430S06001
       United States
       Environmental Protection
       Agency
                                                April 2006
  Executive
        Summary
        of the Inventory of U.S. Greenhouse Gas Emissions and Sinks:  1990-2004
           Central to any study of climate change is the development of an emissions inventory that identifies and quantifies
           a country's primary anthropogenic1 sources and sinks of greenhouse gases. This inventory adheres to both
           (1) a comprehensive and detailed methodology for estimating sources and sinks of anthropogenic greenhouse
gases, and (2) a common and consistent mechanism that enables Parties to the United Nations Framework Convention on
Climate Change (UNFCCC) to compare the relative contribution of different emission sources and greenhouse gases to
climate change.
    In 1992, the United States  signed and ratified the UNFCCC. As stated in Article 2 of the UNFCCC, "The ultimate
objective of this Convention and any related legal instruments that the Conference of the Parties may adopt is to achieve, in
accordance with the relevant provisions of the Convention, stabilization of greenhouse gas concentrations in the atmosphere
at a level that would prevent dangerous anthropogenic interference with the climate system. Such a level should be achieved
within a time-frame sufficient to allow ecosystems to adapt naturally to climate change, to  ensure that food production is
not threatened and to enable economic development to proceed in a sustainable manner."2
    Parties to the  Convention,  by ratifying,  "shall develop, periodically update, publish and  make available... national
inventories of anthropogenic emissions by sources and removals by sinks of all greenhouse gases not controlled by the
Montreal Protocol, using comparable methodologies	"3 The United States views the Inventory report as an opportunity
to fulfill these commitments.
    This chapter summarizes the latest information on U.S. anthropogenic greenhouse gas emission trends from 1990 through
2004. To ensure that the U.S. emissions inventory is comparable to those of other UNFCCC Parties, the estimates presented here
were calculated using methodologies consistent with those recommended in the Revised 1996IPCC Guidelines for National
                                              Greenhouse Gas Inventories (IPCC/UNEP/OECD/IEA 1997), the
                                              IPCC Good Practice Guidance and Uncertainty Management in
  All materialtaken from the Inventory of U.S.     National Greenhouse Gas Inventories (IPCC 2000), and the IPCC
  GreenhOUSe GaS  EmiSSionS and Sinks:     Good Practice Guidance for Land Use, Land-Use Change, and
   1990-2004, U.S. Environmental Protection
  Agency,  Office of Atmospheric Programs,
  EPA 430-R-06-002, April  2006. You may
  electronically download the full Inventory
  report from U.S.  EPA's Global Warming
  web page at: www.epa.gov/globalwarming/
  publications/emissions.
1 The term "anthropogenic," in this context, refers to greenhouse gas emissions
and removals that are a direct result of human activities or are the result of natural
processes that have been affected by human activities (IPCC/UNEP/OECD/IEA
1997).

2 Article 2 of the Framework Convention on Climate Change published by the
UNEPAVMO Information Unit on Climate Change. See .

3 Article 4(1 )(a) of the United Nations Framework Convention on Climate Change
(also identified in Article 12). Subsequent decisions by the Conference of the
Parties elaborated the role of Annex I Parties in preparing national inventories. See

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Box ES-1: Recalculations of Inventory Estimates

      Each year, emission and sink estimates are recalculated and revised for all years in the Inventory of U.S. Greenhouse Gas Emissions and
  Sinks, as attempts are made to improve both the analyses themselves, through the use of better methods or data, and the overall usefulness
  of the Inventory report. In this effort, the United States follows the IPCC Good Practice Guidance (IPCC 2000), which states,  regarding
  recalculations of the time series, "It is good  practice to recalculate historic emissions when methods  are changed or refined, when new
  source categories are included in the national inventory, or when errors in the estimates are identified and corrected."
      In each Inventory report, the results of all methodology changes and historical data updates are presented in the "Recalculations and
  Improvements" chapter; detailed descriptions of each recalculation are contained within each source's  description contained in the report,
  if applicable. In general, when methodological changes have been implemented, the entire time series has been recalculated to reflect the
  change,  per IPCC Good Practice Guidance.  Changes in historical data are generally the result of changes in statistical data supplied by
  other agencies. References for the data are provided for additional information.
Forestry (IPCC 2003). The structure of the Inventory report
is consistent with the UNFCCC guidelines for inventory
reporting.4 For most source categories, the Intergovernmental
Panel on Climate Change  (IPCC) methodologies were
expanded, resulting in a more comprehensive and detailed
estimate of emissions.

ES.1.  Background  Information

    Naturally occurring greenhouse gases include water
vapor, carbon dioxide (CC«2), methane (CHO, nitrous oxide
(N2O), and ozone (63). Several classes of halogenated
substances that contain fluorine, chlorine, or bromine are also
greenhouse  gases, but they are, for the most part, solely a
product of industrial activities. Chlorofluorocarbons (CFCs)
and hydrochlorofluorocarbons (HCFCs) are halocarbons that
contain chlorine, while halocarbons that contain bromine
are referred to as bromofluorocarbons (i.e.,  halons). As
stratospheric ozone depleting substances,  CFCs, HCFCs,
and halons  are covered under the Montreal Protocol on
Substances  that Deplete the Ozone Layer. The UNFCCC
defers to this earlier international treaty. Consequently,
Parties are not required to include these gases in their national
greenhouse  gas emission inventories.5 Some other fluorine-
containing  halogenated substances—hydrofluorocarbons
(HFCs),  perfluorocarbons (PFCs), and sulfur hexafluoride
(SF6)—do not deplete stratospheric ozone but are potent
greenhouse  gases. These latter substances are addressed by
the UNFCCC and accounted for in national greenhouse gas
emission inventories.
    There are also several gases that do not have a direct
global warming effect but indirectly affect terrestrial and/or
solar radiation absorption by influencing  the formation or
destruction of greenhouse gases, including tropospheric and
stratospheric ozone. These gases include carbon monoxide
(CO), oxides of nitrogen (NOX), and non-CtLt volatile organic
compounds (NMVOCs). Aerosols, which are extremely small
particles or liquid droplets, such as those produced by sulfur
dioxide (862) or elemental carbon emissions, can also affect
the absorptive characteristics of the atmosphere.
    Although the  direct  greenhouse gases CC>2, CFLt, and
N2O occur naturally in the atmosphere,  human activities
have  changed  their atmospheric  concentrations.  From
the pre-industrial  era  (i.e., ending about 1750) to  2004,
concentrations  of  these greenhouse gases have increased
globally by 35, 143, and 18 percent, respectively (IPCC
2001, Hofmann 2004).
    Beginning in  the  1950s, the use of CFCs and other
stratospheric ozone depleting substances  (ODS) increased
by nearly 10 percent per year until the mid-1980s, when
international concern about ozone depletion led to the
entry into force of the Montreal Protocol. Since then, the
production of ODS is being phased out. In recent years, use
of ODS substitutes such as HFCs and PFCs has grown as
they begin to be phased in as replacements for CFCs and
4 See .
5 Emissions estimates of CFCs, HCFCs, halons and other ozone-depleting substances are included in the annexes of the Inventory report for informational
purposes.
2  Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004

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HCFCs. Accordingly, atmospheric concentrations of these
substitutes have been growing (IPCC 2001).

Global Warming  Potentials
    Gases in the atmosphere can contribute to the greenhouse
effect both directly and indirectly. Direct effects occur when
the gas itself absorbs radiation. Indirect radiative forcing
occurs when chemical transformations of the substance
produce other greenhouse gases, when a gas influences
the atmospheric lifetimes of other gases, and/or when a
gas affects atmospheric processes that alter the radiative
balance of the earth (e.g., affect cloud formation or albedo).6
The IPCC developed the Global Wanning Potential (GWP)
concept to compare the ability of each greenhouse gas to trap
heat in the atmosphere relative to another gas.
    The GWP of a greenhouse gas is defined as the ratio of
the time-integrated radiative forcing from the instantaneous
release of 1 kg of a trace substance relative to that of 1 kg of
a reference gas (IPCC 2001). Direct radiative effects occur
when the gas itself is a greenhouse gas. The reference gas
used is CC>2, and therefore GWP-weighted emissions are
measured in teragrams of CC«2 equivalent (Tg COa Eq.).7
All gases in this Executive Summary are presented in units
ofTgCO2Eq.
    The UNFCCC reporting  guidelines  for national
inventories were updated in 2002,8 but continue to require
the use of GWPs from the IPCC Second Assessment Report
(SAR). This requirement ensures that current estimates of
aggregate greenhouse gas emissions for 1990  to 2004 are
consistent with estimates developed prior to the publication
of the IPCC Third Assessment  Report (TAR). Therefore,
to comply with international reporting standards  under
the UNFCCC, official emission estimates are  reported by
the United States using SAR GWP values. All estimates
are provided throughout the Inventory report in both CC>2
equivalents and unweighted units. A comparison of emission
values using the SAR GWPs versus the TAR GWPs  can be
found in Chapter 1 and, in more detail, in Annex 6.1 of the
Inventory. The GWP values used in the Inventory report are
listed below in Table ES-1.
Table ES-1: Global Warming Potentials (100-Year Time
Horizon) Used in the Inventory Report
Gas
C02
CH4*
N20
HFC-23
HFC-32
HFC-125
HFC-1343
HFC-143a
HFC-152a
HFC-227ea
HFC-236fa
HFC-4310mee
CF4
C2F6
C4Fio
CeFi4
SF6
GWP
1
21
310
11,700
650
2,800
1,300
3,800
140
2,900
6,300
1,300
6,500
9,200
7,000
7,400
23,900
  Source: IPCC (1996)
  * The CH4 GWP includes the direct effects and those indirect effects
  due to the production of tropospheric ozone and stratospheric water
  vapor. The indirect effect due to the production of C02 is not included.
    Global warming potentials are not provided for CO, NOX,
NMVOCs, SO2, and  aerosols because there is no agreed-
upon method to estimate the contribution of gases that are
short-lived in the atmosphere, spatially variable, or have only
indirect effects on radiative forcing (IPCC 1996).

ES.2. Recent Trends in  U.S.
Greenhouse Gas Emissions
and Sinks
    In 2004, total U.S. greenhouse gas emissions  were
7,074.4 Tg CO2 Eq. Overall, total U.S. emissions have risen
by 15.8 percent from 1990 to 2004, while the U.S. gross
domestic product has increased by 51 percent over the same
period (BEA 2005). Emissions rose from 2003 to 2004,
increasing by 1.7 percent (115.3 Tg CO2 Eq.). The following
factors were primary contributors to this increase: (1) robust
economic growth in 2004, leading to increased demand
for electricity and fossil fuels, (2) expanding industrial
6 Albedo is a measure of the Earth's reflectivity, and is defined as the fraction of the total solar radiation incident on a body that is reflected by it.
7 Carbon comprises 12/44thi of carbon dioxide by weight.
8 See .
                         Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004 3

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Figure ES-1
         U.S. Greenhouse Gas Emissions by Gas
      8,000 -
      7,000 -
      6,000
      5,000-
      4,000
      3,000 -
      2,000 -
      1,000 -
        o-
HFCs, PFCs, & SFS   Methane
Nitrous Oxide    • Carbon Dioxide
           3 S
                                        oooo
Figure ES-2
 Annual Percent Change in U.S. Greenhouse Gas Emissions
      3.5-
      3.0-
      2.5-
      2.0-
      1.5-
      1.0-
      0.5-
      o.o-
     -0.5-
     -1.0-
     -1.5-
         II
      1.7% •         H         17%

   illiJil   nil
           8
production in energy-intensive industries, also increasing
demand for electricity and fossil fuels, and (3) increased
travel, leading to higher rates of consumption of petroleum
fuels.
    Figure ES-1 through Figure ES-3 illustrate the overall
trends in total U.S. emissions by gas, annual changes, and
cumulative change since 1990. Table ES-2 provides a detailed
summary of U.S.  greenhouse gas emissions and sinks for
1990 through 2004.
    Figure ES-4 illustrates the relative contribution of the
direct greenhouse gases to total U.S. emissions in 2004.
The  primary greenhouse gas emitted by human activities
in the United States was CO2, representing approximately
85 percent of total greenhouse gas emissions. The largest
source of  CO2,  and of overall greenhouse gas emissions,
was  fossil fuel  combustion. CFU emissions,  which have
steadily declined since 1990, resulted primarily  from
decomposition of  wastes in landfills, natural gas systems,
and enteric fermentation associated with domestic livestock.
Agricultural soil management and mobile source fossil fuel
combustion were the major sources of N2O emissions. The
emissions  of substitutes for ozone depleting substances and
emissions  of HFC-23  during the production of HCFC-22
were the primary contributors to aggregate HFC emissions.
Electrical transmission and distribution systems accounted
for most SF6 emissions, while PFC emissions resulted from
semiconductor manufacturing and as a by-product of primary
aluminum production.
Figure ES-3
                                        Figure ES-4
       Cumulative Change in U.S. Greenhouse Gas
              Emissions Relative to 1990
    1,000 -

     800-

     600-
   r
   j1 400-
   »
     200 -

       0

     -200 -J
          T- 01  co  ^-  w
             SO>  01  01  o>
             O>  O»  O)  O>
                                    3
                                       i—  CM  CO
                                                2004 Greenhouse Gas Emissions by Gas
                                               MFCs, PFCs, & SFB
                                                        N20
                                                        CH,
                                                        CO,
                                                                         84.6%
4  Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004

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Table ES-2: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg C02 Eq.)
Gas/Source
C02
Fossil Fuel Combustion
Non-Energy Use of Fuels
Iron and Steel Production
Cement Manufacture
Municipal Solid Waste Combustion
Ammonia Manufacture and Urea Application
Lime Manufacture
Limestone and Dolomite Use
Natural Gas Flaring
Aluminum Production
Soda Ash Manufacture and Consumption
Petrochemical Production
Titanium Dioxide Production
Phosphoric Acid Production
Ferroalloy Production
C02 Consumption
Zinc Production
Lead Production
Silicon Carbide Consumption
Net CDs Flux from Land Use, Land-Use
Change and Forestry3
International Bunker Fuels'1
Wood Biomass and Ethanol Consumption"
CH4
Landfills
Natural Gas Systems
Enteric Fermentation
Coal Mining
Manure Management
Wastewater Treatment
Petroleum Systems
Rice Cultivation
Stationary Combustion
Abandoned Underground Coal Mines
Mobile Combustion
Petrochemical Production
Iron and Steel Production
Field Burning of Agricultural Residues
Silicon Carbide Production
International Bunker Fuels"
N20
Agricultural Soil Management
Mobile Combustion
Manure Management
Nitric Acid Production
Human Sewage
Stationary Combustion
Settlements Remaining Settlements
Adipic Acid Production
N20 Product Usage
Municipal Solid Waste Combustion
Field Burning of Agricultural Residues
Forest Land Remaining Forest Land
International Bunker Fuels"
MFCs, PFCs, and SF6
Substitution of Ozone Depleting Substances
HCFC-22 Production
Electrical Transmission and Distribution
Semiconductor Manufacture
Aluminum Production
Magnesium Production and Processing
Total
Net Emissions (Sources and Sinks)
+ Does not exceed 0.05 Tg CO? Eq.
1990 1
5,005.3 1
4,696.6 1
117.2 1
85.0 I
33.3 I
10.9 1
19.3 1
11.2 1
5.5 1
5.8 I
7.0 I
4.1 I
2.2 I
1-3 1
1-5 I
2.0 1
0.9 1
0.9 1
0.3 1
0.1 1
1
(9W.4) I
113.5 I
2/6.7 1
618.1 I
172.3 1
126.7 i
117.9 I
81.9 i
31.2 i
24.8 1
34.4 I
7-1 1
7-9 i
6.0 1
4.7 1
1-2 1
1.3 1
0.7 i
+ 1
0.2 1
394.9 1
266.1 I
43.5 I
16.3 •
17.8 1
12.9 1
12.3 i
5-6 i
15.2 I
4.3 I
0.5 1
0-4 1
0.1 •
1.0 I
90.8 I
0.4 i
35.0 1
28.6 I
2-9 1
18.4 1
5^4 i
6,109F|
5.198.6 1

• 	 1998
1 M20.2
1 5.271.8
i 152.8
I 67.7
1 39.2
I 17-1
i 21.9
1 13'9
i 7.4
I 6.6
i 6.4
1 4.3
i 3-0
i 1.8
1 1-6
1 2.0
1 0.9
1 1-1
1 °-3
I 0.2
1
1 (744-°)
I ?'4.6
I 2/7.2
1 579-5
I 144.4
1 125.4
1 116.7
i 62.8
1 38.8
I 32.6
I 29.7
1 7.9
I 6.8
I 6.9
1 3-8
i 1.7
1 1-2
1 0.8
1 +
1 °-2
I 440.6
I 301.1
1 54-8
1 17-4
1 20.9
I 14.9
1 13.4
1 6.2
1 6.0
i 4-8
1 0.4
i 0.5
i 0.4
I 1-°
I 133.4
I 54.5
i 40-1
I 16.7
1 7.1
1 9-1
1 	 5.8
1 6J73.7
I 6.029.6

1999
5,695.0
5,342.4
160.6
63.8
40.0
17.6
20.6
13.5
8.1
6.9
6.5
4.2
3.1
1.9
1.5
2.0
0.8
1.1
0.3
0.1

(765.7)
105.2
222.3
569.0
141.6
121.7
116.8
58.9
38.1
33.6
28.5
8.3
7.0
6.9
3.6
1.7
1.2
0.8
+
0.1
419.4
281.2
54.1
17.4
20.1
15.4
13.4
6.2
5.5
4.8
0.4
0.4
0.5
0.9
131.5
62.8
30.4
16.1
7.2
9.0
6.0
6,814.9
6,049.2

2000
5,864.5
5,533.7
140.7
65.3
41.2
17.9
19.6
13.3
6.0
5.8
6.2
4.2
3.0
1.9
1.4
1.7
1.0
1.1
0.3
0.1

(759.5)
101.4
226.8
566.9
139.0
126.7
115.6
56.3
38.0
34.3
27.8
7.5
7.3
7.2
3.5
1.7
1.2
0.8
+
0.1
416.2
278.2
53.1
17.8
19.6
15.5
13.9
6.0
6.0
4.8
0.4
0.5
0.4
0.9
134.7
71.2
29.8
15.3
6.3
9.0
3.2
6,982.3
6,222.8

3 Parentheses indicate negative values or sequestration. The net C02 flux total includes both emissions and
2001
5,795.2
5,486.9
131.0
57.8
41.4
18.6
16.7
12.8
5.7
6.1
4.5
4.1
2.8
1.9
1.3
1.3
0.8
1.0
0.3
0.1

(768.0)
97.8
200.5
560.3
136.2
125.6
114.6
55.5
38.9
34.7
27.4
7.6
6.6
6.6
3.3
1.4
1.1
0.8
+
0.1
412.8
282.9
50.0
18.1
15.9
15.6
13.5
5.8
4.9
4.8
0.5
0.5
0.4
0.9
124.9
78.6
19.8
15.3
4.5
4.0
2.6
6,893.1
6,125.1

sequestration,
2002
5,815.9
5,501.8
136.5
54.6
42.9
18.9
18.5
12.3
5.9
6.2
4.6
4.1
2.9
2.0
1.3
1.2
1.0
0.9
0.3
0.1

(768.6)
89.5
794.4
559.8
139.8
125.4
114.7
52.5
39.3
35.8
26.8
6.8
6.2
6.0
3.2
1.5
1.0
0.7
+
o.r
407.4
277.8
47.5
18.0
17.2
15.6
13.2
6.0
5.9
4.8
0.5
0.4
0.4
0.8
132.7
86.2
19.8
14.5
4.4
5.3
2.6
6,915.8
6,147.2

2003
5,877.7
5,571.1
133.5
53.3
43.1
19.4
15.3
13.0
4.7
6.1
4.6
4.1
2.8
2.0
1.4
1.2
1.3
0.5
0.3
0.1

(774.8)
84.1
202.1
564.4
142.4
124.7
115.1
54.8
39.2
36.6
25.9
6.9
6.5
5.8
3.0
1.5
1.0
0.8
+
0.1
386.1
259.2
44.8
17.5
16.7
15.8
13.6
6.2
6.2
4.8
0.5
0.4
0.4
0.8
131.0
93.5
12.3
14.0
4.3
3.8
3.0
6,959.1
6,184.3

2004
5,988.0
5,656.6
153.4
51.3
45.6
19.4
16.9
13.7
6.7
6.0
4.3
4.2
2.9
2.3
1.4
1.3
1.2
0.5
0.3
0.1

(780.7)
94.5
277.2
556.7
140.9
118.8
112.6
56.3
39.4
36.9
25.7
7.6
6.4
5.6
2.9
1.6
1.0
0.9
+
0.7
386.7
261.5
42.8
17.7
16.6
16.0
13.7
6.4
5.7
4.8
0.5
0.5
0.4
0.9
143.0
103.3
15.6
13.8
4.7
2.8
2.7
7,074.4
6,294.3

and constitutes a sink in the
United States. Sinks are only included in net emissions total.
b Emissions from International Bunker Fuels and Wood Biomass and Ethanol Consumption are not included in totals.
Note: Totals may not sum due to independent rounding.
                         Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004 5

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    Overall, from 1990 to 2004, total emissions of CO2
increased by 982.7 Tg CO2 Eq. (20 percent), while CH4 and
N2O emissions decreased by 61.3 Tg CO2 Eq. (10 percent)
and 8.2 Tg  CO2 Eq. (2 percent), respectively. During the
same period, aggregate weighted emissions of HFCs, PFCs,
and SF6 rose by 52.2 Tg CO2 Eq. (58 percent). Despite being
emitted in smaller quantities relative to the other principal
greenhouse  gases, emissions of HFCs, PFCs, and SFe are
significant because many of them have extremely high global
warming potentials and, in the cases of PFCs and SFe, long
atmospheric lifetimes. Conversely, U.S. greenhouse gas
emissions were  partly offset  by carbon sequestration in
forests, trees in urban areas, agricultural soils, and landfilled
yard trimmings and food scraps, which, in aggregate, offset
11 percent of total emissions in 2004. The following sections
describe each gas's contribution to total U.S. greenhouse gas
emissions in more detail.

Carbon Dioxide Emissions
    The global carbon cycle  is made  up of large carbon
flows and reservoirs. Billions of tons of carbon in the form
of CO2 are  absorbed by oceans and living biomass (i.e.,
sinks)  and are emitted to the atmosphere annually through
natural processes (i.e., sources). When in equilibrium,
carbon fluxes among these various reservoirs are roughly
balanced. Since the Industrial Revolution (i.e., about 1750),
global atmospheric concentrations of CO2 have risen about
35 percent (IPCC 2001, Hofmann 2004), principally due
to the combustion of fossil fuels. Within the United  States,
fuel combustion accounted for 94 percent of CO2 emissions
in 2004. Globally, approximately 25,575 Tg of CO2 were
added  to the atmosphere through the combustion of fossil
fuels in 2002, of which the United States accounted for about
23 percent.9 Changes in land use and forestry practices can
also emit CO2 (e.g., through conversion of forest  land to
agricultural  or urban use) or can act as a sink for CO2 (e.g.,
through net  additions to forest biomass).
    U.S. anthropogenic sources of CO2 are shown in
Figure ES-5. As the largest source of U.S. greenhouse gas
emissions, CO2 from fossil fuel combustion has accounted
for approximately 80 percent of GWP weighted emissions
since  1990, growing slowly from 77 percent of total
GWP-weighted emissions in 1990 to 80 percent in 2004.
Figure ES-5
                  2004 Sources of CO?
              Fossil Fuel Combustion •
             Iron and Steel Production •
               Cement Manufacture •
        Municipal Solid Waste Combustion •
   Ammonia Manufacture and Urea Application •
                Lime Manufacture •
           Limestone and Dolomite Use •
                Natural Gas Flaring •
               Aluminum Production •
     Soda Ash Manufacture and Consumption •
           Titanium Dioxide Production I
           Phosphoric Acid Production I
               Ferroalloy Production I
           Carbon Dioxide Consumption I
                  Zinc Production I
                 Lead Production I
           Silicon Carbide Consumption |
                                                 5,656.6
COZ as a Portion
ol all Emissions
                              10   20   30   40   50
                                   TgCO,Eq.
Emissions of CO2 from fossil fuel combustion increased at
an average annual rate of 1.3 percent from 1990 to 2004.
The fundamental factors influencing this trend include (1) a
generally growing domestic economy over the last 14 years,
and (2) significant growth in emissions from transportation
activities and electricity generation. Between 1990 and 2004,
CO2 emissions from fossil fuel combustion increased from
4,696.6 Tg CO2 Eq. to 5,656.6 Tg CO2  Eq.—a 20 percent
total increase over  the fourteen-year period. Historically,
changes in emissions from fossil fuel combustion have been
the dominant factor affecting U.S. emission trends.
    From 2003 to 2004, these emissions increased by 85.5
Tg CO2 Eq.  (1.5 percent). A number of factors  played a
major role in the magnitude of this increase. Strong growth
in the U.S. economy and industrial production, particularly
in energy-intensive industries,  caused  an increase in  the
demand for electricity and fossil fuels.  Demand for travel
was also higher, causing an increase in petroleum consumed
for transportation. In contrast, the wanner winter conditions
led to decreases in demand for heating fuels in both  the
residential and commercial sectors. Moreover, much of the
increased electricity demanded was generated by natural gas
consumption and nuclear power, rather than  more carbon
intensive coal, moderating the increase in CO2 emissions
from electricity generation. Use of renewable fuels rose very
slightly due to increases in the use of biofuels.
9 Global COi emissions from fossil fuel combustion were taken from Marland et al. (2005) .
6  Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004

-------
    The four major end-use sectors contributing to
emissions from fossil fuel  combustion are industrial,
transportation, residential,  and commercial. Electricity
generation also  emits CC>2, although these emissions are
produced as fossil fuel is consumed to provide electricity to
one of the four end-use sectors. For the discussion below,
electricity generation emissions have been distributed to each
end-use sector on the basis of each sector's share of aggregate
electricity consumption. This method  of distributing
emissions assumes that  each end-use sector consumes
electricity  that is generated from the national average mix
of fuels according to their carbon intensity. Emissions from
electricity generation are also addressed separately after the
end-use sectors have been discussed.
    Note that emissions from U.S. territories are calculated
separately due to a lack of specific consumption data for the
individual end-use sectors.
    Figure ES-6, Figure ES-7, and Table ES-3 summarize
CO2 emissions  from  fossil  fuel combustion by end-use
sector.
    Transportation End-Use Sector. Transportation activities
(excluding international bunker fuels) accounted for 33
percent of COa  emissions from fossil fuel combustion in
2004.10 Virtually all of the energy consumed in this end-use
                                              Figure ES-7
Figure ES-6
     2,500 -|
     2,000 -
   s1'500"
   s
   f!
     1,000 -
      500 -
           2004 C02 Emissions from Fossil Fuel
           Combustion by Sector and Fuel Type
Relative Contribution
  by Fuel Type
         Residential  Commercial Industrial  Transportation  Electricity   U.S.
                                        Generation  Territories
  Note Electricity generation also includes emissions of less tnan
  1 To COz Ep from geothermal-based electricity generation
                                                     2004 End-Use Sector Emissions of C02 from
                                                               Fossil Fuel Combustion
                                                   2,000 -i
                                                   1,500
                                                o  1,000 -
                                                    500 -
                                                      o -1
                                                            II From Electricity
                                                              Consumption
                                                            • From Direct Fossil
                                                              Fuel Combustion
                                                         Residential   Commercial   Industrial  Transportation    U.S.
                                                                                            Territories
sector came from petroleum products. Over 60 percent of the
emissions resulted from gasoline consumption for personal
vehicle use.  The remaining emissions came from other
transportation activities, including the combustion of diesel
fuel in heavy-duty vehicles and jet fuel in aircraft.
    Industrial End-Use Sector. Industrial COa emissions,
resulting both directly from the combustion of fossil fuels and
indirectly from the generation of electricity that is consumed
by industry, accounted for 28 percent of CO2 from fossil fuel
combustion in 2004. About half of these emissions resulted
from direct fossil fuel combustion to produce steam and/or
heat for industrial processes. The other half of the emissions
resulted from consuming electricity  for motors, electric
furnaces, ovens, lighting,  and other applications.
    Residential and Commercial End-Use Sectors. The
residential and commercial end-use sectors accounted for
21 and 17 percent,  respectively, of CC>2 emissions from
fossil fuel combustion in  2004. Both sectors relied heavily
on electricity for meeting energy demands, with 68 and
77 percent, respectively,  of their emissions attributable to
electricity consumption for lighting, heating, cooling, and
operating appliances. The remaining emissions were due to
the consumption of  natural gas and petroleum for heating
and cooking.
    Electricity Generation. The United States relies on
electricity to meet a significant portion of its energy demands,
10 If emissions from international bunker fuels are included, the transportation end-use sector accounted for 34 percent of U.S. emissions from fossil fuel
combustion in 2004.
                           Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004 7

-------
Table ES-3: C02 Emissions from Fossil Fuel Combustion by End-Use Sector (Tg C02 Eq.)
  End-Use Sector
  Transportation
    Combustion
    Electricity
  Industrial
    Combustion
    Electricity
  Residential
    Combustion
    Electricity
  Commercial
    Combustion
    Electricity
  U.S. Territories
  Total
  Electricity Generation
  1990
1,464.4
1,461.4
    3.0
1,528.3
 851.1
 677.2
 922.8
 338.0
 584.8
 753.1
 222.6
 530.5
   28.0
4,696.6
1,795.5
  1998
1999
2000
2001
2002
2003
2004
1,663.4
1,660.3
3.1
1,634.5
871.9
762.6
1,044.5
333.5
711.0
895.9
217.7
678.2
33.5
1,725.6
1,722.4
3.2
1,613.5
849.0
764.5
1,064.0
352.3
711.7
904.8
218.6
686.2
34.5
1,770.3
1,766.9
3.4
1,642.8
862.6
780.3
1,123.2
369.9
753.3
961.6
229.3
732.4
35.8
1,757.0
1,753.6
3.5
1,574.9
861.2
713.7
1,123.2
361.5
761.7
983.3
224.9
758.4
48.5
1,802.2
1,798.8
3.4
1,542.8
842.1
700.7
1,139.8
360.0
779.8
973.9
224.3
749.6
43.1
1,805.4
1,801.0
4.3
1,572.4
844.6
727.7
1,166.6
378.8
787.9
978.1
235.8
742.2
48.7
1,860.2
1,855.5
4.7
1,595.0
863.5
731.5
1,166.8
369.6
797.2
983.1
226.0
757.2
51.4
5,271.8    5,342.4     5,533.7    5,486.9    5,501.8     5,571.1     5.656.6
2,154.9    2,165.6     2,269.3    2,237.3    2,233.5     2,262.2     2,290.6
  Note: Totals may not sum due to independent rounding. Combustion-related emissions from electricity generation are allocated based on aggregate
  national electricity consumption by each end-use sector.
especially for lighting, electric motors,  heating, and air
conditioning. Electricity generators consumed 34 percent of
U.S. energy from fossil fuels and emitted 40 percent of the
CO2 from fossil fuel combustion in 2004. The type of fuel
combusted by electricity generators has a significant effect
on their emissions. For example, some electricity is generated
with low CC>2 emitting energy technologies, particularly non-
fossil options such as nuclear, hydroelectric, or geothermal
energy. However, electricity generators rely on coal for over
half of their total energy requirements and accounted for 94
percent of all coal consumed for energy in the United States
in 2004. Consequently, changes in electricity demand have
a significant  impact on coal consumption and  associated
CO2 emissions.
    Other significant CO2 trends included the following:
•   CO2 emissions from iron and steel production decreased
    to 51.3 Tg CO2 Eq. in 2004, and have declined by 33.7
    Tg CO2 Eq. (40 percent) from 1990 through 2004, due
    to reduced domestic production of pig iron,  sinter, and
    coal coke.
•   CO2 emissions from cement production increased to 45.6
    Tg CO2 Eq. in 2004, a 37 percent increase in emissions
     since 1990. Emissions mirror growth in the construction
    industry. In contrast to many other manufacturing
     sectors, demand for domestic cement remains strong
    because it is not cost-effective to transport cement far
     from its point of manufacture.
                              •    CO2 emissions from municipal solid waste combustion
                                  (19.4 Tg CO2 Eq. in 2004) increased by 8.4 Tg CO2 Eq.
                                  (77 percent) from 1990 through 2004, as the volume of
                                  plastics and other fossil carbon-containing materials in
                                  municipal solid waste grew.
                              •    Net CO2  sequestration from Land Use, Land-Use
                                  Change, and Forestry decreased by 130.3 Tg CO2 Eq.
                                  (14 percent) from 1990 through 2004. This decline
                                  was primarily due to a decline in the rate of net carbon
                                  accumulation in forest carbon stocks. Annual carbon
                                  accumulation in landfilled yard trimmings and food
                                  scraps  also slowed over this period, while the rate of
                                  carbon accumulation in agricultural soils and urban trees
                                  increased.

                              Methane Emissions
                                  According to the IPCC, CFLj is more than 20 times as
                              effective as CO2 at trapping heat in the atmosphere. Over
                              the last two hundred and fifty years, the concentration of
                              CH4 in the atmosphere increased by 143 percent (IPCC
                              2001,  Hofmann 2004). Experts believe that over half
                              of this atmospheric increase was due to emissions from
                              anthropogenic sources,  such as landfills, natural gas and
                              petroleum  systems,  agricultural activities,  coal mining,
                              wastewater treatment, stationary and mobile combustion,
                              and certain industrial processes (see Figure ES-8).
 8 Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004

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Figure ES-8
                2004 U.S. Sources of CH4
                   Landfills
            Natural Gas Systems
            Enteric Fermentation
                 Coal Mining
            Manure Management
           Wastewater Treatment
             Petroleum Systems
               Rice Cultivation
           Stationary Combustion
   Abandoned Underground Coal Mines
             Mobile Combustion
         Petrochemical Production I
         Iron and Steel Production I
   Field Burning of Agricultural Residues I
         Silicon Carbide Production <0.05
                             30
                                  60    90
                                   TgCO^q.
                                             120
                                                   I
                                                  150
    Some significant trends in U.S. emissions of CHt include
the following:
•   Landfills are the largest anthropogenic  source of CH4
    emissions in the United  States. In 2004, landfill CH4
    emissions were 140.9  Tg CO2 Eq. (approximately 25
    percent of total CH4 emissions), which represents a
    decline of 31.4 Tg CO2 Eq., or 18 percent, since 1990.
    Although the amount of solid waste landfilled each year
    continues to climb,  the amount of CH4 captured  and
    burned at landfills has increased dramatically, countering
    this trend.
•   CH4 emissions from natural gas systems were 118.8 Tg
    COa Eq. in 2004; emissions have declined by 7.9 Tg CO2
    Eq. (6 percent) since 1990. This decline has been due to
    improvements in technology and management practices,
    as well as some replacement of old equipment.
•   Enteric fermentation was also a significant source of
    CH4, accounting for 112.6 Tg CO2 Eq. in 2004. This
    amount has declined by 5.3 Tg CO2 Eq. (4 percent) since
    1990, and by 10.4 Tg CO2 Eq. (8 percent) from a high in
    1995. Generally, emissions have been decreasing since
    1995, mainly due to decreasing populations of both beef
    and dairy cattle and improved feed quality for  feedlot
    cattle.

Nitrous Oxide Emissions
    N2O is produced by biological processes that occur in
soil and water and by a variety of anthropogenic activities
in the agricultural, energy-related, industrial, and waste
management fields. While total N2O emissions are much
lower than CO2 emissions, N2O is approximately 300 times
more powerful than CO2 at trapping heat in the atmosphere.
Since 1750, the global atmospheric concentration of N2O has
risen by approximately 18 percent  (IPCC 2001, Hofmann
2004). The main anthropogenic activities producing N2O
in the United States are agricultural soil management,
fuel combustion in motor vehicles, manure management,
nitric acid production, human sewage, and  stationary fuel
combustion (see Figure ES-9).
     Some significant trends in U.S. emissions of N2O include
the following:
•    Agricultural soil management activities such as fertilizer
     application and other cropping practices were the largest
     source of U.S. N2O emissions, accounting for 68 percent
     (261.5 Tg CO2 Eq.) of 2004 emissions. N2O emissions
     from  this source have not shown any significant long-
     term  trend, as they are highly sensitive  to such factors
     as temperature and precipitation, which have generally
     outweighed changes in the amount of nitrogen  applied
     to soils.
•    In 2004, N2O emissions from mobile combustion were
     42.8 Tg CO2 Eq. (approximately 11 percent of U.S. N2O
     emissions). From 1990 to  2004, N2O emissions from
     mobile combustion decreased by 1 percent. However,
     from  1990 to 1998 emissions increased by 26 percent,
Figure ES-9
                2004 U.S. Sources of N?0
       Agricultural Soil Management
             Mobile Combustion
            Manure Management
            Nitric Acid Production
               Human Sewage
           Stationary Combustion
    Settlements Remaining Settlements |
           Adipic Acid Production |
             N,0 Product Usage |
    Municipal Solid Waste Combustion |
   Field Burning of Agricultural Residues |
    Forest Land Remaining Forest Land |
261.5
                            10
                                 20   30
                                 Tg C02 Eq.
                                           40   50
                           Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004 9

-------
    due to control technologies that reduced CH4 emissions
    while increasing N2O emissions. Since 1998, new
    control technologies have led to a steady decline in N2O
    from this source.

HFC, PFC, and  SF6 Emissions
    HFCs and PFCs are families of synthetic chemicals
that are being used as alternatives to the ODSs, which are
being phased out under the Montreal Protocol and Clean Air
Act Amendments of 1990. HFCs and PFCs do not deplete
the stratospheric ozone layer, and are therefore acceptable
alternatives under the Montreal Protocol.
    These compounds, however,  along with  SF6, are
potent greenhouse  gases.  In addition to having high
global  warming potentials, SFe and  PFCs have extremely
long atmospheric lifetimes, resulting in their essentially
irreversible accumulation in the atmosphere once emitted.
Sulfur  hexafluoride is the most potent greenhouse gas the
IPCC has evaluated.
    Other emissive sources of these gases include HCFC-22
production, electrical transmission and distribution systems,
semiconductor manufacturing, aluminum production, and
magnesium production and processing (see Figure ES-10).
    Some significant trends in U.S. HFC, PFC,  and SFe
emissions include the following:
•   Emissions  resulting from the substitution of ozone
    depleting substances (e.g., CFCs) have been increasing
    from small amounts in  1990 to 103.3 Tg CO2 Eq. in
    2004. Emissions from substitutes  for ozone depleting
    substances are both the largest and the fastest growing
    source of HFC, PFC and SFe emissions. These emissions
    have been increasing as phase-outs required under the
    Montreal Protocol come into effect, especially after
    1994  when full  market  penetration was made for the
    first generation of new technologies featuring ODS
    substitutes.
•   The increase in  ODS emissions is offset substantially
    by decreases in emission of HFCs, PFCs, and SFe from
    other sources. Emissions from  aluminum production
    decreased by 85 percent (15.6 Tg CO2 Eq.) from 1990
    to 2004, due to both industry emission reduction efforts
    and lower domestic aluminum production.
•   Emissions from the production of HCFC-22 decreased
    by 55 percent (19.4 Tg CO2 Eq.), due to a steady decline
    in the emission rate of HFC-23 (i.e., the amount of HFC-
    23 emitted per kilogram of HCFC-22 manufactured) and
    the use of thermal oxidation at some plants to reduce
    HFC-23 emissions.
•   Emissions from electric power  transmission and
    distribution systems decreased by 52 percent (14.8 Tg
    CO2 Eq.) from 1990 to 2004, primarily because of higher
    purchase prices for SFg and efforts by industry to reduce
    emissions.

ES.3. Overview of  Sector  Emissions
and Trends
    In accordance with the Revised 1996 IPCC Guidelines
for National Greenhouse Gas Inventories (IPCC/UNEP/
OECD/IEA 1997), and the 2003 UNFCCC Guidelines on
Reporting  and Review (UNFCCC 2003),  the Inventory
of U.S. Greenhouse Gas Emissions and Sinks report is
segregated  into six sector-specific  chapters. Figure ES-11
and Table  ES-4 aggregate emissions and sinks by these
chapters.

Energy
    The Energy chapter contains emissions of all greenhouse
gases resulting from stationary and mobile energy activities
including fuel  combustion and fugitive fuel emissions.

Figure ES-10
        2004 U.S. Sources of HFCs, PFCs, and SF6
    Substitution of Ozone
    Depleting Substances
    HCFC-22 Production
   Electrical Transmission
       and Distribution
       Semiconductor •
         Manufacture •
    Aluminum Production I
   Magnesium Production I
                 I
                               HFCs, PFCs, and
                              SF, as a Portion of
                                all Emissions
and Processing
                     20
                          40   60   80
                             Tg C02 Eq.
                                         100   120
10  Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004

-------
Figure ES-11
    U.S. Greenhouse Emissions by Chapter/IPCC Sector
                      Industrial Processes
                                      Agriculture
              land Us«, Land-Un CHUM, and Forestry Mnu
 Note Relatively smaller amounts of GWP-weighted emissions are also emitted from the
 Land-Use Change and Forestry sector and the Solvent and Other Product Use sector
                                                            Overall, emission sources in the Energy chapter account
                                                            for a combined 86  percent of total U.S. greenhouse gas
                                                            emissions in 2004.

                                                            Industrial Processes
                                                                The Industrial Processes chapter contains by-product
                                                            or fugitive emissions of greenhouse gases from industrial
                                                            processes not directly related to energy activities such  as
                                                            fossil fuel combustion. For example, industrial processes

                                                            Figure ES-12
                                                                 2004 U.S. Energy Consumption by Energy Source
                                                                         Renewable
                                                                           Nuclear

                                                                         Natural Gas
                                                                             Coal
                                                                         Petroleum
                                                                                                22.5%
                                                                                                40.1%
Energy-related activities, primarily fossil fuel combustion,
accounted for the vast majority of U.S. CCh emissions for
the period of 1990 through 2004. In 2004, approximately
86 percent of the energy consumed in  the United States
was produced through the combustion of fossil fuels. The
remaining 14 percent came from other energy sources such
as hydropower, biomass, nuclear, wind, and solar energy
(see Figure  ES-12). Energy related  activities are also
responsible for CH4 and N2O emissions (39 percent and 15
percent of total U.S. emissions of each gas, respectively).
Table ES-4: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector (Tg C02 Eq.)
                                                                                                23.0%
  Chapter/IPCC Sector
1998
5,752.3
335.1
4.8
483.2
6.5
191.8
6,773.7
(744.0)
6,029.6
1999
5,822.3
327.5
4.8
463.1
6.7
190.7
6,814.9
(765.7)
6,049.2
2000
5,994.3
329.6
4.8
458.4
6.4
188.8
6,982.3
(759.5)
6,222.8
2001
5,931.6
300.7
4.8
463.4
6.2
186.4
6,893.1
(768.0)
6,125.1
2002
5,944.6
310.9
4.8
457.8
6.4
191.3
6,915.8
(768.6)
6,147.2
2003
6,009.8
304.1
4.8
439.1
6.6
194.8
6,959.1
(774.8)
6,184.3
2004
6,108.2
320.7
4.8
440.1
6.8
193.8
7,074.4
(780.1)
6,294.3
  Energy
  Industrial Processes
  Solvent and Other Product Use
  Agriculture
  Land Use, Land-Use Change, and
    Forestry (Emissions)
  Waste	
  Tola!	
  Net COz Flux from Land Use,
    Land-Use Change, and Forestry*
  Net Emissions
    (Sources and Sinks)	
  * The net C02 flux total includes both emissions and sequestration, and constitutes a sink in the United States. Sinks are only included in net emissions
  total.
  Note: Totals may not sum due to independent rounding.
                          Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004  11

-------
can chemically transform raw materials, which often release
waste gases such as CO2, CH4j and N2O. The processes
include iron and steel production, lead and zinc production,
cement manufacture, ammonia manufacture and urea
application, lime manufacture, limestone and dolomite
use (e.g.,  flux stone, flue gas  desulfurization, and glass
manufacturing),  soda ash manufacture and  use, titanium
dioxide production, phosphoric acid production, ferroalloy
production, CO2 consumption, aluminum production,
petrochemical production, silicon carbide production, nitric
acid production,  and adipic acid production. Additionally,
emissions from industrial processes release HFCs,  PFCs
and SFe- Overall, emission sources in the Industrial Process
chapter account  for 4.5 percent of  U.S. greenhouse gas
emissions in 2004.

Solvent and  Other Product  Use
    The Solvent and Other Product Use chapter contains
greenhouse gas emissions that are produced as a by-product
of various solvent and other product uses. In the United
States, emissions from N2O product usage, the only source
of greenhouse gas emissions from this sector, accounted for
less than 0.1 percent of total U.S. anthropogenic greenhouse
gas emissions on a carbon equivalent basis in 2004.

Agriculture
    The Agriculture chapter contains anthropogenic
emissions from agricultural activities (except fuel combustion,
which is addressed in the Energy chapter). Agricultural
activities contribute  directly to emissions of greenhouse
gases through a variety of processes, including the following
source categories: enteric fermentation in domestic livestock,
livestock manure management, rice cultivation, agricultural
soil management, and field burning of agricultural residues.
CH4 and N2O were the primary greenhouse  gases emitted
by  agricultural  activities.  CI-Lt emissions  from enteric
fermentation and manure management represented about
20  percent and  7 percent  of total  CFL; emissions from
anthropogenic activities, respectively, in 2004. Agricultural
soil management activities such as fertilizer  application
and other cropping practices were the largest source of U.S.
N2O emissions in 2004, accounting for 68 percent. In 2004,
emission sources accounted for in the Agriculture chapter
were  responsible for 6.2 percent of  total U.S.  greenhouse
gas emissions.
Land Use, Land-Use Change,  and Forestry
    The Land Use, Land-Use Change, and Forestry chapter
contains emissions  and removals  of  CO2 from forest
management, other land-use activities, and land-use change.
Forest management practices, tree planting in urban areas,
the management of agricultural soils, and the landfilling
of yard trimmings and food  scraps have resulted in a net
uptake (sequestration) of carbon in the United States. Forests
(including vegetation, soils, and harvested wood) accounted
for approximately 82 percent of total 2004  sequestration,
urban trees  accounted for 11 percent,  agricultural soils
(including mineral and organic  soils and the application
of lime) accounted  for 6 percent,  and landfilled yard
trimmings and food scraps accounted for 1 percent of the
total sequestration in 2004. The net forest sequestration is a
result of net forest growth and increasing  forest area, as well
as a net accumulation of carbon stocks in harvested wood
pools. The net sequestration in urban forests is a result of net
tree growth in these areas. In agricultural  soils, mineral soils
account for a net carbon sink that is almost two times larger
than the sum of emissions from organic soils and liming. The
mineral soil carbon sequestration is largely due to conversion
of cropland  to permanent pastures and  hay production, a
reduction in summer fallow areas in semi-arid  areas, an
increase  in the  adoption of conservation tillage  practices,
and an increase in the amounts  of organic fertilizers (i.e.,
manure and sewage sludge) applied to agriculture lands. The
landfilled yard trimmings and food scraps  net sequestration is
due to the long-term accumulation of yard trimming carbon
and food scraps in landfills.
    Land use, land-use change, and forestry activities in
2004 resulted in a net carbon sequestration of 780.1 Tg CO2
Eq. (Table ES-5). This represents an offset of approximately
13 percent of total U.S. CO2 emissions, or 11 percent of total
greenhouse gas emissions in 2004. Total land use, land-use
change, and forestry  net carbon sequestration declined by
approximately 14 percent between 1990 and 2004, which
contributed to an increase in net U.S.  emissions (all sources
and sinks) of 21 percent from 1990  to 2004. This decline
was primarily due to a decline in the rate  of net carbon
accumulation in forest carbon stocks. Annual carbon
accumulation in landfilled yard trimmings and food scraps
and agricultural soils also slowed over this period.  However,
the rate of annual carbon accumulation increased in both
agricultural soils and urban trees.
12 Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004

-------
Table ES-5: Net C02 Flux from Land Use, Land-Use Change, and Forestry (Tg C02 Eq.)
  Sink Category
  Forest Land Remaining Forest Land
    Changes in Forest Carbon Stocks
  Cropland Remaining Cropland
    Changes in Agricultural Soil Carbon
      Stocks and Liming Emissions
  Land Converted to Cropland
    Changes in Agricultural Soil Carbon Stocks
  Grassland Remaining Grassland
    Changes in Agricultural Soil Carbon Stocks
  Land Converted to Grassland
    Changes in Agricultural Soil Carbon Stocks
  Settlements Remaining Settlements
    Urban Trees
    Landfilled Yard Trimmings and Food Scraps
  Total
  1998    1999     2000    2001    2002     2003    2004
(618.8)  (637.9)
(618.8)  (637.9)
 (24.6)   (24.6)
 (24.6)
  (2.8)
  (2.8)
   7.5
   7.5
 (21.1)
 (21.1)
 (84.2)
 (73.3)
 (10.9)
(24.6)
 (2.8)
 (2.8)
  7.5
  7.5
(21.1)
(21.1)
(86.8)
(77.0)
 (9.8)
       (631.0)   (634.0)
       (631.0)   (634.0)
        (26.1)    (27.8)
(26.1)
 (2.8)
 (2.8)
  7.4
  7.4
(21.1)
(21.1)
(85.9)
(77.0)
 (8.9)
(27.8)
 (2.8)
 (2.8)
  7.4
  7.4
(21.1)
(21.1)
(89.7)
(80.7)
 (9.0)
(634.6)
(634.6)
 (27.5)

 (27.5)
  (2.8)
  (2.8)
   7.4
   7.4
 (21.1)
 (21.1)
 (89.9)
 (80.7)
  (9.3)
                        (635.8)  (637.2)
                        (635.8)  (637.2)
                         (28.7)    (28.9)
(28.7)
 (2.8)
 (2.8)
  7.3
  7.3
(21.1)
(21.1)
(93.8)
(84.3)
 (9.4)
(28.9)
 (2.8)
 (2.8)
  7.3
  7.3
(21.1)
(21.1)
(97.3)
(88.0)
 (9.3)
(744.0)  (765.7)   (759.5)  (768.0)  (768.6)   (774.8)  (780.1)
  Note: Totals may not sum due to independent rounding. Parentheses indicate net sequestration.
     Land use, land-use change, and forestry activities in
2004 also resulted in emissions of N2O (6.8 Tg CO2 Eq.).
Total N2O emissions from the application of fertilizers
to forests and settlements increased by approximately 20
percent between 1990 and 2004.


Waste
     The Waste chapter contains emissions from  waste
management activities (except  municipal  solid  waste
incineration, which is  addressed  in the Energy chapter).
Landfills were the largest source of anthropogenic CI-Lt
emissions, accounting for 25 percent of total U.S. CH4
emissions.11 Additionally, wastewater treatment accounts
for 7 percent of U.S. CH4 emissions. N2O emissions from
the discharge of wastewater treatment effluents into aquatic
environments were estimated, as were N2O emissions from
the treatment process itself, using a simplified methodology.
Wastewater treatment systems are a potentially  significant
source  of N2O emissions; however, methodologies are
not currently available to develop a complete estimate.
N2O emissions from the treatment of the human sewage
component of wastewater were estimated, however, using a
simplified methodology. Overall, in 2004, emission sources
   accounted for in the Waste chapter generated 2.7 percent of
   total U.S. greenhouse gas emissions.


   ES.4.  Other Information


   Emissions by Economic Sector
       Throughout the  Inventory  of  U.S.  Greenhouse Gas
   Emissions and Sinks report, emission estimates are grouped
   into six sectors (i.e., chapters) defined by the IPCC: Energy;
   Industrial Processes;  Solvent Use; Agriculture; Land Use,
   Land-Use Change, and  Forestry; and Waste. While it is
   important to use this  characterization for consistency with
   UNFCCC reporting guidelines, it is also useful to allocate
   emissions into more commonly used sectoral categories. This
   section reports emissions by the following economic sectors:
   residential, commercial, industrial, industry, transportation,
   electricity generation, agriculture, and U.S. territories.

       Table ES-6  summarizes emissions from each of these
   sectors, and Figure ES-13 shows the trend in emissions by
   sector from 1990 to 2004.
1' Landfills also store carbon, due to incomplete degradation of organic materials such as wood products and yard trimmings, as described in the Land-
Use, Land-Use Change, and Forestry chapter of the Inventory report.
                         Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004 13

-------
Table ES-6: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors (Tg C02 Eq.)
Economic Sector
Electricity Generation
Transportation
Industry
Agriculture
Commercial
Residential
U.S. Territories
Total
Net COa Flux from Land Use,
Land-Use Change, and Forestry*
Net Emissions (Sources and Sinks)
1990
1,846.4
1,520.3
1,438.9
486.3
433.6
349.4
33.8
6,109.0
(910.4)
5,198.6
1998
2,202.4
1,753.4
1,452.4
541.6
428.0
353.3
42.7
6,773.7
(744.0)
6,029.6
1999
2,213.3
1,819.3
1,411.0
523.9
430.6
372.6
44.2
6,814.9
(765.7)
6,049.2
2000
2,315.9
1,866.9
1,409.7
509.5
443.0
390.4
46.9
6,982.3
(759.5)
6,222.8
2001
2,284.4
1,852.7
1,366.6
514.4
439.5
381.6
54.0
6,893.1
(768.0)
6,125.1
2002
2,280.1
1,898.0
1,346.7
511.0
447.5
380.1
52.4
6,915.8
(768.6)
6,147.2
2003
2,308.5
1,898.9
1,342.7
484.2
466.5
399.8
58.6
6,959.1
(774.8)
6,184.3
2004
2,337.8
1,955.1
1,377.3
491.3
459.9
391.1
61.9
7,074.4
(780.1)
6,294.3
  * The net CO? flux total includes both emissions and sequestration, and constitutes a sink in the United States. Sinks are only included In net emissions
  total.
  Note: Totals may not sum due to independent rounding. Emissions include C02, CH4, NzO, MFCs, PFCs, and SFe.
  See Table 2-14 for more detailed data.
Figure ES-13
Emissions Allocated to Economic Sectors
2,500 ~

2,000 -
d- 1,500-
LLJ
3
1,000-
500-
o-
Note Does not me
Electricity Generation
^x*"- -««"""'""
fif^" Transportation
"^ 	 Uu»l"
s Agriculture
lf/_i^' Commercial


ude U S termones
    Using this categorization, emissions from electricity
generation accounted for the largest portion (33  percent)
of U.S. greenhouse gas emissions in 2004. Transportation
activities, in aggregate,  accounted for the second largest
portion (28 percent). Emissions from industry accounted
for 19 percent of U.S. greenhouse gas emissions in 2004. In
contrast to electricity generation and transportation, emissions
from industry have in general declined over the past decade,
although there was an increase in industrial emissions in
2004 (up 3 percent from 2003 levels). The long-term decline
in these emissions has been due to structural changes in the
U.S. economy (i.e.,  shifts from a manufacturing-based to
a service-based  economy), fuel switching, and efficiency
improvements. The remaining 20 percent of U.S. greenhouse
gas emissions were contributed by the residential, agriculture,
and commercial sectors, plus emissions from U.S. territories.
The residential sector  accounted for  about 6 percent, and
primarily consisted of CC>2 emissions from fossil fuel
combustion. Activities related to agriculture accounted for
roughly 7 percent of U.S. emissions; unlike other economic
sectors, agricultural sector emissions were dominated by
N2O emissions from agricultural soil management and CI-Lt
emissions from enteric fermentation,  rather than CC>2 from
fossil fuel combustion.  The commercial sector accounted for
about 1 percent of emissions, while U.S. territories accounted
for 1 percent.
    CO2 was also  emitted and  sequestered by a variety
of activities related to forest management practices, tree
planting in urban areas, the management of agricultural soils,
and landfilling of yard trimmings.
    Electricity  is ultimately consumed in the economic
sectors described above. Table  ES-7 presents  greenhouse
gas emissions from economic sectors with emissions related
to electricity generation distributed into end-use categories
(i.e., emissions from electricity generation are  allocated to
the economic sectors in which the electricity is consumed).
 12 Emissions were not distributed to U.S. territories, since the electricity generation sector only includes emissions related to the generation of electricity
 in the 50 states and the District of Columbia.
 14 Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004

-------
            Table ES-7: U.S Greenhouse Gas Emissions by Economic Sector with Electricity-Related Emissions Distributed (Tg
            C02 Eq.)
Economic Sector
Industry
Transportation
Commercial
Residential
Agriculture
U.S. Territories
Total
Net C02 Flux from Land Use,
Land-Use Change, and Forestry*
Net Emissions (Sources and Sinks)
1990
2,074.6
1,523.4
979.2
950.8
547.2
33.8
6,109.0
(910.4)
5,198.6
1998
2,210.3
1,756.5
1,102.0
1,060.0
602.4
42.7
6,773.7
(744.0)
6,029.6
1999
2,174.4
1,822.5
1,115.8
1,083.2
575.0
44.2
6,814.9
(765.7)
6,049.2
2000
2,186.1
1,870.3
1,171.8
1,140.0
567.2
46.9
6,982.3
(759.5)
6,222.8
2001
2,073.6
1,856.2
1,190.8
1,136.2
582.6
54.0
6,893.1
(768.0)
6,125.1
2002
2,042.0
1,901.4
1,191.4
1,154.1
574.5
52.4
6,915.8
(768.6)
6,147.2
2003
2,066.0
1,903.2
1,204.3
1,182.9
544.3
58.6
6,959.1
(774.8)
6,184.3
2004
2,103.0
1,959.8
1,211.0
1,181.9
556.9
61.9
7,074.4
(780.1)
6,294.3
              * The net C02 flux total includes both emissions and sequestration, and constitutes a sink in the United States. Sinks are only included in net emissions
              total.
              See Table 2-16 of the Inventory report for more detailed data.
            To distribute electricity emissions among end-use sectors,
            emissions from the source categories assigned to electricity
            generation  were allocated to the residential, commercial,
            industry, transportation, and agriculture economic sectors
            according  to retail  sales of electricity.12 These source
            categories include CO2 from fossil fuel combustion and the
            use of limestone and dolomite for flue gas desulfurization,
            CC>2 and N2O from waste combustion, CH4 and N2O from
            Figure ES-14
                      Emissions with Electricity Distributed
                              to Economic Sectors
                  2,500-


                  2,000-

               s
               a 1,SOD-
               S'

                  1,000-


                   500-
                       Transporlation
                        Commercial
                        •ii "i""
                         Residential
5
->„,.*"
 •* IO  o> o> o>
                                                 §

Agriculture
S  S
              Note- Does not include U S territories
stationary sources, and SFe from electrical transmission and
distribution systems.
    When emissions from electricity are distributed among
these sectors, industry accounts for the largest share of U.S.
greenhouse gas emissions (30 percent) in 2004. Emissions
from the residential and commercial sectors also  increase
substantially when emissions from electricity are included, due
to their relatively large share of electricity consumption (e.g.,
lighting, appliances, etc.). Transportation activities remain the
second largest contributor to total U.S. emissions (28 percent).
In all sectors except agriculture, COi accounts for more than
80 percent of greenhouse gas emissions, primarily from the
combustion of fossil fuels. Figure ES-14 shows the trend in
these emissions by sector from 1990 to 2004.

Indirect Greenhouse Gases (CO, NOX,
NMVOCs, and S02)
    The reporting requirements of the UNFCCC13 request
that information be provided on indirect greenhouse gases,
which include CO, NOX, NMVOCs, and SO2. These gases do
not have a direct global warming effect, but indirectly affect
terrestrial radiation absorption by influencing the formation
and destruction of tropospheric and stratospheric ozone, or,
in the case of SO2, by affecting the absorptive characteristics
of the atmosphere. Additionally,  some of these gases may
            13 See .
            14 NOX and CO emission estimates from field burning of agricultural residues were estimated separately, and therefore not taken from EPA (2005).
                                     Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004 15

-------
Box ES-2: Recent Trends in Various U.S. Greenhouse Gas Emissions-Related Data

      Total emissions can be compared to other economic and social indices to highlight changes overtime. These comparisons include: (1)
  emissions per unit of aggregate energy consumption, because energy-related activities are the largest sources of emissions; (2) emissions
  per unit of fossil fuel consumption, because almost all energy-related emissions involve the combustion of fossil fuels; (3) emissions per
  unit of electricity consumption, because the electric power industry—utilities and nonutilities combined—was the largest source of U.S.
  greenhouse gas emissions in 2004; (4) emissions per unit of total gross domestic product as a measure of national economic activity; or
  (5) emissions per capita.
      Table ES-8 provides data on various statistics related to U.S. greenhouse gas emissions normalized to 1990 as a baseline year. Greenhouse
  gas emissions in the United States have grown at an average annual rate of 1.1 percent since 1990. This rate is slower than that for total
  energy or fossil fuel consumption and much slower than that for either electricity consumption or overall gross domestic product. Total U.S.
  greenhouse gas emissions have also grown more slowly than national population since 1990 (see Figure ES-15). Overall, global atmospheric
  CO? concentrations—a function of many complex anthropogenic and natural processes—are increasing at 0.4 percent per year.

  Table ES-8: Recent Trends in Various U.S. Data (Index  1990 = 100) and  Global Atmospheric C02  Concentration
Variable
Greenhouse Gas Emissions3
Energy Consumption11
Fossil Fuel Consumption11
Electricity Consumption11
GDP^
Population11
Atmospheric CO? Concentration6
1991 -»'
0.0. F %
yy ™*^ ^"fc
100 *£
qq Ij^r*1
99 its
102 Tl
100 ? ^f
101 ii
100 £* '
1998
111
112
113
121
127
110
103
1999
112
114
114
123
133
112
104
2000
114
117
117
127
138
113
104
2001
113
114
115
125
139
114
105
2002
113
116
116
128
141
115
105
2003
114
116
117
129
145
116
106
2004
116
118
118
131
151
117
106
Growth
Rate*
1.1%
1.2%
1.2%
2.0%
3.0%
1.1%
0.4%
  a GWP weighted values
  b Energy content weighted values (EIA 2004)
  c Gross Domestic Product in chained 2000 dollars (BEA 2005)
  o U.S. Census Bureau (2005)
  «Hofmann (2004)
  ' Average annual growth rate
                                Figure ES-15
                                        U.S. Greenhouse Gas Emissions Per Capita
                                         and Per Dollar of Gross Domestic Product
                                                                                 Real GDP
                                 Source BEA (2005), U S Census Bureau (2005), and emission estimates in the
                                 Inventory report
16  Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004

-------
Table ES-9: Emissions of NO,, CO, NMVOCs, and S02 (Gg)
  Gas/Activity	
  NO,                                  2;
    Stationary Fossil Fuel Combustion         :
    Mobile Fossil Fuel Combustion          1
    Oil and Gas Activities
    Waste Combustion
    Industrial Processes
    Solvent Use
    Agricultural Burning
    Waste
  CO                                  13
    Stationary Fossil Fuel Combustion
    Mobile Fossil Fuel Combustion         11'
    Oil and Gas Activities
    Waste Combustion
    Industrial Processes
    Solvent Use
    Agricultural Burning
    Waste
  NMVOCs                              2
    Stationary Fossil Fuel Combustion
    Mobile Fossil Fuel Combustion          1i
    Oil and Gas Activities
    Waste Combustion
    Industrial Processes                    ;
    Solvent Use
    Agricultural Burning
    Waste
  S02                                  2
    Stationary Fossil Fuel Combustion        1
    Mobile Fossil Fuel Combustion
    Oil and Gas Activities
    Waste Combustion
    Industrial Processes
    Solvent Use
    Agricultural Burning
    Waste	
  Source: (EPA 2005) except for estimates from field burning of agricultural residues.
  + Does not exceed 0.5 Gg
  NA (Not Available)
  Note: Totals may not sum due to independent rounding.
1998
21,964
9,419
11,592
130
145
637
3
35
3
98,984
3,927
87,940
332
2,826
3,163
1
789
5
16,403
1,016
7,742
440
326
2,047
4,671
NA
161
17,189
15,191
665
310
30
991
1
NA
1
1999
20,530
8,344
11,300
109
143
595
3
34
3
94,361
5,024
83,484
145
2,725
2,156
46
767
13
15,869
1,045
7,586
414
302
1,813
4,569
NA
140
15,917
13,915
704
283
30
984
1
NA
1
2000
20,288
8,002
11,395
111
114
626
3
35
2
92,895
4,340
83,680
146
1,670
2,217
46
790
8
15,228
1,077
7,230
389
257
1,773
4,384
NA
119
14,829
12,848
632
286
29
1,031
1
NA
1
2001
19,414
7,667
10,823
113
114
656
3
35
2
89,329
4,377
79,972
147
1,672
2,339
45
770
8
15,048
1,080
6,872
400
258
1,769
4,547
NA
122
14,452
12,461
624
289
30
1,047
1
NA
1
2002
18,850
7,522
10,389
135
134
630
6
33
2
87,428
4,020
78,574
116
1,672
2,286
46
706
8
14,217
923
6,560
340
281
1,723
4,256
NA
133
13,928
11,946
631
315
24
1,009
1
NA
1
2003
17,995
7,138
9,916
135
134
631
6
34
2
87,518
4,020
78,574
116
1,672
2,286
46
796
8
13,877
922
6,212
341
282
1,725
4,262
NA
134
14,208
12,220
637
315
24
1,009
1
NA
1
2004
17,076
6,662
9,465
135
134
632
6
39
2
87,599
4,020
78,574
116
1,672
2,286
46
877
8
13,556
922
5,882
341
282
1,727
4,267
NA
134
13,910
11,916
644
315
24
1,009
1
NA
1
react with other chemical compounds in the atmosphere to

form compounds that are greenhouse gases.

     Since 1970, the United States has published estimates

of annual emissions of CO, NOX, NMVOCs, and SO2 (EPA

2005),14 which are regulated under the Clean Air Act. Table

ES-9 shows that fuel combustion accounts for the majority

of emissions of these indirect greenhouse gases. Industrial

processes—such as the manufacture of chemical and allied
products, metals processing, and industrial uses of solvents—
are also significant sources of CO, NOX, and NMVOCs.


Key Categories
    The IPCC's Good Practice Guidance (IPCC  2000)
defines a key category  as a "[source or sink category] that
is prioritized within the national inventory system because
its estimate has a significant influence on a country's total
inventory of direct greenhouse gases in terms of the absolute
15 See Chapter 7 "Methodological Choice and Recalculation" m IPCC (2000). 
                          Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004 17

-------
level of emissions, the trend in emissions, or both."15 By
definition, key categories are sources or sinks that have the
greatest contribution to the absolute overall level of national
emissions in any of the years covered by the time series. In
addition, when an entire time series of emission estimates
is prepared, a thorough investigation of key categories must
also account for the influence of trends of individual source
and sink categories. Finally, a qualitative evaluation of key
categories should be performed, in order to capture any key
categories that were not identified in either of the quantitative
analyses.
    Figure  ES-16 presents  2004 emission estimates for
the 2004 key categories as defined by a level analysis (i.e.,
the contribution of each source or sink category to the total
inventory level). The UNFCCC reporting guidelines request
that key category analyses be reported at an appropriate
level of disaggregation, which may lead to source and sink
category names which differ from those used elsewhere in
the Inventory  report. For more information regarding key
categories, see section 1.5 and Annex 1 of the Inventory
report.
Quality Assurance  and Quality Control
(QA/QC)
    The United States seeks to continually improve the
quality, transparency, and credibility of the Inventory of
U.S. Greenhouse Gas Emissions and Sinks. To assist in these
efforts, the United States implemented a systematic approach
to QA/QC. While QA/QC has always been an integral part
of the U.S. national system for inventory development, the
procedures followed for the current inventory, as described
in the Introduction chapter of the Inventory report, have
been formalized in accordance with the QA/QC plan  and
the  UNFCCC reporting guidelines.

Uncertainty Analysis of Emission
Estimates
    While the current U.S. emissions inventory provides a
solid foundation for the development of a more detailed and
comprehensive national inventory, there are uncertainties
associated with the emission estimates. Some of the current
estimates, such as  those  for CC>2 emissions from energy-
related  activities and cement processing,  are considered
to have low  uncertainties. For some other categories
of emissions, however,  a lack of data or an incomplete
Figure ES-16
                                   2004 Key Categories - Tier 1 Level Assessment
               CO, Emissions from Stationary Combustion - Coal
            C02 Emissions from Mobile Combustion: Road & Other
               CO, Emissions from Stationary Combustion - Gas
                C02 Emissions from Stationary Combustion - Oil
               C02 Emissions from Mobile Combustion: Aviation
                  Direct N20 Emissions from Agricultural Soils
                  C02 Emissions from Non-Energy Use of Fuels
                 CM, Emissions from Solid Waste Disposal Sites
             CH, Fugitive Emissions from Natural Gas Operations
      CH, Emissions from Enteric Fermentation in Domestic Livestock
         Emissions from Substitutes for Ozone Depleting Substances
          Indirect N20 Emissions from Nitrogen Used in Agriculture
            CH4 Fugitive Emissions  from Coal Mining and Handling
                C02 Emissions from Mobile Combustion: Marine
                 CO; Emissions from Iron and Steel Production
                      CO, Emissions from Cement Production
            N20 Emissions from Mobile Combustion: Road & Other
                                                            I            I           1
                                                           500         1,000       1,500
                                                                     2004 Emissions (Tg C02 Eq.)
                                  2,000
2,500
 Note For a complete discussion of the key category analysis see Annex 1 of the full Inventory report.
18 Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004

-------
understanding of how emissions are generated increases the
uncertainty associated with the estimates presented. Acquiring
a better understanding of the uncertainty associated with
inventory estimates is an important step in helping to prioritize
future work and improve the overall quality of the inventory.
Recognizing the  benefit of conducting an uncertainty
analysis, the UNFCCC  reporting guidelines follow the
recommendations of the IPCC Good Practice Guidance (IPCC
2000) and require that countries provide single estimates of
uncertainty for source and sink categories.
    Currently, a qualitative discussion of uncertainty is
presented for all source and sink categories in Annex 7 of
the Inventory report. Within the discussion of each emission
source of the Inventory report, specific factors affecting the
uncertainty  surrounding the  estimates are  discussed. Most
sources also contain a quantitative uncertainty assessment, in
accordance with UNFCCC reporting guidelines.


References
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EIA (2004) Monthly Energy Review,  July  2004 and
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EPA (2005) Air Emissions Trends—Continued Progress
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Hofmann, D (2004) Long-lived Greenhouse Gas Annual
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IPCC (2000) Good Practice Guidance and Uncertainty
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Marland, G., T.A. Boden, and R. J. Andres (2005) "Global,
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                       Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004 19

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o-EPA
United States
Environmental Protection
Agency

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