million)
1985 2000 2025 2060 2075 2100
YEAR
5
£ 3000
(Parti Par Billion)
1185 2000 2025 2050 2075 2100
YEAR
NITROUS OXIDE
(Partf Par Sllllon)
2025 20*0
VIAR
2075 2100
CHLOROFLUOROCARBONS
(Parti Par Trillion of CFC-12 Equivalent)
u 2000
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Policy Options for Stabilizing Global Climate -- Review Draft Chapter V
CO2 concentrations reach twice their preindustrial levels (570 ppm) in about 2080 in the Slowly
Changing World scenario; this level is reached by 2055 in the Rapidly Changing World and
concentrations more than three times preindustrial values are reached by 2100 (Figure 5-18). Despite
declining CO2 emissions in the policy scenarios, CO2 concentrations continue to increase throughout
the projection period, reaching almost 440 ppm in the SCWP case and 470 ppm in the RCWP case
by 2100. It is interesting to note that the fraction of total CO2 emissions during the 21st Century
that remain in the atmosphere in 2100 is 46% in the RCW case and 39% in the RCWP case, so that
emission reductions have a more than linear impact on concentrations.
CH4 concentrations increase by almost a factor of 2 in the SCW and a factor of 2.6 in the
RCW, with the most rapid growth occurring between 1985 and 2050 (Figure 5-18). Interestingly, the
2050 concentration obtained in the SCW is similar to the result of linearly extrapolating the currently
observed growth rate of 1% per year, whereas the RCW value is close to an exponential extrapolation
of current growth; the 2100 values lie substantially below a continuation of such extrapolations for
another 50 years. In the policy cases CH4 concentrations increase by 13-18% between 1985 and 2025,
after which they level off and decline to roughly 1985 levels by 2100. CH4 concentrations are affected
by temperature feedbacks on atmospheric chemistry: Increasing the climate sensitivity of the model
from 2.0°C to 4.0°C reduces concentrations by 100 ppb in the SCWP and 210 ppb in the RCW.
By contrast with methane, N2O concentrations increase gradually in all the scenarios as a result
of the current imbalance between sources and sinks (Figure 5-18). The concentration increase is
about 80 ppb in the SCW and 100 ppb in the RCW compared with 50 ppb in the Stabilizing Policy
cases. Thus, the policy assumptions reduce the .concentration growth by 40-50%.
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CFC concentrations increase dramatically in the No Response scenarios despite the assumption
that at least 65% of developing countries and 95% of industrialized countries participate in the
Montreal Protocol (Figure 5-18). The total concentration of CFCs weighted by their contribution to
the greenhouse effect increases by a factor of 4.2 and 6.5 in the SCW and RCW scenarios,
respectively. On the other hand, the phase-out assumed in the policy cases does stabilize CFC
concentrations (other than HCFC-22) by 2025, but their total greenhouse forcing still increases by
1.5-2.4 times over current levels.
It is interesting to compare the concentration changes calculated here, on the basis of explicit
assumptions linking emissions with activities, to recent studies that have made less formal estimates
based primarily on current trends in concentrations and/or emissions (Table 5-9). Our No Response
estimates of future concentrations are in good agreement with those of Ramanathan et al. (1985) for
2030 and Dickinson and Cicerone (1986) for 2050. A notable exception is CFCs, for which we expect
significantly lower concentrations as a result of the recent Montreal Protocol to control production
of these compounds. In addition, our 2030 estimates of N2O concentrations are somewhat below the
lower end of the range given by Ramanathan et al., although they fall within the lower end of
Dickinson and Cicerone's range for 2050. The differences between the SCW and RCW scenarios are
significantly less than the ranges suggested by these authors for all the compounds listed in Table 5-
9~at least partially because the only differences between the SCW and RCW scenarios are
assumptions about activity levels and technology, whereas the estimated ranges from the literature also
consider uncertainties in current sources, atmospheric chemistry, and ocean carbon uptake
(uncertainties in these factors are considered in Chapter VI). Also, the Slowly Changing World and
Rapidly Changing World scenarios are not intended to completely bound future possibilities;
significant reductions in emissions per unit of GNP are built into the No Response scenarios-if this
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Chapter V
TABLE 5-9
Comparison of Estimates of Trace-Gas Concentrations in 2030 and 2050
Concentrations in
Trace-Gas
C02 (ppm)
CH4 (ppm)
Trop-O3 (%)
N20 (ppb)
CFC-11 (ppb)
CFC-12 (ppb)
HCFC-22 (ppb)
Ramanathan
et al. (1985)
450
2.3(1.8-3.3)
12.5
375(350-450)
1.1(0.5-2.0)
1.8(0.9-3.5)
0.9(0.4-1.9)
GISS
A B
443 427
3.5 2.5
* 0
381 352
2.3 0.8
3.9 1.4
C
368
1.9
0
314
0.2
0.5
sew
440
2.5
19
340
0.5
1.0
0.4
Concentrations in
Trace-Gas
C02 (ppm)
CH4 (ppm)
Trop-C-3 (%)
N20 (ppb)
CFC-11 (ppb)
CFC-12 (ppb)
HCFC-22 (ppb)
Dickinson &
Cicerone (1985)
400-600
2.1-4.0
15-50
350-450
0.7-3.0
2.0-4.8
GISS
A B
513 465
4.7 2.7
* 0
480 376
4.2 1.0
7.3 1.8
C
368
1.9
0
314
0.2
0.4
sew
490
2.8
23
360
1.2
1.2
0.6
2030
RCW
450
2.6
18
340
0.5
1.1
0.7
2050
RCW
540
3.1
26
360
1.4
1.4
1.7
SCWP
400
1.8
-1
330
0.3
0.7
0.3
SCWP
410
1.8
-2
330
0.3
0.6
0.5
RCWP
400
1.9
-1
330
0.4
0.8
0.6
RCWP
420
1.9
0
340
0.3
0.7
1.6
* In this scenario the effect of O3 and other trace gas changes is approximated by doubling the radiative forcing
contributed by CFC-11 and CFC-12.
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fails to materialize, and/or if economic growth is more rapid than assumed here, concentrations of
a number of greenhouse gases could be significantly higher than is estimated in these scenarios.
Global Temperature Increases
Evaluating the consequences of alternative climate change scenarios is beyond the scope of this
report (a variety of potential effects are examined in the companion report Potential Effects of Global
Climate Change on the United States; Smith and Tirpak, 1988), but an indicator of the relative
magnitude of change is needed as a basis for comparing the scenarios considered here. Analysts of
trace-gas emissions have often emphasized the date at which carbon dioxide concentrations (or the
equivalent combination of trace gases) can be expected to reach twice their pre-industrial level of
2xCO2. In the absence of policies to reduce emissions, however, climate change is potentially open-
ended. Atmospheric composition and climate would continue to change after the 2xCO2 level were
reached and the ecological and social consequences may depend as much on what happens after CO2
doubles (if it does) as on when this benchmark occurs. More relevant to ecological and social
systems are the average and maximum rate of climatic change. In order to compare scenarios, we
therefore focus on the average rate at which global temperature may increase during the next century
as well as the maximum rate of change. We emphasize that these parameters are only indicators of
global change; changes at the regional level will vary in both magnitude and tuning and changes in
precipitation may be as important as changes in temperature. Nonetheless, the global quantities
calculated here can be used to compare the scenarios presented here among themselves and with
results of more detailed climate models.
The changes in concentrations shown in Figure 5-18 produce the estimated global temperature
changes shown in Figure 5-19 for a range of climate sensitivity (2.0-4.0°C equilibrium increase in
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Chapter V
FIGURE 5-19
REALIZED AND EQUILIBRIUM WARMING
(Degrees Celsius; 2.0 - 4.0 Degree Climate Sensitivity)
REALIZED WARMING EQUILIBRIUM WARMING
Slowly Changing Scenarios
SCWP
19BS 2000 2026 2060 2075 2100
REALIZED WARMING
Rapidly Changing Scenarios
Slowly Changing Scenarios
1385 2000 202E 2050 2075 2100
EQUILIBRIUM WARMING
1385 2000
2075 2100
Rapidly Changing Scenarios
1385 2000
2025 2050
YEAR
2075 2100
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global temperature from doubling the atmospheric concentration of CO2; see Chapter III). Both the
"equilibrium wanning commitment" and the "realized warming" are presented as a function of time.
The equilibrium warming commitment for any given year is the temperature increase that would
occur in equilibrium if the atmospheric composition was fixed in that year. Because the oceans have
a large heat capacity the temperature change realized hi the atmosphere lags considerably behind the
equilibrium level. Realized warming has been estimated with a simple model of ocean heat uptake
as discussed hi Chapter III. Because the response of the climate system to changes in greenhouse
gas concentrations is quite uncertain we also consider a range of "climate sensitivities". Climate
sensitivity is defined as the equilibrium warming commitment due to doubling the concentration of
carbon dioxide from preindustrial levels. Given a particular emissions scenario and climate sensitivity,
the realized warming is much more uncertain than the equilibrium wanning commitment because the
effective heat storage capacity of the ocean is not known. On the other hand, because the amount
of unrealized warming increases with increasing climate sensitivity, for a given scenario realized
warming depends less on climate sensitivity than does warming commitment.
Both the SCW and RCW scenarios lead to substantial global warming. In the SCW, estimated
realized warming increases 1.0-1.5°C between 2000 and 2050, and 2-3°C between 2000 and 2100
(Figure 5-19). The maximum decadal rate of change associated with this scenario is 0.2-0.3°C
sometime in the middle of the next century. The total equilibrium warming commitment is
substantially higher, reaching 3-6°C by 2100 relative to preindustrial levels. The equilibrium warming
commitment equivalent to doubling the concentration of CO2 from preindustrial levels is reached by
about 2040 in the SCW scenario.
The rate of change during the next century would be more than 50% greater in the RCW
scenario, which shows a global temperature increase from 2000 of 1.2-1.9°C by 2050 and 3-5°C by
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2100 (Figure 5-19). In this case the maximum rate of change is 0.4-0.6°C per decade, which occurs
sometime between 2070 and 2100. The equilibrium warming commitment is 5-10°C by 2100 in this
scenario and the 2xCO2 equivalent level is reached by about 2030.11
By contrast, the rate of climatic change in the Stabilizing Policy scenarios would be less than
1.6°C per century. Global temperatures in the SCWP case increase by 0.4-0.8°C from 2000 to 2050
and 0.6-1.1°C from 2000 to 2100; corresponding values are 0.5-0.9°C and 0.8-1.4°C in the RCWP case.
The maximum rate of change in these scenarios is less than 03°C per decade and occurs before 2010,
largely as a result of warming to which the world may already be committed. (Indeed, the 0.3°C per
decade figure, obtained assuming a climate sensitivity of 4°C, occurs at the very beginning of the
simulation and may be an artifact of how the model is initialized.) In these cases the additional
commitment to warming is greater between 2000 and 2050 than it is between 2050 and 2100: 0.3-
0.9°C versus 0.1-0.4°C. Total equilibrium warming commitment reaches 1.4-2.8°C in the SCWP and
1.7-3.3°C in the RCWP. While not without some risk, the rate of change represented by the
Stabilizing Policy scenarios would give societies and ecosystems much more time to adapt to climatic
change than would be the case in the No Response scenarios.
Carbon dioxide accounts for more than 65% of increased commitments to global warming
between 2000 and 2100 in all of the scenarios analyzed in this report (Figure 5-20). This represents
a significantly higher estimate of the role of CO2 compared to roughly 50% in the last few decades
and in Ramanathan et al.'s scenario for 2030. Much of this difference is due to smaller increases
in CFCs in our scenarios due to our assumption that the Montreal Protocol comes into force. In
addition, growth in emissions of CH4 and N2O is projected to be slower than that of CO2, particularly
11 Estimates of equilibrium warming commitments greater than 6°C represent extrapolations
beyond the range tested in most climate models, and this warming may not be fully realized because
the strength of some positive feedback mechanisms may decline as the Earth warms.
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Chapter V
FIGURE 5-20
RELATIVE CONTRIBUTION TO
WARMING BY 2100
(Percent)
SLOWLY CHANGING WORLD
66'-
4%
12%
13%
SLOWLY CHANGING WORLD
WITH POLICY
73%
10%
6%
11%
RAPIDLY CHANGING WORLD
68%,,
4%
12%
12%
RAPIDLY CHANGING WORLD
WITH POLICY
71%
0.3V.I
13%
11%
Carbon Dioxide
Methane
Nitrous Oxide
CFCs
5%
Ozone
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after 2030. The role of CO2 is greatest in the policy scenarios because our assumptions lead to
relatively stable concentrations of CH4 and tropospheric ozone, while CO2 concentrations continue
to increase gradually.
Comparison with General Circulation Model Results
Hansen et al. (1988) analyzed three transient trace-gas scenarios using the GISS GCM. The
GISS A scenario, based on exponentially extrapolating current greenhouse gas trends, most closely
resembles our RCW with 4.0°C climate sensitivity (the climate sensitivity of the GISS GCM is 4.2°C).
Indeed, both the equilibrium and realized global warming in these cases are within 0.1°C in 2025.12
By 2050 the continuation of exponential growth in trace-gas concentrations in the GISS A scenario
leads to an equilibrium warming commitment that is about 40% higher than in the RCW, with a
corresponding realized warming of 3.4 versus 2.8°C (all references to realized warming -in the GISS
scenarios are based on 5-year running means, Figure 3b in Hansen et al., 1988a). By 2060, the end
of the GISS simulation, the realized warming in the GISS A scenario is 4.2°C compared with 3.3°C
in the RCW. The GISS B scenario, which is based on linearly increasing trace-gas concentrations
at current rates, is most similar to the RCWP case (with 4.0°C climate sensitivity). These two cases
have very similar equilibrium warming commitment and realized warming in 2030 (the end of the
GISS simulation for this scenario). The final scenario examined by GISS (case C) assumes that
atmospheric composition is stable after 2000, which leads to realized warming of about 0.9°C by 2040.
The policy cases examined here do not achieve this result; realized warming reaches 1.3-1.4°C by 2025
if the climate sensitivity is 4.0°C. Thus, the GISS scenarios bracket the range of the scenarios
12 The path to 2025, however, is not identical. The GISS scenarios are referenced to the
atmospheric composition of 1960, whereas our scenarios are referenced to the estimated preindustrial
atmospheric composition. Thus, the warming commitment in 2000 is already 2.1°C in the RCW,
whereas it is only 1.9°C in the GISS A scenario.
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developed here, and may provide some indication of the regional differences in the rates and
magnitude of change that might be associated with our cases.
Relative Effectiveness of Selected Strategies
The major assumptions that distinguish the Worlds where governments provide leadership in
pursuing stabilizing policies from the Worlds with no such policies have been grouped into eleven
categories in order to examine the relative importance of different policy strategies. Each set of
options was applied individually to the RCW case; the combination of all the strategies represents
the RCWP case. Figure 5-21 presents the results in terms of the effect of each policy strategy in
reducing the equilibrium warming commitment in 2050 and 2100. This analysis suggests that
accelerated energy efficiency improvements, reforestation, modernization of biomass use, and carbon
emissions fees could have the largest near-term impact on the rate of climatic change. In the long
run, advances in solar technology and biomass plantations also play an essential role.
CONCLUSIONS
While the future will never be anticipated with certainty, it is useful to explore the
consequences of alternative plausible scenarios. The results of this exercise suggest that even with
sluggish rates of economic growth and optimistic assumptions regarding technical innovation, the
world could experience significant rates of climatic change during the next century. Temperature
increases reach 3-4°C by 2100 under our assumptions; and the world would be committed to an
additional wanning of up to 2°C at this date. With higher rates of economic growth, certainly the
goal of most governments, significantly more rapid rates of climatic change are possible. With our
assumptions, which involve lower global energy use than considered in many previous studies, a
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Chapter V
FIGURE 5-21
STABILIZING POLICY STRATEGIES:
DECREASE IN EQUILIBRIUM WARMING COMMITMENT
Percent Reduction Relative to RCW Scenario
1. CFC Phaseout
2. Reforestation
3. Improved Transportation
Efficiency0
4. Other Efficiency Gains
5. Energy Emissions Fee8
f
6. Promote Natural Gas
7. Emission Controls*
8. Solar Technologies
9. Commercialized Blomass
10. Agriculture, Landfills,
and Cement
11. Promote Nuclear
Power
RCWP (Simultaneous
Implementation of 1-11)
L
j_
i
0 5
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10 15
Percent
V-83
2050
2100
45V.
65%
20 25
February 16, 1989
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Policy Options for Stabilizing Global Climate -- Review Draft Chapter V
FIGURE 5-21 -- NOTES
Impact Of Stabilizing Policies On Global Warming
a A 100% phaseout of CFCs by 2003 and a freeze on methyl chloroform is imposed. There is 100%
participation by industrialized countries and 94% participation in developing countries.
b The terrestrial biosphere becomes a net sink for carbon by 2000 through a rapid reduction in
deforestation and a linear increase in the area of reforested land and biomass plantations. Net CO2
uptake by 2025 is 0.7 Pg C.
c The average efficiency of new cars in the U.S. reaches 40 mpg (5.9 liters/100 km) by 2000. Global
fleet-average automobile efficiency reaches 50 mpg by 2025 (4.7 liters/100 km).
d The rate of energy efficiency improvements in the residential, commercial, and industrial sectors
are increased about 0.1-0.2 percentage points by 2025 compared to the RCW, and about 0.3-0.4
percentage points annually from 2025-2100.
" Emission fees are placed on fossil fuels in proportion to carbon content. Maximum production fees
(1985$) were $0.50/GJ for coal S0.36/GJ for oil, and S0.23/GJ for natural gas. Maximum
consumption fees were 28% for coal, 20% for oil, and 13% for natural gas. These fees increased
linearly from zero, with maximum consumption fee changed by 2025 and maximum production fee
by 2050.
' Assumes that economic incentives accelerate exploration and production of natural gas, reducing
the cost of locating and producing natural gas by an annual rate of .5% relative to the RCW
scenario. Incentives for gas use for electricity generation increases gas share by 5% in 2025 and 10%
thereafter.
* Assume more stringent NOX and CO controls on mobile and stationary sources including all gas
vehicles using three-way catalysts in OECD countries by 2000 and in the rest of the world by 2025
(new light duty vehicles in the rest of the world uses oxidation catalysts from 2000 to 2025); from
2000 to 2025 conventional coal boilers used for electricity generation are retrofit with low NOX
burners with 85% retrofit in the developed countries and 40% in developing countries; starting in
2000 all new combustors used for electricity generation and all new industrial boilers require selective
catalytic reduction hi the developed countries and low NOX burners in the developing countries and
after 2025 all new combustors of these types require selective catalytic reduction; other new industrial
non-boiler combustors such as Kilns and Dryers require low NOX burners after 2000.
h Assumes that low cost solar technology is available by 2025 at costs as low as 4.6 cents/kwh.
1 Assumes the cost of producing and converting biomass to modern fuels reaches $4.00/gigajoule
(1985$) for gas and $6.00/gigajoule (1985$) for liquids. The maximum amount of liquid or gaseous.
fuel available from biomass is 210 exajoules.
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FIGURE 5-21 -- NOTES (continued)
Impact Of Stabilizing Policies On Global Warming
J Assumes that research and improved agricultural practices result in an annual decline of 0.5% in
the emissions from rice production, enteric fermentation, and fertilizer use. CH4 emissions from
landfills assumed to decline at an annual rate of 2% in developed countries due to policies aimed
at reducing waste and landfill gas recovery, emissions in developing countries continue to grow until
2025 then remain flat due to incorporation of the source policies. Technological improvements reduce
demand for cement by 25%.
k Assumes that technological improvements in nuclear design reduce cost by about $4/gigajoule
(1985$) by 2050 (about 0.5% per year efficiency improvement).
1 Impact on warming when all the above measures are implemented simultaneously. The sum of
each individual reduction in warming is not precisely equal to the difference between the RCW and
RCWP cases because not all the strategies are strictly additive.
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wanning of 4-6°C could be expected by 2100, with an additional commitment of 1-4°C by that date.
On the other hand, by vigorously pursuing a variety of technical and policy options simultaneously,
it would be possible to reduce the average rate of warming during the next century by more than
60%. Chapters VII-IX of this report explore these options in more detail.
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Hansen, J., A. Lacis, D. Rind, G. Russel, P. Stone, I. Fung, R. Ruedy, and J. Lerner. 1984. Climate
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EPA, Office of Air and Radiation, Washington, D.C.
Houghton, R. 1988. Reply to S. Idso, Comment on "Biotic changes consistent with increased seasonal
amplitude of atmospheric CO2 concentrations." Journal of Geophysical Research 93:1747-1748.
Houghton, R., J. Hobbie, J. Melillo, B. Moore, B. Peterson, G. Shaver, and G. Woodwell. 1983.
Changes in the carbon content of terrestrial biota and soils between 1860 and 1980: A net release
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Kahane, A. 1986. Industrial Electrification: Case Studies of Four Industries. Berkeley: LBL 22098.
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Prather, Michael J. 1988. An Assessment Model for Atmospheric Composition. NASA Conference
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CHAPTER VI
SENSITIVITY ANALYSES
FINDINGS VI-3
INTRODUCTION VT-12
ASSUMPTIONS ABOUT THE MAGNITUDE AND TIMING OF GLOBAL CLIMATE
STABILIZATION STRATEGIES VT-12
No Participation by the Developing Countries VI-13
Delay in Adoption of Policies VI-17
ASSUMPTIONS AFFECTING RATES OF TECHNOLOGICAL CHANGE VI-18
Availability of Non-Fossil Technologies VI-18
Cost and Availability of Fossil Fuels VI-22
High Coal Prices VI-22
Alternative Oil and Natural Gas Supply Assumptions VI-24
Availability of Methanol-Fueled Vehicles VI-29
ATMOSPHERIC COMPOSITION: COMPARISON OF MODEL RESULTS TO ESTIMATES
OF HISTORICAL CONCENTRATIONS VI-30
ASSUMPTIONS ABOUT TRACE-GAS SOURCES AND STRENGTHS VI-34
Methane Sources VI-35
Nitrous Oxide Emissions From Fertilizer VI-38
Anhydrous Ammonia VI-38
N2O Leaching From Fertilizer VI-39
N2O Emissions From Combustion VI-39
UNCERTAINTIES IN THE GLOBAL CARBON CYCLE VI-41
Unknown Sink In Carbon Cycle VI-43
Amount of CO2 From Deforestation VI-44
Alternative CO2 Models of Ocean Chemistry and Circulation VI-47
ASSUMPTIONS ABOUT CLIMATE SENSITIVITY AND TIMING VI-50
Sensitivity of the Climate System VI-50
Rate of Heat Diffusion VI-53
ASSUMPTIONS ABOUT ATMOSPHERIC CHEMISTRY: A COMPARISON OF
MODELS VI-54
Model Descriptions VI-56
Assessment Model for Atmospheric Composition VI-56
Isaksen Model VI-57
Thompson et al. Model VI-58
Results from the Common Scenarios VI-59
EVALUATION OF UNCERTAINTIES USING AMAC VI-67
Atmospheric Lifetime of CFC-11 VI-67
Interaction of Chlorine with Column Ozone VI-69
Sensitivity of Tropospheric Ozone to CH4 Abundance VI-69
Sensitivity of OH to NO, VI-71
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BIOGEOCHEMICAL FEEDBACKS VI-72
Ocean Circulation VI-72
Methane Feedbacks VI-73
Combined Feedbacks VI-75
REFERENCES VI-78
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FINDINGS
• The degree of participation by developing countries in policies to limit
warming is one of the most important factors affecting equilibrium
temperatures in the year 2100. If only industrialized countries adopt policy
measures, equilibrium temperatures could increase by 40% or more relative
to scenarios with global cooperation. This suggests that, despite uncertainties
about future economic growth rates, developing countries will be a significant
determinant in the ultimate level of global warming.
• Delaying any response to global warming by OECD and East Bloc Countries
until the year 2010 and by developing countries until 2025 might increase the
equilibrium warming commitment in 2050 by 30-40%.
• The sensitivity of the climate system to a given increase in greenhouse gases
is among the most important causes for uncertainty about the ultimate
magnitude of global warming. For most of the analysis in this report, we
have assumed that the climate sensitivity to doubling CO2 is 2.0 to 4.0°C;
broadening the range of climate sensitivity to between 1.5 and 5.5°C for a
CO2 doubling causes the estimated range for equilibrium warming in 2050
to become 2.0-7.4°C in the Rapidly Changing World scenario. The impact
on realized warming is less: the estimated range for 2050 increases from
1.9-2.8°C to 1.5-3.2°C. This uncertainty has important implications for the
timing and stringency of policy responses. Even the lower values, when
considered with information on the impacts of global warming, suggests a
need for caution about future emissions.
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• Uncertainties in biogeochemical feedbacks appear to be the potentially most
important reason to suspect that global warming may ultimately be greater
than predicted by current general circulation models. Changes in the ocean
circulation, methane releases from hydrates, bogs, and rice cultivation and
other positive feedbacks could amplify realized warming in 2100 by 20-40%
for a climate sensitivity of 2.0-4.0°C. These estimates are speculative, they
are based on the fragmentary evidence currently available, and these positive
feedbacks may not occur or may be delayed until the later part of the next
century, but the potentially large impact on the magnitude of warming
suggests that even more drastic policy measures than those considered in the
Rapidly Changing World with Stabilizing Policies scenario might be needed.
• Sensitivity analyses with four ocean models for CO2 uptake suggest that the
path of atmospheric concentrations could follow somewhat different
trajectories, but very little difference is observed in equilibrium warming for
the year 2100. These equilibrium temperatures differ by at most 10%
depending on the type of ocean model. More complex ocean circulation
models currently in the research stage could broaden or decrease this range
in the future.
• Assumptions about the total supply of oil and gas are among the least
significant factors affecting global warming in the year 2100. While gas may
be desirable as a transition fuel, sensitivity tests that assume very optimistic
estimates of oil and gas availability at each price level suggest only small
changes in global warming. A larger impact could occur if policy measures
were adopted to take advantage of the assumed increases in gas resources.
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Policy Options for Stabilizing Global Climate - Review Draft Chapter VI
• The sources of methane are subject to considerable uncertainty. Estimates
of some individual emission sources vary by a factor of two to three.
Sensitivity tests that consider extreme assumptions about anthropogenic
methane emission sources suggest that uncertainties in this budget could
cause equilibrium warming commitments in 2100 to vary by about 5%.
These results should not be interpreted to mean that methane is not an
important greenhouse gas, but simply that uncertainties in the current budget
do not greatly affect the ultimate temperatures derived in this report.
• A comparison between current atmospheric concentrations and growth rates
for the greenhouse gases and those calculated with the atmospheric
composition model, based on estimates of preindustrial concentrations and
past emissions, show good agreement. The largest discrepancies are for
relatively short-lived gases that have been increasing rapidly in recent years,
such as HCFC-22 and carbon tetrachloride.
• Non-greenhouse gases such as NOD CO, and Non-methane hydrocarbons
(NMHC) affect the lifetimes and concentrations of tropospheric ozone and
methane. A comparison of different chemistry models suggests that
increases in methane concentrations may vary by approximately a factor of
two for similar assumptions about NOj/CO/NMHC. This range may be
attributed to differences in initial budgets and modeling approaches and may
ultimately increase or decrease as other models become available.
• The most important determinant of future atmospheric concentrations of
methane appears to be the growth rate of methane sources. While NOX and
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Policy Options for Stabilizing Global Climate -- Review Draft Chapter VI
CO affect the lifetime of methane, model studies suggest that assumptions
about the emissions of these gases are less important than assumptions
about the direct emissions of methane. However, considerable research is
needed to further our understanding of the chemistry of the atmosphere.
• There is considerable uncertainty about future concentrations of tropospheric
ozone and about changes in composition at different altitudes. While model
comparisons all suggest that increases in ozone are likely, the effect of these
changes in global temperatures is difficult to predict.
• For the major sensitivity analyses presented in this chapter, Table 6-1
summarizes the impact on realized warming and equilibrium warming by
2050 and 2100 (assuming a 3.0°C climate sensitivity). Throughout this
chapter results are discussed for 2.0-4.0°C climate sensitivities for the Rapidly
Changing World Scenario, with any figures using the midpoint of this range,
i.e., a 3.00C climate sensitivity, unless stated otherwise. Other assumptions
would not change the basic findings, only the absolute size of the impacts;
these are presented in Appendix C.
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Chapter VI
TABLE 6-1
Impact of Sensitivity Analyses on Realized Warming
and Equilibrium Warming
(degrees Celsius--3.0°C climate sensitivity)
2050
2100
Realized Equilibrium Realized Equilibrium
Rapidly Changing World - No Response (RCW) 2.4°
Rapid Changing World - Stabilizing Policies (RCWP) 1.5
4.0°
2.2
4.T
1.9
IT*
2.5
Sensitivity Case Assumptions
No Participation by Developing Countries3
Global Delay in Adopting Policies'1
Non-Fossil Technology'
Fossil Resources
High Coal Prices'*
High Oil Supply6
High Gas Supply*
Methane Budget8
High CO Emissions
N2O From Fertilizer
Anhydrous Ammoniah
N2O Leaching'
N2O From Combustion1
2.1
2.4
2.4
2.3-2.5
2.4
2.4
2.4
3.1-3.3
3.1
3.4-3.7
3.5
4.0
4.0
3.9-4.2
To
4.0
4.0
4.0
3.2-3.7
2.6
3.6-4.0
3.6
4.7
4.7
4.6-5.0
be added
4.7
4.7
4.7
,53
71*
72*
7.0-7.6*
72*
72*
72*
* Estimates of equilibrium warming commitment greater than 6°C represent extrapolations beyond
the range tested in most climate models; this warming may not be fully realized because the strength
of some positive feedback mechanisms may decline as the Earth warms.
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VI-7
February 21, 1989
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Policy Options for Stabilizing Global Climate - Review Draft Chapter VI
TABLE 6-1 (continued)
Impact of Sensitivity Analyses on Realized Warming
and Equilibrium Warming
(degrees Celsius~3.0°C climate sensitivity)
2050 2100
Realized Equilibrium Realized Equilibrium
CO2 From Biomass*
CO2 Models'
Oeschger et al.m
Bolin et al."
Bjorkstrom"
Siegenthalerp
Unknown Sinkq
1.5-5.5°C Sensitivity
Heat Diffusion5
Prather Model
CFC-11 Lifetime'
Chlorine/Col O3U
Trop 03/CH4V
OH/NO,"
Feedbacks
Ocean Circulation*
Methane5'
CO2/CH4/Uptakez
2.4
-
-
-
-
2.2-2.5
1.5-3.2
1.9-2.7
2.4
2.3
2.4
2.4
3.1
2.7
3.0
4.1
3.9
3.9
3.9
3.9
3.7-4.2
2.0-7.4
4.0
4.0
3.8
4.1
4.0-4.1
4.1
4.6
4.9
4.8
-
-
-
-
4.1-4.9
2.9-6.6
3.9-5.3
4.7
4.4
4.8
4.7-4.8
7.1*
5.5
6.3
73*
69
69
69
63
6.2-7.4*
3.6-13.2*
7.2*
7.2*
6.6
7.3*
7.1-7.3*
7.6*
8.4*
8.9*
* Estimates of equilibrium warming commitments greater than 6°C represent extrapolations beyond
the range tested in most climate models, and this warming may not be fully realized because the
strength of some positive feedback mechanisms may decline as the Earth warms.
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Policy Options for Stabilizing Global Climate -- Review Draft Chapter VI
TABLE 6-1 -- NOTES
a Developing countries were assumed to not participate in climate stabilization policies. The range
represents uncertainty in the rate of technological diffusion, i.e., even if developing countries do
not participate, they will indirectly benefit from technological improvements as a result of
stabilization policies among the developed countries.
b Impact if developed countries do not respond to global warming until 2010; developing countries
delay to 2025.
c These ranges represent modest to optimistic assumptions about future commercial availability
of non-fossil technologies, e.g., solar photovoltaics, advanced nuclear power designs, and synthetic
fuel production from biomass. Solar photovoltaic costs decline to 4.6 cents/kwh (1985$) by 2020
in the optimistic scenario and by 2050 in the modest assumptions. Nuclear costs decline 0.5%
annually with the optimistic assumptions and remain relatively flat in the modest assumptions.
The cost of producing and converting biomass to modern fuels reaches $4.00/gigajoule for gas
and $6.00 (gigajoule) for liquids by 2020 in the optimistic assumptions and by 2050 in the modest
assumptions. The total amount of fuel available from biomass is 210 EJ.
d The impact of an escalation in coal prices above the RCW case by about 1% annually from
1985 to 2100.
e The impact of an increase in global oil resources to 25,000 EJ, more than double the estimate
in the RCW case, assuming proportionate increases in resource availability at each cost level.
f The impact of an increase in global natural gas resources to 27,000 EJ, more than 2.5 times the
estimate in the RCW case, assuming proportionate increases in resource availability at each cost
level.
g These ranges represent assumptions about the relative sizes of anthropogenic versus non-
anthropogenic sources of methane emissions, thereby affecting growth in emissions over time,
i.e., high emission levels (373 Tg CH4) from anthropogenic activities such as fuel production and
landfilling with low emission levels (137 Tg CH4) from natural processes such as oceans and
wetlands, versus low anthropogenic emissions (245 Tg CH4) with high natural emissions (265 Tg
CH4).
h The impact of elevating the emission coefficient for the anhydrous ammonia fertilizer type (the
percent of N evolved as N2O) from 0.5% to 2.0%.
The impact of assuming additional N2O emissions from fertilizer leaching into surface water and
ground water, modeled by increasing all the fertilizer emission coefficients by 1 percentage point.
J The impact of higher emission coefficients for N2O from combustion; assumes that N2O
emissions are about 25% of NO^ emissions and the N2O emissions from combustion sources in
1985 equaled 2.3 Tg N, over 2 tunes the level assumed in the RCW case.
k The impact of assuming a higher estimate for the amount of carbon initially contained in forest
vegetation and soils (roughly a 50-100% increase) and a more rapid rate of change in land-use,
resulting in emissions of carbon of 281 Pg from 1980 and 2100 compared to 188 Pg C in the
RCW scenario.
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TABLE 6-1 -- NOTES (continued)
Realized warming was not calculated in these tests.
This box-diffusion model represents carbon turnover below 75 meters as a purely diffusive
process.
This is a 12-compartment regional model which divides the Atlantic and Pacific-Indian Oceans
into surface-, intermediate-, deep-, and bottom-water compartments and divides the Arctic and
Antarctic Oceans into surface- and deep-water compartments.
This is an advective-diffusive model which divides the ocean into cold and warm compartments;
water downwells directly from the cold surface compartment into intermediate and deep layers.
An outcrop-diffusion model that allows direct ventilation of the intermediate and deep oceans
in high latitudes by incorporating an outcrop connecting all sublayers to the atmosphere.
These ranges represent the impact of alternative assumptions about the "unknown carbon sink"
that absorbs the unaccounted-for carbon in the carbon cycle. Two sensitivities were analyzed:
1) a high case, where the size of the unknown sink increases at the same rate as atmospheric
CO2 levels compared with preindustrial levels; and 2) a low case, where the si2e decreases to
zero exponentially at 2% per year.
Atmospheric response to a doubling of CO2 was varied from 1.5-5.5°C.
Heat diffusion in the oceans is modeled as a purely diffusive process. To capture some of the
uncertainty regarding actual heat uptake, the base case eddy-diffusion coefficient of 0.55X10"4
m2/sec was increased to 2X1CT4 and decreased to 2xlO"5 m2/sec.
The atmospheric lifetime of CFC-11, 65 years in the RCW case, was varied from 55 to 75 years.
Increases or decreases in the atmospheric concentration of CFC-11, however, tend to be offset
by corresponding decreases or increases in atmospheric concentrations of other trace gases, such
as other CFCs and CH4.
The amount of stratospheric ozone depletion due to chlorine contained in CFCs was increased
from a 0.03% to 0.20% decline in total column ozone/(ppb)2 of stratospheric chlorine.
The rate at which tropospheric ozone forms as a result of CH4 abundance was increased. In
the RCW case, this variable for the Northern Hemisphere is a 0.2% change in tropospheric
ozone for each percentage change in CH4 concentration; it was changed to 0.4% hi the sensitivity
analysis.
Tropospheric OH formation is affected by the level of NOX emissions. A 0.1% OH change for
every 1% change in NO, emissions for the Northern Hemisphere was assumed in the RCW
case; in the sensitivity analysis, a range of 0.05% to 0.2% was evaluated.
For this analysis we assumed that a 2°C increase in realized warming would alter ocean
circulation patterns sufficiently to shut off net uptake of CO2 and heat by the oceans.
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TABLE 6-1 -- NOTES (continued)
We assumed that with each 1°C increase in temperature, an additional 110 Tg CH4 from
methane hydrates, 12 Tg CH4 from bogs, and 7 Tg CH4 from rice cultivation would be released.
This case illustrates the combined impact of several types of biogeochemical feedbacks: 1)
methane emissions from hydrates, bogs, and rice cultivation (see footnote above); 2) increased
stability of the thermocline, thereby slowing the rate of heat and CO2 uptake of the deep ocean
by 30% due to less mixing; 3) vegetation albedo, which is a decrease in global albedo as a result
of changes in the distribution of terrestrial ecosystems by 0.06% per 1°C warming; 4) disruption
of existing ecosystems, resulting in transient reductions in biomass and soil carbon at the rate
of 0.5 Pg C per year per 1°C warming; and 5) CO2 fertilization, which is an increase in the
amount of carbon stored in the biosphere in response to higher CO2 concentrations by 0.3 Pg
C per ppm.
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INTRODUCTION
The Rapidly Changing World and Slowly Changing World scenarios presented in Chapter V
describe two significantly different futures for the global community. Although these two potential
paths capture a wide range of uncertainty, they do not represent all possible outcomes. Alternative
assumptions are clearly possible for many of the parameters specified in these scenarios; these
alternative specifications could alter the timing and magnitude of global climate change described in
the Rapidly Changing World and Slowly Changing World scenarios. To understand the importance
of these alternative assumptions, this chapter examines how changes in key parameters affect our
portrayal of the rate and magnitude of global climate change. These sensitivity analyses include
alternative assumptions about: the magnitude and timing of global policies to combat climate change,
rates of technological change, trace-gas source strengths and emission coefficients, the carbon cycle,
sensitivity of the climate system, atmospheric chemistry, and feedbacks.
The sensitivity analyses discussed in this chapter are generally run relative to the Rapidly
Changing World scenario, unless specified otherwise. Overviews of each case are provided to describe
the basic results for the reader; more detailed discussion of the sensitivity analyses are provided in
Appendix C.
ASSUMPTIONS ABOUT THE MAGNITUDE AND TIMING OF GLOBAL CLIMATE
STABILIZATION STRATEGIES
The analyses of the Stabilizing Policy scenarios presented in this Report are based on the
assumption that the global community takes immediate, concerted action to contend with the
consequences of climate change. Potential actions, which are discussed in Chapters VII - IX, include
reducing the amount of energy required to meet the world's increasing needs, developing alternative
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technologies that do not require the consumption of fossil fuels, halting deforestation, and making
changes in agricultural practices, among others. For many reasons, however, the world may not
respond to the threat of climate change in a timely fashion. This section explores the consequences
of other possibilities, particularly the unwillingness or inability of some countries to participate hi
climate stabilization programs and the implications of delaying global action until a later date.
No Participation by the Developing Countries
Most of the greenhouse gas emissions currently committing the world to climate change can
be traced to activities by the industrialized countries. Although the quantity of emissions generated
by developing countries has been increasing, the argument is sometimes made that since the
greenhouse problem has been largely caused by the industrialized countries, these countries should
be responsible for solving the problem. Also, despite the potential environmental consequences of
global climate change, other problems facing the developing countries, such as poverty, inadequate
health care, and other apparently more pressing environmental problems may make it difficult for
developing countries to commit any resources to climate stabilization policies.
Regardless of the merits of these arguments, for this sensitivity analysis we have assumed
that developing countries do not participate in any climate stabilization activities; that is, only
developed countries adopt policies to limit global climate change. For this analysis the developing
countries include China and Centrally-planned Asian economies, the Middle East, Africa, Latin
America, and South/Southeast Asia. We have assumed that industrialized countries (i.e., the U.S.,
the rest of the OECD countries, and the USSR and Eastern Europe) follow the path assumed in the
Rapidly Changing World with Stabilizing Policies (RCWP) scenario, while developing countries follow
the path assumed in the Rapidly Changing World No Response (RCW) case, in which the entire
global community does not respond to climate change.
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Policy Options for Stabilizing Global Climate - Review Draft Chapter VI
Even if developing countries do not participate in global stabilization policies, however,
policies adopted by the industrialized countries are likely to lead to technological advancements,
altered market conditions, etc., that indirectly reduce emissions in the developing countries as well.
For example, advancements by the developed countries in automobile fuel efficiency or fuel supply
technologies may be partly adopted by the developing countries, tangentially allowing for some
climate stabilization benefits. If the developing countries do not participate, however, they may tend
to adopt technological advances more slowly and at a higher cost than if they had participated from
the start. This slower rate of technological diffusion could occur for many reasons ~ for example,
if the industrialized countries take actions that prevent easy access to improved technologies or they
are unwilling or unable to make the necessary capital available for investment, or if developing
countries decide to invest their limited resources in other areas.
Since we cannot be certain of the direction that non-participation by the developing countries
might take, we analyzed two cases to capture the potential range of likely possibilities. In the first
case, little technological diffusion was assumed, resulting in a future path of energy consumption and
investment trends for developing countries similar to those assumed in the RCW scenario. In the
second case, developing countries were assumed to have greater access to the efficiency improvements
and technological advances assumed for the RCWP case as a result of policies by the industrialized
countries to make these improvements available and extend the credit necessary for investment by
the developing countries in these improvements.
In this analysis key assumptions for the developing countries included the following: (1)
rates of energy efficiency improvements for all sectors are the same as in the RCW case or midway
between the RCW and RCWP case; for example, automobile efficiency levels, which by 2050 in
developing countries were 5.9 liters/100 km (40 mpg) in the RCW case and 3.1 liters/100 km (75
mpg) in the RCWP case, were varied from 5.9-4.1 liters/100 km (40-58 mpg); (2) CFCs are not
DRAFT - DO NOT QUOTE OR CITE VI-14 February 21, 1989
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Policy Options for Stabilizing Global Climate -- Review Draft Chapter VI
phased out (although compliance with the Montreal Protocol would still occur); (3) agricultural
practices that cause methane emissions from rice and enteric fermentation and nitrous oxides from
fertilizers do not change or would show modest improvements; (4) deforestation continues as in the
RCW case with an exponential decline in forest area; (5) non-fossil energy supply technologies
developed by the industrialized countries are available to developing countries at a later date and a
higher cost than assumed in the RCWP case; for example, technological diffusion of biomass
gasification technology would occur 10 years later than it would in the RCWP case, but feedstock
costs would remain high due to a lack of investment by the developing countries in highly productive
energy plantations; and (6) no additional incentives are provided for increased use of natural gas.
Without the participation of the developing countries to stabilize the atmosphere, the amount
of greenhouse gas emissions will increase substantially: In the analysis considered here, CO2
emissions are 3-4 Pg C higher than in the RCWP case by 2050 and 6-13 Pg C higher by 2100
(emissions by 2100 are 8.1 to 14.7 Pg C lower than in the RCW case since industrialized countries
adopt climate stabilization policies);l other greenhouse gas emissions are also higher. These emission
increases are sufficient to increase realized warming by 0.4-0.6°C in 2050 compared with the RCWP
case and 1.0-2.1°C by 2100 (see Figure 6-1), with equilibrium warming by 2100 up to 1.4-4.1°C higher.
Figure 6-1 also shows the results for the Slowly Changing World scenarios. In this scenario, emission
increases are sufficient to increase realized warming by 0.3-0.5°C in 2050 compared with the SCWP
case and 0.6-1.10C by 2100, with equilibrium warming by 2100 up to 0.8-2.0°C higher.
The implications of these results are clear: even if the industrialized countries adopt very
stringent policies to counteract the effects of climate change, the atmosphere continues to warm at
a rapid rate. As a result, unilateral action by the industrialized countries can significantly slow the
1 Pg = petagram; 1 petagram = 1015 grams.
DRAFT - DO NOT QUOTE OR CITE VI-15 February 21, 1989
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Policy Options for Stabilizing Global Climate - Review Draft
Chapter VI
FIGURE 6-1
INCREASE IN REALIZED WARMING
WHEN DEVELOPING COUNTRIES DO NOT PARTICIPATE
(Degrees Celsius; Based on 3.0 Degree Sensitivity)
I 3
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a
SLOWLY CHANGING WORLD
No Participation
by Developing
Countries
SCW ,'
SCWP
RAPIDLY CHANGING WORLD
RCW .'
No Participation
by Developing
Countries
RCWP
1985 2000 2025 2050 2075 2100 1985 2000 2025 2050 2075 2100
YEAR YEAR
DRAFT - DO NOT QUOTE OR CITE VI-16
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Policy Options for Stabilizing Global Climate « Review Draft Chapter VI
rate and magnitude of climate change, but because of the growing impact that developing countries
have on the global climate, without the participation of the developing countries, substantial global
warming is unavoidable. Because most of the world's population resides in these countries, their role
in climate stabilization becomes increasingly important as the demand for resources to feed and
clothe their growing population and improve their standard of living expands.
Delay in Adoption of Policies
For the Stabilizing Policy cases presented in Chapter V it is assumed that the global
community takes immediate action to respond to the dangers posed by climate change. For this
sensitivity analysis we have assumed that the global community delays any response to the threat of
climate change, with developed countries (i.e, the United States, the rest of the OECD countries, the
USSR and centrally-planned European economies) delaying action until 2010, and the developing
countries delaying action until 2025. Additionally, once regions do initiate action to combat global
warming, they do so at a slower rate than assumed in the RCWP case. This slower approach
assumes a minimum 25-year delay in attaining the policy goals of the RCWP case; that is, levels of
technological improvement, availability of alternative energy supply technologies, etc., will be achieved
at least 25 years later. For example, in the RCWP case, automobile efficiency reaches 3.1 liters/100
km (75 mpg) by 2050; in the Delay case industrialized countries reach 3.9 liters/100 km (60 mpg)
by 2050, while developing countries reach 4.7 liters/100 km (50 mpg); the rate of energy efficiency
improvement for the residential, commercial, and industrial sectors is unchanged from the rates
assumed in the RCW case, through 2010 for industrialized countries and 2025 for developing
countries. After these years, energy efficiency improvements occur at the same rate assumed in the
RCWP case; and the implementation of production and consumption taxes on fossil fuels from the
RCWP case was delayed until 2010 for developed countries and 2025 for developing countries.
DRAFT - DO NOT QUOTE OR CITE VI-17 February 21, 1989
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Policy Options for Stabilizing Global Climate - Review Draft Chapter VI
Delaying the adoption of policies to stabilize the atmosphere significantly increases the
Earth's commitment to global warming. With delay by the industrialized countries until 2010 and by
the developing countries until 2025, the increase in realized warming compared to that assumed in
the RCWP case is 0.4-0.6°C by 2050 and 0.5-0.9°C by 2100; equilibrium warming is 0.6-1.2°C higher
by 2050 and 0.7-1.4°C higher by 2100 (based on climate sensitivities of 2.0-4.0°C; see Figure 6-2).
Figures 6-2 also shows the results for the Slowly Changing World scenarios. If global delays do
occur, the increase in realized warming compared to that assumed in the SCWP case is 0.3-0.5°C
by 2050 and 0.3-0.6°C by 2100; equilibrium warming is 0.5-0.9°C higher by 2050 and 0.4-0.8°C higher
by 2100 (based on climate sensitivities of 2.0-4.0°C).
ASSUMPTIONS AFFECTING RATES OF TECHNOLOGICAL CHANGE
The extent of global warming will depend on the availability of energy supplies and
technologies that minimize dependence on carbon-based fuels, nitrogen-based fertilizers, and other
sources of greenhouse gas emissions. The availability of non-fossil fuel technologies and the
development of new production methods that significantly increase the supply of natural gas could
have an impact on the rate of change in greenhouse gas emissions. Alternative assumptions regarding
these factors are presented below.
Availability of Non-Fossil Technologies
Most technologies in use currently rely on fossil fuels to' supply their energy needs. In the
Rapidly Changing World, fossil-fuel-based technologies continue to dominate throughout the next
century: by 2100 fossil fuels still supply over 70% of primary energy needs. However, if non-fossil
technologies can be commercialized earlier, the magnitude of global climate change can be reduced
because these technologies do not emit the greenhouse gases that cause global warming. To evaluate
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Policy Options for Stabilizing Global Climate - Review Draft
Chapter VI
V)
I 3
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FIGURE 6-2
INCREASE IN REALIZED WARMING
DUE TO GLOBAL DELAY IN POLICY ADOPTION
(Degrees Celsius; Based on 3.0 Degree Sensitivity)
Slowly Changing World
SCW
Global Delay
SCWP
Rapidly Changing World
RCW '-
Global Delay /
RCWP
1985 2000 2025 2050 2075 2100 1985 2000 2025 2050 2075 2100
YEAR YEAR
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Policy Options for Stabilizing Global Climate - Review Draft Chapter VI
the implications of the availability of non-fossil technologies, two different scenarios were analyzed:
(1) an Early Non-Fossil case, in which non-fossil technologies, specifically solar photovoltaics,
advanced nuclear power designs, and production of synthetic fuels from biomass, are commercially
available by 2000 at a rate faster than that assumed in the RCWP case; and (2) an Intermediate
Non-Fossil case, in which non-fossil technologies are widely available by the middle of the next
century (i.e., greater use of non-fossil technologies than in the RCW case, but less than in the RCWP
case). The intent of these two cases is to capture a range of possible roles for non-fossil
technologies, with the first case reflecting very optimistic assumptions on non-fossil availability and
the second case reflecting more modest assumptions.
In the Early Non-Fossil case, non-fossil energy sources increase their share of total primary
energy supply from 12% in 1985 to about 50% by 2025 and 65% by 2100, while in the Intermediate
Non-Fossil case the share for non-fossil technologies increases to 21% by 2025 and about 60% by
2100 (see Figure 6-3a). As shown in Figure 6-3a, in the near term the non-fossil share of total
energy could be greater than reflected in the RCWP case if commercial availability is achieved at an
earlier date. In this sensitivity analysis, however, the non-fossil share is lower in the long run
compared with the share in the RCWP case because other policies that were included in the RCWP
case to discourage the use of fossil fuels were not included in this case. In both cases, however, an
increased role for non-fossil technologies can affect the amount of global warming. As shown in
Figure 6-3b, for the two cases presented here the amount of realized warming compared with the
RCW case could be reduced about 0.1-0.3°C by 2050 and 0.6-1.3°C by 2100; equilibrium warming
could be reduced about 0.2-0.9°C by 2050 and 0.9-2.50C by 2100 (based on 2.0-4.0°C climate
sensitivities).
DRAFT - DO NOT QUOTE OR CITE VI-20 February 21, 1989
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Policy Options for Stabilizing Global Climate - Review Draft
Chapter VI
FIGURE 6-3
AVAILABILITY OF NON-FOSSIL ENERGY OPTIONS
(a) Non-Fossil Share Of Total Primary Energy Supply
(Percent)
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a.
(b)
v>
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ui
UJ
cc
O
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o
100
80
60
40
20
RCWP
Non-Fossil
Energy Options
RCW
1985 2000
2025 2050
YEAR
2075
2100
Increase In Realized Warming
(Degrees Celsius; Based on 3.0 Degree Sensitivity)
RCW
Non-Fossil
Energy Options
RCWP
1985 2000
2100
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VI-21
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Policy Options for Stabilizing Global Climate -- Review Draft Chapter VI
Cost and Availability of Fossil Fuels
As discussed in Chapters IV and VII, there is significant uncertainty over the amount of
fossil-fuel resources available globally and the cost at which these resources could be produced. The
development of the fossil energy resource estimates and the associated extraction costs used in this
analysis are documented in ICF (1988). Given the uncertainties about the cost and availability of
fossil energy supplies, several sensitivity cases were analyzed. These are discussed below.
High Coal Prices
In the RCW case from 1985 to 2050 there was no real escalation in coal prices. Given the
vast quantity of coal resources available worldwide, and the rate of productivity improvements in coal
extraction that have helped to contain cost increases, coal prices may not escalate in real terms (e.g.,
from 1949 to 1987, U.S. coal prices declined an average of 0.2% annually [U.S. DOE, 1988]). Since
the longer-term price path for coal is highly uncertain, however, we analyzed the impacts of a high
price coal case where coal prices escalated about 1 percent annually from 1985 to 2100.
As illustrated in Figure 6-4a, increasing coal prices have a significant impact on the amount
of primary energy consumed; for example, by 2100 total primary energy demand is more than 20%
lower compared with this demand in the RCW case. Most of this reduction in energy demand is
due to the decline in coal use as consumers respond to the escalating prices. Because coal is a
major energy resource for electricity production and synthetic fuel production, the impact on the level
of greenhouse gas emissions is fairly substantial. For example, CO2 emissions are reduced nearly
50% by 2100. The reductions in greenhouse gas emissions have a significant impact on global
warming, as shown in Figure 6-4b, which indicates a decline in realized warming from the RCW case
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Policy Options for Stabilizing Global Climate - Review Draft
Chapter VI
FIGURE 6-4
IMPACT OF 1% PER YEAR REAL ESCALATION IN COAL PRICES
(a)
Total Primary Energy Demand
(Exajoules)
1500
1250
LU 1000
O
75°
500
250
RCW
High Coal Prices
RCWP
1985 2000
2025
2050
YEAR
2075
2100
(b)
Increase In Realized Warming
(Degrees Celsius; Based on 3.0 Degree Sensitivity)
v>
CO
co
111
LU
CC.
a
LU
O
RCW
High Coal Prices
RCWP
1985 2000
2025 2050
YEAR
2075
2100
DRAFT - DO NOT QUOTE OR CITE
VI-23
February 21, 1989
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Policy Options for Stabilizing Global Climate - Review Draft Chapter VI
of 0.2-0.3°C by 2050 and 0.9-1.3°C by 2100 (assuming 2.0-4.0°C climate sensitivities). The
corresponding decrease in equilibrium warming by 2100 is 1.3-2.5°C.
Alternative Oil and Natural Gas Supply Assumptions
There are many uncertainties concerning the amount of oil and natural gas supplies available
worldwide. As discussed in Chapter VII, for example, the viability of increased use of natural gas
as a near-term option for reducing greenhouse gas emissions critically depends on the amount of
natural gas available, its price, the length of time over which adequate supplies can be secured, etc.
To explore how sensitive the level of greenhouse gas emissions may be to the amount of oil and
natural gas supplies, two sensitivity cases assuming higher global supplies have been analyzed: (1)
a high oil resource case and (2) a high natural gas resource case. The higher oil resource estimates
were derived from Grossling and Nielsen (1985), who indicated that resources may be more than
double the estimates used in the base case analyses (which were about 12,000 EJ of conventional oil
resources).2 For this analysis we assumed conventional oil resources of about 25,000 EJ. Natural
gas estimates were derived from Hay et al. (1988), which assumed in-place resources of about 150,000
EJ. For purposes of this sensitivity case, we assumed that technological improvements in gas
extraction would permit an additional 10% of in-place resources to be economically recovered. This
amount was added to the baseline estimates of proved reserves and economically recoverable
resources, for a total resource base of about 27,000 EJ. We must emphasize that these sensitivity
cases do not examine policy options that encourage greater oil and natural gas use; rather, they only
attempt to examine how current uncertainties concerning the size of the resource base for these
energy supplies can directly affect the rate and magnitude of global climate change. Policy options
EJ = exajoule; 1 exajoule = 10 joules.
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Policy Options for Stabilizing Global Climate -- Review Draft Chapter VI
encouraging greater use of these fuels in conjunction with higher resource estimates would have a
substantially different impact.
High Oil Resources. An increase in global oil resources to 25,000 EJ is more than double
the resource estimates assumed in the RCW case. These additional resources were assumed to be
available at the same economic costs, such that the amount of oil available at any given price was
twice the amount assumed in the RCW case. This increase in oil resources had two major impacts:
(1) the amount of synthetic production of liquid fuels from coal declined substantially since
conventional oil supplies were available at a competitive price to meet this demand; and (2) total
demand for energy, mainly oil, increased as consumers responded to the increased availability of oil
supplies at the same price (since twice the amount of oil was available at a price equal to that in the
RCW case). The net effect of these impacts is a small increase in total primary energy demand (a
4% increase by 2050), a major shift from coal (primarily for synthetic fuel production) to oil, and a
decrease in the portion of total primary energy supplied by non-fossil resources since oil is more
plentiful and competitive; for example, non-fossil fuels supply about 22% of all energy by 2050
compared with 24% in the RCW case (see Figure 6-5). The net effect of these factors is an increase
in CO2 emissions of 0.4 Pg C by 2050 and 2.2 Pg C by 2100. The decline in coal production,
however, lowered methane (CH4) emissions since the amount of CH4 emitted during coal mining
decreased substantially (e.g., by 2100 CH4 emissions from fuel production declined from about 360
Tg in the RCW case to 210 Tg), resulting in a modest decline of less than 0.1°C in realized warming
by 2100 compared with the RCW case warming (assuming 2.0-4.0°C climate sensitivities).3
High Natural Gas Resources. For the high natural gas resource case, natural gas resources
were increased from about 10,000 EJ to 27,000 EJ. As in the high oil resource case, higher natural
3 Tg = teragram; 1 teragram = 1012 grams.
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Policy Options for Stabilizing Global Climate - Review Draft
Chapter VI
FIGURE 6-5
IMPACT OF HIGHER OIL RESOURCES
ON TOTAL PRIMARY ENERGY SUPPLY
(Exajoules)
RCW
Crt
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4
X
1500
1250
1000
750
500
250
_
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Policy Options for Stabilizing Global Climate -- Review Draft Chapter VI
gas resource estimates result in two major impacts: (1) an increase in demand for energy,
particularly for gas, since natural gas is more plentiful compared with the amount available in the
RCW case; and (2) a decline in the conversion of coal to synthetic gas, since natural gas supplies are
available to meet the demand.
Overall, by the end of the 21st century the amount of primary energy consumed changes very
little from the RCW case. In the near term energy demand increases slightly compared with the
RCW case, since natural gas is more plentiful (e.g., by 2025 energy demand is about 2.5% higher
compared with the RCW case; see Figure 6-6). However, the total amount of energy required in the
long run is less because a greater portion of end-use energy demand is met with natural gas rather
than with synthetic gas from coal. This increase in conventional natural gas consumption reduces
the total primary energy required to satisfy demand because the decline in synthetic fuel demand
from the RCW case reduces the amount of energy required for synthetic fuel conversion, although
this impact is small: by 2100 primary energy demand is lower by less than 1%.
The amount of natural gas consumed does increase significantly; for example, in 2050 natural
gas consumption increases to 215 EJ compared with 100 EJ in the RCW case. However, the
increased availability of natural gas also reduces the portion of energy supplied by non-fossil fuels;
for example, by 2050 non-fossil energy sources supply about 20% of total demand compared with
24% in the RCW case. The net impact on CO2 emissions due to these factors is quite small: no
change in emissions by 2050 and a decline of 0.7 Pg C by 2100. The impact on realized and
equilibrium warming is negligible (less than 0.1°C).
DRAFT - DO NOT QUOTE OR CITE Vl-27 February 21, 1989
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Chapter VI
FIGURE 6-6
IMPACT OF HIGHER NATURAL GAS RESOURCES ON
TOTAL PRIMARY ENERGY SUPPLY
(Exajoules)
RCW
1500
1250 -
Biomass
Solar
1985 2000
2025 2050
YEAR
2075
2100
1500
High Natural Gas Resources
1985 2000
2025
2050
2075
2100
YEAR
DRAFT - DO NOT QUOTE OR CITE
VI-28
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Policy Options for Stabilizing Global Climate -- Review Draft Chapter VI
Availability of Methanol-Fueled Vehicles
The transportation sector throughout the world is heavily dependent on petroleum-based
fuels. This dependence, particularly on gasoline and diesel fuel, produces substantial quantities of
greenhouse gases (see Chapter IV). A variety of non-petroleum-based alternatives are under
development, including the use of methanol. There are many potential advantages to using methanol
as a transportation fuel rather than gasoline; according to recent research, advanced methanol-fueled
vehicles could be 20-40% more energy efficient, emit much lower levels of CO, and reduce non-
methane hydrocarbon (NMHC) reactivity up to 95% (Gray, 1987). Methanol's potential to reduce
NMHC reactivity could reduce levels of urban ozone, which would improve ambient air quality in
urban areas. These reductions could be on the order of about 5-20% of peak ozone levels (DeLuchi
et al., 1988). However, it is not clear how reductions in urban ozone levels may translate to
reductions in average tropospheric ozone and, therefore, changes in radiative forcing. Current
understanding of these atmospheric processes attributes urban ozone changes primarily to NMHC
and NOX flux, while tropospheric ozone changes depend primarily on (in descending order of
importance) CH4, CO, NOX flux, and NMHC flux (Prather, 1988). Interactions between urban air
quality and the rest of the troposphere cannot be evaluated with the aggregate model used here.
Since the ability of methanol to affect tropospheric ozone levels cannot be reliably estimated,
we cannot reflect all of the potential advantages of using methanol as a transportation fuel. It is
useful to note, however, that in addition to reducing emissions of CO and other gases, methanol can
be produced from different types of feedstocks, such as natural gas, coal, or biomass. When biomass
is the feedstock, the carbon emitted during the combustion process is recycled from the environment
as the biomass is grown. As a result, the net CO2 emissions are zero when biomass is used.
Greenhouse gas emissions from methanol, however, can be greater than those from gasoline if coal
is used as the feedstock because additional emissions will be generated during the methanol
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production process. According to one analysis, methanol production from coal would generate about
twice the amount of CO2-equivalent emissions (based on their radiative effect) compared to gasoline
from crude oil, while methanol from natural gas would only be slightly better (about 3%) than
petroleum-based fuels (DeLuchi et al. 1988). From a global warming perspective, DeLuchi et al.
concluded that only biomass-derived methanol would substantially reduce the amount of radiative
forcing from transportation fuels, although as mentioned above, this argument does not incorporate
any potential benefits from reductions in urban ozone levels.
ATMOSPHERIC COMPOSITION: COMPARISON OF MODEL RESULTS TO ESTIMATES OF
HISTORICAL CONCENTRATIONS
The atmospheric composition model was applied to estimates of historical emissions of trace
gases and the results compared to historical data on atmospheric composition. This exercise provides
insight on how the model performed under conditions much different from the reference year, 1985,
and provided one mechanism to validate the model. The exercise included the development of a
single scenario of historical emissions of trace gases and application of the model using different
assumptions on climate sensitivity and chemistry parameters in the model.
The scenario of historical emissions of trace gases is based on estimates of natural sources
from the Atmospheric Stabilization Framework described in Chapter V, estimates from a study by
Darmstadter et al. (1987) on historical emissions from various anthropogenic sources, and estimates
of historical CO2 emissions from Rotty (1987) and Houghton (1988). For natural emission sources,
historical emissions were assumed to be constant from 1870 to 1985 at the levels assumed in the
scenarios described in Chapter V. The exception is emissions of CH4 from wetlands, which were
assumed to be larger in 1870 by 50% and to decline to current levels due to destruction of wetlands.
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The estimates of historical emissions of CFCs and Halons were taken from EPA's Regulatory Impact
Analysis on Stratospheric Ozone Protection (U.S. EPA, 1988).
The alternative scenarios of historical atmospheric composition and global warming reflect
a range of assumptions concerning the climate sensitivity and the first and second order relationships
assumed in the model. Figure 6-7 illustrates the increase in realized warming projected from 1840
to 1985, which ranges from 0.4°C to 0.8°C based on a range of climate sensitivities (from 1.5 to
5.5°C for doubled CO2). These results compare well with results from Wigley et al. (1986), who
estimated a global temperature increase of 0.3-0.7°C in the last century, and Hansen et al. (1988),
who estimated a global temperature increase of 0.4-0.8°C during the same period. The model
produced estimates of atmospheric concentrations of CO,, CH4, N2O, CO, and CFC-12 within 1.5%
and estimates of concentrations of CFC-11 within 3.5% of observed values in 1985. In addition, the
pattern of estimated atmospheric concentrations over tune conformed well with historical
measurements for CO2, N2O, and CH4. Estimates of concentrations of some gases such as CFC-22
varied from the historical measurements to a greater extent, which reflects their more recent
introduction and rapid growth in atmospheric concentrations. Table 6-2 summarizes the results for
the long-lived gases.
For CO2, the atmospheric concentration over time matched the Mauna Loa and Ice Core
measurements by design through the use of the unknown sink in the model (see Unknown Sink in
Carbon Cycle). The unknown sink is zero through 1940 and then slowly rises to 1.9 Pg C per year
by 1985, which represents about one-third of the estimated anthropogenic emissions.
The estimates of CH4 concentrations match atmospheric and ice core measurements well,
especially given the uncertainties in the emissions estimates and the historical measurements. The
model shows somewhat higher than expected growth in the late 19th century, which may reflect the
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Policy Options for Stabilizing Global Climate - Review Draft
Chapter VI
FIGURE 6-7
REALIZED WARMING THROUGH 1985
(Degrees Celsius; Based on 1.5-5.5 Degree Climate Sensitivity)
co
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ui
a:
o
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0.9 -
0.8 -
0.7 -
0.6 -
0.5 -
0.4 -
0.3 -
0.2 -
0.1 -
1840 1865 1890
1915
YEAR
1940 1965 1985
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Chapter VI
TABLE 6-2
Comparison of Model Results to Concentrations in 1986
Trace Gas (units^
C02 (ppm)
N20 (ppb)
CH, (ppb)'
CFC-11 (ppt)
CFC-12 (ppt)
HCFC-22 (ppt)
CC1< (ppt)b
CH3CC13 (ppt)
Halon 1211 (ppt)
Model Atmospheric Observed
Model Results Growth Rates Measurements Growth Rates
346
314
1650-1750
212-222
391
37
70
186
0.4
0.4%
0.27%
1%
4%
4%
14%
0.6%
12%
100%
346
310
1675
226
392
100
121
125
2
0.4%
0.2-0.3%
1%
4%
4%
7%
1.3%
6%
>10%
1 1987 value.
b 1982 value.
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uncertainties surrounding the scenario of historical emissions. Using the reference assumptions, the
model achieves an atmospheric concentration of 1671 ppb in 1985 compared to the observed value
of 1675 ppb. The CH4 concentrations vary considerably in the sensitivity analyses and range from
1650 ppb to 1750 ppb for alternative chemistry parameters.
Of the three dominant greenhouse gases, the estimates of N2O concentrations vary the most
from historical measurements. The model predicts concentrations of 314 ppb in 1985 compared to
308 to 310 ppb cited in the literature. From 1979 to 1986, the model estimates growth in N2O
concentrations of 310 to 314 ppb compared to measurement data that suggests growth of 303 to 310
ppb. One of the possible explanations of these results is that the relative share of emissions of
N2O from anthropogenic sources is larger than estimated in the model. A larger anthropogenic
source combined with lower natural emissions or a shorter atmospheric life would be needed to
reduce the overall concentrations and obtain the growth in concentrations seen from 1979 to 1986.
These results suggest that the model may underestimate future atmospheric concentrations of N2O.
The model "predicts" very little deviation from current levels for the short-lived gases,
including OH, O3, and CO. The results for levels in 1870 include higher levels of OH by 14-26%,
lower levels of tropospheric O3 by 19-29%, lower concentrations of CO by approximately 50%, and
increased levels of upper stratospheric ozone by 4.5%.
ASSUMPTIONS ABOUT TRACE-GAS SOURCES AND STRENGTHS
Among the various greenhouse gases there is some uncertainty over the quantity of emissions
that can be attributed to specific sources and the ability of these gases to modify the atmosphere.
The most critical of these uncertainties are examined below.
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Methane Sources
The available evidence on CH4 indicates that annual production ranges from 400-640 Tg of
methane (based on known sources and sinks, its atmospheric lifetime, and current atmospheric
concentrations). Within this budget, however, there is much dispute over the size of individual
sources. For example, research indicates that current CH4 emissions from rice paddies could be 60-
170 Tg; similarly, estimated emissions from biomass burning range from 50-100 Tg (Cicerone and
Oremland, 1989).
To account for these uncertainties, the initial CH4 budget was varied to construct two cases:
(1) a high anthropogenic impact case, where the starting methane budget was biased toward
anthropogenic sources by assuming that anthropogenic activities such as fuel production and landfilling
caused higher emission levels than assumed in the RCW case, while lower emission estimates were
assumed from natural processes such as oceans, wetlands, wildfires, and wild ruminants; and (2) a
low anthropogenic impact case, by assuming lower emissions from anthropogenic activities such as
fuel production, enteric fermentation, and rice cultivation, with corresponding emission increases from
natural processes such as oceans and wetlands. The specific emission assumptions for the starting
budget are summarized in Table 6-3.
The alternative starting budgets in Table 6-3 result in different growth paths for CH4, since
emissions from anthropogenic sources increase by different amounts over time. These differences
alter the atmospheric concentration of CH4: by 2100 the atmospheric concentration is about 3500-
3700 ppb in the Low Impact case and 5200-5500 ppb in the High Impact case (compared with 4100-
4400 ppb in the RCW case). The increase (decrease) in CH4 also increases (decreases) the amount
of tropospheric ozone. The impact on realized warming is summarized in Figure 6-8, which indicates
a decline of 0.1-0.2°C by 2100 in the Low Impact case compared with the RCW case and an increase
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Chapter VI
TABLE 6-3
Low and High Anthropogenic Impact Budgets For Methane
(teragrams/year as of 1985)
Source of Methane
Fuel Production
Enteric Fermentation
Rice Cultivation
Landfills
Oceans
Wetlands
Biomass Burning— Anthropogenic
Biomass Burning— Natural
Wild Ruminants
Other Sources
TOTAL
Low Impact
50
70
60
30
40
150
35
20
44
11
510
RCW
60
75
110
30
15
115
35
20
44
__6
510
High Impact
95
75
110
58
6
100
35
15
10
_6
510
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Chapter VI
FIGURE 6-8
oo
2
oo
_J
LU
O
W
UJ
UJ
tc.
O
UJ
0
INCREASE IN REALIZED WARMING
DUE TO CHANGES IN THE METHANE BUDGET
(Degrees Celsius; Based on 3.0 Degree Sensitivity)
j_
High Methane
Low Methane
RCWP
1985 2000
2025
2050
YEAR
2075
2100
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of 0.2-0.3°C by 2100 in the High Impact case. The corresponding effects on equilibrium warming
by 2100 are a decline of 0.2-0.3°C in the Low Impact case and an increase of 0.3-0.5°C in the High
Impact case.
Nitrous Oxide Emissions From Fertilizer
N2O is naturally produced in soils by microbial processes during denitrification and
nitrification. When nitrogen-based fertilizers are applied, N2O emissions from the soil can increase
as a result of the additional nitrogen source. The amount of fertilizer nitrogen evolved as N2O is
highly variable and uncertain. We have used the emission estimates developed by Galbally (1985)
in our base cases: 0.5% for anhydrous ammonia, 0.1% for ammonium nitrate, 0.1% for ammonium
salts, 0.5% for urea, and 0.05% for nitrates. Alternative assumptions are explored below.
Anhydrous Ammonia
One of the key uncertainties concerns the emission coefficient for anhydrous ammonia. A
review of the scientific literature on measurements of N2O emissions by fertilizer type indicates that
the percentage of anhydrous ammonia evolved as N ranges from 0.05-6.84%, with most measurements
ranging from 0.5-2.0% (Eichner, 1988). The impact of this uncertainty was evaluated by changing
the anhydrous ammonia coefficient from 0.5% to 2.0%. This change Increased the amount of N2O
from fertilizer applications by 0.1 Tg of N annually, an increase in 2025 from 0.7 to 0.8 Tg, which
was too small to affect the amount of global warming.
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H,O Leaching From Fertilizer
As discussed above, in the RCW case N2O emissions from fertilizer were based on estimates
by Galbally (1985). One N2O emission pathway not included in this estimate is leaching from the
fields into the ground water or surface water due to the application of the fertilizer. The rate of
emissions related to leaching is highly uncertain; Conrad et al. (1983) and Kaplan et al. (1978) have
suggested that the amount of N2O evolved due to leaching may be as large as N2O from the
denitrification/nitrification processes in the soil. The impact of leaching on total N2O emissions and
the resulting global warming was analyzed by increasing all of Galbally's emission coefficients by one
percentage point. The higher rate of N2O from fertilizer due to leaching resulted in an increase in
emissions of about 1.0-1.5 Tg annually.
Atmospheric N2O concentrations increase about 20 ppb by 2100 compared with the RCW
case (from 403 to 424 ppb; see Figure 6-9). While N2O concentrations increase when leaching is
assumed, the impact on global warming is not as certain. In this case, global warming was slightly
reduced (less than 0.1°C) due to the chemical interactions that occur with increased N2O levels.
Specifically, higher N2O levels in the stratosphere reduce the amount of stratospheric ozone, which
in turn allows more ultraviolet (UV) radiation to penetrate to lower elevations. The increased UV
radiation increases the amount of CFC destruction, which reduces the contribution of CFCs to global
warming. None of these reactions are very strong, since the change in N2O emissions due to leaching
does not have a major effect on atmospheric concentrations, but they are sufficient to counteract the
warming effect of higher N2O concentrations alone.
N2O Emissions From Combustion
During the combustion process, chemical interactions downstream from the combustion
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Policy Options for Stabilizing Global Climate « Review Draft
Chapter VI
FIGURE 6-9
CHANGE IN ATMOSPHERIC CONCENTRATION OF N20
DUE TO LEACHING
(Parts Per Billion; 3.0 Degree Celsius Sensitivity)
500
400 -
tr
iu
a.
cc
<.
a.
300
200
I
I
Leaching
RCW
1985 2000
2025 2050
YEAR
2075
2100
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Policy Options for Stabilizing Global Climate - Review Draft Chapter VI
chamber can lead to N2O formation from nitrogen oxides. The rate of this formation is highly
uncertain, although recent evidence indicates that it is likely to be fairly small. In the RCW case
these low emission coefficients were assumed (see Chapter II). To ascertain the impact of higher
emission coefficients, N2O coefficients from combustion were increased such that emissions from
energy in 1985 were 2.3 Tg N rather than 1.1 Tg N as obtained in the RCW case. The higher N2O
emission levels increased atmospheric concentrations about 50 ppb by 2100 (as shown in Figure 6-
10); the resulting impact on global warming was negligible (less than 0.1°C) for the same reasons
discussed above under leaching from fertilizer.
UNCERTAINTIES IN THE GLOBAL CARBON CYCLE
The global carbon cycle, which regulates the flow of carbon through the environment,
including the atmosphere, biosphere, and hydrosphere, was discussed in Chapters II and III.
Uncertainties in the size of the various sources and sinks for carbon and the interactions that govern
the flow of carbon increase the difficulty of estimating the impact of anthropogenic activities on
global climate. In this section the major uncertainties in the global carbon cycle are evaluated. The
first part focuses on the impact of deforestation on CO2 emissions. The second part discusses the
ability of the oceans to absorb CO2 and heat. Currently, the oceans are the dominant sink for
anthropogenic CO2 emissions, with the mixed layer alone containing about as much carbon as the
atmosphere. The oceans' ability to operate as a net sink for carbon and heat is an important
component of the global climate system; any changes in this absorption ability could have profound
effects on global climate (see Chapter III).
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Chapter VI
FIGURE 6-10
CHANGE IN ATMOSPHERIC CONCENTRATION OF N20
DUE TO COMBUSTION
(Parts Per Billion; Based on 3.0 Celsius Degree Sensitivity)
500
450
400
CO
OL
01
O.
w 350
cc.
<
a.
300
250
200
Combustion
RCW
1985 2000
2025
2050
YEAR
2075
2100
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Unknown Sink In Carbon Cycle
Atmospheric CO2 concentrations have changed historically due to an imbalance between the
sources and sinks for carbon. If the production of carbon exceeds the ability of the various carbon
sinks to absorb it, then the atmospheric CO2 concentrations will increase (and vice versa). When
analyzing the amount of carbon produced from various sources in the past, atmospheric scientists
have been unable to balance the carbon cycle. That is, given current estimates of carbon sources,
it would appear that atmospheric CO2 concentrations would have to be higher than currently
measured, since all known sinks do not appear to be able to absorb all of the carbon produced. To
account for this imbalance, we have assumed the existence of an "unknown sink" that absorbs the
unaccounted-for' carbon. The size of this unknown sink depends on the assumed magnitude of
known sources and sinks-by definition, the unknown sink is simply: sources minus sinks minus
atmospheric accumulation.
For our base cases, the size of the unknown sink was kept constant at 1.6 Pg annually based
on its calculated value (from the model) for 1975-1985. However, alternative assumptions are
plausible. To capture these uncertainties, two sensitivities were analyzed: (1) a high case, where the
size of the unknown sink increases at the same rate as atmospheric CO2 levels compared with
preindustrial levels (this increase might occur, e.g., because the size of the unknown sink is related
to the fertilization of terrestrial ecosystems by increasing CO2); and (2) a low case, where the size
decreases to zero exponentially at 2% per year (e.g., because the process responsible for the unknown
sink has a limited capacity).
When the unknown sink is assumed to increase in proportion to CO2 concentrations in the
RCW case, the amount of carbon absorbed by the unknown sink increases to 11.6 Pg annually by
2100. This rate of carbon absorption results in a decline in CO2 concentrations relative to the
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Policy Options for Stabilizing Global Climate -- Review Draft Chapter VI
Rapidly Changing World, which reduces realized warming by 0.1-0.2°C in 2050 and 0.5-0.7°C in 2100;
equilibrium warming is reduced in 2050 by 0.2-0.5°C and in 2100 by 0.7-1.4°C (based on 2.0-4.0°C
climate sensitivities).
In the low case, that is, when the unknown sink decreases to zero, the estimated impact on
warming is significantly lower, since the unknown sink was only 1.6 Pg annually to start. As a result,
CO2 concentrations do increase, but the increase in realized warming is less than 0.1°C in 2050 and
0.1-0.2°C in 2100 (based on 2.0-4.0°C climate sensitivities; see Figure 6-11).
Amount of CO2 From Deforestation
Estimates of the amount of CO2 emitted from deforestation activities vary due to different
assumptions on the rate of deforestation, the fate of the deforested lands, and the amount of carbon
contained in the forest vegetation and soils. In the base cases we used the lower carbon estimates
(i.e., lower biomass estimates) given by Houghton (1988); for 1980 the resulting net flux of carbon
to the atmosphere was about 0.4 Pg of carbon. Higher estimates of initial biomass have also been
analyzed by Houghton (1988); with these estimates the net flux of carbon to the atmosphere in 1980
would have been about 2.2 Pg. These higher biomass estimates are evaluated here for the three
deforestation scenarios discussed in Chapter V. The net flux of carbon for each of these scenarios
is presented in Figure 6-12.
In the RCW case the rate of CO2 emissions from deforestation was based on an exponential
decline in forest area using the lower biomass assumptions. If the higher biomass estimates are used,
the total carbon flux from deforestation from 1980 to 2100 is 281 Pg compared with 118 Pg using the
low estimates of carbon stocks (Houghton 1988). Similarly, in the population-based deforestation
scenario the total carbon flux to the atmosphere from 1980 to 2100 is about 138 Pg using the lower
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Policy Options for Stabilizing Global Climate •- Review Draft
Chapter VI
FIGURE 6-11
IMPACT ON REALIZED WARMING DUE TO
SIZE OF UNKNOWN SINK
(Degrees Celsius; Based on 3.0 Degree Sensitivity)
v>
UJ
Ul
cc.
a
UJ
a
1985 2000
2025
2050
YEAR
2075
2% Decline
RCW
Proportional
Increase
2100
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Chapter VI
FIGURE 6-12
C02 FROM DEFORESTATION ASSUMING HIGH BIOMASS
(Petagrams of Carbon/Year)
3 -
O
CO
cc.
«t
O
u.
O
<£
a
«t
t-
UJ
CL
-1
SCW /
//I
/ I
RCW
Stabilizing
Policy Scenarios
I
1950 1980
2010 2040
YEAR
2070 2100
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biomass estimates and 324 Pg using the higher biomass estimates. In the reforestation scenario, the
total accumulation of carbon from the atmosphere was 38 Pg using the lower biomass estimates and
59 Pg using the higher biomass estimates.
Despite the substantial increase in the amount of carbon from deforestation when the higher
biomass estimates are used (e.g., by 2050 CO2 emissions from deforestation are 2.3 Pg compared
with 1.0 Pg in the RCW with the lower estimates), the resulting atmospheric concentration of CO2
is only slightly higher (see Figure 6-13 for the differences in the RCW case, i.e., forest area declines
exponentially). This result is due to the larger size of the "unknown carbon sink" in our model when
higher deforestation emissions are assumed (see Unknown Sink above). In our analysis the increase
in the size of the unknown sink was sufficient to absorb some of the additional carbon when the
higher biomass estimates are used, assuming that the size of the unknown sink remains constant at
its average 1975-1985 value (i.e., 2.6 Pg C with high biomass vs. 1.6 Pg C with low biomass). The
additional increase in CO2 increased realized warming and equilibrium warming less than 0.1°C by
2100 compared with the RCW case warming (assuming 2.0-4.0°C climate sensitivities).
Alternative CO2 Models of Ocean Chemistry and Circulation
In the RCW case ocean chemistry was represented using a diffusion model of the ocean (the
Modified GISS model) based on the model described by Hansen et al. (1988). Several other
approaches have also been developed and adopted for the EPA framework by W. Emmanuel and B.
Moore. These include:
• Box-Diffusion Model introduced by Oeschger et al. (1975), which represents the
turnover of carbon below 75 meters as a purely diffusive process.
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Chapter VI
FIGURE 6-13
IMPACT OF HIGH BIOMASS ASSUMPTIONS ON
ATMOSPHERIC CONCENTRATION OF C02
(Parts Per Million; Based on 3.0 Degree Celsius Sensitivity)
1000
800 -
UJ
a.
v>
-------
Policy Options for Stabilizing Global Climate -- Review Draft Chapter VI
• 12-Compartment Regional Model by Bolin et al. (1983), which divides the Atlantic
and Pacific-Indian Oceans into surface-, intermediate-, deep-, and bottom-water
compartments and divides the Arctic and Antarctic Oceans into surface- and deep-
water compartments.
• Advective-Diffusive Model by Bjorkstrom (1979), which divides the surface ocean into
cold and warm compartments; water downwells directly from the cold surface
compartment into intermediate and deep layers.
• Outcrop-Diffusion Model by Siegenthaler (1983), which allows direct ventilation of
the intermediate and deep oceans at high latitudes by incorporating outcrops
connecting all sublayers to the atmosphere.
Because each of these models uses a different approach to evaluate ocean chemistry, the
resulting impact on atmospheric CO2 concentrations could vary from one approach to the next. To
determine how comparable these models were, the RCW case was evaluated using each model in
turn.
The estimates of future CO2 concentrations from each model are summarized in Figure 6-
14a. These results indicate that the Modified GISS model tends to project higher atmospheric CO2
concentrations than the other models; for example, by 2100 CO2 concentrations are about 6-7%
higher than concentrations estimated by Oeschger et al., Bolin et al., or Bjorkstrom, and about 19%
higher than those estimated by Siegenthaler. There are two basic reasons for these differences: (1)
The Modified GISS model, unlike the other models, incorporates temperature feedback that alters
ocean carbonate chemistry; that is, as the mixed layer of the oceans warms due to atmospheric
warming, the amount of carbon that can be absorbed by the oceans decreases; and (2) The Modified
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GISS model does not incorporate any heat or CO2 transfer between the thermocline and the deep
ocean (below 1,000 meters); to the extent heat or CO2 is transported to the ocean depths in the long
run, the Modified GISS model understates the oceans' absorption capacity.
Siegenthaler's Outcrop-Diffusion Model estimates lower CO2 concentrations than any of the
other models. This result is anticipated because the Outcrop-Diffusion Model allows CO2 to be
absorbed from the atmosphere to the deep layers rather than diffuse through the intervening layers,
so that, in this model, carbon is absorbed more quickly in the oceans than in the other models. By
2100 equilibrium warming using Siegenthaler's model is 0.6-1.2°C lower than the RCW case (see
Figure 6-14b for warming estimates from all five models).
ASSUMPTIONS ABOUT CLIMATE SENSITIVITY AND TIMING
Sensitivity of the Climate System
A general benchmark for comparing atmospheric models is their response to a doubling of
CO2 concentrations (2xCO2; see Chapter III). Put simply, this benchmark describes how much
warming would be expected once the atmosphere stabilizes following a two-fold increase in CO2
concentrations. In our analyses we have used the range from 2.0-4.0°C. As discussed in Chapter
III, there is a great deal of uncertainty about the strength of internal climate feedbacks, and, in some
cases, whether a feedback will be positive or negative. If cloud and surface albedo changes produce
large positive feedbacks, as suggested by some analyses, the climate sensitivity could be 5.5°C or
greater. On the other hand, these feedbacks could be weak and cloud feedbacks could be negative,
resulting in a climate sensitivity as low as 1.5°C. For the sensitivity analysis, therefore, we have
evaluated the extent of global warming using 1.5 and 5.5°C as lower and upper bounds, respectively.
The impact of these assumptions on realized warming is summarized in Figure 6-15 for the RCW
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Chapter VI
FIGURE 6-14
COMPARISON OF DIFFERENT OCEAN MODELS
CO2 CONCENTRATIONS
(Parts Per Million)
z
o
900
800
d 700
a.
LU
o- 600
500
400
300
Siegenthaler
Oeschger
Bolin
Bjorkstrom
1985 2000
2025
2050
YEAR
2075
2100
IMPACT ON EQUILIBRIUM WARMING
(Based on 3.0 Degree Celsius Sensitivity)
2
v>
ui
o
w .
u 4
UJ
cc.
o
ui
a
Siegenthaler
Oeschger
Bolin
Bjorkstrom
1985 2000
2025
2050
YEAR
2075
2100
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Chapter VI
FIGURE 6-15
IMPACT OF CLIMATE SENSITIVITY ON
REALIZED WARMING
(Degrees Celsius; 1.5-5.5 Degree Climate Sensitivity)
Slowly Changing World Scenario
Rapidly Changing World Scenario
1985 2000 2025 2050 2075 2100 1985 2000 2026 2050 2075 2100
YEAR YEAR
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and SCW cases. In the RCW case, the range of realized warming for a 1.5-5.5°C climate sensitivity
would be 1.5-3.2°C by 2050 and 2.9-6.6°C by 2100, compared with a range of 1.9-2.8°C by 2050 and
3.6-5.6°C by 2100 when the climate sensitivity is bounded by 2.0-4.0°C. The corresponding values
for equilibrium warming for a 1.5-5.5'C climate sensitivity are 2.0-7.4°C by 2050 and 3.6-13.2°C by
2100, compared with 2.7-5.4°C by 2050 and 4.8-9.6°C by 2100 for a 2.0-4.0°C climate sensitivity. In
the SCW case, the range of realized warming for a 1.5-5.5°C climate sensitivity would be 1.3-2.9°C
by 2050 and 2.0-4.7°C by 2100, compared with a range of 1.6-2.5°C by 2050 and 2.5-4.0°C by 2100
when the range of climate sensitivity is 2.0-4.0°C. The corresponding values for equilibrium warming
for a 1.5-5.5°C climate sensitivity are 1.7-6.1°C by 2050 and 2.4-8.6°C by 2100, compared with 2.2-
4.5°C by 2050 and 3.1-6.3°C for a 2.0-4.0°C climate sensitivity.
Rate of Heat Diffusion
CO2 and heat are currently transferred from the atmosphere to the oceans and within the
ocean itself as a result of many complex chemical and physical interactions. One of these interactions
is the transfer of heat from the mixed layer to the thermocline, thereby delaying global warming.
Additionally, changes in ocean mixing and circulation patterns as a result of climate change could
alter the capacity of the oceans to absorb heat (see BIOGEOCHEMICAL FEEDBACKS for further
discussion). The rate at which heat is absorbed only affects the rate of realized warming, not the
rate of equilibrium warming, because the oceans cannot absorb heat indefinitely.
In our model the rate at which mixing occurs between the mixed layer and the thermocline
is parameterized with an eddy-diffusion coefficient (see Chapter III). The value of the eddy-diffusion
coefficient in the base cases was assumed to be 0.55 x 10"4 m2/sec. For purposes of this sensitivity
analysis alternative values of 2 x 10'5 and 2 x 10"4 have been evaluated.
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As shown in Figure 6-16 the rate at which the oceans absorb heat can noticeably affect the
amount of realized warming by 2100. If the rate of heat absorption is greater than that assumed in
the base cases (i.e., if the eddy-diffusion coefficient is 2 x 10"4 m2/sec), realized warming by 2100
would be 0.5-1.2°C less than in the RCW case (assuming 2.0-4.0°C climate sensitivities). For the
smaller eddy-diffusion coefficient of 2 x 10"5 m2/sec, realized warming by 2100 would be 0.3-0.9°C
higher.
ASSUMPTIONS ABOUT ATMOSPHERIC CHEMISTRY: A COMPARISON OF MODELS
As discussed in Chapters II and III, the chemistry of the future troposphere is one of the
uncertainties in the prediction of atmospheric composition. The principal factors contributing to this
uncertainty are: (1) the complexity and tremendous natural variability of chemistry in the
troposphere, especially regarding oxidant formation; (2) the range of interactions between tropospheric
chemistry and radiation perturbed by climate change and changes in stratospheric composition; and
(3) the range of uncertainties in future emissions of CH4> CO, NOn and non-methane hydrocarbons
(NMHC). This section focuses on the first two aspects of uncertainty in atmospheric composition.
Recognizing the uncertainty in tropospheric chemistry, EPA sponsored a workshop on
atmospheric composition to discuss recent modelling efforts among members of the atmospheric
sciences community and to construct a parameterized atmospheric chemistry model that would
incorporate the latest scientific findings. The end result was the Assessment Model for Atmospheric
Composition (AMAC), the model used to obtain the findings discussed in this report. AMAC was
developed by Prather of NASA/GISS as a result of the workshop, which was held in January 1988
(see Prather, 1988). To obtain insight into the uncertainties introduced by the physical simplifications
made by the AMAC and to ensure results that are comparable to current, more-detailed modeling
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Chapter VI
FIGURE 6-16
4 -
3 -
v>
55
_i
UJ
o
v>
UJ
O 2
UJ '
1 -
CHANGE IN REALIZED WARMING
DUE TO RATE OF OCEAN HEAT UPTAKE
(Degrees Celsius; Based on 3.0 Degree Sensitivity)
1985 2000
2025
2050
2075
YEAR
2x 10
RCW
-5
2x 10
-4
2100
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efforts, a set of common scenarios of CH4, CO, and NOX emissions were analyzed in the AMAC,
as well as in two current research models: a 2-D tropospheric chemistry model developed by Isaksen
(Isaksen and Hov, 1987); and a multi-box photochemical model of the global troposphere developed
by Thompson and co-workers at NASA/Goddard (Thompson et al., 1988).
Model Descriptions
Each of these models is briefly described below.
Assessment Model for Atmospheric Composition
The focus of interest in tropospheric composition is on O3, CH<, and OH, because the two
former gases are key greenhouse absorbers and OH (together with ozone) determines the oxidizing
capacity of the atmosphere and the abundance of many gases such as methane, carbon monoxide,
methyl chloroform, and HCFC-22 (CHF2C1).
For the simulation of the troposphere in this model, the Northern and Southern Hemispheres
(NH & SH) are treated separately because significant asymmetries are observed in many of the
important shorter-lived gases, such as CO and NO,. These species play a major role in the budgets
for CH4, O3, and OH in each hemisphere.
In the AMAC, tropospheric OH can be treated as a steady-state variable as it responds
immediately to the annual average values of the trace gases. To derive perturbations to OH, a non-
linear system is solved equating a "production" term to a "loss" term. OH losses are partitioned
among the predicted gases (CH4, CO), the specified fluxes (NMHC), and self-reactions (OH). The
production side of the equation includes a positive response to increases in UV radiation (i.e., loss
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in column ozone) and in tropospheric H2O, O3 and NOX fluxes. Coefficients for variations in either
the production or loss terms with respect to column O3) tropospheric water vapor, trop-O3, CO,
CH4, and fluxes of both NMHCs and NOX are based on results from 1-D and 2-D models (Liu et
al., 1987; Thompson and Cicerone, 1986; Is'aksen and Hov, 1987; Isaksen et al., 1988). Major sources
of uncertainty in calculating OH are the spatial averaging for this highly variable constituent and the
nonlinearity in perturbation coefficients, especially with respect to NOX distribution.
Perturbations to tropospheric ozone affect both tropospheric temperatures and the long-lived
source gases controlled by OH. A significant fraction of tropospheric ozone originates in the
stratosphere and is destroyed by surface deposition; it is sufficiently short-lived (a few months) that
the AMAC calculates ozone perturbations separately for each hemisphere. Changes in tropospheric
ozone are associated with perturbations to the total ozone column, and to tropospheric chemical
reactions, which are evaluated with sensitivity coefficients, dln(O3)/dln(X), ascribed to the precursor
gases (Column -O3, 0.8; CH4, O.2; CO, O.I; NO., flux, O.I; NMHC flux, O.I). The coefficients are
based on detailed photochemical models for typical tropospheric air parcels (Liu et al., 1987;
Thompson et al., 1988), but their uncertainties are large, approximately a factor of 2. Also, the
efficiency of O3 production varies widely with the NO, levels (Liu et al., 1987), which in turn cannot
be adequately characterized throughout the entire troposphere due to their large dynamic range. A
similar concern applies to the simplified treatment of non-methane hydrocarbons.
Isaksen Model
The Isaksen model is a 2-D transport model that calculates absolute concentrations for O3
and OH (and several dozen other trace chemical constituents in the troposphere) as functions of
altitude and latitude, as emissions are varied over time (Isaksen and Hov, 1987; Isaksen et al., 1988).
Unlike the AMAC, this model resolves latitudinal and altitude distributions, and emission changes
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are introduced with latitudinal discrimination. The transport of longer-lived constituents can locate
key areas of tropospheric ozone and OH change that the AMAC will miss; because a 2-D model
resolves altitude, the effects of high-altitude aircraft emissions on NOX and ozone or cloud
perturbations to radiation fields, for example, can be explored. The Isaksen model differs from the
AMAC in that the troposphere is not coupled to the stratosphere, so that the impact of changing
climate or perturbations on stratospheric ozone are not included. Methane flux changes are included
in annual updates of the model.
Thompson et al. Model
The Thompson model couples the result of 1-D model calculations of the time history for
eight chemically coherent global regions, which are then averaged to estimate net global changes.
A steady-state method is used: emissions are specified in simulations to represent conditions at 5-
year intervals. This is somewhat inadequate for lifetime changes, tending to underestimate
tropospheric ozone increases (up to 30% in one test case) and to overestimate increases in CH4
concentrations.
The description of chemically coherent regions offers insight into regional variability, a
feature lacking in the version of the Isaksen model used here, which does not have the longitudinal
variation needed to treat emissions that are restricted to the source area but that can have extensive
effects on ozone and OH. Like the Isaksen model, the Thompson model includes a more complete
set of chemical constituents than does the AMAC and can identify other effects and interactions of
climate perturbation. For example, the oxidants that contribute to sulfuric acid formation in clouds
and rain (HO2 and H2O2) are very sensitive to changes in stratospheric ozone and tropospheric
water vapor. At the 2035 year point, stratospheric ozone depletion and climate change (temperature
and water vapor) effects are added to calculations with perturbed CH4, CO, and NO,, emissions.
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Results from the Common Scenarios
It is not easy to compare the models because the structure, input, and derived quantities
from the three models are not treated comparably. Nevertheless, insights into uncertainties can be
obtained by comparing selected results from each model. EPA supplied eight scenarios of alternative
estimates of CH4, CO, and NO, for evaluation in each model. In this section one of these scenarios
is discussed (EPA Scenario #2), which assumes low CH4, low CO, and high NO^ growth in sources,
a rapid growth scenario for CO2 and N2O from combustion, and a CFC and halon scenario consistent
with the Montreal Protocol. Table 6-4 summarizes the emission estimates for this scenario and
compares them to estimates from the Rapidly Changing World (RCW) and Slowly Changing World
(SCW) scenarios (Appendix C provides more detail for all eight scenarios). The RCW and SCW
cases could not be explicitly included for this model comparison because the development of these
cases occurred simultaneously with the model comparison. Table 6-5 summarizes the results for all
eight scenarios.
The EPA #2 emission estimates are in the same range as those of the other two cases
except for CO emissions. These estimates are much lower than both the RCW and SCW cases and
are similar to the Stabilizing Policy cases due to stringent control assumptions on transport vehicle
emissions. For the other emissions, the CO2 emission estimates in EPA #2 fall between the RCW
and SCW cases for most of the time periods, approaching the RCW estimates by 2100. The CH4
and NO, estimates are similar to those for the SCW case, except that NO,, estimates after 2050 fall
between the RCW and SCW estimates.
The AMAC's troposphere is basically a parameterized 2-box model: it reports mean
tropospheric values (ppb) for CH4, and separate perturbations (% change) to OH and O3 in each
hemisphere. For the global average perturbation to OH and O3, Northern and Southern Hemisphere
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Chapter VI
TABLE 6-4
Comparison of Emission Estimates For EPA #2,
RCW, and SCW cases
(in teragrams, unless indicated otherwise)
Emissions Estimates bv Year
Trace Gas
C02 (Pg C)
EPA #2
RCW
SCW
CO (as C)
EPA #2
RCW
SCW
CH,
EPA #2
RCW
SCW
NOX (as N)
EPA #2
RCW
SCW
1985
6.3
5.9
5.9
316
502
502
500
510
510
59
53
53
2000
7.3
7.6
7.2
226
571
616
548
577
569
59
60
59
2025
10.4
11.5
9.2
194
699
842
640
712
676
64
73
69
2050
13.7
16.6
9.8
192
895
859
721
880
740
72
92
71
2075
17.9
22.2
9.7
178
1050
595
779
1025
773
82
108
65
2100
25.2
25.5
11.4
192
1207
604
809
1089
816
104
118
70
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Chapter VI
TABLE 6-5
Comparison of Results From Atmospheric Chemistry Models for the Year 2050
Compared to 1985
Model
Increases in Methane (ppb)
Low NO,,
Prather
Isaksen
Thompson et al.
High NO,
Prather
Isaksen
Thompson et al.
Percent Change in CO
Low NOX
Prather
Isaksen
Thompson et al.
High NOX
Prather
Isaksen
Thompson et al.
LOW
Low CO
EPA#1
806
400
750
EPA#2
801
350
870
EPA#1
16
-13
-10
EPA#2
17
-9
-2.4
Test Case
CH4
High CO
EPA#3
1031
720
1000
EPA#4
1048
550
1240
EPA#3
43
8
25
EPA#4
44
8
58
Results
HIGH C
Low CO
EPA#5
1750
950
1710
EPA#6
2082
1010
2210
EPA#5
55
0
16
EPA#6
70
8
35
High CO
EPA#7
2022
1220
1890
EPA#8
2242
1200
2560
EPA#7
82
17
52
EPA#8
99
19
97
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Chapter VI
TABLE 6-5 (continued)
Comparison of Results From Atmospheric Chemistry Models for the Year 2050
Compared to 1985
Model
Percent Change in OH
Low NOX
Prather
Isaksen
Thompson et al.
High NOX
Prather
Isaksen
Thompson et al.
Percent Change in O3
Low NO,
Prather
Isaksen
Thompson et al.
High NOX
Prather
Isaksen
Thompson et al.
LOW
Low CO
EPA#1
-9
5
-9.4
EPA#2
- 2
8
-8.9
EPA#1
1
-1
3.7
EPA#2
5
5
10.0
Test Case
CH4
High CO
EPA#3
-14
- 1
-15.5
EPA#4
- 9
4
-17.4
EPA#3
8
2
9.9
EPA#4
13
8
17.5
Results
HIGH
Low CO
EPA#5
-23
1
-20.1
EPA#6
-22
5
-22.1
EPA#5
21
3
13.2
EPA#6
33
10
23.1
CH.
High CO
EPA#7
-26
- 2
-27.6
EPA#8
-25
3
-23.9
EPA#7
27
5
18.3
EPA#8
39
13
29.0
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results are averaged with equal weight. In addition to the perturbed species discussed here with the
tropospheric chemical models, the AMAC calculated other significant perturbations, such as a 12%
decrease in column ozone, a 2°K rise in mean tropospheric temperature, along with a 10% increase
in tropospheric water vapor. These perturbations have an impact on tropospheric OH, O3, CO, and
CH4. Additionally, unlike the other two models, which provide point estimates, the AMAC produces
a range of trace-gas scenarios in response to specified uncertainties in the model coefficients (only
the mean of each range is provided in Table 6-5).
The Thompson model averages over eight "chemically coherent regions" (Appendix C gives a
description of how the EPA scenarios were assigned to the regions). This approach is probably
adequate for short-lived species such as OH, and possibly for tropospheric O3. However, it makes
it difficult to interpret CH4 calculations, which predict different CH4 concentrations among the boxes,
when in fact the long lifetime of CH4 ensures that it is well mixed throughout the troposphere. The
methane results in Table 6-5 have been averaged over the eight regions and scaled to account for
the CH4 lifetime changes occurring in the perturbed atmosphere. Also summarized are percent
changes in CO (surface mixing ratios) and OH and O3 (column-integrated from 0-15 km). The CH4
and CO changes obtained by the Thompson model are very similar to those obtained with the
AMAC. Although not shown in the global averages in Table 6-5, the most useful results of the
regional calculations are localized estimates of OH and O3 changes in each chemically defined region
where CO and NOj growth rates may differ considerably. The differences between areas with
controlled emissions (Urban 1) and without controlled emissions (Urban 2) are very striking (Figure
6-17).
The Isaksen model calculates perturbations as a function of latitude, altitude (0-16 km), and
time of year. The increase in CH4 is distributed uniformly throughout the troposphere as expected.
There is a problem with the implementation of the EPA #2 scenario in that the CH4 concentrations
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Chapter VI
FIGURE 6-17
REGIONAL DIFFERENCES FOR URBAN AREAS
WITH DIFFERENT EMISSIONS OF CO AND NO
Fraction Change: 1965-2050
Fraction Change: 1985-2050
-I
9.1
3
t.S
2
1.6
W
I '
i •!.»
0
-OA
-I
I
Fraction Chance: 1965-2060
Fraction Ch»nj«. 1085- 3050
UrUn-l
1 2 3 4 S « 7 6
Urban-2
Source: Thompson, 1988.
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decline at the beginning of the model integration. This may be due to the low estimate of global
CO flux. Initial fluxes were scaled in the AMAC and Thompson models to obtain a steady-state of
current concentrations. CH4 does not recover to its initial concentration for at least 20 years into
the scenario, and this is probably the reason for the Isaksen model predicting such a small increase
in CH4. The patterns for OH and O3 perturbations are distinct (Figure 6-18). The greatest changes
in O3 are below 2 km altitude: there is a large increase between 0° and 35°N and a small decrease
centered at 50°N. The spatial pattern of changes in OH are interesting: in the upper troposphere
between 12 and 16 km the OH increases by 10-30% in the Northern Hemisphere, whereas throughout
most of the Southern Hemisphere OH decreases. Both of these changes may be driven by increases
in CH4. In the dry upper troposphere in the presence of NOB CH4 increases the OH concentration
during its atmospheric oxidation, but in the lower troposphere the CH4 provides merely a sink for
OH.
Overall, all three models predict similar increases in tropospheric O3. The Thompson et al.
and AMAC models predict decreases in tropospheric OH, while the Isaksen model reports a globally
averaged increase. This discrepancy may be explained by the large increases in OH above 12 km as
noted above, something that is also calculated by the Thompson model. However, most of the
difference in OH levels seems attributable to the lower CO and CH4 concentrations calculated by
Isaksen compared with the other two models. As shown in Table 6-5 for all eight scenarios, none
of the increases in CO by 2050 are more than 15-20% in Isaksen, whereas Thompson et al. and
AMAC show CO increases up to 100% (see scenario #8). Some of the CO and OH differences
between Isaksen et al. and the other two models are due to the difference in initialization described
above, but most of the OH difference may be due to how CO behaves in each model. This may be
one of the more prominent uncertainties in predicting future tropospheric composition. CO has a
moderate lifetime (typically about a few months) with considerable spatial variability that is not well
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Chapter VI
FIGURE 6-18
OH AND OZONE PERTURBATIONS
IN THE ISAKSEN AND HOV MODEL
(Percent Change)
Ozone
is.oo-;
2.00-!
S3 80 10 50 SO
50 JO
OH
0 -<0 -20 -SO -*0 -SO -SO -10 -90 -SO
LATITUCE
30 SO 10 SO SO *0 SO 20 '0
3 -'0 -20 -SO ~*0 -50 -€0 -10 -90 -90
LATITUCE
Figure 6-18. Perturbation in O3 and OH from the Isaksen model using the EPA #2 emission
estimates. Solid line indicates an increase in the parameter; dashed line indicates a decrease. O3
shows large increase between 0° and 35°N; OH shows increases up to 30% in Northern Hemisphere,
and decreases in Southern Hemisphere. Source: Isaksen and Hov, 1987.
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resolved in any of the models. Perhaps the Isaksen model gives a lower limit to CO and OH
changes, and the other two models estimate the largest expected changes.
EVALUATION OF UNCERTAINTIES USING AMAC
Comparing the results of AMAC to other models given identical scenarios provides one approach
to evaluating uncertainties related to atmospheric chemistry. Valuable information can also be
obtained by testing the robustness of the AMAC results to changes in critical model parameters.
This section examines these impacts by varying key parameters within AMAC and then comparing
the results to the RCW scenario.
Atmospheric Lifetime of CFC-11
The assumed atmospheric lifetime for CFC-11 in the AMAC for the RCW case was 65 years.
Its atmospheric lifetime, however, may range from 55 to 75 years (Prather 1988); these estimates
were evaluated to determine the impact on atmospheric chemistry. The changes in atmospheric
concentration for CFC-11 are summarized in Figure 6-19, which indicates that concentration levels
may vary from about 650 to 810 ppt by 2100. Increases (decreases) in the atmospheric concentration
of CFC-11, however, tend to be offset by corresponding decreases (increases) in atmospheric
concentrations of other trace gases, such as other CFCs and CH4. That is, the increase (decrease)
in the lifetime of CFC-11 increases (decreases) the amount of stratospheric ozone depletion, which
increases (decreases) the amount of UV radiation; these higher (lower) UV levels increase (decrease)
the rate of destruction of these other gases. As a result, the impacts on global warming are
negligible (less than 0.1°C).
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Chapter VI
FIGURE 6-19
SENSITIVITY OF ATMOSPHERIC CONCENTRATION
OF CFC-11 TO ITS LIFETIME
(Parts Per Trillion; Based on 3.0 Degree Celsius Sensitivity)
800 -
700 -
600 -
O
-J 500 -
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Policy Options for Stabilizing Global Climate - Review Draft Chapter VI
Interaction of Chlorine with Column Ozone
Chlorine in the stratosphere has a negative, non-linear impact on total column ozone. This
chemical interaction is one of the primary causes of stratospheric ozone depletion due to the chlorine
contained in CFCs; this interaction has been included in the AMAC, however, primarily for its ability
to affect the rate of tropospheric ozone formation. In the RCW case this relationship was defined
as a 0.03% decline in total column ozone/(ppb)2 of stratospheric chlorine. A higher value, 0.20%,
was evaluated, which would increase the rate of column ozone destruction.
With the 0.20% assumption, total column ozone depletion was 45-47% by 2050 (assuming 2.0-
4.0°C climate sensitivities) compared with a total column ozone depletion of 16.8% with the lower
value (i.e., the -0.03% value used in the RCW case). The increase in total column ozone depletion
has a positive feedback on the tropospheric OH levels due to the increase in UV radiation. The
resulting OH interactions with other trace gases substantially reduces the atmospheric concentration
of CH4, HCFC-22, methyl chloroform, and methyl chloride, and reduces the rate of tropospheric
ozone formation. (The role of O3 is problematic, O3 at 10-12 km probably would increase. At this
altitude, O3 probably has the largest greenhouse effect. See Chapter II) These impacts reduce the
amount of global warming; as shown in Figure 6-20, the decline in realized warming is 0.1°C by
2050, compared with the RCW case, and 0.3-0.4°C by 2100; the decline in equilibrium warming by
2100 is 0.4-0.8°C (assuming 2.0-4.0°C climate sensitivities).
Sensitivity of Tropospheric Ozone to CH4 Abundance
Tropospheric ozone formation is affected by the amount of CH4 present, although the rate at
which tropospheric ozone forms as a result of CH4 abundance is subject to some uncertainty. In
the RCW case, this variable for the Northern Hemisphere was assumed to be a 0.2% change in
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Chapter VI
FIGURE 6-20
CHANGE IN REALIZED WARMING
DUE TO RATE OF INTERACTION OF CLx WITH OZONE
(Degrees Celsius; Based on 3.0 Degree Sensitivity)
w
UJ
i
g
1986 2000
2025
2050
YEAR
2075
RCW
CLx/Ozone
Interaction
2100
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tropospheric ozone for each percentage change in CH4 concentration; other evidence suggests that
a higher value, 0.4%, is possible (Prather, 1988).
Using this higher value increases the change in tropospheric ozone in 2100 by about 50% over
the RCW case (tropospheric ozone increases by about 63% compared with about 43% when a value
of 0.2% is assumed). The increase in tropospheric ozone indirectly results in a decrease in CH4
concentrations since the tropospheric ozone increase also increases OH formation, which destroys
CH4. Due to this partially offsetting effect, the increase in global warming is less than 0.1°C.
Sensitivity of OH to NOX
Tropospheric OH formation is affected by the level of NO, emissions, although the rate of OH
formation is uncertain. In the RCW case, we assumed a 0.1% OH change for every 1.0% change
in NO, emissions for the Northern Hemisphere. We evaluated a range of uncertainty from 0.05%
to 0.2%.
An increase (decrease) in the amount of OH due to a higher (lower) sensitivity to NOX emissions
results in less (more) tropospheric ozone formation as well as lower (higher) levels of CO and CH<.
The higher sensitivity value of 0.2% reduces realized warming about 0.1"C by 2100 compared with
the RCW case (assuming 2.0-4.0°C climate sensitivities; equilibrium warming is about 0.2°C lower by
2100), while the lower sensitivity value of 0.05% increases realized warming less than 0.1°C by 2100
(equilibrium warming increases a maximum of 0.1°C by 2100).
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BIOGEOCHEMICAL FEEDBACKS
The sensitivity of the climate system to anthropogenic perturbations is determined by a
combination of feedbacks that amplify or dampen the direct radiative effects of increasing
concentrations of greenhouse gases. Several important internal climate feedbacks, such as those
resulting from changes in water vapor, clouds, and sea ice albedo, are included in the estimates of
climate sensitivity discussed throughout this Report. There are a number of feedbacks of a
biogeochemical origin, however, that may also play an important role in climatic change that were
not included in the analyses on which this range is based. Biogeochemical sources of feedback
include releases of methane hydrates; changes in ocean chemistry, biology, and circulation; and
changes in the albedo of the global vegetation.
Any attempt to quantify the impact of biogeochemical feedbacks is necessarily quite speculative
at this time; however, it does appear that they could have an important impact on global climate.
For example, Lashof (1989) has estimated that the gain from biogeochemical feedbacks ranges from
0.05-0.29 compared with a 0.17-0.77 gain from internal climate feedbacks. (The gain is defined as
the portion of global equilibrium temperature change attributable to the feedback divided by the total
global equilibrium temperature when the feedback is included). Some of these key feedbacks were
incorporated into the AMAC for these sensitivity cases to determine the magnitude of their impact
on global warming.
Ocean Circulation
As mentioned above, the oceans are currently a major sink for heat and CO2. Concerns have
been raised, however, that the basic circulation patterns that allow these processes to continue could
be significantly altered as the global climate changes. This possibility is suggested by the rapid rate
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of atmospheric CO2 change during past periods of climate change (e.g., see Chapter III). If
circulation patterns did change, it is plausible that the oceans would no longer be a net sink for heat
and CO2.
It is not known at what point ocean circulation would be altered. For this analysis we assumed
that a 2°C increase in realized warming would alter ocean circulation patterns sufficiently to shut off
net uptake of CO2 and heat by the oceans. This would increase atmospheric CO2 concentrations
from 10-25% by 2100, and would reduce the difference between realized and equilibrium warming
as the atmosphere warmed more quickly due to the oceans' inability to continue to act as a heat
sink. As shown in Figure 6-21, this feedback is sufficient to increase realized warming up to 1.4°C
by 2050 and 1.3-3.5°C by 2100 compared with the warming estimated for the RCW case.
Methane Feedbacks
Increases in global temperature could increase the amount of CH4 emissions due to several
feedback processes: (1) release of methane from hydrates, which are methane compounds contained
in continental slope sediments, as increasing temperatures destabilize the formations; (2) additional
methane from high-latitude bogs due to longer growing seasons and higher temperatures; and (3)
increased rate of methanogenesis from rice cultivation. The amount of CH4 that could be released
from each of these feedback processes, and the rate at which any releases might occur, are highly
speculative. For each process we have assumed that the rate of CH4 release is linearly related to
the increase in temperature, with each 1°C increase leading to an additional 110 Tg from methane
hydrates, 12 Tg from bogs, and 7 Tg from rice cultivation (Lashof, 1989). These methane feedbacks
could have a major impact on atmospheric CH4 concentrations: by 2100 concentrations would
increase to about 7000-8350 ppb, compared with 4150-4400 ppb in the RCW case. As shown in
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Chapter VI
FIGURE 6-21
CO
3
55
UJ
o
v>
ui
iu
o
INCREASE IN REALIZED WARMING
DUE TO CHANGE IN OCEAN CIRCULATION
(Degrees Celsius; Based on 3.0 Degree Sensitivity)
1985 2000
2025
2050
YEAR
Ocean
Circulation
RCW
2075
2100
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Figure 6-22, this increase in CH4 would be sufficient to increase realized warming relative to the
RCW case about 0.2-0.4°C by 2050 and 0.4-1.0°C by 2100 (assuming 2.0-4.0°C climate sensitivities).
Combined Feedbacks
In addition to the two separate feedbacks discussed above, we analyzed the combined impact
of several types of biogeochemical feedbacks. The following specific feedbacks were included: (1)
methane from hydrates, bogs, and rice cultivation, as previously discussed; (2) increased stability of
the thermocline, thereby slowing the rate of heat and CO2 uptake by the deep ocean by 30% due
to less mixing; (3) vegetation albedo, which is a decrease in global albedo as a result of changes in
the distribution of terrestrial ecosystems by 0.06% per 1°C warming; (4) disruption of existing
ecosystems, resulting in transient reductions in biomass and soil carbon at the rate of 0.5 Pg C per
year per 1°C warming; and (5) CO2 fertilization, which is an increase in the amount of carbon stored
in the biosphere in response to higher CO2 concentrations at the rate of 0.3 Pg C per ppm. See
Lashof (1989) for further discussion.
The combined impact of these feedbacks on realized warming is an increase of 0.4-0.9°C by
2050 and 0.8-2.5°C by 2100 relative to the RCW case (assuming 2.0-4.0°C climate sensitivities; see
Figure 6-23); the increase in equilibrium warming is 0.4-1.7°C by 2050 and 0.7-3.2°C by 2100. These
preliminary analyses strongly suggest that biogeochemical feedbacks could have a major impact on
the rate of climatic change during the next century.
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Chapter VI
FIGURE 6-22
5 -
4 -
M 3 _
LLJ
U
W
\u
1U
oc
o
IU
o
2 -
INCREASE IN REALIZED WARMING
DUE TO METHANE FEEDBACKS
(Degrees Celsius; Based on 3.0 Degree Sensitivity)
1985 2000
2025
2050
2075
YEAR
Methane
Feedbacks
RCW
2100
DRAFT - DO NOT QUOTE OR CITE VI-76
February 21, 1989
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Policy Options for Stabilizing Global Climate - Review Draft
Chapter VI
FIGURE 6-23
INCREASE IN REALIZED WARMING
DUE TO CHANGE IN COMBINED FEEDBACKS
(Degrees Celsius; Based on 3.0 Degree Sensitivity)
v>
tu
o
(A
IU
O
£
1965 2000
2026
2050
YEAR
2075
Combined
Feedbacks
RCW
2100
DRAFT - DO NOT QUOTE OR CITE VI-77
February 21, 1989
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Policy Options for Stabilizing Global Climate - Review Draft Chapter VI
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