United states
Environmental Protection
i Agency
Climate Change Indicators
in the United States, 2014
Third Edition
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Contents
Acknowledgments 2
Introduction 3
Understanding Greenhouse Gases 7
Summary of Key Points 8
Greenhouse Gases 12 |yjp Snow an(j |ce 56
U.S. Greenhouse Gas Emissions 14 Arctic Sea Ice 58
Sources of Data on U.S. Greenhouse Glaciers 60
GasEmissions 16 Lake Ice 62
Global Greenhouse Gas Emissions 18 Community Connection: Ice Breakup in
Atmospheric Concentrations of Greenhouse Gases..20 Two Alaskan Rivers 64
Climate Forcing 24 Snowfall 66
Snow Cover 68
Snowpack 70
'Weather and Climate 26
U.S. and Global Temperature 28 I^i
High and Low Temperatures 30 ^£[^jj Health and Society 72
U.S. and Global Precipitation 34 Heating and Cooling Degree Days 74
Heavy Precipitation 36 Heat-Related Deaths 76
Drought 38 Lyme Disease 78
A Closer Look: Temperature and Drought in the Length of Growing Season 80
Southwest 40 Ragweed Pollen Season 82
Tropical Cyclone Activity 42
^n^ Ecosystems 84
Oceans 44 Wildfires 86
0ceanHeat 46 Streamflow 88
Sea Surface Temperature 48 Great Lakes Water Levels and Temperatures 90
SeaLevel 50 Bird Wintering Ranges 92
A Closer Look: Land Loss Along the Leaf and Bk)om Dates 94
Atlantic Coast 52
Community Connection: Cherry Blossom Bloom
0ceanAcidity 54 Dates in Washington, D.C 96
Climate Change Resources 98
Endnotes 99
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Acknowledgments
EPA wishes to thank various federal government agencies, nongovernmental organizations, and other institutions for their
commitment, contribution, and collaboration on this report. EPA looks forward to continuing to work with coordinating
bodies such as the U.S. Global Change Research Program and with other agencies, organizations, and individuals to collect
useful data; inform policies and programs; and explore additional opportunities for updating, improving, and communicating
climate-related indicators.
DATA CONTRIBUTORS AND INDICATOR REVIEWERS
U.S. Governmental Organizations
• Centers for Disease Control and Prevention: C. Ben
Beard, Paul Mead, Ambarish Vaidyanathan
• National Aeronautics and Space Administration: Joey
Comiso, Stacey Frith
• National Oceanic and Atmospheric Administration
- Climate Prediction Center: Gerry Bell
- Coastal Services Center: Nate Herold
- Earth System Research Laboratory: Ed
Dlugokencky, Steve Montzka
- National Climatic Data Center: Deke Arndt, Karin
Gleason, Boyin Huang
- National Ocean Service: Chris Zervas
• U.S. Department of Agriculture
- Agricultural Research Service: Lewis Ziska
- Forest Service: Jennifer Lecker, Karen Short
• U.S. Geological Survey
- Alaska Science Center: Shad O'Neel, Louis Sass
- Maine Water Science Center: Robert Dudley, Glenn
Hodgkins
- New York Water Science Center: Mike McHale
- Washington Water Science Center: Matt
Bachmann
Universities, Nongovernmental Organizations,
and International Institutions
• Bermuda Institute of Ocean Sciences: Nick Bates
• Commonwealth Scientific and Industrial Research
Organisation: John Church, Catia Domingues, Neil
White
• Georgia Institute of Technology: Ray Wang
• Japan Agency for Marine-Earth Science and Technolo-
gy: Masayoshi Ishii
• Massachusetts Institute of Technology: Kerry Emanuel
• National Audubon Society: Justin Schuetz, Candan
Soykan
• North Carolina State University: Ken Kunkel
• Oregon State University, The Oregon Climate Change
Research Institute: Philip Mote, Darrin Sharp
• Rutgers University Global Snow Lab: David Robinson
• Scripps Institution of Oceanography: Tim Arnold
• University of Colorado: Mark Tschudi
• University of Montana: John Dore
• University of Nebraska-Lincoln: Song Feng
• Universidad de las Palmas de Gran Canaria: Melchor
Gonzalez-Davila
• University of Wisconsin-Madison: Corinna Cries
• University of Wisconsin-Milwaukee: Mark Schwartz
• USA National Phenology Network: Jake Weltzin
• Woods Hole Oceanographic Institution: Ivan Lima
• World Glacier Monitoring Service: Michael Zemp
• World Resources Institute: Tom Damassa
PEER REVIEW
The report was peer reviewed by 12 external, independent experts: Michael Oppenheimer, Connie Roser-Renouf,Tanja
Srebotnjak, Scott C. Doney, Alexa McKerrow, Noah Molotch, Ron Neilson, Nicholas H. Ogden, Michael J. Prather, Terry
L. Root, Claudia Tebaldi, and David G.Victor.
REPORT DEVELOPMENT AND PRODUCTION
Support for content development, data analysis, and report design and production was provided by Eastern Research
Group, Inc. (ERG). Abt Associates also provided analytical and content development support.
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Introduction
.*-.
he Earth's climate is changing. Temperatures are rising, snow and rainfall
patterns are shifting, and more extreme climate events—like heavy rain-
storms and record high temperatures—are already taking place. Scientists
are highly confident that many of these observed changes can be linked to the
climbing levels of carbon dioxide and other greenhouse gases in our atmosphere,
which are caused by human activities.
The climate change indicators in this report look at the composition of the
atmosphere, fundamental measures of climate, and the extent to which several
climate-sensitive aspects of the oceans, snow and ice, human health, society, and
ecosystems are changing.Together, these indicators present compelling evidence
that climate change is happening now in the United States and around the world.
HOW IS THE CLIMATE CHANGING?
Since the Industrial Revolution began in the 1700s, people have added a signifi-
cant amount of greenhouse gases into the atmosphere, largely by burning fossil
fuels to generate electricity, heat and cool buildings, power vehicles—as well as
by clearing forests. The major greenhouse gases that people have added to the
atmosphere are carbon dioxide, methane, nitrous oxide, and fluorinated gases.
When these gases are emitted into the atmosphere, many remain there for long
time periods, ranging from a decade to thousands of years. Past emissions affect
our atmosphere in the present day; current and future emissions will continue to
increase the levels of these gases in our atmosphere for the foreseeable future.
"Greenhouse gases" got their name because they trap heat (energy) in the lower
part of the atmosphere (see "The Greenhouse Effecf'on p. 4). As more of these
gases are added to the atmosphere, more heat is trapped. This extra heat leads
to higher air temperatures near the Earth's surface, alters weather patterns, and
raises the temperature of the oceans.
These observed changes affect people and the environment in important ways.
For example, sea levels are rising, glaciers are melting, and plant and animal life
cycles are changing.These types of changes can bring about fundamental disrup-
tions in ecosystems, affecting plant and animal populations, communities, and
biodiversity. Such changes can also affect society and traditional ways of life for
WHY USE INDICATORS?
One important way to track and
communicate the causes and ef-
fects of climate change is through
the use of indicators. An indicator
represents the state or trend of
certain environmental or societal
conditions over a given area and
a specified period of time. For
example, long-term measurements
of temperature in the United
States and globally are used as
an indicator to track and better
understand the effects of changes
in the Earth's climate.
HOW DO THE
INDICATORS RELATE TO
CLIMATE CHANGE?
All of the indicators in this report
relate to either the causes or
effects of climate change. Some
indicators show trends that can be
more directly linked to human-in-
duced climate change than others.
Collectively, the trends depicted in
these indicators provide important
evidence of "what climate change
looks like."
WHAT IS CLIMATE CHANGE?
Climate change refers to any substantial change in measures of climate (such as temperature or precipitation)
lasting for an extended period (decades or longer). Climate change may result from natural factors and process-
es or from human activities.
Global warming is a term often used interchangeably with the term "climate change," but they are not entire-
ly the same thing. Global warming refers to an average increase in the temperature of the atmosphere near the
Earth's surface. Global warming is just one aspect of global climate change, though a very important one.
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fe
certain communities, including where people can live, what kinds of crops are
most viable, and what kinds of businesses can thrive in certain areas.
Although the climate is continually changing, not every climate change indicator
will show a smooth pattern of steady change.The Earth is a complex system, and
there will always be natural variations from one year to the next—for example, a
very warm year followed by a colder year. The Earth's climate also goes through
other natural cycles that can play out over a period of several years or even de-
cades. Individual years or even individual decades can deviate from the long-term
trend.1 Thus, EPA's indicators present trends for as many years as the underlying
data allow.
ABOUT THIS REPORT
EPA publishes this report to communicate information about the science and
impacts of climate change, assess trends in environmental quality, and inform de-
cision-making. Climate Change Indicators in the United States, 2014, is the third
edition of a report first published by the U.S. Environmental Protection Agency
(EPA) in 2010 and updated in 2012. This report presents 30 indicators to help
readers understand observed long-term trends related to the causes and effects
of climate change, the significance of these changes, and their possible conse-
quences for people, the environment, and society. Although each indicator has a
connection to climate change, this report is not intended to identify the extent to
which a certain indicator is driving climate change, nor the relative role of climate
change in causing a trend in an observed indicator. Connections between human
activities, climate change, and observed indicators are explored in more detail
elsewhere in the scientific literature.
This report and the accompanying detailed technical documentation have been
designed to ensure that the science and underlying peer-reviewed data sup-
porting the indicators are presented and documented transparently. This report
consists of peer-reviewed, publicly available data from a number of government
agencies, academic institutions, and other organizations. EPA also received
feedback from scientists, researchers, and communications experts in nongov-
ernmental and private sectors. This feedback helped to inform the content and
new features of this 2014 report. The entire report, including its technical support
document, was peer-reviewed by independent technical experts.
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About the Indicators in This Report
The indicators in this report were chosen using a set of criteria that considered
usefulness, data quality, and relevance to climate change.The report is a compila-
tion of key data sets for communication purposes; in addition to being published
here, these data sets have been published in the scientific literature and in other
government or academic reports.
Trends relevant to climate change are best viewed at broad geographic scales and
over long time horizons, rather than at localized scales or over a few years or a
season. The indicators in this report are based on historical records that go back
in time as far as possible without sacrificing data quality. Most of the indicators
in this report focus on the United States. However, some include global trends to
provide context or a basis for comparison, or because they are intrinsically global
in nature, such as atmospheric concentrations of greenhouse gases, which are
influenced by global activities. The geographic extent and timeframe that each
indicator represents largely depend on data availability and the nature of what is
being measured.
All of the indicators discussed in this report relate to either the causes or effects
of climate change. Some indicators are directly linked to human activities that
cause climate change, such as Global Greenhouse Gas Emissions. Changes depict-
ed by other indicators, such as U.S. and Global Temperature, have been confidently
linked with the increase in greenhouse gases caused by human activity. Some of
the trends in other indicators, such as Wildfires, although consistent with what
one would expect in a warming climate, cannot yet be firmly attributed to hu-
man-induced climate change for various reasons (for example, limitations in the
historical data, or other factors in addition to climate change that may influence
the trend). A few indicators do not yet show any significant trend over the period
for which data are available.
A Roadmap to the Report
The indicators are divided into six chapters: Greenhouse Gases, Weather and Cli-
mate, Oceans, Snow and Ice, Health and Society, and Ecosystems. Some chapters
also include a "Community Connection" or "A Closer Look" feature that high-
lights a specific region, data record, or area of interest. Each indicator features
five elements:
• One or more graphics depicting changes over time. Some indicators consist of
a single metric, while others present multiple metrics (for example, the Drought
indicator shows two different ways of calculating drought).
• Key points about what the indicator shows.
• Background on how the indicator relates to climate change.
• Information about how the indicator was developed.
• Important notes concerning interpretation of the indicator.
EPA has compiled an accompanying technical support document containing
more detailed information about each indicator, including data sources, data
collection methods, calculations, statistical considerations, and sources of un-
certainty. This document also describes EPA's approach and criteria for selecting
indicators for the report. This information is available on EPA's website at:
www.epa.gov/climatechange/indicators.
Additional resources that can provide readers with more information appear at
the end of the report (see Climate Change Resources on p. 98).
WHO IS THIS REPORT FOR?
Climate Change Indicators in the
United States, 2014, is written
with the primary goal of informing
readers' understanding of climate
change. It is also designed to be
useful for the public, scientists, an-
alysts, decision-makers, educators,
and others who can use climate
change indicators as a tool for:
O Effectively communicating
relevant climate science informa-
tion in a sound, transparent, and
easy-to-understand way.
O Assessing trends in environmen-
tal quality, factors that influence
the environment, and effects on
ecosystems and society.
O Informing science-based deci-
sion-making.
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LOOKING AHEAD
Indicators of climate change are
expected to become even more
numerous and to depict even
clearer trends in the future. EPA
will continue to work in partner-
ship with coordinating bodies,
such as the U.S. Global Change
Research Program, and with
other agencies, organizations, and
individuals to collect and commu-
nicate useful data and to inform
policies and programs based
on this knowledge. As new and
more comprehensive indicator
data become available, EPA will
continually update the indicators
presented in this report.
WHAT'S NEW IN 2014?
The 2014 report reflects the following new features and changes:
• Four new indicators: Heating and Cooling Degree Days, Lyme Disease,
Wildfires, and Great Lakes Water Levels and Temperatures. These additions
provide further evidence of climate change and its effects on people, society, and
ecosystems.
• Expanded indicators: Atmospheric Concentrations of Greenhouse Gases
was expanded to cover global concentrations of ozone, and Climate Forcing was
expanded to show the influence of ozone and other short-lived climate forcers.
New metrics were added to the High and Low Temperatures and Streamflow
indicators. Maps were added to Sea Surface Temperature and Leaf and Bloom
Dates to show how changes over time vary by region.
• Updated indicators: Nearly all indicators have been updated with additional years
of data that have become available since the last report.
• "Community Connection" and "A Closer Look" content: Four chapters high-
light observed data for particular areas to provide a local or regional perspective on
relevant topics. The data for these features meet the same data quality criteria as
EPA's national indicators, but are focused on highlighting specific, more localized
areas or topics of interest.
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Understanding Greenhouse Gases
MAJOR GREENHOUSE GASES ASSOCIATED WITH HUMAN ACTIVITIES
The major greenhouse gases emitted into the atmosphere are carbon dioxide,
methane, nitrous oxide, and fluorinated gases (see the table below). Some of
these gases are produced almost entirely by human activities; others come from a
combination of natural sources and human activities.
Many of the major greenhouse gases can remain in the atmosphere for tens to
hundreds of years after being released. They become globally mixed in the lower
part of the atmosphere, called the troposphere (the first several miles above
the Earth's surface), reflecting the combined contributions of emissions sources
worldwide from the past and present. Due to this global mixing, concentrations of
these gases will be fairly similar no matter where in the world they are measured.
Some other substances have much shorter atmospheric lifetimes (i.e., less than
a year) but are still relevant to climate change. Important short-lived substances
that affect the climate include water vapor, ozone in the troposphere, pollutants
that lead to ozone formation, and aerosols (atmospheric particles) such as black
carbon and sulfates. Water vapor, tropospheric ozone, and black carbon contribute
to warming, while other aerosols produce a cooling effect.
Several factors determine how strongly a particular greenhouse gas will affect the
Earth's climate. One factor is the length of time that the gas remains in the atmo-
sphere. A second factor is each gas's unique ability to absorb energy. By consid-
ering both of these factors, scientists calculate a gas's global warming potential,
which measures how much a given amount of the greenhouse gas is estimated
to contribute to global warming over a specific period of time (for example, 100
years) after being emitted. For purposes of comparison, global warming potential
values are calculated in relation to carbon dioxide, which is assigned a global
warming potential equal to 1 .The table below describes sources, lifetimes, and
global warming potentials for several important long-lived greenhouse gases.
GASES AND SUBSTANCES
INCLUDED IN THIS REPORT
This report focuses on most of
the major, well-mixed green-
house gases that contribute to
the vast majority of warming
of the climate. It also includes
certain substances with shorter
atmospheric lifetimes (i.e., less
than a year) that are relevant to
climate change. In addition to
several long-lived greenhouse
gases, the Atmospheric Concen-
trations of Greenhouse Gases
indicator tracks concentrations of
ozone in the layers of the Earth's
atmosphere, while Figure 2 of the
Climate Forcing indicator shows
the influence of a variety of short-
lived substances.
Major Long-Lived Greenhouse Gases and Their Characteristics
Greenhouse
Carbon
dioxide
Methane
Nitrous oxide
Fluorinated
gases
How it's produced
Emitted primarily through the burning of fossil fuels (oil, natural gas, and coal), solid waste,
and trees and wood products. Changes in land use also play a role. Deforestation and soil
degradation add carbon dioxide to the atmosphere, while forest regrowth takes it out of the
atmosphere.
Emitted during the production and transport of coal, natural gas, and oil. Methane emissions
also result from livestock and agricultural practices and from the anaerobic decay of organic
waste in municipal solid waste landfills.
Emitted during agricultural and industrial activities, as well as during combustion of fossil
fuels and solid waste.
A group of gases that contain fluorine, including hydrofluorocarbons, perfluorocarbons, and
sulfur hexafluoride, among other chemicals. These gases are emitted from a variety of indus-
trial processes and commercial and household uses, and do not occur naturally. Sometimes
used as substitutes for ozone-depleting substances such as chlorofluorocarbons (CFCs).
Average
lifetime in the
atmosphere
see below*
12 years
121 years
A few weeks to
thousands of
years
100-year global
warming
potential
1
265
Varies (the
highest is sulfur
hexafluoride at
23,500)
'his table shows 100-year global warming potentials, which describe the effects that occur over a period of 100 years after a particular mass of a gas is emitted. Global
warming potentials and lifetimes come from the Intergovernmental Panel on Climate Change's Fifth Assessment Report.'
* Carbon dioxide's lifetime cannot be represented with a single value because the gas is not destroyed over time, but instead moves among different parts of the ocean-
atmosphere-land system. Some of the excess carbon dioxide will be absorbed quickly (for example, by the ocean surface), but some will remain in the atmosphere for
thousands of years, due in part to the very slow process by which carbon is transferred to ocean sediments.
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Summary of Key Points
1
U.S. Greenhouse Gas Emissions. In the United States, greenhouse gas emissions caused by human
activities increased by 5 percent from 1990 to 2012. However, since 2005, total U.S. greenhouse gas
emissions have decreased by 10 percent. Carbon dioxide accounts for most of the nation's emissions and
most of the increase since 1990. Electricity generation is the largest source of greenhouse gas emissions in
the United States, followed by transportation. Emissions per person have decreased slightly in the last few
years.
Global Greenhouse Gas Emissions. Worldwide, net emissions of greenhouse gases from human
activities increased by 35 percent from 1990 to 2010. Emissions of carbon dioxide, which account for
about three-fourths of total emissions, increased by 42 percent over this period. As with the United States,
the majority of the world's emissions result from electricity generation, transportation, and other forms of
energy production and use.
Atmospheric Concentrations of Greenhouse Gases. Concentrations of carbon dioxide and other
greenhouse gases in the atmosphere have increased since the beginning of the industrial era. Almost all
of this increase is attributable to human activities. Historical measurements show that current levels of
many greenhouse gases are higher than any levels recorded for hundreds of thousands of years, even after
accounting for natural fluctuations.
Climate Forcing. Climate forcing refers to a change in the Earth's energy balance, leading to either a
warming or cooling effect. An increase in the atmospheric concentrations of greenhouse gases produces a
positive climate forcing, or warming effect. From 1990 to 2013, the total warming effect from greenhouse
gases added by humans to the Earth's atmosphere increased by 34 percent. The warming effect associated
with carbon dioxide alone increased by 27 percent.
U.S. and Global Temperature. Average temperatures have risen across the contiguous 48 states since
1901, with an increased rate of warming over the past 30 years. Seven of the top 10 warmest years on re-
cord have occurred since 1998. Average global temperatures show a similar trend, and the top 10 warmest
years on record worldwide have all occurred since 1998. Within the United States, temperatures in parts of
the North, the West, and Alaska have increased the most.
High and Low Temperatures. Many extreme temperature conditions are becoming more common.
Since the 1970s, unusually hot summer temperatures have become more common in the United States,
and heat waves have become more frequent—although the most severe heat waves in U.S. history remain
those that occurred during the "Dust Bowl" in the 1930s. Record-setting daily high temperatures have
become more common than record lows. The decade from 2000 to 2009 had twice as many record highs
as record lows.
U.S. and Global Precipitation. Total annual precipitation has increased in the United States and over
land areas worldwide. Since 1901, precipitation has increased at an average rate of 0.5 percent per decade
in the contiguous 48 states and 0.2 percent per decade over land areas worldwide. However, shifting
weather patterns have caused certain areas, such as Hawaii and parts of the Southwest, to experience less
precipitation than usual.
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Heavy Precipitation. In recent years, a higher percentage of precipitation in the United States has come
in the form of intense single-day events. Nationwide, nine of the top 10 years for extreme one-day precip-
itation events have occurred since 1990.The occurrence of abnormally high annual precipitation totals (as
defined by the National Oceanic and Atmospheric Administration) has also increased.
Drought. Average drought conditions across the nation have varied since records began in 1895. The
1930s and 1950s saw the most widespread droughts, while the last 50 years have generally been wetter
than average. However, specific trends vary by region. A more detailed index developed recently shows
that between 2000 and 2013, roughly 20 to 70 percent of the United States experienced drought at any
given time, but this index has not been in use for long enough to compare with historical drought patterns.
A Closer Look: Temperature and Drought in the Southwest. The southwestern United
States is particularly sensitive to changes in temperature and thus vulnerable to drought, as
even a small decrease in water availability in this already arid region can threaten natural
systems and society.
Tropical Cyclone Activity. Tropical storm activity in the Atlantic Ocean, the Caribbean, and the Gulf of
Mexico has increased during the past 20 years. Increased storm intensity is closely related to variations in
sea surface temperature in the tropical Atlantic. However, changes in observation methods over time make
it difficult to know for sure whether a long-term increase in storm activity has occurred. Records collected
since the late 1800s suggest that the actual number of hurricanes per year has not increased.
Ocean Heat. Three separate analyses show that the amount of heat stored in the ocean has increased
substantially since the 1950s. Ocean heat content not only determines sea surface temperature, but also
affects sea level and currents.
Sea Surface Temperature. Ocean surface temperatures increased around the world over the 20th
century. Even with some year-to-year variation, the overall increase is clear, and sea surface tempera-
tures have been higher during the past three decades than at any other time since reliable observations
began in the late 1800s.
Sea Level. When averaged over all the world's oceans, sea level has increased at a rate of roughly
six-tenths of an inch per decade since 1880. The rate of increase has accelerated in recent years to more
than an inch per decade. Changes in sea level relative to the land vary by region. Along the U.S. coast-
line, sea level has risen the most along the Mid-Atlantic coast and parts of the Gulf coast, where some
stations registered increases of more than 8 inches between 1960 and 2013. Sea level has decreased
relative to the land in parts of Alaska and the Northwest.
A Closer Look: Land Loss Along the Atlantic Coast. As sea level rises, dry land and
wetland can turn into open water. Along many parts of the Atlantic coast, this problem is
made worse by low elevations and land that is already sinking. Between 1996 and 2011,
the coastline from Florida to New York lost more land than it gained.
Ocean Acidity. The ocean has become more acidic over the past few centuries because of increased
levels of atmospheric carbon dioxide, which dissolves in the water. Higher acidity affects the balance of
minerals in the water, which can make it more difficult for certain marine animals to build their skeletons
and shells.
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Arctic Sea Ice. Part of the Arctic Ocean is covered by ice year-round. The area covered by ice is typically
smallest in September, after the summer melting season. The minimum extent of Arctic sea ice has de-
creased over time, and in September 2012 it was the smallest on record. Arctic ice has also become thinner,
which makes it more vulnerable to additional melting.
Glaciers. Glaciers in the United States and around the world have generally shrunk since the 1960s, and
the rate at which glaciers are melting has accelerated over the last decade. The loss of ice from glaciers has
contributed to the observed rise in sea level.
Lake Ice. Most lakes in the northern United States are freezing later and thawing earlier compared with
the 1800s and early 1900s. Freeze dates have shifted later at a rate of roughly half a day to one day per
decade, while thaw dates for most of the lakes studied have shifted earlier at a rate of half a day to two
days per decade.
Community Connection: Ice Breakup in Two Alaskan Rivers. Regions in the far north are
warming more quickly than other parts of the world.Two long-running contests on theTa-
nana and Yukon rivers in Alaska—where people guess the date when the river ice will break
up in the spring—provide a century's worth of evidence revealing that the ice on these rivers
is generally breaking up earlier in the spring than it used to.
Snowfall. Total snowfall—the amount of snow that falls in a particular location—has decreased in most
parts of the country since widespread records began in 1930. One reason for this decline is that more than
three-fourths of the locations studied have seen more winter precipitation fall in the form of rain instead
of snow.
Snow Cover. Snow cover refers to the area of land that is covered by snow at any given time. Between
1972 and 2013, the average portion of North America covered by snow decreased at a rate of about 3,500
square miles per year, based on weekly measurements taken throughout the year. However, there has been
much year-to-year variability.
Snowpack. The depth or thickness of snow on the ground (snowpack) in early spring decreased at about
three-fourths of measurement sites in the western United States between 1955 and 2013. However, other
locations saw an increase in spring snowpack. The average change across all sites for this time period
amounts to about a 14 percent decline.
Heating and Cooling Degree Days. Heating and cooling degree days measure the difference between
outdoor temperatures and the temperatures that people find comfortable indoors. As the U.S. climate has
warmed in recent years, heating degree days have decreased and cooling degree days have increased
overall, suggesting that Americans need to use less energy for heating and more energy for air condition-
ing. This pattern stands out the most in the North and West, while much of the Southeast has experienced
the opposite results.
Heat-Related Deaths. Over the past three decades, nearly 8,000 Americans were reported to have
died as a direct result of heat-related illnesses such as heat stroke. The annual death rate is higher when
accounting for other deaths in which heat was reported as a contributing factor. Considerable year-to-year
variability in the data and certain limitations of this indicator make it difficult to determine whether the
United States has experienced long-term trends in the number of deaths classified as "heat-related."
Lyme Disease. Lyme disease is a bacterial illness spread by ticks that bite humans.Tick habitat and popu-
lations are influenced by many factors, including climate. Nationwide, the rate of reported cases of Lyme
disease has approximately doubled since 1991. Lyme disease is most common in the Northeast and the
upper Midwest, where some states now report 50 to 90 more cases of Lyme disease per 100,000 people
than they did in 1991.
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Length of Growing Season. The average length of the growing season in the contiguous 48 states
has increased by nearly two weeks since the beginning of the 20th century. A particularly large and steady
increase has occurred over the last 30 years. The observed changes reflect earlier spring warming as well
as later arrival of fall frosts. The length of the growing season has increased more rapidly in the West than
in the East.
Ragweed Pollen Season. Warmer temperatures and later fall frosts allow ragweed plants to produce
pollen later into the year, potentially prolonging the allergy season for millions of people.The length of
ragweed pollen season has increased at 10 out of 11 locations studied in the central United States and
Canada since 1995.The change becomes more pronounced from south to north.
Wildfires. Since 1983, the United States has had an average of 72,000 recorded wildfires per year. Of
the 10 years with the largest acreage burned, nine have occurred since 2000, with many of the largest
increases occurring in western states. The proportion of burned land suffering severe damage each year
has ranged from 5 to 22 percent.
Streamflow. Changes in temperature, precipitation, snowpack, and glaciers can affect the rate of
streamflow and the timing of peak flow. Over the last 73 years, minimum, maximum, and average flows
have changed in many parts of the country—some higher, some lower. Nearly half of the rivers and
streams measured show peak winter-spring runoff happening at least five days earlier than it did in the
mid-20th century.
Great Lakes Water Levels and Temperatures. Water levels in most of the Great Lakes have declined
in the last few decades. Water levels in lakes are influenced by water temperature, which affects evapo-
ration rates and ice formation. Since 1995, average surface water temperatures have increased by a few
degrees for Lakes Superior, Michigan, Huron, and Ontario. Less of a temperature change has been observed
in Lake Erie.
Bird Wintering Ranges. Some birds shift their range or alter their migration habits to adapt to changes
in temperature or other environmental conditions. Long-term studies have found that bird species in North
America have shifted their wintering grounds northward by an average of more than 40 miles since 1966,
with several species shifting by hundreds of miles. On average, bird species have also moved their winter-
ing grounds farther from the coast, consistent with inland winter temperatures becoming less severe.
Leaf and Bloom Dates. Leaf growth and flower blooms are examples of natural events whose timing
can be influenced by climate change. Observations of lilacs and honeysuckles in the contiguous 48 states
suggest that first leaf dates and bloom dates show a great deal of year-to-year variability. Leaf and bloom
events are generally happening earlier throughout the North and West but later in much of the South.
Community Connection: Cherry Blossom Bloom Dates in Washington, D.C. "Peak"
bloom dates of the iconic cherry trees in Washington, D.C., recorded since the 1920s, indicate
that cherry trees are blooming slightly earlier than in the past. Bloom dates are key to plan-
ning the Cherry Blossom Festival, one of the region's most popular spring attractions.
11
ft
-------
REENHOUS
GASES
Greenhouse gases from human activities are
the most significant driver of observed climate
change since the mid-20th century.1 The indicators
in this chapter characterize emissions of the
major greenhouse gases resulting from human
activities, the concentrations of these gases
in the atmosphere, and how emissions and
concentrations have changed over time. When
comparing emissions of different gases, these
indicators use a concept called "global warming
potential" to convert amounts of other gases into
carbon dioxide equivalents.
WHY DOES IT MATTER?
As greenhouse gas emissions from human activities increase, they build
up in the atmosphere and warm the climate, leading to many other chang-
es around the world—in the atmosphere, on land, and in the oceans. The
indicators in other chapters of this report illustrate many of these changes.
These changes have both positive and negative effects on people, society,
and the environment—including plants and animals. Because many of the
major greenhouse gases stay in the atmosphere for tens to hundreds of
years after being released, their warming effects on the climate persist over
a long time and can therefore affect both present and future generations.
-------
Summary of Key Points
ions. In the United States, greenhouse gas emissions caused
by human activities increased by 5 percent from 1990 to 2012. However, since 2005, total U.S.
greenhouse gas emissions have decreased by 10 percent. Carbon dioxide accounts for most of the
nation's emissions and most of the increase since 1990. Electricity generation is the largest source
of greenhouse gas emissions in the United States, followed by transportation. Emissions per person
have decreased slightly in the last few years.
Greenhouse Gas Emissions. EPA has two
key programs that provide data on greenhouse gas emissions in the United States: the
Inventory of U.S. Greenhouse Gas Emissions and Sinks and the Greenhouse Gas Reporting
Program. The programs are complementary, providing both a higher-level perspective
on the nation's total emissions and detailed information about the sources and types of
emissions from individual facilities.
Global Greenhouse Gas Emissions. Worldwide, net emissions of greenhouse gases
from human activities increased by 35 percent from 1990 to 2010. Emissions of carbon dioxide,
which account for about three-fourths of total emissions, increased by 42 percent over this period.
As with the United States, the majority of the world's emissions result from electricity generation,
transportation, and other forms of energy production and use.
Atmospheric Concentrations of Greenhouse Gases. Concentrations of carbon
dioxide and other greenhouse gases in the atmosphere have increased since the beginning of the
industrial era. Almost all of this increase is attributable to human activities.2 Historical measure-
ments show that current levels of many greenhouse gases are higher than any levels recorded for
hundreds of thousands of years, even after accounting for natural fluctuations.
Climate Forcing. Climate forcing refers to a change in the Earth's energy balance, leading to
either a warming or cooling effect. An increase in the atmospheric concentrations of greenhouse
gases produces a positive climate forcing, or warming effect. From 1990 to 2013, the total warm-
ing effect from greenhouse gases added by humans to the Earth's atmosphere increased by 34
percent. The warming effect associated with carbon dioxide alone increased by 27 percent.
i
13
l: I,
-------
KEY POINTS
In 2012, U.S. greenhouse gas emis-
sions totaled 6,526 million metric
tons (14.4 trillion pounds) of carbon
dioxide equivalents. This 2012 total
represents a 5 percent increase since
1990 but a 10 percent decrease since
2005 (see Figure 1).
For the United States, during the pe-
riod from 1990 to 2012 (see Figure 1):
- Emissions of carbon dioxide, the
primary greenhouse gas emitted
by human activities, increased by
5 percent.
- Methane emissions decreased by
11 percent, as reduced emissions
from landfills, coal mines, and
natural gas systems were greater
than increases in emissions
from activities such as livestock
production.3
- Nitrous oxide emissions, pre-
dominantly from agricultural soil
management practices such as
the use of nitrogen as a fertilizer,
increased by nearly 3 percent.
- Emissions of fluorinated gases
(hydrofluorocarbons, perfluoro-
carbons, and sulfur hexafluoride),
released as a result of commer-
cial, industrial, and household
uses, increased by 83 percent.
Electricity generation is the largest
U.S. emissions source, accounting for
32 percent of total greenhouse gas
emissions since 1990. Transportation
is the second-largest source of green-
house gas emissions, accounting for
27 percent of emissions since 1990
(see Figure 2).
Emissions sinks, the opposite of emis-
sions sources, absorb carbon dioxide
from the atmosphere. In 2012,15
percent of U.S. greenhouse gas emis-
sions were offset by sinks resulting
from land use and forestry practices
(see Figure 2). One major sink is the
net growth of forests, which remove
(Continued on p. 15)
A number of factors influence the quantities of greenhouse gases released into the atmosphere, including
economic activity, population, consumption patterns, energy prices, land use, and technology. There are
several ways to track these emissions, such as by measuring emissions directly, calculating emissions
based on the amount of fuel that people burn, and estimating other activities and their associated emissions.
EPA has two key programs that provide data on greenhouse gas emissions in the United States: the Inventory
of U.S. Greenhouse Gas Emissions and Sinks and the Greenhouse Gas Reporting Program. See "Sources of
Data on U.S. Greenhouse Gas Emissions" (on p. 16) to learn more about these programs.
ABOUT THE INDICATOR
This indicator focuses on emissions of carbon dioxide, methane, nitrous oxide, and several fluorinated gases—
all important greenhouse gases that are influenced by human activities. These particular gases are covered un-
der the United Nations Framework Convention on Climate Change, an international agreement that requires
participating countries to develop and periodically submit an inventory of greenhouse gas emissions. Data and
analysis for this indicator come from EPA's annual inventory submission, the Inventory of U.S. Greenhouse Gas
Emissions and Sinks: 1990-2012.4~[h\s indicator is restricted to emissions associated with human activities.
Each greenhouse gas has a different lifetime (how long it stays in the atmosphere) and a different ability
to trap heat in our atmosphere. To allow different gases to be compared and added together, emissions are
converted into carbon dioxide equivalents. This step uses each gas's 100-year global warming potential,
which measures how much a given amount of the gas is estimated to contribute to global warming over a
period of 100 years after being emitted. Carbon dioxide is assigned a global warming potential equal to 1.
This analysis uses global warming potentials from the Intergovernmental Panel on Climate Change's (IPCC's)
Second Assessment Report. In that report, methane has a global warming potential of 21, which means a
ton of methane emissions contributes 21 times as much warming as a ton of carbon dioxide emissions over
100 years, and that ton of methane emissions is therefore equal to 21 tons of carbon dioxide equivalents.
See the table on p. 7 for comparison with global warming potentials from IPCC's Fifth Assessment Report. For
additional perspective, this indicator also shows greenhouse gas emissions in relation to economic output and
population.
Figure 1. U.S. Greenhouse Gas Emissions by Gas, 1990-2012
8,000
7,000
2 Jj 6,000
oJ '5 5,000
£ o-
c *
O -g 4,000
:= °5
J. ~ 3,000
MFCs, PFCs, and SF6"
Nitrous oxide
Carbon dioxide
M 5
£ u
2,000
1,000
1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012
Year
This figure shows emissions of carbon dioxide, methane, nitrous oxide, and several fluorinated gases in the United States from
1990 to 2012. For consistency, emissions are expressed in million metric tons of carbon dioxide equivalents.
* HFCs are hydrofluomcarbons, PFCs are perfluomcarbons, and SF6 is sulfur hexafluoride.
Data source: U.S. EPA, 2014s
14
-------
Figure 2. U.S. Greenhouse Gas Emissions and Sinks by Economic Sector, 1990-2012
8,000
Agriculture
Commercial
Residential
7,000
-1,000
-2,000
Land use, land-use change, and forestry (sinks)
1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012
Year
Figure 3. U.S. Greenhouse Gas Emissions per Capita and per Dollar of
GDP, 1990-2012
180
160
40
1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012
Year
This figure shows trends in greenhouse gas emissions from 1990 to 2012 per capita (heavy orange line), based on the total U.S. popu-
lation (thin orange line). It also shows trends in emissions per dollar of real GDP (heavy blue line). Real GDP (thin blue line) is the value
of all goods and services produced in the country during a given year, adjusted for inflation. All data are indexed to 1990 as the base
year, which is assigned a value of 100. For instance, a real GDP value of 173 in the year 2012 would represent a 73 percent increase
since 1990.
Data source: U.S. EPA, 20147
INDICATOR NOTES
While this indicator includes the major greenhouse gases emitted by human activities, it does not include other
greenhouse gases and substances that are not covered under the United Nations Framework Convention on
Climate Change but that still affect the Earth's energy balance and climate (see the Climate Forcing indicator
on p. 24 for more details). For example, this indicator excludes ozone-depleting substances such as chlorofluo-
rocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), which have high global warming potentials, as these
gases have been or are currently being phased out under an international agreement called the Montreal Pro-
tocol. This indicator also excludes black carbon and aerosols, which most greenhouse gas emissions inventories
do not cover. There are also many natural greenhouse gas emissions sources; however, this indicator includes
only emissions that are associated with human activities—those that are most responsible for the observed
buildup of these gases in our atmosphere.
This figure shows greenhouse gas emissions and
sinks (negative values) by source in the United States
from 1990 to 2012. For consistency, emissions are
expressed in million metric tons of carbon dioxide
equivalents. All electric power emissions are grouped
together in the "Electricity generation"sector, so
other sectors such as "Residential" and "Commer-
cial" are only showing non-electric sources, such as
burning oil or gas for heating. Totals do not match
Figure 1 exactly because the economic sectors shown
here do not include emissions from U.S. territories
outside the 50 states.
Data source: U.S. EPA, 20146
DATA SOURCES
Data for this indicator came from EPA's
Inventory of U.S. Greenhouse Gas
Emissions and Sinks: 1990-2012. This
report is available online at: www.epa.
gov/dimatechange/ghgemissions/usinven-
toryreport.html. The calculations in Figure
3 are based on GDP and population data
provided by the U.S. Bureau of Economic
Analysis and the U.S. Census, respectively.
KEY POINTS
ontinued from p. 14)
carbon from the atmosphere. Other
carbon sinks are associated with how
people use the land, including the
practice of depositing yard trim-
mings and food scraps in landfills.
Emissions increased at about the
same rate as the population from
1990 to 2007, which caused emis-
sions per capita to remain fairly level
(see Figure 3). Total emissions and
emissions per capita declined from
2007 to 2009, due in part to a drop
in U.S. economic production during
this time. Emissions decreased again
from 2010 to 2012, largely due to
the growing use of natural gas to
generate electricity in place of more
carbon-intensive fuels.8
From 1990 to 2012, greenhouse
gas emissions per dollar of goods
and services produced by the U.S.
economy (the gross domestic product
or GDP) declined by 39 percent (see
Figure 3). This change may reflect
a combination of increased energy
efficiency and structural changes in
the economy.
15
-------
Sources of Data on U.S.
Greenhouse Gas Emissions
EPA has two key programs that provide data on greenhouse gas
emissions in the United States: the Inventory of U.S. Greenhouse Gas
Emissions and Sinks and the Greenhouse Gas Reporting Program.
ERA'S INVENTORY OF GREENHOUSE GAS EMISSIONS
AND SINKS
EPA develops an annual report called the Inventory of U.S. Greenhouse Gas Emissions and Sinks (or
the Greenhouse Gas Inventory). This report tracks trends in total annual U.S. emissions by source (or
sink), economic sector, and greenhouse gas going back to 1990. EPA uses national energy data, data on
national agricultural activities, and other national statistics to provide a comprehensive accounting of
total greenhouse gas emissions for all man-made sources in the United States. This inventory fulfills the
nation's obligation to provide an annual emissions report under the United Nations Framework Conven-
tion on Climate Change.
ERA'S GREENHOUSE GAS REPORTING PROGRAM
EPA's Greenhouse Gas Reporting Program collects annual emissions data from industrial sources that
directly emit large amounts of greenhouse gases. Generally, facilities that emit more than 25,000 metric
tons of carbon dioxide equivalents per year are required to report. The program also collects data from
entities known as "suppliers" that supply certain fossil fuels and industrial gases that will emit green-
house gases into the atmosphere if burned or released—for example, refineries that supply petroleum
products such as gasoline. The Greenhouse Gas Reporting Program only requires reporting; it is not an
emissions control program. This program helps EPA and the public understand where greenhouse gas
emissions are coming from, and will improve our ability to make informed policy, business, and regulato-
ry decisions. This program:
• Covers carbon dioxide, methane, nitrous oxide, and fluorinated gases.
• Represents 85 to 90 percent of U.S. greenhouse gas emissions.
• Covers 41 industrial categories (for example, power plants, oil and gas producers, landfills, and
other industrial facilities).
• Collects greenhouse gas data from more than 8,000 entities.
Visit: www.epa.gov/ghgreporting to learn more about the sources that report data.
EPA's Greenhouse Gas Reporting Program provides facility-level information and allows people to track
changes in greenhouse gas emissions in various industries, geographic areas, and industrial facilities. EPA
has now verified three years of data and made them publicly available.
Data from the Greenhouse Gas Reporting Program are easily accessible from EPA's website at: www.epa.
gov/ghgreporting/ghgdata/index.html. Visitors can explore data by facility, industry, location, or gas using
a data visualization and mapping tool called FLIGHT.
16
-------
Facilities That Directly Emit Greenhouse Gases and Report to
EPA's Greenhouse Gas Reporting Program (2012)
Facility emissions
(metric tons of carbon
dioxide equivalents)
V ' '?wR*sj'' -*.; • °-500
. ' • 500,0(
. 2,000,
COMPARING THE SOURCES
EPA's Inventory of U.S. Greenhouse Gas Emissions and Sinks presents
annual national-level greenhouse gas emissions estimates from 1990
to the present. It estimates the total greenhouse gas emissions across
all sectors of the economy using national-level data. This inventory
and its 20+ years of data serve as the basis for this report's U.S.
Greenhouse Gas Emissions indicator (p. 14).
In contrast, the Greenhouse Gas Reporting Program is a relatively
new program that began collecting data in 2010. The reporting
program collects detailed emissions data from the largest greenhouse
gas emitting facilities in the United States.
While the inventory provides high-level perspective needed to under-
stand the United States' total emissions or "carbon footprint," the
Greenhouse Gas Reporting Program provides detailed information
that helps us better understand the sources and types of greenhouse
gas emissions at individual facilities. The inventory provides a more
complete estimate of total U.S. emissions because it accounts for
some sources that the reporting program does not cover (see diagram
at right). Thus, the inventory and the reporting program are comple-
mentary tools.
0-500,000
500,000-2,000,000
2,000,000-5,000,000
5,000,000-10,000,000
>10,000,000
Comparing EPA's Two Sources of Greenhouse Gas
Emissions Data
This includes:
• Agricultural sources
• Emissions from land-use
changes (e.g., forestry)
This includes:
• Mobile sources
• Fuel use at stationary
sources with small
emissions (residential,
commercial, industrial)
• Industrial gases
This includes:
• Power plants
• Large industrial facilities
• Landfills
offift
a
0 o
a. « £
^ * e -9.. e . -
p * _ 5 *
-
e
o
i
Facility
Level
Information on
Greenhouse gases
Tool
17
-------
Global Greenhouse Ga
Emissions
is indicator describes
KEY POINTS
3 In 2010, estimated worldwide emis-
sions from human activities totaled
nearly 46 billion metric tons of green-
house gases, expressed as carbon
dioxide equivalents. This represents
a 35 percent increase from 1990 (see
Figures 1 and 2). These numbers rep-
resent net emissions, which include
the effects of land use and forestry.
3 Between 1990 and 2010, global emis-
sions of all major greenhouse gases
increased (see Figure 1). Net emis-
sions of carbon dioxide increased
by 42 percent, which is particularly
important because carbon dioxide
accounts for about three-fourths of
total global emissions. Nitrous oxide
emissions increased the least—9
percent—while emissions of methane
increased by 15 percent. Emissions of
fluorinated gases more than doubled.
3 Energy production and use (including
fuels used by vehicles) represent the
largest source of greenhouse gas
emissions worldwide (about 71 per-
cent of the total in 2010), followed by
agriculture (13 percent in 2010) (see
Figure 2). While land-use change and
forestry represent a net sink for emis-
sions in the United States, absorbing
carbon dioxide and offsetting emis-
sions from other sources (see the U.S.
Greenhouse Gas Emissions indicator
on p. 14), these activities are a net
source of emissions on a global scale,
largely because of deforestation.9
3 Carbon dioxide emissions are increas-
ing faster in some parts of the world
(for example, Asia) than in others
(see Figure 3). The majority of emis-
sions come from three regions: Asia,
Europe, and the United States, which
together accounted for 82 percent of
total global emissions in 2011.
Increasing emissions of greenhouse gases due to human activities worldwide have led to a substantial
increase in atmospheric concentrations of long-lived and other greenhouse gases (see the Atmospheric
Concentrations of Greenhouse Gases indicator on p. 20). Every country around the world emits greenhouse
gases into the atmosphere, meaning the root cause of climate change is truly global in scope. Some countries
produce far more greenhouse gases than others, and several factors—such as economic activity, population,
income level, land use, and climatic conditions—can influence a country's emissions levels. Tracking green-
house gas emissions worldwide provides a global context for understanding the United States' and other
nations' roles in climate change.
ABOUT THE INDICATOR
Like the U.S. Greenhouse Gas Emissions indicator (on p. 14), this indicator focuses on emissions of gases
covered under the United Nations Framework Convention on Climate Change: carbon dioxide, methane, nitrous
oxide, and several fluorinated gases. These are all important greenhouse gases that are influenced by human
activities, and the Convention requires participating countries to develop and periodically submit an inventory
of these emissions.
Data and analysis for this indicator come from the World Resources Institute's Climate Analysis Indicators Tool
(CAIT), which compiles data from peer-reviewed and internationally recognized greenhouse gas inventories de-
veloped by EPA and other government agencies worldwide. Global estimates for carbon dioxide are published
annually, but estimates for other gases, such as methane and nitrous oxide, are available only every fifth year.
CAIT includes estimates of emissions and sinks associated with land use and forestry activities, which come
from global estimates compiled by the Food and Agriculture Organization of the United Nations.
Each greenhouse gas has a different lifetime (how long it stays in the atmosphere) and a different ability
to trap heat in our atmosphere. To allow different gases to be compared and added together, emissions are
converted into carbon dioxide equivalents. This step uses each gas's 100-year global warming potential, which
measures how much a given amount of the gas is estimated to contribute to global warming over a period of
100 years after being emitted. Carbon dioxide is assigned a global warming potential equal to 1.
Figure 1. Global Greenhouse Gas Emissions by Gas, 1990-2010
50,000
40,000
MFCs, PFCs, and SF6"
Nitrous oxide
Methane
Carbon dioxide
1990
1995
2000
Year
2005
2010
This figure shows worldwide emissions of carbon dioxide, methane, nitrous oxide, and several fluorinated gases from 1990 to
2010. For consistency, emissions are expressed in million metric tons of carbon dioxide equivalen ts. These totals include emissions
and sinks due to land-use change and forestry.
* HFCs are hydmfluomcarbons, PFCs are perfluomcarbons, and SF6 is sulfur hexafluoride.
Data source: WRI, 2014;'»FAO, 2014"
18
-------
Figure 2. Global Greenhouse Gas Emissions by Sector, 1990-2010
50,000
C £
O C 40,000
*•
-------
of Greenhouse Gases
This indicator describes how the levels of major greenhouse gases in the atmosphere hav
KEY POINTS
0 Global atmospheric concentrations
of carbon dioxide, methane, nitrous
oxide, and certain manufactured
greenhouse gases have all risen sig-
nificantly over the last few hundred
years (see Figures 1, 2,3, and 4).
0 Historical measurements show tha.
the current global atmospheric
concentrations of carbon dioxide,
methane, and nitrous oxide are
unprecedented compared with the
past 800,000 years (see Figures 1,2,
and 3).
O
WATER VAPOR AS A
GREENHOUSE GAS
Water vapor is the most abundant
greenhouse gas in the atmosphere. Hu-
man activities have only a small direct
influence on atmospheric concentra-
tions of water vapor, primarily through
irrigation and deforestation, so it is
not included in this indicator. However,
the surface warming caused by human
production of other greenhouse gases
leads to an increase in atmospheric
water vapor, because warmer tem-
peratures make it easier for water to
evaporate and stay in the air in vapor
form. This creates a positive "feedback
loop" in which warming leads to more
warming.
O o
Since the Industrial Revolution began in the 1700s, people have added a substantial amount of green-
house gases into the atmosphere by burning fossil fuels, cutting down forests, and conducting other
activities (see the U.S. and Global Greenhouse Gas Emissions indicators on pp. 14 and 18). When
greenhouse gases are emitted into the atmosphere, many remain there for long time periods ranging from a
decade to many millennia. Over time, these gases are removed from the atmosphere by chemical reactions or
by emissions sinks, such as the oceans and vegetation, which absorb greenhouse gases from the atmosphere.
However, as a result of human activities, these gases are entering the atmosphere more quickly than they are
being removed, and thus their concentrations are increasing.
Carbon dioxide, methane, nitrous oxide, and certain manufactured gases called halogenated gases (gases that
contain chlorine, fluorine, or bromine) become well mixed throughout the global atmosphere because of their
relatively long lifetimes and because of transport by winds. Concentrations of these greenhouse gases are
measured in parts per million (ppm), parts per billion (ppb), or parts per trillion (ppt) by volume. In other words,
a concentration of 1 ppb for a given gas means there is one molecule of that gas in every 1 billion molecules
of air. Some halogenated gases are considered major greenhouse gases due to their very high global warming
potentials and long atmospheric lifetimes even if they only exist at a few ppt (see table on p. 7).
Ozone is also a greenhouse gas, but it differs from other greenhouse gases in several ways. The effects of
ozone depend on its altitude, or where the gas is located vertically in the atmosphere. Most ozone naturally
exists in the layer of the atmosphere called the stratosphere, which ranges from approximately 6 to 30 miles
above the Earth's surface. Ozone in the stratosphere has a slight net warming effect on the planet, but it is
good for life on Earth because it absorbs harmful ultraviolet radiation from the sun, preventing it from reaching
the Earth's surface. In the troposphere—the layer of the atmosphere near ground level—ozone is an air
pollutant that is harmful to breathe, a main ingredient of urban smog, and an important greenhouse gas that
contributes to climate change (see the Climate Forcing indicator on p. 24). Unlike the other major greenhouse
gases, tropospheric ozone only lasts for days to weeks, so levels often vary by location and by season.
ABOUT THE INDICATOR
This indicator describes concentrations of greenhouse gases in the atmosphere. It focuses on the major green-
house gases that result from human activities.
For carbon dioxide, methane, nitrous oxide, and halogenated gases, recent measurements come from moni-
toring stations around the world, while measurements of older air come from air bubbles trapped in layers of
ice from Antarctica and Greenland. By determining the age of the ice layers and the concentrations of gases
trapped inside, scientists can learn what the atmosphere was like thousands of years ago.
This indicator also shows data from satellite instruments that measure ozone density in the troposphere, the
stratosphere, and the "total column," or all layers of the atmosphere. These satellite data are routinely com-
pared with ground-based instruments to confirm their accuracy. Ozone data have been averaged worldwide
for each year to smooth out the regional and seasonal variations.
; indicator looks at global average levels of ozone in both the
stratosphere and troposphere. For trends in ground-level ozone
concentrations within the United States, see EPA's National Air
Quality Trends Report at: www.epa.gov/airtrends.
20
-------
Figure 1. Global Atmospheric Concentrations of Carbon
Dioxide Over Time
800,000 BC to 2013 AD
1950 to 2013 AD
-800,000 -600,000 -400,000 -200,000 0
Year (negative values = BC)
1950 1960 1970 1980 1990 2000 2010 2020
Year
This figure shows concentrations of carbon dioxide in the atmosphere from hundreds of thousands of years ago through
2013, measured in parts per million (ppm). The data come from a variety of historical ice core studies and recent air moni-
toring sites around the world. Each line represents a different data source.
Data source: Compilation of 10 underlying datasets16
3 Carbon dioxide concentrations have
increased steadily since the begin-
ning of the industrial era, rising from
an annual average of 280 ppm in the
late 1700s to 396 ppm at Mauna Loa
in 2013—a 41 percent increase (see
Figure 1). Almost all of this increase
is due to human activities.18
3 The concentration of methane in the
atmosphere has more than doubled
since preindustrial times, reaching
approximately 1,800 ppb in 2013 (see
the range of measurements in Figure
2). This increase is predominantly
due to agriculture and fossil fuel
Figure 2. Global Atmospheric Concentrations of Methane Over Time
800,000 BC to 2013 AD 1950 to 2013 AD
2,000
Q.
.2
+*
IB
1,500
-------
KEY POINTS
0 Over the past 800,000 years, con-
centrations of nitrous oxide in the
atmosphere rarely exceeded 280 ppb.
Levels have risen since the 1920s,
however, reaching a new high of 326
ppb in 2013 (average of three sites in
Figure 3). This increase is primarily
due to agriculture.20
5 Concentrations of many of the
halogenated gases shown in Figure 4
were essentially zero a few decades
ago but have increased rapidly as
they have been incorporated into
industrial products and processes.
Some of these chemicals have bee
or are currently being phased out
use because they are ozone-deple1
substances, meaning they also cause
harm to the Earth's protective ozone
layer. As a result, concentrations of
many major ozone-depleting gases
have begun to stabilize or decline
(see Figure 4, left panel). Concen-
trations of other halogenated gases
have continued to rise, however,
especially where the gases have
emerged as substitutes for ozone-de-
pleting chemicals (see Figure 4, right
panel).
Figure 3. Global Atmospheric Concentrations of Nitrous Oxide Over Time
800,000 BC to 2013 AD 1950 to 2013 AD
-800,000 -600,000 -400,000 -200,000 0 1950 1960 1970 1980 1990 2000 2010 2020
Year (negative values = BC) Year
This figure shows concentrations of nitrous oxide from hundreds of thousands of years ago through 2013, measured in parts per
billion (ppb). The data come from a variety of historical ice core studies and recent air monitoring sites around the world. Each line
represents a different data source.
Data source: Compilation of six underlying datasets21
INDICATOR NOTES
This indicator includes several of the most important halogenated gases, but some others are not shown.
Many other halogenated gases are also greenhouse gases, but Figure 4 is limited to a set of common exam-
ples that represent most of the major types of these gases. The indicator also does not address certain other
pollutants that can affect climate by either reflecting or absorbing energy. For example, sulfate particles can
reflect sunlight away from the Earth, while black carbon aerosols (soot) absorb energy. Data for nitrogen
trifluoride (Figure 4) reflect modeled averages based on measurements made in the Northern Hemisphere and
some locations in the Southern Hemisphere, to represent global average concentrations over time. The global
averages for ozone only cover the area between 50°N and 50°S latitude (77 percent of the Earth's surface),
because at higher latitudes the lack of sunlight in winter creates data gaps and the angle of incoming sunlight
during the rest of the year reduces the accuracy of the satellite measuring technique.
DATA SOURCES
Global atmospheric concentration measurements for carbon dioxide (Figure 1), methane (Figure 2), and
nitrous oxide (Figure 3) come from a variety of monitoring programs and studies published in peer-reviewed
literature. Global atmospheric concentration data for selected halogenated gases (Figure 4) were compiled by
the Advanced Global Atmospheric Gases Experiment, the National Oceanic and Atmospheric Administration,
and a peer-reviewed study on nitrogen trifluoride. A similar figure with many of these gases appears in the
Intergovernmental Panel on Climate Change's Fifth Assessment Report.22 Satellite measurements of ozone
were processed by the National Aeronautics and Space Administration and validated using ground-based
measurements collected by the National Oceanic and Atmospheric Administration.
22
-------
Figure 4. Global Atmospheric Concentrations of Selected
Halogenated Gases, 1978-2012
Ozone-depleting substances Other halogenated gases
1,000 1,000
CFC-12
_ 100
o
I
o
u
10
1975
2005
2015
This figure shows concentrations of several halogenated gases (which contain fluorine, chlorine, or bromine) in the
atmosphere, measured in parts per trillion (ppt). The data come from monitoring sites around the world. Note that the
scale increases by factors of 10. This is because the concentrations of different halogenated gases can vary by a few orders
of magnitude. The numbers following the name of each gas (e.g., HCFC-22) are used to denote specific types of those
particular gases.
Data sources: AGAGE, 2014;23 Arnold, 2013;24 NOAA, 201325
Figure 5. Global Atmospheric Concentrations of Ozone, 1979-2013
350
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Total column
Stratosphere
1980
Troposphere
1985 1990
1995
Year
2000
2005
2010
This figure shows the average amount of ozone in the Earth s atmosphere each year, based on satellite measurements. The
total represents the "thickness" or density of ozone throughout all layers of the Earth's atmosphere, which is called total
column ozone and measured in Dobson units. Higher numbers indicate more ozone. For most years, Figure 5 shows how
this ozone is divided between the troposphere (the part of the atmosphere closest to the ground) and the stratosphere.
From 1994 to 1996, only the total is available, due to limited satellite coverage.
Data sources: NASA, 2013,26 20142"8
3 Overall, the total amount of ozone In
the atmosphere decreased by about
3 percent between 1979 and 2013
(see Figure 5). All of the decrease
happened In the stratosphere, with
most of the decrease occurring
between 1979 and 1994. Changes in
stratospheric ozone reflect the effect
of ozone-depleting substances. These
chemicals have been released Into
the air for many years, but recently,
international efforts have reduced
emissions and phased out their use.
S Globally, the amount of ozone in the
troposphere increased by about 4
percent between 1979 and 2013 (see
Figure 5).
23
-------
Climate Fore
is indicator measures
i/eTo
or neatmq
KEY POINTS
3 In 2013, the Annual Greenhouse Gas
Index was 1.34, which represents a
34 percent increase in radiative forc-
ing (a net warming influence) since
1990 (see Figure 1).
0 Of the greenhouse gases shown in
Figure 1, carbon dioxide accounts for
by far the largest share of radiative
forcing since 1990, and its contribu-
tion continues to grow at a steady
rate. Carbon dioxide alone would
account for a 27 percent increase in
radiative forcing since 1990.
3 Although the overall Annual Green-
house Gas Index continues to rise,
the rate of increase has slowed
somewhat over time. This change has
occurred in large part because meth-
ane concentrations have increased
at a slower rate in recent years and
because chlorofluorocarbon (CFC)
concentrations have been declining,
as production of CFCs has been
phased out globally due to the harm
they cause to the ozone layer (see
Figure 1).
3 Greenhouse gases produced by hu-
man activities have caused an overall
warming influence on the Earth's
climate since 1750. The largest con-
tributor to warming has been carbon
dioxide, followed by methane, and
black carbon. Although aerosol pollu-
tion and certain other activities have
caused cooling, the net result is that
human activities on the whole have
warmed the Earth (see Figure 2).
When energy from the sun reaches the Earth, the planet absorbs some of this energy and radiates
the rest back to space as heat. The Earth's surface temperature depends on this balance between
incoming and outgoing energy. Average conditions tend to remain stable unless the Earth experienc-
es a force that shifts the energy balance. A shift in the energy balance causes the Earth's average temperature
to become warmer or cooler, leading to a variety of other changes in the lower atmosphere, on land, and in the
oceans.
A variety of physical and chemical changes can affect the global energy balance and force changes in the
Earth's climate. Some of these changes are natural, while others are influenced by humans. These changes are
measured by the amount of warming or cooling they can produce, which is called "radiative forcing." Changes
that have a warming effect are called "positive" forcing, while changes that have a cooling effect are called
"negative" forcing. When positive and negative forces are out of balance, the result is a change in the Earth's
average surface temperature.
Changes in greenhouse gas concentrations in the atmosphere affect radiative forcing (see the Atmospheric
Concentrations of Greenhouse Gases indicator on p. 20). Greenhouse gases absorb energy that radiates
upward from the Earth's surface, re-emitting heat to the lower atmosphere and warming the Earth's surface.
Human activities have led to increased concentrations of greenhouse gases that can remain in the atmosphere
for decades, centuries, or longer, so the corresponding warming effects will last for a long time.
ABOUT THE INDICATOR
Figure 1 of this indicator measures the average total radiative forcing of 20 long-lived greenhouse gases,
including carbon dioxide, methane, and nitrous oxide. The results were calculated by the National Oceanic and
Atmospheric Administration based on measured concentrations of the gases in the atmosphere, compared with
the concentrations that were present around 1750, before the Industrial Revolution began. Because each gas
has a different ability to absorb and emit energy, this indicator converts the changes in greenhouse gas con-
centrations into a measure of the total radiative forcing (warming effect) caused by each gas. Radiative forcing
is calculated in watts per square meter, which represents the size of the energy imbalance in the atmosphere.
The National Oceanic and Atmospheric Administration also translates the total radiative forcing of these mea-
sured gases into an index value called the Annual Greenhouse Gas Index (right side of Figure 1).This number
compares the radiative forcing for a particular year with the radiative forcing in 1990, which is a common
baseline year for global agreements to track and reduce greenhouse gas emissions.
For reference, this indicator also presents an estimate of the total radiative forcing associated with a variety of
human activities from 1750 to the present. Figure 2 shows the influence of:
• Tropospheric ozone, a short-lived greenhouse gas.
• Emissions that indirectly lead to greenhouse gases through chemical reactions in the atmosphere. For
example, methane emissions also lead to an increase in tropospheric ozone.
• Aerosol pollution, which consists of solid and liquid particles suspended in the air that can reflect incom-
ing sunlight.
• Black carbon (soot), which can make the Earth's surface darker and less reflective when it is deposited
on snow and ice.
• Several other factors, like land use change, that affect radiative forcing.
24
-------
Figure 1. Radiative Forcing Caused by Major Long-Lived Greenhouse
Gases, 1979-2013
Carbon dioxide
Methane
Nitrous oxide
CFC-12
CFC-11
15 other gases
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0.4
0.2
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1980
2010
This figure shows the amount of radiative forcing caused by various greenhouse gases, based on the change in concen-
tration of these gases in the Earth's atmosphere since 1750. Radiative forcing is calculated in watts per square meter,
which represents the size of the energy imbalance in the atmosphere. On the right side of the graph, radiative forcing
has been converted to the Annual Greenhouse Gas Index, which is set to a value of 1.0 for 1990.
Data source: NOAA, 201429
Figure 2. Radiative Forcing Caused by Human Activities Since 1750
Long-lived
greenhouse •<
gases
Carbon dioxide
Methane
Halogenated gases
Nitrous oxide
(negative radiative forcing is because
some of these gases destroy ozone)
Short-lived gases that create
ozone or create or destroy
other greenhouse gases
Aerosols (solid or liquid particles)
Changes in clouds due to aerosols
Change in albedo* due to land use
I
3^ Change in energy from the sun
IB
Net total due to human activities
black carbon (soot)
reflectiveness of Earth's surface
Cooling Warming
Radiative forcing (watts per square meter)
This figure shows the total amount of radiative forcing caused by human activities—including indirect effects—between
1750 and 2011. Radiative forcing is calculated in watts per square meter, which represents the size of the energy
imbalance in the atmosphere. Each colored bar represents scientists' best estimate, while the thin black bars indicate the
likely range of possibilities. The natural change in the energy received from the sun over this time period is provided for
reference.
Data source: IPCC, 201330
INDICATOR NOTES
The index in Figure 1 does not include short-
lived greenhouse gases like tropospheric
ozone, reflective aerosol particles, black
carbon (soot), or the indirect influence of
methane through its effects on water vapor
and ozone formation. Figure 2 includes these
and other indirect influences.
DATA SOURCES
Data for Figure 1 were provided by the
National Oceanic and Atmospheric Adminis-
tration. This figure and other information are
available at: www.esrl.noaa.gov/gmd/aggi.
Data for Figure 2 came from the Intergov-
ernmental Panel on Climate Change (www.
ipcc.ch), which publishes assessment reports
based on the best available climate science
data.
25
-------
EATHER
AND CLIMATE
Rising global average temperature is associated
with widespread changes in weather patterns.
Scientific studies indicate that extreme weather
events such as heat waves and large storms
are likely to become more frequent or more
intense with human-induced climate change.
This chapter focuses on observed changes
in temperature, precipitation, storms, and
droughts.
WHY DOES IT MATTER?
Long-term changes in climate can directly or indirectly affect many aspects
of society in potentially disruptive ways. For example, warmer average
temperatures could increase air conditioning costs and affect the spread of
diseases like Lyme disease, but could also improve conditions for growing
some crops. More extreme variations in weather are also a threat to soci-
ety. More frequent and intense extreme heat events can increase illnesses
and deaths, especially among vulnerable populations, and damage some
crops. Similarly, increased precipitation can replenish water supplies and
support agriculture, but intense storms can damage property, cause loss of
life and population displacement, and temporarily disrupt essential services
such as transportation, telecommunications, energy, and water supplies.
I
-------
Summary of Key Points
U.S. and Global Temperature.
Average temperatures have risen across the contig-
uous 48 states since 1901, with an increased rate of
warming over the past 30 years. Seven of the top 10
warmest years on record have occurred since 1998.
Average global temperatures show a similar trend,
and the top 10 warmest years on record worldwide
have all occurred since 1998. Within the United
States, temperatures in parts of the North, the West,
and Alaska have increased the most.
High and Low Temperatures. Many
extreme temperature conditions are becoming more
common. Since the 1970s, unusually hot summer
temperatures have become more common in the
United States, and heat waves have become more
frequent—although the most severe heat waves in
U.S. history remain those that occurred during the
"Dust Bowl" in the 1930s. Record-setting daily high
temperatures have become more common than re-
cord lows. The decade from 2000 to 2009 had twice
as many record highs as record lows.
Weather and Climate
Weather is the state of the atmosphere at any given time and
place. Most of the weather that affects people, agriculture, and
ecosystems takes place in the lower layer of the atmosphere.
Familiar aspects of weather include temperature, precipita-
tion, clouds, and wind that people experience throughout the
course of a day. Severe weather conditions include hurricanes,
tornadoes, blizzards, and droughts.
Climate is the long-term average of the weather in a given
place. While the weather can change in minutes or hours, a
change in climate is something that develops over longer
periods of decades to centuries. Climate is defined not only by
average temperature and precipitation but also by the type,
frequency, duration, and intensity of weather events such as
heat waves, cold spells, storms, floods, and droughts.
While the concepts of climate and weather are often confused,
it is important to understand the difference. For example, the
eastern United States experienced a cold and snowy winter in
2013/2014, but this short-term regional weather phenomenon
does not negate the long-term rise in national and global
temperatures, sea level, or other climate indicators. It may be
helpful to think about the difference between weather and
climate with an analogy: weather influences what clothes
you wear on a given day, while the climate where you live
influences the entire wardrobe you buy.
U.S. and Global Precipitation. Total annu-
al precipitation has increased in the United States and over land areas worldwide. Since 1901, precipitation
has increased at an average rate of 0.5 percent per decade in the contiguous 48 states and 0.2 percent per
decade over land areas worldwide. However, shifting weather patterns have caused certain areas, such as
Hawaii and parts of the Southwest, to experience less precipitation than usual.
Heavy Precipitation. In recent years, a higher percentage of precipitation in the United States has
come in the form of intense single-day events. Nationwide, nine of the top 10 years for extreme one-day
precipitation events have occurred since 1990.The occurrence of abnormally high annual precipitation
totals (as defined by the National Oceanic and Atmospheric Administration) has also increased.
Drought. Average drought conditions across the nation have varied since records began in 1895. The
1930s and 1950s saw the most widespread droughts, while the last 50 years have generally been wetter
than average. However, specific trends vary by region. A more detailed index developed recently shows that
between 2000 and 2013, roughly 20 to 70 percent of the United States experienced drought at any given
time, but this index has not been in use for long enough to compare with historical drought patterns.
A Closer Look: Temperature and Drought in the Southwest. The south-
western United States is particularly sensitive to changes in temperature and thus vulnerable to
drought, as even a small decrease in water availability in this already arid region can threaten
natural systems and society.
Tropical Cyclone Activity. Tropical storm activity in the Atlantic Ocean, the Caribbean, and the Gulf
of Mexico has increased during the past 20 years. Increased storm intensity is closely related to variations in
sea surface temperature in the tropical Atlantic. However, changes in observation methods over time make
it difficult to know for sure whether a long-term increase in storm activity has occurred. Records collected
since the late 1800s suggest that the actual number of hurricanes per year has not increased.
-------
U.S. and Global
Temperatures
indicator describes trends in average surface
KEY POINTS
0 Since 1901, the average surface
temperature across the contiguous
48 states has risen at an average
rate of 0.14°F per decade (see Figure
1). Average temperatures have risen
more quickly since the late 1970s
(0.31 to 0.48°F per decade). Seven of
the top 10 warmest years on record
for the contiguous 48 states have
occurred since 1998, and 2012 was
the warmest year on record.
3 Worldwide, 2001-2010 was the
warmest decade on record since ther-
mometer-based observations began.
Global average surface temperature
has risen at an average rate of 0.15°F
per decade since 1901 (see Figure 2),
similar to the rate of warming within
the contiguous 48 states. Since the
late 1970s, however, the United
States has warmed faster than the
global rate.
3 Some parts of the United States have
experienced more warming than
others (see Figure 3). The North, the
West, and Alaska have seen tem-
peratures increase the most, while
some parts of the Southeast have
experienced little change. However,
not all of these regional trends are
statistically significant.
This figure shows how annual average tempera-
tures in the contiguous 48 states have changed
since 1901. Surface data come from land-based
weather stations. Satellite measurements cover
the lower troposphere, which is the lowest level
of the Earth's atmosphere. "UAH" and "RSS"
represent two different methods of analyzing
the original satellite measurements. This graph
uses the 1901-2000 average as a baseline for
depicting change. Choosing a different baseline
period would not change the shape of the data
over time.
Data source: NOAA, 2014'
Temperature is a fundamental measurement for describing the climate, and the temperature in particular
places can have wide-ranging effects on human life and ecosystems. For example, increases in air tem-
perature can lead to more intense heat waves, which can cause illness and death, especially in vulnerable
populations. Annual and seasonal temperature patterns also determine the types of animals and plants that
can survive in particular locations. Changes in temperature can disrupt a wide range of natural processes,
particularly if these changes occur more quickly than plant and animal species can adapt.
Concentrations of heat-trapping greenhouse gases are increasing in the Earth's atmosphere (see the Atmo-
spheric Concentrations of Greenhouse Gases indicator on p. 20). In response, average temperatures at the
Earth's surface are rising and are expected to continue rising. However, because climate change can shift the
wind patterns and ocean currents that drive the world's climate system, some areas are warming more than
others, and some have experienced cooling.
ABOUT THE INDICATOR
This indicator examines U.S. and global surface temperature patterns from 1901 to the present. U.S. surface
measurements come from weather stations on land, while global surface measurements also incorporate ob-
servations from buoys and ships on the ocean, thereby providing data from sites spanning much of the surface
of the Earth. For comparison, this indicator also displays satellite measurements that can be used to estimate
the temperature of the Earth's lower atmosphere since 1979.
This indicator shows anomalies, which compare recorded annual temperature values against a long-term
average. For example, an anomaly of +2.0 degrees means the average temperature was 2 degrees higher
than the long-term average. This indicator uses the average temperature from 1901 to 2000 as a baseline
for comparison. Annual anomalies are calculated for each weather station, starting from daily and monthly
average temperatures. Anomalies for broader regions have been determined by dividing the country (or the
world) into a grid, averaging the data for all weather stations within the grid, and then averaging the grid cells
together (for Figures 1 and 2) or displaying them on a map (Figure 3). This method ensures that the results are
not biased toward regions that happen to have many stations close together.
Figure 1. Temperatures in the Contiguous 48 States, 1901-2013
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Lower troposphere
Earth's surface (measured by satellite)
• • — UAH RSS
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020
Year
28
-------
Figure 2. Temperatures Worldwide, 1901-2013
-2
-3
Earth's surface Lower troposphere
(land and ocean) (measured by satellite)
• • — UAH RSS
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020
Year
This figure shows how annual average temperatures worldwide have changed since 1901. Surface data come from
a combined set of land-based weather stations and sea surface temperature measurements. Satellite measurements
cover the lower troposphere, which is the lowest level of the Earth's atmosphere. "UAH" and "RSS" represent two
different methods of analyzing the original satellite measurements. This graph uses the 1901-2000 average as a
baseline for depicting change. Choosing a different baseline period would not change the shape of the data over time.
Data source: NOAA, 20142
Figure 3. Rate of Temperature Change in the United States,
1901-2012
INDICATOR
NOTES
Data from the early 20th century are some-
what less precise than more recent data
because there were fewer stations collecting
measurements at the time, especially in the
Southern Hemisphere. However, the overall
trends are still reliable. Where possible, the
data have been adjusted to account for any
biases that might be introduced by factors
such as station moves, urbanization near the
station, changes in measuring instruments,
and changes in the exact times at which
measurements are taken.
DATA SOURCES
The data for this indicator were provided
by the National Oceanic and Atmospheric
Administration's National Climatic Data
Center, which maintains a large collection
of climate data online at: www.ncdc.noaa.
gov/oa/ncdc.html. The surface temperature
anomalies shown here were calculated based
on monthly values from a network of long-
term monitoring stations. Satellite data were
analyzed by two independent groups—the
Global Hydrology and Climate Center at the
University of Alabama in Huntsville (UAH) and
Remote Sensing Systems (RSS)—resulting in
slightly different trend lines.
This figure shows how annual average air temperatures
have changed in different parts of the United States
since the early 20* century (since 1901 for the contigu-
ous 48 states, 1905 for Hawaii, and 1918 for Alaska).
Data source: NOAA, 20133
Rate of temperature change (°F per century):
-4-3-2-101234
Gray interval:-0.1 toO.I'F
-------
m.
!
d Low
Temperatures
This indicator describes trends in
KEY POINTS
0 Since 1901, the average surface
Heatwaves in the 1930s remain the
most severe heat waves in the U.S.
historical record (see Figure 1). The
spike in Figure 1 reflects extreme,
persistent heat waves in the Great
Plains region during a period known
as the "Dust Bowl." Poor land use
practices and many years of intense
drought contributed to these heat
waves by depleting soil moisture and
reducing the moderating effects of
evaporation.4
3 Nationwide, unusually hot summer
days (highs) have become more
common over the last few decades
(see Figure 2). The occurrence of
unusually hot summer nights (lows)
has increased at an even faster rate.
This trend indicates less "cooling off"
at night.
5 The 20th century had many win-
ters with widespread patterns
of unusually low temperatures,
including a particularly large spike
in the late 1970s (see Figure 3). Since
the 1980s, though, unusually cold
winter temperatures have become
less common—particularly very cold
nights (lows).
Unusually hot or cold temperatures can result in prolonged extreme weather events like summer heat
waves or winter cold spells. Heat waves can lead to illness and death, particularly among older adults,
the very young, and other vulnerable groups (see the Heat-Related Deaths indicator on p. 76). People
can also die from exposure to extreme cold (hypothermia). In addition, prolonged exposure to excessive heat
and cold can damage crops and injure or kill livestock. Extreme heat can lead to power outages as heavy de-
mands for air conditioning strain the power grid, while extremely cold weather increases the need for heating
fuel.
Record-setting daily temperatures, heat waves, and cold spells are a natural part of day-to-day variation in
weather. However, as the Earth's climate warms overall, heat waves are expected to become more frequent,
longer, and more intense.5-6Higher heat index values (which combine temperature and humidity to describe
perceived temperature) are expected to increase discomfort and aggravate health issues. Conversely, cold
spells are expected to decrease. In most locations, scientists expect daily minimum temperatures—which
typically occur at night—to become warmer at a faster rate than daily maximum temperatures.7This change
will provide less opportunity to cool off and recover from daytime heat.
ABOUT THE INDICATOR
This indicator examines trends in unusual temperatures from several perspectives:
• The size and frequency of prolonged heatwave events (Figure 1).
• Unusually hot summer temperatures and cold winter temperatures nationwide (Figures 2 and 3).
• The change in the number of days with unusually hot and cold temperatures at individual weather
stations (Figures 4 and 5).
• Changes in record high and low temperatures (Figure 6).
The data come from thousands of weather stations across the United States. National patterns can be
determined by dividing the country into a grid and examining the data for one station in each cell of the grid.
This method ensures that the results are not biased toward regions that happen to have many stations close
together.
Figure 1. U.S.Annual Heatwave Index, 1895-2013
This figure shows the annual values of the U.S.
Heat Wave Index from 1895 to 2013. These data
cover the contiguous 48 states. Interpretation:
An index value of 0.2 (for example) could mean
that 20 percent of the country experienced one
heat wave, 10 percent of the country experienced
two heat waves, or some other combination of
frequency and area resulted in this value.
Data source: Kunkel, 20148
1.4
1.2
X
91
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-------
Figure 1 shows the U.S. Annual Heat Wave Index, which tracks the occurrence of heatwave conditions across
the contiguous 48 states from 1895 to 2013. While there is no universal definition of a heatwave, this index
defines a heat wave as a period lasting at least four days with an average temperature that would only be ex-
pected to occur once every 10 years, based on the historical record. The index value for a given year depends
on how often heat waves occur and how widespread they are.
Figures 2 and 3 show trends in the percentage of the country's area experiencing unusually hot temperatures
in the summer and unusually cold temperatures in the winter. These graphs are based on daily maximum tem-
peratures, which usually occur during the day, and daily minimum temperatures, which usually occur at night.
At each station, the recorded highs and lows are compared with the full set of historical records. After aver-
aging over a particular month or season of interest, the coldest 10 percent of years are considered "unusually
cold" and the warmest 10 percent are "unusually hot." For example, if last year's summer highs were the 10*
warmest on record for a particular location with more than 100 years of data, that year's summer highs would
be considered unusually warm. Data are available from 1910 to 2013 for summer (June through August) and
from 1911 to 2014 for winter (December of the previous year through February).
(Continued on next page)
Figure 2. Area of the Contiguous 48 States with Unusually Hot Summer Temperatures, 1910-2013
90
80
70
Hot daily highs
^— Hot daily highs (smoothed)
— Hot daily lows
^— Hot daily lows (smoothed)
This graph shows the percentage of the land area
of the contiguous 48 states with unusually hot daily
high and low temperatures during the months of June,
July, and August. The thin lines represent individual
years, while the thick lines show a nine-year weighted
average. Red lines represent daily highs, while orange
lines represent daily lows. The term "unusual" in this
case is based on the long-term average conditions at
each location.
Data source: NOAA, 2014'
1910 1920
2010 2020
Figure 3. Area of the Contiguous 48 States with Unusually Cold Winter Temperatures, 1911-2014
90
80
70
Cold daily highs
^— Cold daily highs (smoothed)
Cold daily lows
^— Cold daily lows (smoothed)
This graph shows the percentage of the land area of
the contiguous 48 states with unusually cold daily high
and low temperatures during the months of December,
January, and February. The thin lines represent individu-
al years, while the thick lines show a nine-year weight-
ed average. Blue lines represent daily highs, while
purple lines represent daily lows. The term "unusual" in
this case is based on the long-term average conditions
at each location.
Data source: NOAA, 201410
1910 1920
2000 2010 2020
-------
m.
!
High and Low Temperatures
Continued
KEY POINTS
0 The two maps show where changes
in the number of days with unusually
hot (above the 95th percentile) and
cold (below the 5th percentile) days
have occurred since 1948. Unusually
high temperatures have increased in
the western United States and in sev-
eral areas along the Gulf and Atlantic
coasts, but decreased in much of the
middle of the country (see Figure 4).
The number of unusually cold days
has generally decreased throughout
the country (see Figure 5).
3 If the climate were completely stable,
one might expect to see highs and
lows each accounting for about 50
percent of the records set. However,
since the 1970s, record-setting daily
high temperatures have become more
common than record lows across the
United States (see Figure 6). The most
recent decade had twice as many
record highs as record lows.
(Continued from previous page)
Figures 4 and 5 show how trends in unusually hot and cold daily temperatures throughout the year vary by
location. These maps cover 1,119 weather stations that have operated since 1948. Figure 4 was created by re-
viewing all daily maximum temperatures from 1948 to 2013 and identifying the 95th percentile temperature (a
temperature that one would only expect to exceed in five days out of every 100) at each station. Next, for each
year, the total number of days with maximum temperatures higher than the 95th percentile (that is, unusually
hot days) was determined. The map shows how the total number of unusually hot days per year at each station
has changed over time. Figure 5 is similar except that it looks at unusually cold days, based on the 5th percentile
of daily minimum temperatures.
Many people are familiar with record daily high and low temperatures, which are frequently mentioned in
weather reports. Figure 6 depicts trends in these records by comparing the number of record-setting highs
with the number of record-setting lows by decade. These data come from a set of weather stations that have
collected data consistently since 1950.
Figure 4. Change in Unusually Hot Temperatures in the Contiguous 48
States, 1948-2013
o °'
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Change in number of days hotter than 95th percentile:
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15 to
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5 to
15
5 fewer
to 5 more
5 to
15
15 to
25
More
than 25
Decrease
Increase
This map shows trends in unusually hot temperatures at individual weather stations that have operated consistently since 1948.
In this case, the term "unusually hot" refers to a daily maximum temperature that is hotter than the 95th percentile temperature
during the 1948-2013 period. Thus, the maximum temperature on a particular day at a particular station would be considered
"unusually hot" if it falls within the warmest 5 percent of measurements at that station duringthe 1948-2013 period. The map
shows changes in the total number of days per year that were hotter than the 95th percentile. Red upward-pointing symbols
show where these unusually hot days are becoming more common. Blue downward-pointing symbols show where unusually
hot days are becoming less common.
Data source: NCAA, 2014
32
-------
Figure 5. Change in Unusually Cold Temperatures in the Contiguous 4
States, 1948-2013
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Change in number of days colder than 5th percentile:
T T O A A
More
than 25
15 to
25
5 to
15
5 fewer
to 5 more
5 to
15
15 to
25
More
than 25
Decrease
Increase
This map shows trends in unusually cold temperatures at individual weather stations that have operated consistently since 1948.
In this case, the term "unusually cold" refers to a daily minimum temperature that is colder than the 5th percentile temperature
during the 1948-2013 period. Thus, the minimum temperature on a particular day at a particular station would be considered
"unusually cold" if it falls within the coldest 5 percent of measurements at that station during the 1948-2013 period. The map
shows changes in the total number of days per year that were colder than the 5* percentile. Blue upward-pointing symbols show
where these unusually cold days are becoming more common. Red downward-pointing symbols show where unusually cold days
are becoming less common.
Data source: NOAA, 201412
Figure 6. Record Daily High and Low Temperatures in the Contiguous 48
States, 1950-2009
100
75
50
£ 25
INDICATOR NOTES
Temperature data are less certain for the
early part of the 20th century because
fewer stations were operating at that
time. In addition, measuring devices
and methods have changed over time,
and some stations have moved. The
data have been adjusted to the extent
possible to account for some of these
influences and biases, however, and
these uncertainties are not sufficient to
change the fundamental trends shown
in the figures.
DATA SOURCES
The data for this indicator are based on
measurements from weather stations
managed by the National Oceanic and
Atmospheric Administration. Figure 1
uses data from the National Weather
Service Cooperative Observer Net-
work. Figures 2 and 3 come from the
U.S. Climate Extremes Index, which is
based on a smaller group of long-term
weather stations that are tracked by
the National Climatic Data Center
and referred to as the U.S. Historical
Climatology Network. Figures 4 and
5 use data from a somewhat larger
set of stations tracked by the National
Climatic Data Center, known as the
Global Historical Climatology Network.
Figure 6 uses National Weather Service
data processed by Meehl et al. (2009)."
All of these weather station records are
available online at: www.ncdc.noaa.
gov, and information about the Climate
Extremes Index can be found at: www.
ncdc.noaa.gov/extremes/cei.
Record highs B Record lows
0 —
-25
-50
-75
-100
This figure shows the percentage of daily temperature
records set at weather stations across the contiguous
48 states by decade. Record highs (red) are compared
with record lows (blue).
Data source: Meehl et al., 200913
1950s 1960s 1970s 1980s
Decade
1990s
2000s
-------
U.S. and Global
Precipitation
ndicator describes trends in average precipit;
KEY POINTS
On average, total annual precipita-
tion has Increased over land areas In
the United States and worldwide (see
Figures 1 and 2). Since 1901, global
precipitation has Increased at an av-
erage rate of 0.2 percent per decade,
while precipitation In the contiguous
48 states has Increased at a rate of
0.5 percent per decade.
Some parts of the United States
have experienced greater Increases
In precipitation than others. A few
areas such as Hawaii and parts of the
Southwest have seen a decrease In
precipitation (see Figure 3).
Precipitation can have wide-ranging effects on human well-being and ecosystems. Rainfall, snowfall, and
the timing of snowmelt can all affect the amount of water available for drinking, irrigation, and industry,
and can also determine what types of animals and plants (including crops) can survive in a particular
place. Changes in precipitation can disrupt a wide range of natural processes, particularly if these changes
occur more quickly than plant and animal species can adapt.
As average temperatures at the Earth's surface rise (see the U.S. and Global Temperature indicator on p.
28), more evaporation occurs, which, in turn, increases overall precipitation. Therefore, a warming climate is
expected to increase precipitation in many areas. However, just as precipitation patterns vary across the world,
so will the precipitation effects of climate change. By shifting the wind patterns and ocean currents that drive
the world's climate system, climate change will also cause some areas to experience decreased precipitation. In
addition, higher temperatures lead to more evaporation, so increased precipitation will not necessarily increase
the amount of water available for drinking, irrigation, and industry (see the Drought indicator on p. 38).
ABOUT THE INDICATOR
This indicator examines U.S. and global precipitation patterns from 1901 to the present, based on rainfall and
snowfall measurements from land-based weather stations worldwide.
This indicator shows annual anomalies, or differences, compared with the average precipitation from 1901
to 2000. These anomalies are presented in terms of percent change compared with the baseline. Annual
anomalies are calculated for each weather station. Anomalies for broader regions have been determined by
dividing the country (or the world) into a grid, averaging the data for all weather stations within each cell of
the grid, and then averaging the grid cells together (for Figures 1 and 2) or displaying them on a map (Figure
3). This method ensures that the results are not biased toward regions that happen to have many stations close
together.
Figure 1. Precipitation in the Contiguous 48 States, 1901-2012
This figure shows how the total
annual amount of precipitation
in the contiguous 48 states has
changed since 1901. This graph
uses the 1901-2000 average as
a baseline for depicting change.
Choosing a different baseline
period would not change the
shape of the data over time.
Data source: NOAA, 201315
-20
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020
Year
34
-------
Figure 2. Precipitation Worldwide, 1901-2012
20
15
-20
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020
Year
This figure shows how the total annual amount of precipitation over land worldwide has changed since 1901. This graph
uses the 1901-2000 average as a baseline for depicting change. Choosing a different baseline period would not change the
shape of the data over time.
Data source: NOAA, 2013"
Figure 3. Rate of Precipitation Change in the United States, 1901-2012
)R NOTI
INDICATOR NOTES
Data from the early 20th century are some-
what less precise because there were
fewer stations collecting measurements
at the time. To ensure that overall trends
are reliable, the data have been adjusted
where possible to account for any biases
that might be introduced by factors such
as station moves or changes in measure-
ment instruments.
DATA SOURCES
The data for this indicator were provided
by the National Oceanic and Atmospheric
Administration's National Climatic Data
Center, which maintains a large collection
of climate data online at: www.ncdc.
noaa.gov/oa/ncdc.html. The precipitation
anomalies shown here were calculated
based on monthly values from a network
of long-term monitoring stations.
This figure shows the rate of change in
total annual precipitation in different parts
of the United States since the early 20*
century (since 1901 for the contiguous
48 states, 1905 for Hawaii, and 1918 for
Alaska).
Data source: NOAA, 201317
Rate of change in precipitation (% per century):
-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60
Gray interval: -2 to 2%
-------
Heavy Precipitation
This indicator tracks the frequency of heavy precipitation events in the United States.
KEY POINTS
0 In recent years, a larger percentage
of precipitation has come In the form
of Intense single-day events. Nine of
the top 10 years for extreme one-day
precipitation events have occurred
since 1990 (see Figure 1).
J> The prevalence of extreme single-day
precipitation events remained fairly
steady between 1910 and the 1980s,
but has risen substantially since then.
Over the entire period from 1910
to 2013, the portion of the country
experiencing extreme single-day
precipitation events increased at a
rate of about half a percentage point
per decade (see Figure 1).
0 The percentage of land area expe-
riencing much greater than normal
yearly precipitation totals increased
between 1895 and 2013. However,
there has been much year-to-year
variability. In some years there were
no abnormally wet areas, while a few
others had abnormally high precipi-
tation totals over 10 percent or more
of the contiguous 48 states' land area
(see Figure 2). For example, 1941 was
extremely wet in the West, while
1982 was very wet nationwide.18
0 Figures 1 and 2 are both consistent
with other studies that have found
an increase in heavy precipitation
over timeframes ranging from single
days to 90-day periods to whole
years.19 For more information on
trends in overall precipitation levels,
see the U.S. and Global Precipitation
indicator.
H
• eavy precipitation refers to instances during which the amount of precipitation experienced in a location
substantially exceeds what is normal. What constitutes a period of heavy precipitation varies according
to location and season.
Climate change can affect the intensity and frequency of precipitation. Warmer oceans increase the amount of
water that evaporates into the air. When more moisture-laden air moves over land or converges into a storm
system, it can produce more intense precipitation—for example, heavier rain and snow storms.20 The potential
impacts of heavy precipitation include crop damage, soil erosion, and an increase in flood risk due to heavy
rains. In addition, runoff from precipitation can impair water quality as pollutants deposited on land wash into
water bodies.
Heavy precipitation does not necessarily mean the total amount of precipitation at a location has increased—
just that precipitation is occurring in more intense events. However, changes in the intensity of precipitation,
when combined with changes in the interval between precipitation events, can also lead to changes in overall
precipitation totals.
ABOUT THE INDICATOR
Heavy precipitation events can be measured by tracking their frequency, examining their return period (the
chance that the event will be equaled or exceeded in a given year), or directly measuring the amount of precip-
itation in a certain period (for example, inches of rain falling in a 24-hour period).
One way to track heavy precipitation is by calculating what percentage of a particular location's total precip-
itation in a given year has come in the form of extreme one-day events—or, in other words, what percentage
of precipitation is arriving in short, intense bursts. Figure 1 of this indicator looks at the prevalence of extreme
single-day precipitation events over time.
For added insight, this indicator also tracks the occurrence of unusually high total yearly precipitation. It does
so by looking at the Standardized Precipitation Index (SPI), which compares actual yearly precipitation totals
with the range of precipitation totals that one would typically expect at a specific location, based on histor-
ical data. If a location experiences less precipitation than normal during a particular period, it will receive a
negative SPI score, while a period with more precipitation than normal will receive a positive score. The more
precipitation (compared with normal), the higher the SPI score. The SPI is a useful way to look at precipitation
totals because it allows comparison of different locations and different seasons on a standard scale. Figure 2
shows what percentage of the total area of the contiguous 48 states had an annual SPI score of 2.0 or above
(well above normal) in any given year.
-------
Figure 1. Extreme One-Day Precipitation Events in the Contiguous
48 States, 1910-2013
25
1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020
Year
This figure shows the percentage of the land area of the contiguous 48 states where a much greater than normal portion
of total annual precipitation has come from extreme single-day precipitation events. The bars represent individual years,
while the line is a nine-year weighted average.
Data source: NOAA, 201421
NDICATOR NOTES
Weather monitoring stations tend to be
closer together in the eastern and central
states than in the western states. In areas
with fewer monitoring stations, heavy pre-
cipitation indicators are less likely to reflect
local conditions accurately.
DATA SOURCES
The data used for this indicator come
from a large national network of weather
stations and were provided by the National
Oceanic and Atmospheric Administration's
National Climatic Data Center. Figure 1 is
based on Step #4 of the National Oceanic
and Atmospheric Administration's U.S.
Climate Extremes Index; for data and a
description of the index, see: www.ncdc.
noaa.gov/extremes/cei. Figure 2 is based on
the U.S. SPI, which is shown in a variety of
maps available online at: www.ncdc.noaa.
gov/oa/dimate/research/prelim/drought/spi.
html. The data used to construct these maps
are available from the National Oceanic
and Atmospheric Administration at: ftp://ftp.
ncdc.noaa.gov/pub/data/cirs.
Figure 2. Unusually High Annual Precipitation in the Contiguous 48
States, 1895-2013
I
-------
Drought
This indies
leasures drought conditions
KEY POINTS
0 Average drought conditions across
the nation have varied since records
began in 1895. The 1930s and 1950s
saw the most widespread droughts,
while the last 50 years have generally
been wetter than average (see
Figure 1).
0 Over the period from 2000 through
2013, roughly 20 to 70 percent of the
U.S. land area experienced conditions
that were at least abnormally dry
at any given time (see Figure 2). The
years 2002-2003 and 2012-2013 had
a relatively large area with at least
abnormally dry conditions, while
2001, 2005, and 2009-2011 had
substantially less area experiencing
drought.
0 Both drought figures indicate that in
2012, the United States experienced
the driest conditions in more than
a decade. During the latter half of
2012, more than half of the U.S. land
area was covered by moderate or
greater drought (see Figure 2). In
several states, 2012 was among the
driest years on record.23 See Tempera-
ture and Drought in the Southwest
(p. 40) for a closer look at trends in
one of the hardest-hit regions.
This chart shows annual values of the Palmer
Drought Severity Index, averaged over the entire
area of the contiguous 48 states. Positive values
represent wetter-than-average conditions, while
negative values represent drier-than-average
conditions. A value between -2 and -3 indicates
moderate drought, -3 to -4 is severe drought, and
-4 or below indicates extreme drought. The thicker
line is a nine-year weighted average.
Data source: NOAA, 201426
There are many definitions and types of drought. Meteorologists generally define drought as a prolonged
period of dry weather caused by a lack of precipitation that results in a serious water shortage for some
activity, population, or ecological system. Drought can also be thought of as an extended imbalance
between precipitation and evaporation.
As average temperatures have risen because of climate change, the Earth's water cycle has sped up through
an increase in the rate of evaporation. An increase in evaporation makes more water available in the air for
precipitation, but contributes to drying over some land areas, leaving less moisture in the soil. Thus, as the
climate continues to change, many areas are likely to experience increased precipitation (see the U.S. and
Global Precipitation indicator on p. 34) and increased risk of flooding (see the Heavy Precipitation indicator
on p. 36), while areas located far from storm tracks are likely to experience less precipitation and increased
risk of drought. As a result, since the 1950s, some regions of the world have experienced longer and more
intense droughts, particularly in southern Europe and West Africa, while other regions have seen droughts
become less frequent, less intense, or shorter (for example, in central North America).24
Drought conditions can negatively affect agriculture, water supplies, energy production, and many other
aspects of society. The impacts vary depending on the type, location, intensity, and duration of the drought.
For example, effects on agriculture can range from slowed plant growth to severe crop losses, while water
supply impacts can range from lowered reservoir levels and dried-up streams to major water shortages. Low-
er streamflow and groundwater levels can also harm plants and animals, and dried-out vegetation increases
the risk of wildfires.
ABOUT THE INDICATOR
During the 20th century, many indices were created to measure drought severity by looking at precipitation,
soil moisture, stream flow, vegetation health, and other variables.25 Figure 1 shows annual values of the most
widely used index, the Palmer Drought Severity Index, which is calculated from precipitation and tempera-
ture measurements at weather stations. An index value of zero represents the average moisture conditions
observed between 1931 and 1990 at a given location. A positive value means conditions are wetter than
average, while a negative value is drier than average. Index values from locations across the contiguous 48
states have been averaged together to produce the national values shown in Figure 1.
For a more detailed perspective on recent trends, Figure 2 shows a newer index called the Drought Monitor,
which is based on several indices (including Palmer), along with additional factors such as snow water con-
tent, groundwater levels, reservoir storage, pasture/range conditions, and other impacts. The Drought Monitor
uses codes from DO to D4 (see table below Figure 2) to classify drought severity. This part of the indicator
covers all 50 states and Puerto Rico.
Figure 1. Average Drought Conditions in the Contiguous 48 States, 1895-2013
8
Wet
Dry
38
1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020
^xri'iSi^t- Year
-------
Figure 2. U.S. Lands Under Drought Conditions, 2000-2013
100
gg DO Abnormally dry
D1 Moderate drought
D2 Severe drought
D3 Extreme drought
D4 Exceptional drought
IB
£
IB
•o
c
_nj
(/)
J
'c
§
-------
Closer Look
Temperature and Drought in the Southwest
KEY POINTS
5 Every part of the Southwest experi-
enced higher average temperatures
between 2000 and 2013 than the
long-term average (1895-2013). Some
areas were nearly 2°F warmer than
average (see Figure 1).
3 Large portions of the Southwest
have experienced drought condi-
tions since weekly Drought Monitor
records began in 2000. For extended
periods from 2002 to 2005 and from
2012 through 2013, nearly the entire
region was abnormally dry or even
drier (see Figure 2).
0 Based on the long-term Palmer index,
drought conditions in the Southwest
have varied since 1895. The early
1900s and the 1950s experienced
considerable drought, the 1970s
were relatively wet, and the last
decade has seen the most persistent
droughts on record (see Figu" ""
This map shows how the average air temperature
from 2000 to 2013 has differed from the long-
term average (1895-2013). To provide more
detailed information, each state has been divided
into climate divisions, which are zones that share
similar climate features.
Data source: NOAA, 201429
The American Southwest might evoke images of a hot, dry landscape—a land of rock, canyons, and deserts
baked by the sun. Indeed, much of this region has low annual rainfall and seasonally high temperatures
that contribute to its characteristic desert climate. Yet this landscape actually supports a vast array of
plants and animals, along with millions of people who call the Southwest home. All of these plants, animals,
and people need water to survive.
Water is already scarce in the Southwest, so every drop is a precious resource. People in the Southwest are
particularly dependent on surface water supplies like Lake Mead, which are vulnerable to evaporation. Thus,
even a small increase in temperature (which drives evaporation) or a decrease in precipitation in this already
arid region can seriously threaten natural systems and society. Droughts also contribute to increased pest
outbreaks and wildfires, both of which damage local economies.28
While two indicators in this report present information about unusually high or low temperatures and drought
on a national scale (see the High and Low Temperatures indicator on p. 30 and the Drought indicator on p.
38), this feature highlights the Southwest because of its particular sensitivity to temperature and drought. It
focuses on six states that are commonly thought of as "southwestern" and characterized at least in part by
arid landscapes and scarce water supplies: Arizona, California, Colorado, Nevada, New Mexico, and Utah. Tem-
perature and drought data come from a network of thousands of weather stations overseen by the National
Weather Service.
The map in Figure 1 shows how average annual temperatures in the Southwest from 2000 to 2013 differed
from the average over the entire period since widespread temperature records became available (1895-2013).
Figures 2 and 3 show two ways of measuring drought in the Southwest: the Drought Monitor and the Palmer
Drought Severity Index. The Palmer Index is calculated from precipitation and temperature measurements at
weather stations, and has been used widely for many years. The Drought Monitor is a more recent and more
detailed index based on several other indices (including Palmer), along with additional factors such as snow
water content, groundwater levels, reservoir storage, pasture/range conditions, and other impacts. See the
Drought indicator (p. 38) for more information about these indices.
Figure 1. Average Temperatures in the Southwestern United States,
2000-2013 Versus Long-Term Average
\
Temperature increase (°F):
0.8
1.2 1.4 1.6 1.8
40
-------
Figure 2. Southwestern U.S. Lands Under Drought Conditions,
2000-2013
10
10
c
_nj
M—
o
+*
u
1
100
90
80
70
60
50
40
30
20
10
0
2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013
Year
DO Abnormally dry
D1 Moderate drought
| D2 Severe drought
| D3 Extreme drought
| D4 Exceptional drought
Figure 3. Drought Severity in the Southwestern United States,
1895-2013
Wet
Dry
This chart shows the percentage of land area in six
southwestern states classified under drought condi-
tions from 2000 through 2013. This figure uses the
U.S. Drought Monitor classification system, which is de-
scribed in the table in the Drought indicator on p. 38.
Data source: National Drought Mitigation Center,
20143"
NOTES
Natural variability, changes in irrigation
practices, and other diversions of wa-
ter for human use can influence certain
drought-related measurements. Soil
moisture, ground water, and streamflow are
part of Drought Monitor calculations (Figure
2), and they are all sensitive to human
activities.
DATA SOURCES
Data for Figures 1 and 3 were obtained
from the National Oceanic and Atmospheric
Administration's National Climatic Data
Center, which maintains a large collection
of climate data online at: www.ncdc.noaa.
gov/oa/ncdc.html. Data for Figure 2 were
provided by the National Drought Mitiga-
tion Center. Historical data in table form are
available at: http://droughtmonitor.unl.edu/
MapsAndData.aspx.
1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020
This chart shows annual values of the Palmer Drought
Severity Index, averaged over six states in the South-
west. Positive values represent wetter-than-average
conditions, while negative values represent dri-
er-than-average conditions. A value between -2 and -3
indicates moderate drought, -3 to -4 is severe drought,
and -4 or below indicates extreme drought. The thicker
line is a nine-year weighted average.
Data source: NOAA, 201431
-------
('
i
Tropical Cyclone Activity
This indicator examines the frequency, intensity, and duration of hurricanes and other tropical storms in the Atlanth
Ocean, Caribbean, and Gulf of Mexico.
KEY POINTS
0 Since 1878, about six to seven
hurricanes have formed in the North
Atlantic every year. Roughly two
per year make landfall in the United
States. The total number of hurri-
canes (particularly after being adjust-
ed for improvements in observation
methods) and the number reaching
the United States do not indicate a
clear overall trend since 1878 (see
Figure 1).
0 According to the total annual ACE
Index, cyclone intensity has risen no-
ticeably over the past 20 years, and
six of the 10 most active years since
1950 have occurred since the mid-
1990s (see Figure 2). Relatively high
levels of cyclone activity were also
seen during the 1950s and 1960s.
5 The PDI (see Figure 3) shows fluctuat-
ing cyclone intensity for most of the
mid- to late 20th century, followed by
a noticeable increase since 1995 (sim-
ilar to the ACE Index). These trends
are associated with variations in sea
surface temperature in the tropical
Atlantic (see Figure 2).
0 Despite the apparent increases in
tropical cyclone activity in Figures
2 and 3, changes in observation
methods over time make it difficult
to know whether tropical storm ac-
tivity has actually shown a long-term
This graph shows the number of hurricanes that
formed in the North Atlantic Ocean each year
from 1878 to 2013, along with the number that
made landfall in the United States. The blue curve
shows how the total count in the red curve can
be adjusted to attempt to account for the lack of
aircraft and satellite observations in early years. All
three curves have been smoothed using a five-year
average, plotted at the middle year. The most
recent average (2009-2013) is plotted at 2011.
Data source: Knutson, 201435
Hurricanes, tropical storms, and other intense rotating storms fall into a general category called cyclones.
There are two main types of cyclones: tropical and extratropical (those that form outside the tropics).
Tropical cyclones get their energy from warm tropical oceans. Extratropical cyclones get their energy
from the jet stream and from temperature differences between cold, dry air masses from higher latitudes and
warm, moist air masses from lower latitudes.
This indicator focuses on tropical cyclones in the Atlantic Ocean, Caribbean, and Gulf of Mexico. Tropical cy-
clones are most common during the "hurricane season," which runs from June through November. The effects
of tropical cyclones are numerous and well known. At sea, storms disrupt and endanger shipping traffic. When
cyclones encounter land, their intense rains and high winds can cause severe property damage, loss of life, soil
erosion, and flooding. The associated storm surge—the large volume of ocean water pushed toward shore by
the cyclone's strong winds—can cause severe flooding and destruction.
Climate change is expected to affect tropical cyclones by increasing sea surface temperatures, a key factor that
influences cyclone formation and behavior. The U.S. Global Change Research Program and the Intergovernmen-
tal Panel on Climate Change project that, more likely than not, tropical cyclones will become more intense over
the 21st century, with higher wind speeds and heavier rains.3334
ABOUT THE INDICATOR
Records of tropical cyclones in the Atlantic Ocean have been collected since the 1800s. The most reliable
long-term records focus on hurricanes, which are the strongest category of tropical cyclones in the Atlantic,
with wind speeds of at least 74 miles per hour. This indicator uses historical data from the National Oceanic
and Atmospheric Administration to track the number of hurricanes per year in the North Atlantic (north of the
equator) and the number reaching the United States since 1878. Some hurricanes over the ocean might have
been missed before the start of aircraft and satellite observation, so scientists have used other evidence, such
as ship traffic records, to estimate the actual number of hurricanes that might have formed in earlier years.
This indicator also looks at the Accumulated Cyclone Energy (ACE) Index and the Power Dissipation Index (PDI),
which are two ways of monitoring the frequency, strength, and duration of tropical cyclones based on wind
speed measurements.
Figure 1. Number of Hurricanes in the North Atlantic, 1878-2013
12
10
wi
01
c
C 6
O
> Total hurricanes (adjusted)
Total hurricanes (unadjusted)
> Hurricanes reaching the United States
1860
1880
1900
1920
1940
Year
I960
1980
2000
2020
42
-------
Figure 2. North Atlantic Tropical Cyclone Activity According to the
Accumulated Cyclone Energy Index, 1950-2013
300
Above
normal
Near
normal
Below
normal
1950
1960
1970
1980
1990
2000
2010
•
2020
Year
This figure shows total annual Accumulated Cyclone Energy (ACE) Index values, which account for cyclone strength,
duration, and frequency, from 1950 through 2013. The National Oceanic and Atmospheric Administration has defined
"near normal," "above normal," and "below normal" ranges based on the distribution of ACE Index values over the 30
years from 1981 to 2010.
Data source: NOAA, 201436
Figure 3. North Atlantic Tropical Cyclone Activity According to the
Power Dissipation Index, 1949-2013
83.2
82.8
82.4
82.0
81.6
Q.
E
01
+*
s
•£
3
l/l
s
I/I
Sea surface>*
temperature
81.2
1950
1960
1970
1980
1990
2000
2010
2020
Year
This figure presents annual values of the Power Dissipation Index (PDI), which accounts for cyclone strength, duration,
and frequency. Tropical North Atlantic sea surface temperature trends are provided for reference. Note that sea surface
temperature is measured in different units, but the values have been plotted alongside the PDI to show how they
compare. The lines have been smoothed using a five-year weighted average, plotted at the middle year. The most recent
average (2009-2013) is plotted at 2011.
Data source: Emanuel, 201437
an ACE lnd>
:very cyclone has an ACE Index value, which is a
number based on the maximum wind speed mea-
sured at six-hour intervals over the entire time
that the cyclone is classified as at least a tropical
storm (wind speed of at least 39 miles per hour).
Therefore, a storm's ACE Index value accounts for
both strength and duration. The National Oceanic
and Atmospheric Administration calculates the
total ACE Index value for an entire hurricane
season by adding the values for all named storms,
including subtropical storms, tropical storms, and
hurricanes. The resulting annual total accounts
for cyclone strength, duration, and frequency. For
this indicator, the index has been converted to
a scale where 100 equals the median value (the
midpoint) over a base period from 1981 to 2010.
The thresholds in Figure 2 define whether the
ACE Index for a given year is close to normal,
significantly above normal, or significantly below.
Like the ACE Index, the PDI is based on mea-
surements of wind speed, but it uses a different
calculation method that places more emphasis on
storm intensity. This indicator shows the annual
PDI value, which represents the sum of PDI values
for all named storms during the year.
INDICATOR NOTES
Over time, data collection methods have changed
as technology has improved. For example, wind
speed collection methods have evolved substan-
tially over the past 60 years, while aircraft recon-
naissance began in 1944 and satellite tracking
around 1966. Figure 1 shows how older hurricane
counts have been adjusted to attempt to account
for the lack of aircraft and satellite observations.
Changes in data gathering technologies could
substantially influence the overall patterns in Fig-
ures 2 and 3. The effects of these changes on data
consistency over the life of the indicator would
benefit from additional research.
While Figures 2 and 3 cover several different
aspects of tropical cyclones, there are other
important factors not covered here, including the
size of each storm, the amount of rain, and the
height of the storm surge.
DATA SOURCES
Hurricane counts are reported on several National
Oceanic and Atmospheric Administration websites
and were compiled using methods described
in Knutsonetal. (2010).38 The ACE Index data
(Figure 2) came from the National Oceanic and
Atmospheric Administration's Climate Prediction
Center, and are available online at: www.cpc.
noaa.gov/products/outlooks/backgroundjnforma-
tion.shtml. Values for the PDI have been calcu-
lated by Kerry Emanuel at the Massachusetts
Institute of Technology. Both indices are based on
wind speed measurements compiled by the Na-
tional Oceanic and Atmospheric Administration.
-------
OCEANS
Covering about 70 percent of the Earth's surface,
the world's oceans have a two-way relationship with
weather and climate. The oceans influence the weather
on local to global scales, while changes in climate can
fundamentally alter many properties of the oceans.
This chapter examines how some of these important
characteristics of the oceans have changed over time.
WHY DOES IT MATTER?
As greenhouse gases trap more energy from the sun, the oceans are absorb-
ing more heat, resulting in an increase in sea surface temperatures and rising
sea level. Changes in ocean temperatures and currents brought about by cli-
mate change will lead to alterations in climate patterns around the world. For
example, warmer waters may promote the development of stronger storms in
the tropics, which can cause property damage and loss of life. The impacts asso-
ciated with sea level rise and stronger storms are especially relevant to coastal
communities.
Although the oceans help reduce climate change by storing large amounts of
carbon dioxide, increasing levels of dissolved carbon are changing the chemistry
of seawater and making it more acidic. Increased ocean acidity makes it more
difficult for certain organisms, such as corals and shellfish, to build their skele-
tons and shells. These effects, in turn, could substantially alter the biodiversity
and productivity of ocean ecosystems.
Changes in ocean systems generally occur over much longer time periods than
in the atmosphere, where storms can form and dissipate in a single day. Inter-
actions between the oceans and atmosphere occur slowly over many months to
years, and so does the movement of water within the oceans, including the mix-
ing of deep and shallow waters. Thus, trends can persist for decades, centuries, or
longer. For this reason, even if greenhouse gas emissions were stabilized tomor-
row, it would take many more years—decades to centuries—for the oceans to
adjust to changes in the atmosphere and the climate that have already occurred.
;
-------
Summary of Key Points
Ocean Heat. Three separate analyses show that the amount of heat stored in the ocean has increased
substantially since the 1950s. Ocean heat content not only determines sea surface temperature, but also
affects sea level and currents.
Sea Surface Temperature. Ocean surface temperatures increased around the world over the 20th
century. Even with some year-to-year variation, the overall increase is clear, and sea surface temperatures
have been higher during the past three decades than at any other time since reliable observations began
in the late 1800s.
Sea Level. When averaged over all the world's oceans, sea level has increased at a rate of roughly
six-tenths of an inch per decade since 1880. The rate of increase has accelerated in recent years to more
than an inch per decade. Changes in sea level relative to the land vary by region. Along the U.S. coastline,
sea level has risen the most along the Mid-Atlantic coast and parts of the Gulf coast, where some stations
registered increases of more than 8 inches between 1960 and 2013. Sea level has decreased relative to
the land in parts of Alaska and the Northwest.
A Closer Look: Land Loss Along the Atlantic Coast. As sea level rises, dry land
and wetland can turn into open water. Along many parts of the Atlantic coast, this problem is
made worse by low elevations and land that is already sinking. Between 1996 and 2011, the
coastline from Florida to New York lost more land than it gained.
Ocean Acidity. The ocean has become more acidic over the past few centuries because of increased
levels of atmospheric carbon dioxide, which dissolves in the water. Higher acidity affects the balance of
minerals in the water, which can make it more difficult for certain marine animals to build their skeletons
and shells.
t
-------
Ocean Heat
indicator describes trends in
KEY POINTS
0 In three different data analyses,
the long-term trend shows that the
oceans have become warmer since
1955 (see Figure 1).
3 Although concentrations of green-
house gases have risen at a rel-
atively steady rate over the past
few decades (see the Atmospheric
Concentrations of Greenhouse Gases
indicator on p. 20), the rate of change
in ocean heat content can vary from
year to year (see Figure 1). Year-
to-year changes are influenced by
events such as volcanic eruptions and
recurring ocean-atmosphere patterns
such as El Nino.
When sunlight reaches the Earth's surface, the world's oceans absorb some of this energy and store
it as heat. This heat is initially absorbed at the surface, but some of it eventually spreads to deeper
waters. Currents also move this heat around the world. Water has a much higher heat capacity than
air, meaning the oceans can absorb larger amounts of heat energy with only a slight increase in temperature.
The total amount of heat stored by the oceans is called "ocean heat content," and measurements of water
temperature reflect the amount of heat in the water at a particular time and location. Ocean temperature
plays an important role in the Earth's climate system—particularly sea surface temperature (see the Sea
Surface Temperature indicator on p. )—because heat from ocean surface waters provides energy for storms
and thereby influences weather patterns.
Higher greenhouse gas concentrations are trapping more energy from the sun. Because changes in ocean
systems occur over centuries, the oceans have not yet warmed as much as the atmosphere, even though they
have absorbed more than 90 percent of the Earth's extra heat since 1955.u If not for the large heat-storage
capacity provided by the oceans, the atmosphere would grow warmer more rapidly.3 Increased heat absorp-
tion also changes ocean currents because many currents are driven by differences in temperature, which
cause differences in density. These currents influence climate patterns and sustain ecosystems that depend on
certain temperature ranges.
Because water expands slightly as it gets warmer, an increase in ocean heat content will also increase the
volume of water in the ocean, which is one cause of the observed increases in sea level (see the Sea Level
indicator on p. ).
ABOUT THE INDICATOR
This indicator shows trends in global ocean heat content from 1955 to 2013. These data are available for the
top 700 meters of the ocean (nearly 2,300 feet), which accounts for just under 20 percent of the total volume
of water in the world's oceans. The indicator measures ocean heat content in joules, which are units of energy.
The National Oceanic and Atmospheric Administration has calculated changes in ocean heat content based on
measurements of ocean temperatures around the world at different depths. These measurements come from
a variety of instruments deployed from ships and airplanes and, more recently, underwater robots. Thus, the
data must be carefully adjusted to account for differences among measurement techniques and data collec-
tion programs. Figure 1 shows three independent interpretations of essentially the same underlying data.
46
a"dy>r* *v
.1 V - k
-------
Figure 1. Ocean Heat Content, 1955-2013
15
-10
1960
1970
1980
1990
2000
2010
Year
This figure shows changes in ocean heat content between 1955 and 2013. Ocean heat content is measured in joules, a unit of
energy, and compared against the 1971-2000 average, which is set at zero for reference. Choosing a different baseline period would
not change the shape of the data over time. The lines were independently calculated using different methods by three agencies: the
National Oceanic and Atmospheric Administration (NOAA), Australia's Commonwealth Scientific and Industrial Research Organisation
(CSIRO), and Japan Meteorological Agency's Meteorological Research Institute (MRI/JMA). For reference, an increase of 5 units on this
graph (5 x 1022 joules) is equal to approximately 100 times the total amount of energy used by all the people on Earth in a year.4
Data sources: CSIRO, 2014;5 MRI/JMA, 2014;6NOAA, 20147
INDICATOR NOTES
Data must be carefully reconstructed and filtered for biases because of different data collection techniques and
uneven sampling over time and space. Various methods of correcting the data have led to slightly different ver-
sions of the ocean heat trend line. Scientists continue to compare their results and improve their estimates over
time. They also test their ocean heat estimates by looking at corresponding changes in other properties of the
ocean. For example, they can check to see whether observed changes in sea level match the amount of sea level
rise that would be expected based on the estimated change in ocean heat.
DATA SOURCES
Data for this indicator were collected by the National Oceanic and Atmospheric Administration and other orga-
nizations around the world. The data were analyzed independently by researchers at the National Oceanic and
Atmospheric Administration, Australia's Commonwealth Scientific and Industrial Research Organisation, and the
Japan Meteorological Agency's Meteorological Research Institute.
-------
Sea Surface Temperature
KEY POINTS
0 Sea surface temperature increased
over the 20th century and continues
to rise. From 1901 through 2013,
temperatures rose at an average rate
of 0.13°F per decade (see Figure 1).
3 Sea surface temperatures have been
higher during the past three decades
than at any other time since reliable
observations began in 1880 (see
Figure 1).
3 Increases in sea surface temperature
have largely occurred over two key
periods: between 1910 and 1940, and
from about 1970 to the present. Sea
surface temperatures appear to have
cooled between 1880 and 1910 (see
Figure 1).
5 Changes in sea surface temperature
vary regionally. While most parts
of the world's oceans have seen
temperatures rise, a few areas have
actually experienced cooling — for
example, parts of the North Atlantic
(see Figure 2).
This indicator describes global trends in sea surface temperature.
Sea surface temperature — the temperature of the water at the ocean surface — is an important physical
attribute of the world's oceans. The surface temperature of the world's oceans varies mainly with latitude,
with the warmest waters generally near the equator and the coldest waters in the Arctic and Antarctic
regions. As the oceans absorb more heat, sea surface temperatures will increase and the ocean circulation
patterns that transport warm and cold water around the globe will change.
Changes in sea surface temperature can alter marine ecosystems in several ways. For example, variations in
ocean temperature can affect what species of plants, animals, and microbes are present in a location, alter
migration and breeding patterns, threaten sensitive ocean life such as corals, and change the frequency and
intensity of harmful algal blooms such as "red tide."8 Over the long term, increases in sea surface temperature
could also reduce the circulation patterns that bring nutrients from the deep sea to surface waters. Changes in
reef habitat and nutrient supply could dramatically alter ocean ecosystems and lead to declines in fish popula-
tions, which in turn could affect people who depend on fishing for food or jobs.9
Because the oceans continuously interact with the atmosphere, sea surface temperature can also have pro-
found effects on global climate. Increases in sea surface temperature have led to an increase in the amount of
atmospheric water vapor over the oceans.10 This water vapor feeds weather systems that produce precipitation,
increasing the risk of heavy rain and snow (see the Heavy Precipitation and Tropical Cyclone Activity indicators
on pp. and ). Changes in sea surface temperature can also shift storm tracks, potentially contributing to
droughts in some areas.11
Figure 1. Average Global Sea Surface Temperature, 1880-2013
2.0
1.5
-1.5
-2.0
1880
1900
1920
1940 1960
Year
1980
2000
2020
This graph shows how the average surface temperature of the world's oceans has changed since 1880. This graph uses
the 1971 to 2000 average as a baseline for depicting change. Choosing a different baseline period would not change
the shape of the data over time. The shaded band shows the range of uncertainty in the data, based on the number of
measurements collected and the precision of the methods used.
Data source: NOAA, 201412
-------
Figure 2. Change in Sea Surface Temperature, 1901-2012
Change in sea surface temperature (°F):
1 1.5 2 2.5 3
+ = statistically significant trend
3.5
Insufficient
data
This map shows how average sea surface temperatures around the world changed between 190 / and 2012, It is based on
a combination of direct measurements and satellite measurements. A black "+" symbol in the middle of a square on the
map means the trend shown is statistically significant. White areas did not have enough data to calculate reliable long-term
trends.
Data source: IPCC, 201313
ABOUT THE INDICATOR
This indicator tracks average global sea surface temperature from 1880 through 2013. It also includes a map
to show how change in sea surface temperature has varied across the world's oceans since 1901.
Techniques for measuring sea surface temperature have evolved since the 1800s. For instance, the earliest
data were collected by inserting a thermometer into a water sample collected by lowering a bucket from a
ship. Today, temperature measurements are collected more systematically from ships, as well as at stationary
and drifting buoys.
The National Oceanic and Atmospheric Administration has carefully reconstructed and filtered the data in
Figure 1 to correct for biases in the different collection techniques and to minimize the effects of sampling
changes over various locations and times. The data are shown as anomalies, or differences, compared with
the average sea surface temperature from 1971 to 2000. The map in Figure 2 was developed by the Intergov-
ernmental Panel on Climate Change, which calculated long-term trends based on a collection of published
studies.
INDICATOR NOTES
Both components of this indicator are
based on instrumental measurements of
surface water temperature. Due to denser
sampling and improvements in sampling
design and measurement techniques,
newer data are more precise than older
data. The earlier trends shown by this
indicator have less certainty because of
lower sampling frequency and less precise
sampling methods, as shown by the width
of the blue shaded band in Figure 1.
DATA SOURCES
Data for Figure 1 were provided by the Na-
tional Oceanic and Atmospheric Adminis-
tration's National Climatic Data Center and
are available online at: www.ncdc.noaa.
gov/ersst. These data were reconstructed
from measurements of water temperature,
which are available from the National Oce-
anic and Atmospheric Administration at:
http://icoads.noaa.gov/products.html. Figure
2 comes from the Intergovernmental Panel
on Climate Change's Fifth Assessment
Report (www.ipcc.ch/report/arB/wg1), which
gathers data from a variety of studies that
provide the best available information
about climate change.
-------
Sea Level
This indicator describes how sea level has changed over til
sea level changes: absolute and relative.
le. The indicator describes two types
KEY POINTS
0 After a period of approximately
2,000 years of little change (not
shown here), global average sea level
rose throughout the 20th century, and
the rate of change has accelerated in
recent years."When averaged over
all the world's oceans, absolute sea
level increased at an average rate
of 0.06 inches per year from 1880
to 2012 (see Figure 1). Since 1993,
however, average sea level has risen
at a rate of 0.11 to 0.12 inches per
year—roughly twice as fast as the
long-term trend.
0 Relative sea level rose along much of
the U.S. coastline between 1960 and
2013, particularly the Mid-Atlantic
coast and parts of the Gulf coast,
where some stations registered
increases of more than 8 inches (see
Figure 2). Meanwhile, relative sea
level fell at some locations in Alaska
and the Pacific Northwest. At those
sites, even though absolute sea level
has risen, land elevation has risen
more rapidly.
3 While absolute sea level has in-
creased steadily overall, particularly
in recent decades, regional trends
vary, and absolute sea level has
decreased in some places.15 Relative
sea level also has not risen uniform-
ly because of regional and local
changes in land movement and long-
term changes in coastal circulation
patterns.
A
s the temperature of the Earth changes, so does sea level. Temperature and sea level are linked for two
main reasons:
1. Changes in the volume of water and ice on land (namely glaciers and ice sheets) can increase or de-
crease the volume of water in the ocean (see the Glaciers indicator on p. ).
2. As water warms, it expands slightly—an effect that is cumulative over the entire depth of the oceans
(see the Ocean Heat indicator on p. ).
Changing sea levels can affect human activities in coastal areas. Rising sea level inundates low-lying wetlands
and dry land, erodes shorelines, contributes to coastal flooding, and increases the flow of salt water into
estuaries and nearby groundwater aquifers. Higher sea level also makes coastal infrastructure more vulnerable
to damage from storms.
The sea level changes that affect coastal systems involve more than just expanding oceans, however, because
the Earth's continents can also rise and fall relative to the oceans. Land can rise through processes such as
sediment accumulation (the process that built the Mississippi River delta) and geological uplift (for example,
as glaciers melt and the land below is no longer weighed down by heavy ice). In other areas, land can sink
because of erosion, sediment compaction, natural subsidence (sinking due to geologic changes), or engineering
projects that prevent rivers from naturally depositing sediments along their banks. Changes in ocean currents
such as the Gulf Stream can also affect sea levels by pushing more water against some coastlines and pulling
it away from others, raising or lowering sea levels accordingly.
Scientists account for these types of changes by measuring sea level change in two different ways. Relative sea
level change is how the height of the ocean rises or falls relative to the land at a particular location. In con-
trast, absolute sea level change refers to the height of the ocean surface above the center of the earth, without
regard to whether nearby land is rising or falling.
Figure 1. Global Average Absolute Sea Level Change, 1880-2013
12
10
Trend based on tide gauges
> Satellite measurements
1880
1900
1920
1940
1960
1980
2000
2020
Year
This graph shows cumulative changes in sea level for the world's oceans since 1880, based on a combination of long-term tide
gauge measurements and recent satellite measurements. This figure shows average absolute sea level change, which refers to the
height of the ocean surface, regardless of whether nearby land is rising or falling. Satellite data are based solely on measured sea
level, while the long-term tide gauge data include a small correction factor because the size and shape of the oceans are changing
slowly over time. (On average, the ocean floor has been gradually sinking since the last Ice Age peak, 20,000 years ago.) The shad-
ed band shows the likely range of values, based on the number of measurements collected and the precision of the methods used.
Data sources: CSIRO, 2013;16 NOAA, 201417
-------
ABOUT THE INDICATOR
INDICATOR NOTES
This indicator presents trends in sea level based on measurements from tide gauges and from satellites
that orbit the Earth. Tide gauges measure relative sea level change at points along the coast, while satellite
instruments measure absolute sea level change over nearly the entire ocean surface. Many tide gauges have
collected data for more than 1 00 years, while satellites have collected data since the early 1 990s.
Figure 1 shows annual absolute sea level change averaged over the entire Earth's ocean surface. The long-
term trend is based on tide gauge data that have been adjusted to show absolute global trends through cal-
ibration with recent satellite data. This long-term data set has been calculated through 201 2, while satellite
data are now available through the end of 2013. Figure 2 shows trends at a more local scale, highlighting
the 1 960 to 201 3 change in relative sea level at 67 tide gauges along the Atlantic, Pacific, and Gulf coasts of
the United States.
Figure 2. Relative Sea Level Change Along U.S. Coasts, 1960-2013
;
• W J*
1
?
4*
f
f
1
f
Alaska
^P^^HP!
t Hawaii and
Pacific Islands
* 4- *•
Relative sea level change (inches):
-7.99 -5.99 -3.99 -1.99
<-8 to-6 to-4 to-2 toO
t t t 1 I
Relative sea level trends represent
a combination of absolute sea level
change and any local land movement.
Tide gauge measurements such as
those in Figure 2 generally cannot
distinguish between these two dif-
ferent influences without an accurate
measurement of vertical land motion
nearby.
Some changes in relative and absolute
sea level can be due to multi-year
cycles such as El Nino and La Nina,
which affect coastal ocean tempera-
tures, salt content, wind patterns,
atmospheric pressure (and thus storm
tracks), and currents. Obtaining a reli-
able trend can require many years of
data, which is why the satellite record
in Figure 1 has been supplemented
with a longer-term reconstruction
based on tide gauge measurements.
DATA SOURCES
Absolute sea level trends were pro-
vided by Australia's Commonwealth
Scientific and Industrial Research
Organisation and the National Oce-
anic and Atmospheric Administration.
These data are based on measure-
ments collected by satellites and tide
gauges. Relative sea level data are
available from the National Oceanic
and Atmospheric Administration,
which publishes an interactive online
map (http://tidesandcurrents.noaa.gov/
sltrends/sltrends.shtml) with links to
detailed data for each tide gauge.
0.01
to 2
2.01
to 4
4.01
to 6
6.01
to 8
>8
This map shows cumulative changes in relative sea level from 1960 to 2013 at tide gauge stations along U.S. coasts. Relative sea
level reflects changes in sea level as well as land elevation.
Data source: NOAA, 201418
-------
A Closer Look
Land Loss Along the Atlantic Coast
KEY POINTS
5 Roughly 20 square miles of dry land
and wetland were converted to open
water along the Atlantic coast
between 1996 and 2011. (For refer-
ence, Manhattan Is 33 square miles.)
More of this loss occurred In the
Southeast than In the Mid-Atlantic
(see Figure 1).
3 The data suggest that at least half
of the land lost since 1996 has been
tidal wetland. The loss of dry upland
appears to be larger than the loss of
non-tidal wetland. These categories
account for approximately 30 per-
cent and 17 percent of the land lost,
respectively (see Figure 2).
Rising sea level tends to make headlines during extreme events, like the storm surge that caused billions
of dollars in damage during Hurricane Sandy in 2012. Yet rising sea level can also cause permanent
changes in the landscape when it inundates (submerges) low-lying land. The Atlantic coast is particularly
vulnerable because of low elevations and sinking shorelines.
The loss of coastal land can affect a large number of people, as nearly 10 million Americans live in a coastal
floodplain.19 Coastal ecosystems are also at risk. These environments provide habitat for many kinds of plants
and animals, as well as services that ensure people's well-being, ranging from food production to recreation.
Coastal wetlands provide valuable nursery, feeding, breeding, staging, and resting areas for many fish, shell-
fish, mammals, and birds, and they can buffer coastal areas against storm and wave damage.
As sea level rises, dry land can turn into wetland or open water. Existing wetlands can be threatened, too, as
salt marshes, mangrove forests, and other coastal wetlands are at risk of being converted to open water.
The Sea Level indicator (p. x) shows that sea level is rising overall in connection with climate change, but
the rate of change varies by region, as do the effects. To provide a useful regional perspective, this feature
examines the amount of land lost to sea level rise along the Atlantic coast from Florida to New York. It is based
on satellite data that have been collected and analyzed at five-year intervals since 1996. Figure 1 divides the
Atlantic coast into two regions for comparison, while Figure 2 shows the different types of land that have been
lost.
Why is the Atlantic coast
particularly vulnerable to
sea level rise?
Much of the land along the Atlantic
coast is flat and close to sea level—
including thousands of square miles
of marshes and other productive
wetlands, plus many low-lying cities. In
addition, much of the land along the
Atlantic coast is sinking, which magni-
fies the local effect of sea level rise. The
land in North America is actually still
adjusting to the loss of ice after the last
ice age, which peaked about 20,000
years ago. Back then, thick sheets of ice
covered areas of what is now Canada
and the northern United States. The
weight of all that ice depressed the
land beneath it, but caused the land
farther south (particularly the Mid-At-
lantic region from North Carolina to
New York) to bulge upward. After the
ice melted and the extra weight was
lifted, northern areas began to rise, and
the Mid-Atlantic region started to sink.
This very slow process continues today.
-------
Figure 1. Land Loss Along the Atlantic Coast, 1996-2011
25
20
VO
01
Oi
i—
S
• 1S
Mid-Atlantic
Southeast
I/I ~
o £
«J 3
10
>& 5
+*
.5
1 o
u
-5
1996-2001
1996-2006
Time period
1996-2011
This graph shows the net amount of land converted to open water along the Atlantic coast during three time periods:
1996-2001, 1996-2006, and 1996-2011. The results are divided into two regions: the Southeast and the Mid-Atlantic (see
locator map). Negative numbers show where land loss is outpaced by the accumulation of new land.
Data source: NOAA, 201320
Figure 2. Land Submergence Along the Atlantic Coast, 1996-2011
25
•o
01
'£ Ol
BJ «-
11
u *"
_« 20
15
10
Tidal wetland -*•
Open water
Non-tidal wetland
Open water
Dryland -*
Open water
1996-2001
1996-2006
Time period
1996-2011
This graph shows the net amount of land converted to open water along the Atlantic coast during three time periods: 1996-2001,
1996-2006, and 1996-2011. The results are divided into categories to show the type of land that has been converted to open
water.
Data source: NOAA, 201321
Mid-Atlantic
Southeast
NOTES
Measurements of the change in coastal
land depend on land cover and elevation
data, which have significant limits in terms
of accuracy and precision. Some results are
field-checked for accuracy. The coastal land
cover data are routinely compiled by the
National Oceanic and Atmospheric Adminis-
tration's Coastal Change Analysis Program,
and they represent the federal government's
most comprehensive set of data on land use
and land cover in the coastal zone.
Sea level rise is not the only factor that
contributes to the loss of coastal land. In
addition to the natural sinking of the shore-
line in some areas, such as the Mid-Atlantic,
coastal land loss has been made worse by
human activities such as navigation and
flood control structures that block wetland
migration or the movement of sediment;
withdrawal of ground water, oil, or natural
gas in some regions; and boat traffic that
accelerates wetland erosion.22 Natural
processes unrelated to current sea level rise
can also cause shores to erode.
DATA SOURCES
This feature is based on land cover data
from the Coastal Change Analysis Program,
which is coordinated by the National Oce-
anic and Atmospheric Administration. For
more information about this program, visit:
http://csc.noaa.gov/digitalcoast.
-------
KEY POINTS
0 Measurements made over the last
few decades have demonstrated that
ocean carbon dioxide levels have
risen in response to increased carbon
dioxide in the atmosphere, leading
to an increase in acidity (that is, a
decrease in pH) (see Figure 1).
0 Historical modeling suggests that
since the 1880s, increased carbon
dioxide has led to lower aragonite
saturation levels in the oceans
around the world, which makes it
more difficult for certain organisms
to build and maintain their skeletons
and shells (see Figure 2).
0 The largest decreases in aragonite
saturation have occurred in tropi-
cal waters (see Figure 2). However,
decreases in cold areas may be of
greater concern because colder
waters typically have lower aragonite
saturation levels to begin with.23
Ocean Acidity
This indicator describes changes in the chemistry of the ocean, which relate to the amount of
carbon dioxide dissolved in the water.
The ocean plays an important role in regulating the amount of carbon dioxide in the atmosphere. As
atmospheric concentrations of carbon dioxide rise (see the Atmospheric Concentrations of Greenhouse
Gases indicator on p. ), the ocean absorbs more carbon dioxide. Because of the slow mixing time be-
tween surface waters and deeper waters, it can take hundreds to thousands of years to establish this balance.
Over the past 250 years, oceans have absorbed about 28 percent of the carbon dioxide produced by human
activities that burn fossil fuels.24
t
Increasing
acidity
Neutral
Increasing
alkalinity
Battery acid
Lemon juice
Vinegar
Although the ocean's ability to take up carbon dioxide prevents atmospheric levels from climbing even higher,
rising levels of carbon dioxide dissolved in the ocean can have a negative effect on some marine life. Carbon
dioxide reacts with sea water to produce carbonic acid. The resulting increase in acidity (measured by lower
pH values) changes the balance of minerals in the water. This makes it more difficult for corals, some types of
plankton, and other creatures to produce a mineral called calcium carbonate, which is the main ingredient in
their hard skeletons and shells. Thus, declining pH can make it more difficult for these animals to thrive. This
can lead to broader changes in the overall structure of ocean and coastal ecosystems, and can ultimately affect
fish populations and the people who depend on them.25 Signs of damage are already starting to appear in
certain areas.26
While changes in ocean pH and mineral saturation caused by the uptake of atmospheric carbon dioxide gen-
erally occur over many decades, these properties can fluctuate over shorter periods, especially in coastal and
surface waters. For example, increased photosynthesis during the day and during the summer leads to natural
fluctuations in pH. Acidity also varies with water temperature.
ABOUT THE INDICATOR
This indicator describes trends in pH and related properties of ocean water, based on a combination of direct
observations, calculations, and modeling.
Figure 1 shows pH values and levels of dissolved carbon dioxide at three locations that have collected mea-
surements consistently over the last few decades. These data have been either measured directly or calculated
from related measurements, such as dissolved inorganic carbon and alkalinity. Data come from two stations in
the Atlantic Ocean (Bermuda and the Canary Islands) and one in the Pacific (Hawaii).
The global map in Figure 2 shows changes over time in aragonite saturation level. Aragonite is a specific
form of calcium carbonate that many organisms produce and use to build their skeletons and shells, and the
saturation state is a measure of how easily aragonite can dissolve in the water. The lower the saturation level,
the more difficult it is for organisms to build and maintain their skeletons and shells. This map was created
by comparing average conditions during the 1 880s with average conditions during the most recent 1 0 years
(2004-201 3). Aragonite saturation has only been measured at selected loca-
tions during the last few decades, but it can be calculated reliably for different
times and locations based on the relationships scientists have observed among
aragonite saturation, pH, dissolved carbon, water temperature, concentrations
of carbon dioxide in the atmosphere, and other factors that can be measured.
Thus, while Figure 2 was created using a computer model, it is based on mea-
surements.
Acid rain
Adult fish die
Milk
Fish reproduction affected
Normal range of precipitation pH
Normal range of stream pH
Baking soda
Sea water
Milkof magnesia
Ammonia
Lye
pH Scale
Acidity is commonly measured using the pH scale. Pure water has a pH of about 7, which
is considered neutral. A substance with a pH less than 7 is considered to be acidic, while a
substance with a pH greater than 7 is considered to be basic or alkaline. The lower the pH,
the more acidic the substance. Like the well-known Richter scale for measuring earthquakes,
the pH scale is based on powers of 10, which means a substance with a pH of 3 is 10 times
more acidic than a substance with a pH of 4. For more information about pH, visit:
www.epa.gov/acidrain/measure/ph.html.
Source: Environment Canada, 200827
54
-------
Figure 1. Ocean Carbon Dioxide Levels and Acidity, 1983-2012
ndAcid
500
Bermuda
2000 2010 2020 ~-
Canary Islands
O. 400
£ 350
Q. 300
-------
SNOW AN
The Earth's surface contains many forms of snow and
ice, including sea, lake, and river ice; snow cover;
glaciers, ice caps, and ice sheets; and frozen ground.
Climate change can dramatically alter the Earth's
snow- and ice-covered areas because snow and ice
can easily change between solid and liquid states in
response to relatively minor changes in temperature.
This chapter focuses on trends in snow, glaciers, and
the freezing and thawing of oceans and lakes.
WHY DOES IT MATTER?
jduced snowfall and less snow cover on the ground could diminish the bene-
cial insulating effects of snow for vegetation and wildlife, while also affecting
rater supplies, transportation, cultural practices, travel, and recreation for mil-
ons of people. For communities in Arctic regions, reduced sea ice could increase
oastal erosion and exposure to storms, threatening homes and property, while
lawing ground could damage roads and buildings and accelerate erosion. Con-
ersely, reduced snow and ice could present commercial opportunities for others,
icluding ice-free shipping lanes and increased access to natural resources.
uch changing climate conditions can have worldwide implications because
now and ice influence air temperatures, sea level, ocean currents, and storm
atterns. For example, melting ice sheets on Greenland and Antarctica add fresh
rater to the ocean, increasing sea level and possibly changing ocean circulation
lat is driven by differences in temperature and salinity. Because of their light
olor, snow and ice also reflect more sunlight than open water or bare ground,
9 a reduction in snow cover and ice causes the Earth's surface to absorb more
nergy from the sun and become warmer.
56
-------
Summary of Key Points
Arctic Sea Ice. Part of the Arctic Ocean is covered by ice year-round. The area covered by ice is typically
smallest in September, after the summer melting season. The minimum extent of Arctic sea ice has de-
creased over time, and in September 2012 it was the smallest on record. Arctic ice has also become thinner,
which makes it more vulnerable to additional melting.
Glaciers. Glaciers in the United States and around the world have generally shrunk since the 1960s, and
the rate at which glaciers are melting has accelerated over the last decade. The loss of ice from glaciers has
contributed to the observed rise in sea level.
Lake Ice. Most lakes in the northern United States are freezing later and thawing earlier compared with
the 1800s and early 1900s. Freeze dates have shifted later at a rate of roughly half a day to one day per
decade, while thaw dates for most of the lakes studied have shifted earlier at a rate of half a day to two
days per decade.
Community Connection: Ice Breakup in Two Alaskan Rivers. Regions in the
far north are warming more quickly than other parts of the world. Two long-running contests on
the Tanana and Yukon rivers in Alaska—where people guess the date when the river ice will break
up in the spring—provide a century's worth of evidence revealing that the ice on these rivers is
generally breaking up earlier in the spring than it used to.
Snowfall. Total snowfall—the amount of snow that falls in a particular location—has decreased in most
parts of the country since widespread records began in 1930. One reason for this decline is that more than
three-fourths of the locations studied have seen more winter precipitation fall in the form of rain instead of
snow.
Snow Cover. Snow cover refers to the area of land that is covered by snow at any given time. Between
1972 and 2013, the average portion of North America covered by snow decreased at a rate of about 3,500
square miles per year, based on weekly measurements taken throughout the year. However, there has been
much year-to-year variability.
Snowpack. The depth or thickness of snow on the ground (snowpack) in early spring decreased at
about three-fourths of measurement sites in the western United States between 1955 and 2013. However,
other locations saw an increase in spring snowpack. The average change across all sites for this time period
amounts to about a 14 percent decline.
-------
Arctic Sea Ice
This indicator tracks the extent and age of sea ice in the Arctic Ocean.
KEY POINTS
3 September 2012 had the lowest sea
ice extent on record, 49 percent
below the 1979-2000 average for
that month.
5 The September 2013 sea Ice extent
was nearly 700,000 square miles less
than the historical 1979-2000 aver-
age—a difference more than twice
the size of Texas (see Figure 1).
0 Although the annual minimum of
sea ice extent typically occurs in
September, all months have shown
a decreasing trend in sea ice extent
over the past several decades. The
largest decreases have occurred in
the summer and fall.12
O Evidence of the age of Arctic sea ice
suggests an overall loss of multi-year
ice. The proportion of sea ice five
years or older has declined dramati-
cally over the recorded time period,
from more than 30 percent of Sep-
tember ice in the 1980s to 7 percent
in 2013. A growing percentage of
Arctic sea ice is only one or two years
old. This thinning of Arctic ice makes
it more vulnerable to further melting.
Sea ice is an integral part of the Arctic Ocean. During the dark winter months, sea ice essentially covers
the entire Arctic Ocean. In summer, some of this ice melts because of warmer temperatures and long
hours of sunlight. Sea ice typically reaches its minimum thickness and extent in mid-September, when
the area covered by ice is roughly half the size of the winter maximum. The ice then begins expanding again.
The extent of area covered by Arctic sea ice is an Dwindling AfCtJC Sea ICG
important indicator of changes in global climate
because warmer air and water temperatures
are reducing the amount of sea ice present.
Because sea ice is more reflective than liquid
water, it plays a significant role in the Earth's
energy balance and keeping polar regions cool.
(For more information on the effects of surface
color on reflecting sunlight, see the Snow Cover
indicator on p. 68.) Sea ice also keeps the air
cool by forming a barrier between the cold air
above and the warmer water below. As the
amount of sea ice decreases, the Arctic region's
ability to stabilize the Earth's climate is re-
duced, potentially leading to a "feedback loop"
of more absorption of solar energy, higher air
temperatures, and even greater loss of sea ice.
The age of sea ice is also an important indi-
cator of Arctic conditions, because older ice is
generally thicker and stronger than younger ice.
A loss of older ice suggests that the Arctic is
losing ice faster than it is accumulating it.
Changes in sea ice can directly affect the health
of Arctic ecosystems. Mammals such as polar Source: NASA- 20143
bears and walruses rely on the presence of sea
ice for hunting, breeding, and migrating. These animals face the threat of declining birth rates and restricted
access to food sources because of reduced sea ice coverage and thickness. Impacts on Arctic wildlife, as well
as the loss of ice itself, are already restricting the traditional subsistence hunting lifestyle of indigenous Arctic
populations such as the Yup'ik, Ifiupiat, and Inuit.
While diminished sea ice can have negative ecological effects, it can also present commercial opportunities. For
instance, reduced sea ice opens shipping lanes and increases access to natural resources in the Arctic region.
58
ABOUT THE INDICATOR
Figure 1 presents trends in Arctic sea ice extent from 1979, when extensive measurements started, to 2013.
Sea ice extent is defined as the area of ocean where at least 15 percent of the surface is frozen. This threshold
was chosen because scientists have found that it gives the best approximation of the edge of the ice. Data
are collected throughout the year, but for comparison, this indicator focuses on the average sea ice extent
in September of each year. This is because September is typically when the sea ice extent reaches its annual
minimum after melting during the spring and summer. Data for this indicator were gathered by the National
Snow and Ice Data Center using satellite imaging technology.
Figure 2 examines the age of the ice that is present in the Arctic during the week in September with the
smallest extent of ice. By combining daily satellite images, wind measurements, and data from surface buoys
that move with the ice, scientists can track specific parcels of ice as they move over time. This tracking enables
them to calculate the age of the ice in different parts of the Arctic. Although satellites started collecting data in
1979, Figure 2 only shows trends back to 1983 because it is not possible to know the full age distribution until
the ice has been tracked for at least five years.
-------
Figure 1. September Monthly Average Arctic Sea Ice Extent, 1979-2013
91
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91
s
IB
01
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0.5
1975 1980 1985 1990 1995 2000 2005 2010 2015
Year
This figure shows Arctic sea ice extent from 1979 through 2013 using data from September of each year, which is when
the minimum extent typically occurs.
Data source: NSIDC, 20134
Figure 2. Age of Arctic Sea Ice at Minimum September Week, 1983-2013
Second-year ice
^/
Third-year ice
Fourth-year ice
1985 1990 1995 2000
Year
2005
2010
This figure shows the distribution of Arctic sea ice extent by age group during the peak melting week in September of
each year. The total extent in Figure 2 differs from the extent in Figure 1 because Figure 1 shows a monthly average,
while Figure 2 shows conditions during a single week.
Data source: NSIDC, 2013s
INDICATOR NOTES
Increasing temperatures associated with
climate change are not the only factor
contributing to reductions in sea ice.
Other conditions that may be affected by
climate change, such as fluctuations in
oceanic and atmospheric circulation and
typical annual and decadal variability,
also affect the extent of sea ice. Deter-
mining the age of ice is an imperfect
science, as there are cases where a small
amount of older ice might exist within an
area classified as younger, or vice-versa.
DATA SOURCES
The data for this indicator were provided
by the National Snow and Ice Data Cen-
ter. Data for Figure 1 are also available
online at: http://nsidc.org/data/seaicejn-
dex/archives.html, while Figure 2 is based
on an analysis by the University of
Colorado and a map published at: http://
nsidc.org/arcticseaicenews/2013/10. The
National Snow and Ice Data Center pro-
duces a variety of reports and a seasonal
newsletter analyzing Arctic sea ice data.
59
~
-------
Glaciers
This indicator examines the balance between snow accumulation and melting in glaciers, anc
glaciers in the United States and around the world have changed over time.
describes he
KEY POINTS
0 On average, glaciers worldwide have
been losing mass since at least the
1970s (see Figure 1), which in turn has
contributed to observed changes in
sea level (see the Sea Level indicator
on p. 50). A longer measurement
record from a smaller number of
glaciers suggests that they have been
shrinking since the 1940s. The rate
at which glaciers are losing mass
appears to have accelerated over
roughly the last decade.
O All three U.S. benchmark glaciers have
shown an overall decline in mass
balance since the 1950s and 1960s
and an accelerated rate of decline in
recent years (see Figure 2). Year-to-
year trends vary, with some glaciers
gaining mass in certain years (for
example. Wolverine Glacier during the
1980s). However, most of the mea-
surements indicate a loss of glacier
mass over time.
5 Trends for the three benchmark gla-
ciers are consistent with the retreat
of glaciers observed throughout the
western United States, Alaska, and
other parts of the world.6 Observa-
tions of glaciers losing mass are also
consistent with warming trends in U.S.
and global temperatures during this
time period (see the U.S. and Global
Temperature indicator on p. 28).
A glacier is a large mass of snow and ice that has accumulated over many years and is present year-
round. In the United States, glaciers can be found in the Rocky Mountains, the Sierra Nevada, the
Cascades, and throughout Alaska. A glacier flows naturally like a river, only much more slowly. At higher
elevations, glaciers accumulate snow, which eventually becomes compressed into ice. At lower elevations, the
"river" of ice naturally loses mass because of melting and ice breaking off and floating away (iceberg calving)
if the glacier ends in a lake or the ocean. When melting and calving are exactly balanced by new snow accu-
mulation, a glacier is in equilibrium and its mass will neither increase nor decrease.
In many areas, glaciers provide communities and ecosystems with a reliable source of streamflow and drinking
water, particularly in times of extended drought and late in the summer, when seasonal snowpack has melted
away. Freshwater runoff from glaciers also influences ocean ecosystems. Glaciers are important as an indicator
of climate change because physical changes in glaciers—whether they are growing or shrinking, advancing or
receding—provide visible evidence of changes in temperature and precipitation. If glaciers lose more ice than
they can accumulate through new snowfall, they ultimately add more water to the oceans, leading to a rise in
sea level (see the Sea Level indicator on p. 50). The same kinds of changes occur on a much larger scale within
the giant ice sheets that cover Greenland and Antarctica, potentially leading to even bigger implications for
sea level. Small glaciers tend to respond more quickly to climate change than the giant ice sheets. Altogether,
the world's small glaciers are adding roughly the same amount of water to the oceans per year as the ice
sheets of Greenland and Antarctica. During the last two decades, they added more water overall to the oceans
than the ice sheets did.7
ABOUT THE INDICATOR
This indicator is based on long-term monitoring data collected at selected glaciers around the world. Scientists
collect detailed measurements to determine glacier mass balance, which is the net gain or loss of snow and ice
over the course of the year. A negative mass balance indicates that a glacier has lost ice or snow. If cumulative
mass balance becomes more negative over time, it means glaciers are losing mass more quickly than they can
accumulate new snow.
Figure 1 shows trends in mass balance for a set of 37 reference glaciers around the world that have been mea-
sured consistently since the 1970s, including a few that have been measured since the 1940s. Data from these
reference glaciers have been averaged together to depict changes over time. Figure 2 shows trends for three
"benchmark" glaciers: South Cascade Glacier in Washington state, Wolverine Glacier near Alaska's southern
coast, and Gulkana Glacier in Alaska's interior. These three glaciers were chosen because they have been stud-
ied extensively by the U.S. Geological Survey for many years and because they are thought to be representative
of other glaciers nearby.
This indicator describes the change in glacier mass balance, which is measured as the average change in thick-
ness across the surface of a glacier. The change in ice or snow has been converted to the equivalent amount of
liquid water.
Photographs of McCall Glacier, Alaska, 1958 and 2003
60
Sources: Post,
19B8;8 Nolan, 2003
-------
Figure 1. Average Cumulative Mass Balance of "Reference"
Glaciers Worldwide, 1945-2012
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77)/5 figure shows the cumulative change in mass balance of a set of "reference" glaciers worldwide beginning in 1945. The line
on the graph represents the average of all the glaciers that were measured. Negative values indicate a net loss of ice and snow
compared with the base year of 1945. For consistency, measurements are in meters of water eguivalent, which represent changes
in the average thickness of a glacier. The small chart below shows how many glaciers were measured in each year. Some glacier
measurements have not yet been finalized for the last few years, hence the smaller number of sites.
Data sources: WGMS, 2013,'° 2014"
Figure 2. Cumulative Mass Balance of Three U.S. Glaciers, 1958-2012
INDICATOR NOTES
The relationship between climate change
and glacier mass balance is complex, and
the observed changes at specific reference
or benchmark glaciers might reflect a
combination of global and local variations
in temperature and precipitation. Individual
glaciers also vary in their structure, flow,
and response to climate. Slightly different
measurement and analysis methods have
been used at different glaciers, but overall
trends appear to be similar.
Long-term measurements are available for
only a relatively small percentage of the
world's glaciers. This indicator does not
include the Greenland and Antarctic ice
sheets, although two decades of satellite
data suggest that these ice sheets are also
experiencing a net loss of ice.14 Continued
satellite data collection will allow scientists
to evaluate long-term trends in the future.
DATA SOURCES
The World Glacier Monitoring Service
compiled data for Figure 1, based on
measurements collected by a variety of
organizations around the world. The U.S.
Geological Survey Benchmark Glacier
Program provided the data for Figure 2.
These data, as well as periodic reports and
measurements of the benchmark glaciers,
are available on the program's website at:
http://ak.water.usgs.gov/glaciology.
Glaciers Shown in Figure 2
AK Gulkana Glacier
o
Iverine Glacier
1955 I960
2005 2010 2015
This figure shows the cumulative mass balance of the three U.S. Geological Survey "benchmark" glaciers since measure-
ments began in the 1950s or 1960s. For each glacier, the mass balance is set at zero for the base year of 1965. Negative
values indicate a net loss of ice and snow compared with the base year. For consistency, measurements are in meters of
water equivalent, which represent changes in the average thickness of a glacier.
Data sources: O'Neel and Sass, 2013;12 USGS, 201413
-------
Lake Ice
This indicator measures the amount of time that ice is present
KEY POINTS
5 The lakes covered by this indicator
are generally freezing later than they
did in the past. Freeze dates have
shifted later at a rate of roughly half
a day to one day per decade (see
Figure 1).
5 Thaw dates for most of these lakes
show a general trend toward earlier
ice breakup in the spring (see Figure
2). Thaw dates have grown earlier by
up to 23 days in the past 107 years,
except for two lakes that remained
unchanged (see Figure 3). None of
these lakes were found to be thawing
later in the year.
5 The changes in lake freeze and thaw
dates shown here are consistent with
other studies. For example, a broad
study of lakes and rivers throughout
the Northern Hemisphere found that
since the mid-1800s, freeze dates
have occurred later and thaw dates
have occurred earlier, both shifting
at an average rate of 0.8 days to one
day per decade.15
.es in the United States.
The formation of ice cover on lakes in the winter and its disappearance the following spring depends on
climate factors such as air temperature, cloud cover, and wind. Conditions such as heavy rains or snow-
melt in locations upstream or elsewhere in the watershed also affect the length of time a lake is frozen.
Thus, ice formation and breakup dates are key indicators of climate change. If lakes remain frozen for longer
periods, it can signify that the climate is cooling. Conversely, shorter periods of ice cover suggest a warming
climate.
Changes in ice cover can affect the physical, chemical, and biological characteristics of a body of water. For
example, ice influences heat and moisture transfers between a lake and the atmosphere. Reduced ice cover
leads to increased evaporation and lower water levels, as well as an increase in water temperature and sun-
light penetration. These changes, in turn, can affect plant and animal life cycles and the availability of suitable
habitat. Additionally, ice cover affects the amount of heat that is reflected from the Earth's surface. Exposed
water will absorb and retain heat, making the Earth's surface warmer, whereas an ice- and snow-covered lake
will reflect more of the sun's energy and absorb less. (For more information on ice and snow reflecting sunlight,
see the Snow Cover indicator on p. 68.)
The timing and duration of ice cover on lakes and other bodies of water can also affect society—particularly in
relation to shipping and transportation, hydroelectric power generation, and fishing. The impacts can be posi-
tive or negative. For example, reduced ice cover on a large lake could extend the open-water shipping season
but require vessels to reduce their cargo capacity, as increased evaporation leads to lower water levels.
ABOUT THE INDICATOR
This indicator analyzes the dates at which lakes freeze and thaw. Freeze dates occur when a continuous and
immobile ice cover forms over a body of water. Thaw dates occur when the ice cover breaks up and open water
becomes extensive.
Freeze and thaw dates have been recorded through human visual observations for more than 150 years. The
National Snow and Ice Data Center maintains a database with freeze and thaw observations from more than
700 lakes and rivers throughout the Northern Hemisphere. This indicator focuses on 14 lakes within the United
States that have the longest and most complete historical records. The lakes of interest are located in Minneso-
ta, Wisconsin, New York, and Maine.
This figure shows the "ice-on" date, or
date of first freeze, for nine U.S. lakes. The
data are available from as early as 1850
to 2012, depending on the lake, and have
been smoothed using a nine-year moving
average.
Data source: Various organizations"
Figure 1. Date of First Freeze for Selected U.S. Lakes, 1850-2012
November 1
December 1
-------
Figure 2. Date of Ice Thaw for Selected U.S. Lakes, 1850-2012
March 1
IB
•o
1
April 1
Mayl
Junel
1840
1860
1880
1900
1920 1940
Year
I960
1980
2000
2020
Cobbosseecontee Lake
Damariscotta Lake
Detroit Lake
Geneva Lake
Lake George
Lake Mendota
LakeMonona
LakeOsakis
Lake Superior
at Bayfield
Mirror Lake
Moosehead Lake
— Otsego Lake
Sebago Lake
Shell Lake
This figure shows the "ice-off" date, or date of ice thawing and breakup, for 14 U.S. lakes. The data are
available from as early as 1850 to 2012, depending on the lake, and have been smoothed using a nine-year
moving average.
Data source: Various organizations17
Figure 3. Change in Ice Thaw Dates for Selected U.S. Lakes, 1905-2012
Detroit Lake
Odays
Lake Superior at Bayfield
| -23 days
0 • Shell Lake
LakeOsakis -5 days
Odays
LakeMonona
Lake Mendota—«»
-5 days •
Geneva Lake
-5 days
Moosehead Lake
-5 days
Cobbosseecontee Lake
-10 days \ u.a Lake
1»4 -9 days
Mirror Lake* 0K
-7 days V Sebago Lake
\ -16 days
Lake George
Otsego Lake • -5 days
-6 days
Damariscotta
Change in ice thaw date:
• Earlier o No change
This figure shows the change in the "ice-off" date, or date of ice thawing and breakup, for 14 U.S. lakes during the
period from 1905 to 2012. All but two of the lakes have red circles with negative numbers, which represent earlier
thaw dates. The other two lakes have not experienced a significant change in thaw dates. Larger circles indicate larger
changes.
Data source: Various organizations18
*
INDICATOR NOTES
Although there is a lengthy historical re-
cord of freeze and thaw dates for a much
larger set of lakes and rivers, some re-
cords are incomplete, with breaks ranging
from brief lapses to large gaps in data.
This indicator is limited to 14 lakes with
sufficiently complete historical records.
The four Maine lakes and Lake Osakis
only have data for ice thaw, so they do
not appear in Figure 1 (first freeze date).
Data used in this indicator are all based
on visual observations. While the proce-
dures for making observations of lake ice
are consistent over time, visual obser-
vations by individuals are open to some
interpretation and can differ from one
individual to the next. In addition, histor-
ical observations for lakes have typically
been made from a particular spot on the
shore, which might not be representative
of lakes as a whole or comparable to sat-
ellite-based observations. Considerations
for defining the thaw date are specific to
each lake.
DATA SOURCES
Data through 2004 for most lakes were
obtained from the Global Lake and River
Ice Phenology Database, which is main-
tained by the National Snow and Ice Data
Center. These data are available at: http://
nsidc.org/data/lake_river_ice. More recent
data were obtained from state, local,
and other organizations that collected or
compiled the observations.
-------
rv
ommunity Connection
Ice Breakup in Two Alaskan Rivers
KEY POINTS
5 The Tanana and Yukon rivers both
demonstrate long-term trends
toward earlier ice breakup in the
spring. The ice breakup dates for
both the Tanana and Yukon rivers
have shifted earlier by six to seven
days over their respective periods
of record.
3 Despite the overall trend toward
earlier breakup, the most recent
breakup dates for both rivers
are within the range of historical
variation.
Regions in the far north are warming more quickly than other parts of the world, and this pattern is
expected to continue19-20 (see the Arctic Sea Ice indicator on p. 58; for more information about regional
temperature changes, see the U.S. and Global Temperature indicator on p. 28). The Tanana and Yukon
rivers in Alaska provide a particularly unique record of northern climate because, for a century or more, local
citizens have recorded the date when the ice on these rivers starts to move or break up each spring. In fact,
some towns have annual competitions to guess when ice breakup will occur.
Since 1917, the Nenana Ice Classic competition on the Tanana River in central Alaska has paid several million
dollars in winnings to the people who come closest to guessing the exact date and time of day when the river
ice will break up. A similar tradition exists in Dawson City on the Yukon River, just across the border in Canada,
where breakup dates have been recorded since 1896.
River ice breakup is more than just a friendly competition, though. Ice breakup is an important time of
transition for communities that rely on these relatively remote and unmodified (wild) rivers for transportation,
subsistence hunting and fishing, and other needs. In addition, early thawing can lead to severe ice movement,
jamming, damage to infrastructure, and destructive floods.21
The data collected by these communities highlights how the river ice breakup dates in Nenana and Dawson
City have changed over time. Both towns use the same method to measure the exact time of river ice breakup.
Residents place a tripod on the ice in the center of the river. This tripod is attached by a cable to a clock on the
shore. When the ice under the tripod breaks or starts to move, the tripod moves, pulling the cable and stopping
the clock.
<
Figure 1. Ice Breakup Dates for Two Alaskan Rivers, 1896-2014
April 1
Tanana River
Yukon River
0)
+*
a
IB
£
00
April 16
Mayl
May 16
May 31
1880 1900 1920 1940 1960 1980
Year
2000
2020
This figure shows the date each year when ice breaks up at two locations: the town of Nenana on the Tanana River and Dawson
City on the Yukon River.
Data sources: Nenana Ice Classic, 2014;22Yukon River Breakup, 201423
64
-------
*
Locations Shown in Figure 1
NOTES
Besides climate change, natural year-to-year variations and
other factors such as local development and land use patterns
can influence ice breakup dates. The two locations featured
here are relatively remote and undeveloped, so the ice breakup
dates are more likely to reflect natural changes in weather and
climate conditions. However, corresponding measurements
of water conditions and air temperature would be useful to
help understand the connection between changes in river ice
breakup and changes in climate.
DATA SOURCES
Ice breakup dates for the Tanana River at Nenana and the
Yukon River at Dawson City have been recorded and made
publicly available as part of two long-running, community
competitions: the Nenana Ice Classic and the Yukon River
Breakup. The data shown here and other information can be
found online at: http://www.nenanaakiceclassic.com and http://
yukonriverbreakup.com. Data records of ice breakup dates for
these two rivers are also archived by the National Snow and
Ice Data Center at: http://nsidc.org/data/lake_river_ice.
-------
This indicator uses two different measures to show how snowfall has changed in the contiguous 48 states.
KEY POINTS
5 Total snowfall has decreased In many
parts of the country since widespread
observations became available In
1930, with 57 percent of stations
showing a decline (see Figure 1).
Among all of the stations shown, the
average change is a decrease of 0.19
percent per year.
0 In addition to changing the overall
rate of precipitation, climate change
can lead to changes in the type of
precipitation. One reason for the
decline in total snowfall is because
more winter precipitation is falling
in the form of rain instead of snow.
More than three-fourths of the sta-
tions across the contiguous 48 states
have experienced a decrease in the
proportion of precipitation falling as
snow (see Figure 2).
5 Snowfall trends vary by region.
The Pacific Northwest has seen a
decline in both total snowfall and the
proportion of precipitation falling as
snow. Parts of the Midwest have also
experienced a decrease, particularly
in terms of the snow-to-precipita-
tion ratio. A few regions have seen
modest increases, including some
areas near the Great Lakes that now
receive more snow than they used to
(see Figures 1 and 2).
Snowfall is an important aspect of winter in much of the United States. Many people depend on snow to
provide water when it melts in the spring—including 60 million people in the western United States24—
and many communities rely on snow for winter recreation. Some plants and animals also depend on
snow and snowmelt for survival. The amount of snow that falls in a particular area directly influences both
snow cover and snowpack, which refer to snow that accumulates on the ground (see the Snow Cover indicator
on p. 68 and the Snowpack indicator on p. 70).
Warmer temperatures cause more water to evaporate from the land and oceans, which leads to more precipita-
tion, larger storms, and more variation in precipitation in some areas. In general, a warmer climate will cause
more of this precipitation to fall in the form of rain instead of snow. However, some places could see more
snowfall if temperatures rise but still remain below the freezing point, or if storm tracks change. Areas near
large lakes might also experience more snowfall as lakes remain unfrozen for longer periods, allowing more
water to evaporate. In contrast, other areas might experience less snowfall as a result of wintertime droughts.
Changes in the amount and timing of snowfall could affect the spawning of fish in the spring and the amount
of water available for people to use in the spring and summer. Changes in snowfall could also affect winter
recreation activities, like skiing, and the communities that rely on these activities.
ABOUT THE INDICATOR
This indicator tracks total snowfall as well as the percentage of precipitation that falls in the form of snow
versus rain. These data were collected from hundreds of weather stations across the contiguous 48 states.
Total snowfall is determined by the height of snow that accumulates each day. These measured values
commonly appear in weather reports (for example, a storm that deposits 10 inches of snow). Figure 1 shows
how snowfall accumulation totals changed between 1930 and 2007 at more than 400 weather stations. These
stations were selected because they had high-quality data for this entire time period.
Figure 2 shows trends in the proportion of total precipitation that falls in the form of snow during each winter
season. This is called the "snow-to-precipitation" ratio, and it is based on comparing the amount of snowfall
with the total amount of precipitation (snow plus rain) in each year. For this comparison, snow has been con-
verted to the equivalent amount of liquid water. These data are available from 1949 to 2014.
INDICATOR NOTES
Several factors make it difficult to measure snowfall precisely. The snow accumulations shown in Figure 1 are
based on the use of measuring rods. This measurement method is subject to human error, as well as the effects
of wind (drifting snow) and the surrounding environment (such as tall trees). Similarly, snow gauges for Figure
2 may catch less snow than rain because of the effects of wind. However, steps have been taken to limit this
indicator to weather stations with the most consistent methods and the highest-quality data.25 As a result,
some parts of the country have a higher station density than others.
Both figures are limited to the winter season. Figure 1 comes from an analysis of October-to-May snowfall,
while Figure 2 covers November through March. Although these months account for the vast majority of
snowfall in most locations, this indicator might not represent the entire snow season in some areas. Most of
the data shown for mountainous regions come from lower elevations (towns in valleys) because that is where
weather stations tend to be located.
66
DATA SOURCES
This indicator shows trends based on two sets of weather records collected and maintained by the National
Oceanic and Atmospheric Administration. Figure 1 was adapted from an analysis by Kunkel etal. (2009)26
based on records from Cooperative Observer Program weather stations. Figure 2 is an updated version of an
analysis by Feng and Hu (2007)27 using data from the U.S. Historical Climatology Network. Additional informa-
tion about the Cooperative Observer Program is available online at: www.nws.noaa.gov/om/coop. Information
about the U.S. Historical Climate Network can be found at: www.ncdc.noaa.gov/oa/climate/research/ushcn.
-------
Figure 1. Change in Total Snowfall in the Contiguous 48 States, 1930-2007
• •
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This figure shows the average rate of change
in total snowfall from 1930 to 2007 at 419
weatherstations in the contiguous 48 states.
Blue circles represent increased snowfall; red
circles represent a decrease.
Data source: Kunkel et al., 200928
<-1.2
<-
Rate of change (percent per year):
-0.9 -0.6 -0.3 -0.1 -0.1 0.1 0.3 0.6 0.9
to-1.2 to-0.9 to-0.6 to-0.3 to 0.1 to 0.3 to 0.6 to 0.9 to 1.2
Less snowfall
More snowfall
Figure 2. Change in Snow-to-Precipitation Ratio in the Contiguous 48 States, 1949-2014
0
§°
"•
0 0
This figure shows the percentage change
in winter snow-to-precipitation ratio from
1949 to 2014 at261 weatherstations in
the contiguous 48 states. This ratio mea-
sures what percentage of total winter
precipitation falls in the form of snow. A
decrease (red circle) indicates that more
precipitation is falling in the form of rain
instead of snow. Filled circles represent
stations where the trend was statistically
significant.
Data source: NOAA, 201429
Percent change:
<-40
-30
to -40
-20
to -30
-10
to-20
-2
to -10
-2
to 2
2
to 10
10
to 20
20
to 30
30
to 40
>40
Lower percentage of snow
Higher percentage of snow
Filled circles represent statistically significant trends.
Open circles represent trends that are not statistically significant.
-------
Snow Cover
This indies
KEY POINTS
5 When averaged over the entire year,
snow covered an average of 3.25
million square miles of North America
during the period from 1972 to 2013
(see Figure 1).
5 The extent of snow cover has varied
from year to year. The average area
covered by snow has ranged from 3.0
million to 3.6 million square miles,
with the minimum value occurring in
1998 and the maximum in 1978 (see
Figure 1).
O Between 1972 and 2013, the average
extent of North American snow cover
decreased at a rate of about 3,500
square miles per year. The average
area covered by snow during the
most recent decade (2004-2013) was
3.21 million square miles, which is
about 4 percent smaller than the av-
erage extent during the first 10 years
of measurement (1972-1981)—a
difference of 120,000 square miles,
or approximately an area the size of
New Mexico (see Figure 1).
O Decreases in snow cover have largely
occurred in spring and summer,
whereas fall and winter snow cover
have remained fairly steady over the
time period studied (see Figure 2).
Spring and summer snow cover can
have a particularly important influ-
ence on water supplies.
leasures the;
that is covered by snc
Snow cover refers to the amount of land covered by snow at any given time. Naturally, it is influenced by
the amount of precipitation that falls as snow. Air temperature also plays a role because it determines
whether precipitation falls as snow or rain, and it affects the rate at which snow on the ground will melt.
As temperature and precipitation patterns change, so can the overall area covered by snow.
Snow cover is not just something that is affected by climate change; it also exerts an influence on climate.
Because snow is white, it only absorbs a small portion of the sunlight that hits it (10 to 20 percent in the case
of fresh snow), and it reflects the rest back to space. In contrast, darker surfaces such as bare ground and open
water absorb the majority of the energy they receive and heat up more quickly. In this way, the overall amount
of snow cover affects patterns of heating and cooling over the Earth's surface. More snow means more energy
reflects back to space, resulting in cooling, while less snow cover means more energy is absorbed at the Earth's
surface, resulting in warming.
On a more local scale, snow cover is important for many plants and animals. For example, some plants rely on
a protective blanket of snow to insulate them from sub-freezing winter temperatures. Humans and ecosystems
also rely on snowmelt to replenish streams and ground water.
ABOUT THE INDICATOR
This indicator tracks the total area covered by snow across all of North America (not including Greenland) since
1972. It is based on maps generated by analyzing satellite images collected by the National Oceanic and At-
mospheric Administration. The indicator was created by analyzing each weekly map to determine the extent of
snow cover, then averaging the weekly observations together to get a value for each year. Average snow cover
was also calculated for each season: spring (defined as March-May), summer (June-August), fall (September-
November), and winter (December-February).
INDICATOR NOTES
Although satellite-based snow cover maps are available starting in the mid-1960s, some of the early years are
missing data from several weeks during the summer, which would lead to an inaccurate annual average. Thus,
the indicator is restricted to 1972 and later, with all years having a full set of data.
DATA SOURCES
The data for this indicator were provided by the Rutgers University Global Snow Lab, which posts data online
at: http://climate.rutgers.edu/snowcover.The data are based on measurements collected by the National Oceanic
and Atmospheric Administration's National Environmental Satellite, Data, and Information Service at: www.
nesdis.noaa.gov.
-------
Figure 1. Snow-Covered Area in North America, 1972-2013
-
4.0
3.5
3.0
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015
Year
Figure 2. Snow-Covered Area in North America by Season, 1972-2013
77)/5 grap/i shows the average area covered
by snow in a given calendar year, based on
an analysis of weekly maps. The area is mea-
sured in square miles. These data cover all of
North America (not including Greenland).
Data source: Rutgers University Global Snow
Lab, 201430
,
«j £
•O m
V £
O
V C
Winter
December - February)
£ 2
Fall
(September - November)
1970 1975 1980
1985 1990 1995 2000 2005
Year
2010 2015
This graph shows the average area covered
by snow during spring (March-May),
summer (June-August), fall (September-No-
vember), and winter (December-February),
based on an analysis of weekly maps. The
area is measured in square miles. These data
cover all of North America (not including
Greenland).
Data source: Rutgers University Global Snow
Lab, 201431
-------
KEY POINTS
5 From 1955 to 2013, April snowpack
declined at about three-fourths of
the sites measured (see Figure 1).
The average change across all sites
amounts to about a 14 percent
decline.
3 In general, the largest and most
consistent decreases were observed
in Washington, Oregon, and the
northern Rockies.
5 Some areas have seen increases in
snowpack, primarily in the southern
Sierra Nevada of California.
Temperature and precipitation are key factors affecting snowpack, which is the amount or thickness of
snow that accumulates on the ground. In a warming climate, more precipitation will be expected to fall
as rain rather than snow in most areas—reducing the extent and depth of snowpack. Higher tempera-
tures in the spring can cause snow to melt earlier.
Mountain snowpack plays a key role in the water cycle in western North America, storing water in the winter
when the snow falls and releasing it as runoff in spring and summer when the snow melts. Millions of people
in the West depend on the melting of mountain snowpack for power, irrigation, and drinking water. In most
western river basins, snowpack is a larger component of water storage than human-constructed reservoirs.32
Changes in mountain snowpack can affect agriculture, winter recreation, and tourism in some areas, as well as
plants and wildlife. For example, certain types of trees rely on snow for insulation from freezing temperatures,
as do some animal species. In addition, fish spawning could be disrupted if changes in snowpack or snowmelt
alter the timing and abundance of streamflows.
ABOUT THE INDICATOR
This indicator uses a measurement called snow water equivalent to determine trends in snowpack. Snow water
equivalent is the amount of water contained within the snowpack at a particular location. It can be thought of
as the depth of water that would result if the entire snowpack were to melt.
The U.S. Department of Agriculture and other collaborators have measured snowpack since the early 1900s.
In the early years of data collection, researchers measured snow water equivalent manually, but since 1980,
measurements at some locations have been collected with automated instruments. This indicator is based on
data from approximately 700 permanent measurement sites in the western United States. The indicator shows
long-term rates of change for the month of April, which could reflect changes in winter snowfall as well as the
timing of spring snowmelt.
70
-------
Figure 1. Trends in April Snowpack in the Western United States,
1955-2013
<-80
-60
to-80
-40
to-60
Percent change:
-20 0 0 20
to-40 to-20 to 20 to 40
40
to 60
60
to 80
>80
This map shows trends in April snowpack in the western United States, measured in terms of snow water equivalent. Blue circles
represent increased snowpack; red circles represent a decrease.
Data source: Mote and Sharp, 201433
INDICATOR NOTES
EPA selected 1955 as a starting point for
this analysis because many measurement
sites in the Southwest were established
in the early 1950s. Natural variability in
the Earth's climate means that snowpack
trends may vary slightly when measured
over different time periods. For example,
the period from 1945 to 1955 was un-
usually snowy in the Northwest, so if this
indicator were to start in 1945 or 1950, the
Northwest would appear to show larger
decreases over time. However, the general
direction of the trend is the same regard-
less of the start date.
Although most parts of the West have seen
reductions in snowpack—consistent with
overall warming trends shown in the U.S.
and Global Temperature indicator (p. 28)—
snowfall trends may be partially influenced
by non-climatic factors such as observation
methods, land-use changes, and forest
canopy changes.
DATA SOURCES
Data for this indicator came from the
U.S. Department of Agriculture's Natural
Resources Conservation Service Water and
Climate Center and the California Depart-
ment of Water Resources. The map was
constructed using methods described in
Mote etal. (2005).34The U.S. Department
of Agriculture data are available at:
www.wcc.nrcs.usda.gov. The California
Department of Water Resources data
are available at: http://cdec.water.ca.gov/
snow/current/snow/index.html.
-------
HEALTH AND
SOCIETY
ir * *• *
rjix
^
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T •'
Changes in the Earth's climate can affect public health, agriculture,
water supplies, energy production and use, land use and development,
and recreation. The nature and extent of climate change effects, and
whether these effects will be harmful or beneficial, will vary regionally
and over time. This chapter looks at some of the ways that climate change
is affecting human health and society, including changes in Lyme disease,
ragweed pollen season, heat-related deaths, heating and cooling needs,
and the agricultural growing season across the United States.
Because impacts on human health are complex, often indirect, and
dependent on multiple societal and environmental factors, the
development of appropriate health-related climate indicators is
challenging and still emerging. It is important for health-related climate
indicators to be clear, measurable, and timely to better understand the link
between climate change and health effects.
WHY DOES IT MATTER?
Changes in climate affect the average weather conditions to which we are accur
omed. These changes may result in multiple threats to human health and wel-
are. Warmer average temperatures will likely lead to hotter days and more fre-
quent and longer heat waves, which could increase the number of heat-related
llnesses and deaths. Increases in the frequency or severity of extreme weather
events, such as storms, could increase the risk of dangerous flooding, high wind
and other direct threats to people and property. Warmer temperatures could als
reduce air quality by increasing the chemical reactions that produce smog, and,
along with changes in precipitation patterns and extreme events, could enhano
the spread of some diseases.
In addition, climate change could require adaptation on larger and faster scales
than in the past, presenting challenges to human well-being and the economy.
The more extensively and more rapidly the climate changes, the larger the potei
tial effects on society. The extent to which climate change will affect different
regions and sectors of society will depend not only on the sensitivity of those
systems to climate change, but also on their ability to adapt to or cope with
climate change. Vulnerable populations, including the poor, the elderly, those al-
ready in poor health, the disabled, and indigenous populations, are most at risk.
72
-------
Summary of Key Points
Heating and Cooling Degree Days. Heating and cooling degree days measure the difference be-
tween outdoor temperatures and the temperatures that people find comfortable indoors. As the U.S. climate
has warmed in recent years, heating degree days have decreased and cooling degree days have increased
overall, suggesting that Americans need to use less energy for heating and more energy for air conditioning.
This pattern stands out the most in the North and West, while much of the Southeast has experienced the
opposite results.
Heat-Related Deaths. Over the past three decades, nearly 8,000 Americans were reported to have
died as a direct result of heat-related illnesses such as heat stroke. The annual death rate is higher when
accounting for other deaths in which heat was reported as a contributing factor. Considerable year-to-year
variability in the data and certain limitations of this indicator make it difficult to determine whether the
United States has experienced long-term trends in the number of deaths classified as "heat-related."
Lyme Disease. Lyme disease is a bacterial illness spread by ticks that bite humans. Tick habitat and
populations are influenced by many factors, including climate. Nationwide, the rate of reported cases of
Lyme disease has approximately doubled since 1991. Lyme disease is most common in the Northeast and
the upper Midwest, where some states now report 50 to 90 more cases of Lyme disease per 100,000 people
than they did in 1991.
Length of Growing Season. The average length of the growing season in the contiguous 48 states
has increased by nearly two weeks since the beginning of the 20th century. A particularly large and steady
increase has occurred over the last 30 years. The observed changes reflect earlier spring warming as well as
later arrival of fall frosts. The length of the growing season has increased more rapidly in the West than in
the East.
Ragweed Pollen Season. Warmer temperatures and later fall frosts allow ragweed plants to pro-
duce pollen later into the year, potentially prolonging the allergy season for millions of people. The length
of ragweed pollen season has increased at 10 out of 11 locations studied in the central United States and
Canada since 1995. The change becomes more pronounced from south to north.
-------
Heating and Cooling
Degree Days
This indicator examines changing temperatures from the perspective of heating and coolin
KEY POINTS
3 Heating degree days have declined in
recent years as the U.S. climate has
warmed (see Figure 1). This change
suggests that heating needs have
decreased overall.
O Overall, cooling degree days have not
increased significantly over the past
100 years. However, a slight increase
is evident over the past few decades,
suggesting that air conditioning ener-
gy demand has also been increasing
recently (see Figure 1).
0 Heating degree days have generally
decreased in the North and West, and
cooling degree days have generally
increased in the West. The Southeast
has seen the opposite: more heating
degree days and fewer cooling de-
gree days (see Figures 2 and 3).
Outdoor temperatures can affect daily life in many ways. In particular, temperature affects our comfort
level and our demand for heating and air conditioning. Collectively, heating and cooling the spaces
in which we live accounts for 48 percent of the energy that American households use every year.1 As
climate change contributes to an increase in average temperatures, an increase in unusually hot days, and a
decrease in unusually cold days (see the U.S. and Global Temperature and High and Low Temperatures indica-
tors on pp. 28 and 30), the overall demand for heating is expected to decline and the demand for cooling is
expected to increase.
One way to measure the influence of temperature change on energy demand is using heating and cooling
degree days, which measure the difference between outdoor temperatures and the temperatures that people
generally find comfortable indoors. These measurements suggest how much energy people might need to
use to heat and cool their homes and workplaces, thus providing a sense of how climate change could affect
people's daily lives and finances.
ABOUT THE INDICATOR
This indicator uses daily temperature data from thousands of weather stations across the United States to
calculate heating and cooling degree days. A "degree day" is determined by comparing the daily average
outdoor temperature with a defined baseline temperature for indoor comfort (in this case, 65°F). For example,
if the average temperature on a particular day is 78°F, then that day counts as 13 cooling degree days, as a
building's interior would need to be cooled by 13°F to reach 65°F. Conversely, if the average outdoor tempera-
ture is 34°F, then that day counts as 31 heating degree days, as a building's interior would need to be warmed
by 31 °F to reach 65°F. For reference, New York City experiences far more heating degree days than cooling de-
gree days per year—a reflection of the relatively cool climate in the Northeast—while Houston, Texas, has far
more cooling degree days than heating degree days—a reflection of the much warmer climate in the South.2
Figure 1 shows each year's average heating and cooling degree days across the contiguous 48 states. Figures
2 and 3 show how heating and cooling degree days have changed by state, based on a comparison of the
first 59 years of available data (1895-1953) with the most recent 60 years (1954-2013). State and national
averages were calculated by finding the total number of heating and cooling degree days per year at each
weather station, averaging the results from all stations within regions called climate divisions (each state has
up to 10 climate divisions), then calculating state and national averages weighted by the population of each
climate division. With this population-weighting approach, average state and national heating and cooling
degree days more closely reflect the conditions that the average resident would experience.
Figure 1. Heating and Cooling Degree Days in the Contiguous
48 States, 1895-2013
Of
Q
This figure shows the average number of
heating and cooling degree days per year
across the contiguous 48 states.
Data source: NOAA, 20143
6,000
5,000
4,000
3,000
2,000
1
1 Heating degree days (colder than 65°F)
1 Cooling degree days (warmer than 65°F)
74
1890
1910
-------
Figure 2. Change in Annual Heating Degree Days by State,
1954-2013 Versus 1895-1953
Difference in annual heating degree days
<-175
-175 to
-125
-125 to
-75
-75 to
-25
-25 to
25
25 to
75
75 to
125
125 to
175
>175
Warmer (less heating needed)
Colder (more heating needed)
Figure 3. Change in Annual Cooling Degree Days by State,
1954-2013 Versus 1895-1953
Difference in annual cooling degree days
<-175
^
-175 to
-125
-125 to
-75
-75 to
-25
-25 to
25
25 to
75
75 to
125
125 to
175
>175
Jfe.
This map shows how the average number of heating
degree days per year has changed in each state over
time. The map was created by comparing the first
59 years of available data (1895-1953) with the
most recent 60 years (1954-2013). "Warmer" colors
indicate an increase in temperatures between the
two periods, leading to less of a need to turn on the
heat—that is, fewer heating degree days. "Cooler"
colors indicate a decrease in temperatures, leading
to more of a need to turn on the heat—that is, more
heating degree days.
Data source: NOAA, 20144
INDICATOR NOTES
Heating and cooling degree days suggest
how temperature changes affect energy
demand, but they do not necessarily
reflect actual energy use. Many other
factors have influenced energy demand
over time, such as more energy-efficient
heating systems, the introduction and
increasingly widespread use of cooling
technologies, larger but better-insulated
homes, behavior change, and population
shifts (such as more people moving to
warmer regions). All of the popula-
tion-weighting in this indicator is based
on the population distribution according
to the 1990 U.S. Census, so any changes
in heating and cooling degree days over
time in this indicator reflect actual chang-
es in the climate, not the influence of
shifting populations. A nationally applied
baseline—in this case, 65°F—has certain
limitations considering the various climate
regimes across the United States.
DATA SOURCES
The data for this indicator were provided
by the National Oceanic and Atmospheric
Administration's National Climatic Data
Center, which maintains a large collection
of climate data online at: www.ncdc.noaa.
gov/oa/ncdc.html.
This map shows how the average number of cooling
degree days per year has changed in each state over
time. The map was created by comparing the first 59
years of available data (1895-1953) with the most re-
cent 60 years (1954-2013). "Warmer" colors indicate
an increase in temperatures between the two periods,
leading to more demand for air conditioning—that is,
more cooling degree days. "Cooler" colors indicate a
decrease in temperatures, leading to less demand for
air conditioning—that is, fewer cooling degree days.
Data source: NOAA, 2014s
Colder (less air conditioning needed)
Warmer (more air conditioning needed)
-------
w
Heat-Related Deaths
This indicator presents data on deaths classified as "heat-related" in the United States.
KEY POINTS
5 Between 1979 and 2010, the death
rate as a direct result of exposure
to heat (underlying cause of death)
generally hovered around 0.5 deaths
per million population, with spikes in
certain years (see Figure 1). Overall,
a total of nearly 8,000 Americans
suffered heat-related deaths since
1979. This number does not capture
the full extent of heat-related deaths
for several reasons (see example on
p. 77).
5 For years in which the two records
overlap (1999-2010), accounting for
those additional deaths in which
heat was listed as a contributing factor
results in a higher death rate—nearly
double for some years—compared
with the estimate that only includes
deaths where heat was listed as the
underlying cause. However, even this
expanded metric does not necessarily
capture the full extent of heat-relat-
ed deaths.
0 The indicator shows a peak in
heat-related deaths in 2006, a year
that was associated with widespread
heat waves and was the second-hot-
test year on record in the contiguous
48 states (see the U.S. and Global
Temperature indicator on p. 28).
O Considerable year-to-year variability
in the data and certain limitations
of this indicator make it difficult
to determine whether the United
States has experienced a meaningful
increase or decrease in deaths clas-
sified as "heat-related" over time.
Dramatic increases in heat-related
deaths are closely associated with
both the occurrence of hot tem-
peratures and heat waves, though
these deaths may not be reported as
"heat-related" on death certificates.
For example, studies of the 1995 heat
wave event in Chicago (see example
on p. 77) suggest that there may have
been hundreds more deaths than
were actually reported as "heat-re-
lated" on death certificates.
When people are exposed to extreme heat, they can suffer from potentially deadly heat-related illness-
es, such as heat exhaustion and heat stroke. Heat is the leading weather-related killer in the United
States, even though most heat-related deaths are preventable through outreach and intervention (see
EPA's Excessive Heat Events Guidebook at: www.epa.gov/heatisland/about/pdf/EHEguide_final.pdf).
Unusually hot summer temperatures have become more frequent across the contiguous 48 states in recent
decades6 (see the High and Low Temperatures indicator on p. 30), and extreme heat events (heat waves) are
expected to become longer, more frequent, and more intense in the future.7 As a result, the risk of heat-related
deaths and illness is also expected to increase.8
Increases in summertime temperature variability may increase the risk of heat-related death for the elderly
and other vulnerable populations.9 Older adults have the highest risk of heat-related death, although young
children are also sensitive to the effects of heat. Across North America, the population over the age of 65 is
growing dramatically. People with certain diseases, such as cardiovascular and respiratory illnesses, are espe-
cially vulnerable to excessive heat exposure, as are the economically disadvantaged.
Some studies suggest that the number of deaths caused by extremely cold temperatures might drop in certain
areas as the climate gets warmer, while others do not expect the number to change at all.10-11 Any decrease
in cold-related deaths will most likely be substantially less than the increase in summertime heat-related
deaths.12-13-14
ABOUT THE INDICATOR
This indicator shows the annual rate for deaths classified by medical professionals as "heat-related" each year
in the United States, based on death certificate records. Every death is recorded on a death certificate, where a
medical professional identifies the main cause of death (also known as the underlying cause), along with other
conditions that contributed to the death. These causes are classified using a set of standard codes. Dividing the
annual number of deaths by the U.S. population in that year, then multiplying by one million, will result in the
death rates shown in Figure 1.
This indicator shows heat-related deaths using two methodologies. One method shows deaths for which exces-
sive natural heat was stated as the underlying cause of death from 1979 to 2010. The other data series shows
deaths for which heat was listed as either the underlying cause or a contributing cause, based on a broader set
of data that at present can only be evaluated back to 1999. For example, in a case where cardiovascular dis-
ease was determined to be the underlying cause of death, heat could be listed as a contributing factor because
it can make the individual more susceptible to the effects of this disease. Because excessive heat events are
associated with summer months, the 1999-2010 analysis was limited to May through September.
INDICATOR NOTES
Several factors influence the sensitivity of this indicator and its ability to estimate the true number of deaths
associated with extreme heat events. It has been well-documented that many deaths associated with extreme
heat are not identified as such by the medical examiner and might not be correctly coded on the death certifi-
cate. In many cases, the medical examiner might classify the cause of death as a cardiovascular or respiratory
disease, not knowing for certain whether heat was a contributing factor, particularly if the death did not occur
during a well-publicized heat wave. By studying how daily death rates vary with temperature in selected cities,
scientists have found that extreme heat contributes to far more deaths than the official death certificates
might suggest.15 This is because the stress of a hot day can increase the chance of dying from a heart attack,
other heart conditions, or respiratory diseases such as pneumonia.16 These causes of death are much more
common than heat-related illnesses such as heat stroke. Thus, this indicator very likely underestimates the
number of deaths caused by exposure to heat.
Just because a death is classified as "heat-related" does not mean that high temperatures were the only factor
that caused or contributed to the death. Pre-existing medical conditions can significantly increase an individu-
al's vulnerability to heat. Other important factors, such as the overall vulnerability of the population, the extent
to which people have adapted and acclimated to higher temperatures, and the local climate and topography,
can affect trends in "heat-related" deaths. Heat response measures, such as early warning and surveillance
systems, air conditioning, health care, public education, cooling centers, infrastructure standards, and air qual-
ity management, can also make a big difference in death rates. For example, after a 1995 heatwave, the city
76
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Figure 1. Deaths Classified as "Heat-Related" in the United States, 1979-2010
4.0
3.5
3-°
o
a
C 2.5
o
91
a. i.s
2 1.0
.c
+*
S 0.5
1975
' Underlying and contributing
causes of death (May-Sept)
Underlying cause of death
(all year)
1980
1985
2000
2005
This figure shows the annual rates for deaths classified
as "heat-related" by medical professionals in the 50
states and the District of Columbia. The orange line
shows deaths for which heat was listed as the main
(underlying) cause. * The blue line shows deaths for
which heat was listed as either the underlying or con-
tributing cause of death during the months from May to
September, based on a broader set of data that became
available in 1999.
* Between 1998 and 1999, the World Health Organi-
zation revised the international codes used to classify
causes of death. As a result, data from earlier than 1999
cannot easily be compared with data from 1999 and
later.
Data source: CDC, 201418'19
2010
Example: Examining Heat-Related Deaths During the 1995 Chicago Heatwave
wi
£
+*
(0
IB
*-
o
E
3
600
500
400
300
200
100
Cook County, July 11-27, 1995:
Excess deaths compared with this time period during an average year: about 700
Deaths classified as "heat-related" on death certificates (not shown here): 465
Many factors can influence the nature, extent, and
timing of health consequences associated with extreme
heat events.20 Studies of heat waves are one way to
better understand health impacts, but different methods
can lead to very different estimates of heat-related
deaths. For example, during a severe heat wave that
hit Chicago * between July 11 and July 27, 1995, 465
heat-related deaths were recorded on death certificates
in Cook County.2' However, studies that compared the
total number of deaths during this heat wave (regardless
of the recorded cause of death) with the long-term av-
erage of daily deaths found that the heat wave likely led
to about 700 more deaths than would otherwise have
been expected.22 Differences in estimated heat-related
deaths that result from different methods may be even
larger when considering the entire nation and longer
time periods.
* This graph shows data for the Chicago Standard
Metropolitan Statistical Area.
Data sources: CDC, 2012;23 NOAA, 201224
6/11
6/21
7/1
7/11
7/21
7/31
8/10 8/20
8/30
Date
of Milwaukee developed a plan for responding to extreme heat conditions; during the 1999 heatwave, heat-related
deaths were roughly half of what would have been expected.17
Future development related to this indicator should focus on capturing all heat-related deaths, not just those with a
reported link to heat stress, as well as examining heat-related illnesses more systematically.
DATA SOURCES
Data for this indicator were provided by the U.S. Centers for Disease Control and Prevention (CDC). The 1979-2010
underlying cause data are publicly available through the CDC WONDER database at: http://wonder.cdc.gov/mortSQL.
html. The 1999-2010 analysis was developed by CDC's Environmental Public Health Tracking Program, which provides
a summary at: www.cdc.gov/nceh/tracking.
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Lyme Disease
This indicator tracks the rate of reported Lyme disease cases across th
KEY POINTS
3 The incidence of Lyme disease in the
United States has approximately dou-
bled since 1991, from 3.74 reported
cases per 100,000 people to 7.01
reported cases per 100,000 people in
2012 (see Figure 1).
0 Among the states where Lyme dis-
ease is most common. New Hamp-
shire and Delaware have experienced
the largest increases in reported case
rates since 1991, followed by Maine,
Vermont, and Massachusetts. On
average, these five states now report
50 to 90 more cases per 100,000
people than they did in 1991 (see
Figure 2).
0 While not necessarily linked only to
climate change, the incidence and
distribution of reported cases of
Lyme disease appear to be increasing
over time (see example maps).
Lyme disease is a bacterial illness that can cause fever, fatigue, joint pain, and skin rash, as well as more
serious joint and nervous system complications. Lyme disease is the most common vector-borne disease
(that is, a disease transmitted by mosquitoes, ticks, or fleas) in the United States. In recent years, approxi-
mately 20,000-30,000 confirmed cases of Lyme disease per year have been reported to the Centers for Disease
Control and Prevention.25 However, the actual number of illnesses is likely greater than what is reported to
health officials.26 Lyme disease is transmitted through the bite of certain species of infected ticks (referred to
commonly as deer ticks) that carry the bacteria that cause Lyme disease. These ticks live not only on deer, but
also on rodents, birds, and other host animals. Deer do not harbor the bacteria that cause Lyme disease, but
certain other hosts such as white-footed mice do, and ticks pick up the bacteria by feeding on these infected
hosts.
Climate is just one of many important factors that influence the transmission, distribution, and incidence of
Lyme disease. However, studies provide evidence that climate change has contributed to the expanded range
of ticks,27 increasing the potential risk of Lyme disease, such as in areas of Canada where the ticks were previ-
ously unable to survive. The life cycle and prevalence of deer ticks are strongly influenced by temperature.28 For
example, deer ticks are mostly active when temperatures are above 45°F, and they thrive in areas with at least
85 percent humidity. Thus, warming temperatures associated with climate change could increase the range of
suitable tick habitat, and are therefore one of multiple factors driving the observed spread of Lyme disease.29
Because tick activity depends on temperatures being above a certain minimum, shorter winters could also
extend the period when ticks are active each year, increasing the time that humans could be exposed to Lyme
disease. Unlike some other vector-borne diseases, tick-borne disease patterns are generally less influenced by
short-term changes in weather (weeks to months) than by longer-term climate change.
Other factors that affect the number of Lyme disease cases include changes in the populations of host species
(particularly deer), which affect tick population size. The percentage of ticks that are infected depends on the
prevalence and infection rates of white-footed mice and certain other hosts. Host species populations and hab-
itats can be affected by climate change and other ecosystem disturbances. Human exposure to infected ticks is
also influenced by multiple factors, including changes in the proximity of human populations to ticks and other
hosts, increased awareness of Lyme disease, and modified behaviors, such as spending less time outdoors,
taking precautions against being bitten, and checking more carefully for ticks.
ABOUT THE INDICATOR
This indicator looks at the incidence of Lyme disease, which reflects the rate of new cases contracted in a given
geographic area and time period. Incidence is typically calculated as the number of cases per 100,000 people
per year. Annual Lyme disease totals and rates for each state were provided by the Centers for Disease Control
and Prevention. The original data were collected by state and local health departments, which track confirmed
cases of Lyme disease that are diagnosed by health care providers and report these cases to the National
Notifiable Diseases Surveillance System. Nationwide reporting of Lyme disease began in 1991.
Example: Reported Lyme Disease Cases in 1996 and 2012
These maps show the distribution
of reported cases of Lyme disease
in 1996 and 2012. Each dot rep-
resents an individual case placed
according to the patient's county of
residence, which may be different
than the county of exposure. The
year 1996 was chosen as a reason-
able starting point for comparison
with recent years. These maps
focus on the parts of the United
States where Lyme disease is most
common.
Data source: CDC, 201430
78
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Figure 1. Reported Cases of Lyme Disease in the United States,
1991-2012
12
1990
1995
2000
2005
2010
2015
Year
This figure shows the annual incidence of Lyme disease, which is calculated as the number of new cases per
100,000 people. The graph is based on cases that local and state health departments report to CDC's national
disease tracking system.
Data source: CDC, 201431
Figure 2. Change in Reported Lyme Disease Incidence in the
Northeast and Upper Midwest, 1991-2012
Total increase in cases per 100,000 people:
20
40
60
80
100
Trend not able
to be calculated
This map shows how reported Lyme disease incidence has changed by state since 1991, based on the number of new cases
per 100,000 people. The total change has been estimated from the average annual rate of change in each state. This map is
limited to the 14 states where Lyme disease is most common, where annual rates are consistently above 10 cases per 100,000.
Connecticut, New York, and Rhode Island had too much year-to-year variation in reporting practices to allow trend calculation.
Data source: CDC, 201432
Figure 1 shows the national incidence of
Lyme disease since 1991, and Figure 2
shows trends in incidence over time in 14
states that collectively account for about
95 percent of the nation's reported cases.
To provide a simple illustration of changes
over time, example maps show the distri-
bution of reported cases in the years 1996
and 2012.
INDICATOR NOTES
For consistency, this indicator only
includes data for confirmed cases of Lyme
disease that are reported to CDC, not
cases that are considered "probable."
Changes in diagnosing practices and
awareness of the disease over time can
affect trends. Cases are reported based on
the patient's county of residence, which
is not necessarily the place where they
were infected. Risk of infection is focused
in certain regions of the country, and con-
firmed reports from low-incidence states
are often the result of travel to an area
of higher incidence. Evidence suggests
that expanding ranges of ticks in certain
northern states may be more related to a
warming climate than expanding ranges
in southern states.3334 However, because
of the many factors affecting tick pop-
ulations and reporting of Lyme disease,
this indicator does not provide sufficient
information to determine what propor-
tion of the observed changes in Lyme
disease incidence is directly driven by
climate change. Further study is critical to
improving the usefulness of this indicator
and informing decisions affecting public
health. For information on prevention,
symptoms, and treatment of Lyme disease,
see: www.cdc.gov/lyme.
DATA SOURCES
All three figures are based on publicly
available Lyme disease data compiled
by the Centers for Disease Control and
Prevention at: www.cdc.gov/lyme/stats/
index.html. Incidence was calculated using
mid-year population estimates from the
U.S. Census Bureau.35
-------
Mi
Length of Growing
Season
ndicator measures
KEY POINTS
3 The average length of the growing
season In the contiguous 48 states
has Increased by nearly two weeks
since the beginning of the 20th cen-
tury. A particularly large and steady
Increase occurred over the last 30
years (see Figure 1).
3 The length of the growing season
has increased more rapidly in the
West than in the East. In the West,
the length of the growing season has
increased at an average rate of about
22 days per century since 1895, com-
pared with a rate of about eight days
per century in the East (see Figure 2).
5 In recent years, the final spring frost
has been occurring earlier than at
any point since 1895, and the first
fall frosts have been arriving later.
Since 1980, the last spring frost has
occurred an average of three days
earlier than the long-term average,
and the first fall frost has occurred
about two days later (see Figure 3).
The length of the growing season in any given region refers to the number of days when plant growth
takes place. The growing season often determines which crops can be grown in an area, as some crops
require long growing seasons, while others mature rapidly. Growing season length is limited by many dif-
ferent factors. Depending on the region and the climate, the growing season is influenced by air temperatures,
frost days, rainfall, or daylight hours.
Changes in the length of the growing season can have both positive and negative effects on the yield and
prices of particular crops. Overall, warming is expected to have negative effects on yields of major crops, but
some individual locations may benefit.36 A longer growing season could allow farmers to diversify crops or
have multiple harvests from the same plot. However, it could also limit the types of crops grown, encourage
invasive species or weed growth, or increase demand for irrigation. A longer growing season could also disrupt
the function and structure of a region's ecosystems and could, for example, alter the range and types of animal
species in the area.
ABOUT THE INDICATOR
This indicator looks at the impact of temperature on the length of the growing season in the contiguous 48
states, as well as trends in the timing of spring and fall frosts. For this indicator, the length of the growing
season is defined as the period of time between the last frost of spring and the first frost of fall, when the air
temperature drops below the freezing point of 32°F. This is referred to as the frost-free season.
Trends in the growing season were calculated using temperature data from 750 weather stations throughout
the contiguous 48 states. These data were obtained from the National Oceanic and Atmospheric Adminis-
tration's National Climatic Data Center. Growing season length and the timing of spring and fall frosts were
averaged across the nation, then compared with long-term average numbers (1895-2013) to determine how
each year differed from the long-term average.
Figure 1. Length of Growing Season in the Contiguous 48 States,
1895-2013
15
i/i
IB
-------
Figure 2. Length of Growing Season in the Contiguous 48 States,
1895-2013: West Versus East
15
10
o»
m
E
o
*«-
c
o
*M
.5
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Ragweed Pollen Season
This indicator depicts changes in the length of ragweed pollen season in the United States and Canada.
KEY POINTS
3 Since 1995, ragweed pollen season
has grown longer at 10 of the 11
locations studied (see Figure 1).
O The Increase In ragweed season
length generally becomes more
pronounced from south to north.
Ragweed season Increased by 27
days In Saskatoon, Saskatchewan; 22
days In Winnipeg, Manitoba; 21 days
In Minneapolis, Minnesota; and 19
days In Fargo, North Dakota (see Fig-
ure 1). This trend is consistent with
many other observations showing
that climate is changing more rapidly
at higher latitudes.41
0 The trends in Figure 1 are strongly
related to changes in the length of
the frost-free season and the timing
of the first fall frost. Northern areas
have seen fall frosts happening later
than they used to, with the delay
in first frost closely matching the
increase in pollen season. Mean-
while, some southern stations have
experienced only a modest change in
frost-free season length since 1995.42
Allergies are a major public health concern, with hay fever (congestion, runny nose, itchy eyes) ac-
counting for more than 13 million visits to physicians' offices and other medical facilities every year.43
One of the most common environmental allergens is ragweed, which can cause hay fever and trigger
asthma attacks, especially in children and the elderly. An estimated 26 percent of all Americans are sensitive to
ragweed.44
Ragweed plants mature in mid-summer and produce small flowers that generate pollen. Ragweed pollen
season usually peaks in late summer and early fall, but these plants often continue to produce pollen until the
first frost. A single ragweed plant can produce up to a billion pollen grains in one season, and these grains can
be carried long distances by the wind.45
Climate change can affect pollen allergies in several ways. Warmer spring temperatures cause some plants to
start producing pollen earlier (see the Leaf and Bloom Dates indicator on p. 94), while warmer fall tempera-
tures extend the growing season for other plants, such as ragweed (see the Length of Growing Season indi-
cator on p. 80). Warmer temperatures and increased carbon dioxide concentrations also enable ragweed and
other plants to produce more pollen.46This means that many locations could experience longer allergy seasons
and higher pollen counts as a result of climate change.
ABOUT THE INDICATOR
This indicator shows changes in the length of the ragweed pollen season in 11 cities in the central United
States and Canada. These locations were selected as part of a study that looked at trends in pollen season at
sites similar in elevation, but across a range of latitudes from south to north. At each location, air samples have
been collected and examined since at least the 1990s as part of a national allergy monitoring network. Pollen
spores are counted and identified using microscopes.
Pollen counts from each station have been analyzed to determine the start and end dates of each year's
ragweed pollen season. Because the length of ragweed season naturally varies from year to year, statistical
techniques have been used to determine the average rate of change over time. This indicator shows the total
change in season length from 1995 to 2013, which was determined by multiplying the average annual rate of
change by the number of years in the period.
INDICATOR NOTES
This indicator is based on data from a limited number of cities in the central states and provinces. These cities
cover a broad range from north to south, however, which allows researchers to establish a clear connection
between pollen season changes and latitude.
Many factors can influence year-to-year changes in pollen season, including typical local and regional vari-
ations in temperature and precipitation, extreme events such as floods and droughts, and changes in plant
diversity. Adding more years of data would provide a better picture of long-term trends, but widespread data
were not available prior to 1995.
This indicator does not show how the intensity of ragweed pollen season (pollen counts) might also be
changing.
DATA SOURCES
Data for this indicator come from the National Allergy Bureau, which is part of the American Academy of
Allergy, Asthma, and Immunology's Aeroallergen Network. Data were compiled and analyzed by a team of
researchers who published a more detailed version of this analysis in a scientific journal with data through
2009.47
82
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Figure 1. Change in Ragweed Pollen Season, 1995-2013
+27 days
+22 days
I+19 days
days
^+15 days
+13 days
+10 days
| +18 days
+10 days
• +1 day
rvr
Change in length
of pollen season:
Decrease A Increase
• -1 day
This figure shows how the length of ragweed pollen season changed at 11 locations in the central United States and
Canada between 1995 and 2013. Red circles represent a longer pollen season; the blue circle represents a shorter season.
Larger circles indicate larger changes.
Data source: Ziska etal., 201448
' J '•' •••.'.•• «>
• r-lI-V
' ' * *i
i •
•. • : i
83
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ECOSYSTEMS
Ecosystems provide humans with food, clean
water, and a variety of other services that can
be affected by climate change. This chapter
looks at some of the ways that climate change
affects ecosystems, including changes in
wildfires, streams and lakes, bird migration
patterns, and plant growth.
WHY DOES IT MATTER?
Changes in the Earth's climate can affect ecosystems by altering the water
cycle, habitats, animal behavior—such as nesting and migration patterns—and
the timing of natural processes such as flower blooms. Changes that disrupt the
functioning of ecosystems may increase the risk of harm or even extinction for
some species. While wildfires occur naturally, more frequent and more intense
fires can significantly disrupt ecosystems, damage property, put people and
communities at risk, and create air pollution problems even far away from the
While plants and animals have adapted to environmental change for millions
of years, the climate changes being experienced now could require adaptation
on larger and faster scales than current species have successfully achieved in
the past, thus increasing the risk of extinction or severe disruption for many
species.
-------
Summary of Key Points
Wildfires. Since 1983, the United States has had an average of 72,000 recorded wildfires
per year. Of the 10 years with the largest acreage burned, nine have occurred since 2000, with
many of the largest increases occurring in western states. The proportion of burned land suffering
severe damage each year has ranged from 5 to 22 percent.
Streamflow. Changes in temperature, precipitation, snowpack, and glaciers can affect the
rate of streamflow and the timing of peak flow. Over the last 73 years, minimum, maximum, and
average flows have changed in many parts of the country—some higher, some lower. Nearly half
of the rivers and streams measured show peak winter-spring runoff happening at least five days
earlier than it did in the mid-20th century.
Great Lakes Water Levels and Temperatures. Water levels in most of the Great
Lakes have declined in the last few decades. Water levels in lakes are influenced by water tem-
perature, which affects evaporation rates and ice formation. Since 1995, average surface water
temperatures have increased by a few degrees for Lakes Superior, Michigan, Huron, and Ontario.
Less of a temperature change has been observed in Lake Erie.
Bird Wintering Ranges. Some birds shift their range or alter their migration habits to
adapt to changes in temperature or other environmental conditions. Long-term studies have
found that bird species in North America have shifted their wintering grounds northward by an
average of more than 40 miles since 1966, with several species shifting by hundreds of miles. On
average, bird species have also moved their wintering grounds farther from the coast, consistent
with inland winter temperatures becoming less severe.
Leaf and Bloom Dates. Leaf growth and flower blooms are examples of natural events
whose timing can be influenced by climate change. Observations of lilacs and honeysuckles in
the contiguous 48 states suggest that first leaf dates and bloom dates show a great deal of year-
to-year variability. Leaf and bloom events are generally happening earlier throughout the North
and West but later in much of the South.
Community Connection: Cherry Blossom Bloom Dates in
Washington, D.C. "Peak" bloom dates of the iconic cherry trees in Washing-
ton, D.C., recorded since the 1920s, indicate that cherry trees are blooming slightly
earlier than in the past. Bloom dates are key to planning the Cherry Blossom Festi-
val, one of the region's most popular spring attractions.
-------
Wildfires
This indicator tracks the frequency, exit.
vildfires in the United States.
KEY POINTS
5 Since 1983, the National Interagen-
cy Fire Center has documented an
average of 72,000 wildfires per year
(see Figure 1). Compiled data from
the Forest Service suggest that the
actual total may be even higher for
the first few years of nationwide data
collection that can be compared. The
data do not show an obvious trend
during this time.
5 The extent of area burned by wild-
fires each year appears to have
increased since the 1980s. According
to National Interagency Fire Center
data, of the 10 years with the largest
acreage burned, nine have occurred
since 2000 (see Figure 2). This period
coincides with many of the warmest
years on record nationwide (see the
U.S. and Global Temperature indica-
tor on p. 28).
5 The late 1990s were a period of tran-
sition in certain climate cycles that
tend to shift every few decades.1 This
shift—combined with other ongoing
changes in temperature, drought,
and snowmelt—may have contribut-
ed to warmer, drier conditions that
have fueled wildfires in parts of the
western United States.23
3 Of the total area burned each year
from 1984 to 2012, the proportion of
burned land suffering severe damage
has ranged from 5 to 22 percent (see
Figure 3).
O Land area burned by wildfires varies
by state. Fires burn more land in
the western United States than in
the East, and parts of the West and
Southwest show the largest increase
in burned acreage between the first
half of the record (1984-1998) and
the second half (1999-2012) (see
Figure 4).
Together, forests, shrubland, and grassland cover more than half of the land area in the United States.4
These ecosystems are important resources, both environmentally and economically. Although wildfires
occur naturally and play a long-term role in the health of these ecosystems, climate change threatens
to increase the frequency, extent, and severity of fires through increased temperatures and drought (see the
U.S. and Global Temperature and Drought indicators on pp. 28 and 38). Earlier spring melting and reduced
snowpack (see the Snowpack indicator on p. 70) result in decreased water availability during hot summer
conditions, which in turn contributes to an increased wildfire risk, allowing fires to start more easily and burn
hotter. In addition to climate change, other factors—like the spread of insects, land use, and management
practices, including fire suppression—play an important role in wildfire frequency and intensity. All of these
factors influencing wildfires vary greatly by region, as do variations in precipitation, wind, temperature,
vegetation types, and landscape conditions. Therefore, understanding changes in fire characteristics requires a
regional perspective and consideration of many factors.5
Wildfires have the potential to harm property, livelihoods, and human health. The recreation and timber in-
dustries depend on healthy forests, and wildfire smoke has been directly linked to poor air quality and illness,
even in communities far downwind.6 Fire-related threats are increasing, especially as more people live in and
around forests, grasslands, and other natural areas/The United States spends more than $1 billion every year
to fight wildfires,8 and these efforts have resulted in the deaths of hundreds of firefighters since 1910.9
Beyond the human impact, wildfires also affect the Earth's climate. Forests in particular store large amounts
of carbon. When they burn, they release carbon dioxide into the atmosphere, which in turn contributes to
additional climate change.
ABOUT THE INDICATOR
This indicator defines wildfires as "unplanned, unwanted wildland fire[s]" in forests, shrubland, and grassland,
where "the objective is to put the fire out."10 This indicator tracks three aspects of wildfires over time: the to-
tal number of fires (frequency), the total land area burned (extent), and the degree of damage that fires cause
to the landscape (severity). The total area and total number of fires are tracked by the National Interagency
Fire Center, which compiles reports from local, state, and federal agencies that are involved in fighting wild-
fires. The U.S. Forest Service tracked similar data using a different reporting system until 1997. Those data have
been added to this indicator for comparison. Wildfire severity is measured by comparing the "greenness" of
satellite images taken before and after a fire to classify how severely the land has been burned. Burn severity
provides an indication of the ecological damage and how long the effects of wildfires are likely to last.
Although some nationwide fire data have been collected since the early 1900s, this indicator starts in 1983
(Figures 1 and 2) and 1984 (Figures 3 and 4), when nationwide data collection became more complete and
standardized.
INDICATOR NOTES
Many environmental impacts associated with climate change can affect wildfire frequency, extent, or severity,
including changes in temperature, precipitation, and drought. Human activities and land management
practices also affect wildfire activity, and preferred practices in wildfire management have evolved over time,
from older policies that favored complete wildfire prevention to more recent policies of wildfire suppression
and controlled burns. While this indicator is limited to "wildland" fires, it includes fires that encroach on—or
perhaps started in—developed areas. Increased development in previous wild lands could influence trends in
wildfire frequency and extent. The total number of fires may also vary due to reporting irregularities, as fires
that split or merge together across jurisdictional lines may be counted differently.
Along with the influence of ongoing climate change, wildfire patterns can be influenced by natural climate
cycles that tend to shift every few decades. Thus, the approximately 30 years of data shown here may not be
enough to draw conclusions about long-term trends. While a longer record would be ideal, data from before
1983 are not consistent enough nationally to be included in this indicator.
86
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Figure 1. Wildfire Frequency in the United States,
1983-2013
200,000
160,000
120,000
80,000
40,000
Figure 2. Wildfire Extent in the United States,
1983-2013
m
c
_§
•o
c
3
J2
1980 1985 1990 1995 2000 2005 2010 2015
Year
F
Forest Service Wildfire Statistics
' National Interagency Fire Center
This figure shows the total number of wildfires per year from 1983 to 2013. These
totals include all reported wildfires, which can be as small as just a few acres. The two
lines represent two different reporting systems; though Forest Service statistics (orange
line) stopped being compiled in 1997 and will not be updated, they are shown here for
comparison.
Data source: NIFC, 2014;" USDA Forest Service, 201412
1980 1985 1990 1995 2000 2005 2010 2015
Year
Forest Service Wildfire Statistics
1 National Interagency Fire Center
This figure shows annual wildfire-burned area (in millions of acres) from 1983 to 2013.
The two lines represent two different reporting systems; though Forest Service statistics
(orange line) stopped being compiled in 1997 and will not be updated, they are shown
here for comparison.
Data source: NIFC, 2014;13USDA Forest Service, 201414
Figure 3. Damage Caused by Wildfires in the
United States, 1984-2012
_ 10
= 6
| Increased greenness
Unburnedtolow
Low
Moderate
• High
1984 1988 1992
1996 2000
Year
2004 2008 2012
Figure 4. Land Area Burned by Wildfires by State,
1984-2012
(a) Average annual burned
acreage, 1984-2012
(b) Change in annual burned acreage
between 1984-1998 and 1999-2012
Burned acres per
square mile of land area:
Change in burned acres per
square mile of land area:
This figure shows the distribution of acreage burned by large wildfires, based on
the level of damage caused to the landscape—a measure of wildfire severity. Large
wildfires are defined as fires with an area larger than 1,000 acres in the western United
States and 500 acres in the eastern United States. The total acreage shown in Figure
3 is slightly less than the total in Figure 2 because Figure 3 is limited to large fires and
because a few areas did not have sufficient satellite imagery to allow damage to be
assessed.
Data source: MTBS, 201415
These maps show the number of acres burned in each state as a proportion of that
state's total land area. For reference, there are 640 acres in a square mile; therefore,
an average burned area of 6.4 acres per square mile would mean that fires burned 1
percent of a state's total land area. The map on the left shows the average extent of
fires per year from 1984 to 2012. Darker-shaded states have the largest proportion
of acreage burned. The map on the right shows how burned acreage has changed
over time, based on a simple comparison between the first half of the available years
(1984-1998) and the second half (1999-2012).
Data source: MTBS, 201416
DATA SOURCES
The full set of wildfire frequency and burned acreage data in Figures 1 and 2 comes from the National Interagency Fire Center, which compiles wildfire
reports sent from local, state, and federal entities that are involved in fighting fires. These data are available online at: www.nifc.gov/firelnfo/firelnfo_sta-
tistics.html. Additional data were provided by the U.S. Forest Service based on a different set of records, referred to as Smokey Bear Reports. Burn severity
data and state-by-state acreage totals in Figures 3 and 4 come from a multi-agency project called Monitoring Trends in Burn Severity, which maintains a
database of wildfire events across the United States. These data are publicly available at: http://mtbs.gov/data/search.html.
87
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Streamflow
This indicator describes trends in the amount of wat
timing of runoff associated with snowmelt.
KEY POINTS
5 Over the past 73 years, seven-day
low flows have generally Increased In
the Northeast and Midwest (In other
words, on the days of lowest flows,
streams In these areas are carrying
more water than before). In parts of
the Southeast and the Pacific North-
west, low flows have generally de-
creased (that Is, streams are carrying
less water than before). Overall, sites
show more Increases than decreases
(see Figure 1).
5 Three-day high-flow trends vary from
region to region across the country.
For example, streams in the North-
east have generally seen an increase
or little change in high flows since
1940, while some West Coast streams
have seen a decrease and others have
seen an increase. Overall, sites show
more increases than decreases (see
Figure 2).
3 The largest changes in annual aver-
age Streamflow have taken place in
the Northeast and Midwest. Other
regions saw few substantial changes.
Overall, sites show more increases
than decreases (see Figure 3).
3 Nearly half of the streams studied
show winter-spring runoff happen-
ing more than five days earlier than
in the mid-20th century. The largest
changes occurred in the Pacific
Northwest and Northeast
(see Figure 4).
Streamflow is a measure of the rate at which water is carried by rivers and streams, and it represents a
critical resource for people and the environment. Changes in Streamflow can directly influence the supply
of drinking water and the amount of water available for irrigating crops, generating electricity, and other
needs. In addition, many plants and animals depend on Streamflow for habitat and survival.
Streamflow naturally varies over the course of a year. For example, rivers and streams in many parts of the
country have their highest flows when snow melts in the spring and their lowest flows in late summer. The
amount of Streamflow is important because very high flows can cause erosion, flooding, and ecosystem disrup-
tion, while very low flows can diminish water quality, harm fish, and reduce the amount of water available for
people. The timing of high flow is important because it affects the ability of reservoir managers to store water
to meet needs later in the year. In addition, some plants and animals (such as fish that migrate) depend on a
particular pattern of Streamflow as part of their life cycles.
Climate change can affect Streamflow in several ways. Changes in the amount of snowpack and earlier spring
melting (see the Snowpack indicator on p. 70) can alter the size and timing of high streamflows. Because of
the relationship between precipitation and runoff, more precipitation will potentially cause higher average
Streamflow in some places, while heavier storms (see the Heavy Precipitation indicator on p. 36) could lead to
larger peak flows. However, more frequent or severe droughts could reduce Streamflow in certain areas.
ABOUT THE INDICATOR
The U.S. Geological Survey measures Streamflow in rivers and streams across the United States using contin-
uous monitoring devices called stream gauges. This indicator is based on 193 stream gauges located in areas
where trends will not be substantially influenced by dams, reservoir management, wastewater treatment facil-
ities, or land-use change. The indicator also excludes stream gauges with substantially overlapping watershed
areas.
This indicator examines four important measures of Streamflow conditions that occur during the course of a
year. Figure 1 shows trends in low flow conditions, which are commonly calculated by averaging the lowest
seven consecutive days of Streamflow in a year. In many locations, this method captures the year's driest condi-
tions. Figure 2 shows trends in high flow conditions, which are commonly calculated by averaging the highest
three consecutive days of Streamflow in a year. Three days is an optimal length of time to characterize runoff
associated with large storms and peak snowmelt. Figure 3 shows changes in the annual average Streamflow,
which is calculated by averaging daily flows over the entire year.
Figure 4 shows trends in the timing of winter and spring runoff. This measure is limited to 56 stream gauges
in areas where at least 30 percent of annual precipitation falls as snow. Scientists look at the total volume of
water that passes by a gauge between January 1 and June 30, then determine the date when exactly half of
that water has gone by. This date is called the winter-spring center of volume date. A long-term trend toward
an earlier date could be caused by earlier spring snowmelt, more precipitation falling as rain instead of snow,
or other changes in precipitation patterns.
INDICATOR NOTES
Streamflow measurements were used from gauges in areas where Streamflow is not highly affected by human
influences such as dams, land development, or changes in land cover. However, changes in land cover and land
use over time could still influence Streamflow trends at some streams. The gauges used for this indicator are
not evenly distributed across the country.
DATA SOURCES
Streamflow data were collected by the U.S. Geological Survey. These data came from a set of gauges in
watersheds with minimal human impacts, which have been classified as reference gauges.17 Daily average
Streamflow data are stored in the National Water Information System and are publicly available at:
http://waterdata.usgs.gov/nwis.
88
-------
Figure 1. Seven-Day Low Streamflows in the
United States, 1940-2012
Figure 2. Three-Day High Streamflows in the
United States, 1940-2012
C*
8
o o
J
A
/A
o 8
More than 20% to 50% 20% decrease 20% to 50% More than
50% decrease decrease to 20% increase increase 50% increase
This map shows percentage changes in the minimum annual rate of water carried by
rivers and streams across the country, based on the long-term rate of change from
1940 to 2012. Minimum streamflow is based on the consecutive seven-day period
with the lowest average flow during a given year.
Data source: USGS, 201418
°0
"T o
I5.'
6 o°o
»>
44
V T O i A
More than 20% to 50% 20% decrease 20% to 50% More than
50% decrease decrease to 20% increase increase 50% increase
This map shows percentage changes in the maximum annual rate of water carried by
rivers and streams across the country, based on the long-term rate of change from
1940 to 2012. Maximum streamflow is based on the consecutive three-day period
with the highest average flow during a given year.
Data source: USGS, 2014"
Figure 3. Annual Average Streamflow in the
United States, 1940-2012
Figure 4. Timing of Winter-Spring Runoff in the
United States, 1940-2012
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More than 5 to 10 2 to 5 2 days 2 to 5 5 to 10 More than
10 days days days earlier to days days 10 days
earlier earlier earlier 2 days later later later later
This map shows percentage changes in the annual average rate of water carried by
rivers and streams across the country, based on the long-term rate of change from
1940 to 2012. This map is based on daily streamflow measurements, averaged over
the entire year.
Data source: USGS, 201420
This map shows changes in the timing of annual high spring flow carried by rivers
and streams from 1940 to 2012. This analysis focuses on parts of the country where
streamflow is strongly influenced by snowmelt. Trends are based on the winter-spring
center of volume, which is the date when half of the streamflow between January 1
and June 30 of each year has passed a streamflow gauge.
Data source: USGS, 201421
89
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Great Lakes Water Levels
and Temperatures
is indicator measures water levels and surface WE
sratures in the Great Lakes.
KEY POINTS
5 Water levels in the Great Lakes have
fluctuated since 1860. Over the last
few decades, they appear to have
declined for most of the Great Lakes
(see Figure 1). However, the most
recent levels are all within the range
of historical variation.
5 Since 1995, average surface water
temperatures have increased by a
few degrees for Lakes Superior, Mich-
igan, Huron, and Ontario (see Figure
2). Less change has been observed in
water temperature in Lake Erie.
5 Recent increases in water tem-
perature have mostly been driven
by warming during the spring and
summer months (see Figure 2). These
trends could relate in part to an
earlier thawing of winter ice (see the
Lake Ice indicator on p. 62).
The Great Lakes, consisting of Lake Superior, Lake Michigan, Lake Huron, Lake Erie, and Lake Ontario, form
the largest group of freshwater lakes on Earth. These lakes support a variety of ecosystems and play a
vital role in the economy of the eight neighboring states and the Canadian province of Ontario, providing
drinking water, shipping lanes, fisheries, recreational opportunities, and more.
Water level and water temperature are two important and interrelated indicators of weather and climate
change in the Great Lakes. Water level (the height of the lake surface above sea level) is influenced by many
factors, including precipitation, snowmelt runoff, drought, evaporation rates, and people withdrawing water for
multiple uses. Water temperature is influenced by many factors, too, but most directly by air temperature.
In recent years, warmer surface water temperatures in the Great Lakes have contributed to lower water levels
by increasing rates of evaporation and causing lake ice to form later than usual (see the Lake Ice indicator on
p. 62), which extends the season for evaporation.22 Lower water levels in the Great Lakes forced ships to reduce
their cargo tonnage by 5 to 8 percent between 1997 and 2000, which increased shipping costs. Lower water
levels can also affect water supplies, the usability of infrastructure such as docks and piers, and shoreline eco-
systems. These types of disruptions from low water levels are expected to continue as the climate changes.23
Another possible effect of warmer water, reduced ice cover, and increased evaporation is a corresponding
increase in precipitation over nearby land, especially "lake effect" snow (see the Snowfall indicator on p.
66).24 Rising water temperatures are also expected to expand the ranges of and give new advantages to some
invasive species such as the zebra mussel, and to encourage the growth of certain water-borne bacteria that
can make people ill.25-26
ABOUT THE INDICATOR
This indicator analyzes water levels and surface water temperatures in the Great Lakes. Water levels are
recorded by gauges along the shore of each lake, some of which have been operated since the 1800s. Pre-1918
data came from one water level gauge per lake. Data since 1918 have come from a designated set of gauges
in each lake. Figure 1 shows annual water level anomalies, or differences, in feet compared with the average
water levels in each lake from 1860 to 2013. Lakes Michigan and Huron are combined because they are con-
nected at the same water level.
Surface water temperatures are measured by satellites. Figure 2 shows annual average temperatures over
the entire surface of each lake, along with the pattern of daily temperatures over the course of the year. This
indicator begins in 1995, which was the first year with complete satellite data for all five lakes.
• Direction of flow
Lake
Superior
Lake
Huron
Lake
Ontario
Lake
Michigan
Lake
Erie
INDICATOR NOTES
While climate change influences water levels, human activities such as dredging can also play
a role. For example, the St. Clair river opening was enlarged in the 1910s, 1930s, and 1960s,
contributing to greater outflows from Lakes Michigan and Huron."Similarly, natural year-to-
year variability and other factors such as human use and wastewater discharges can influence
water temperatures.
DATA SOURCES
Water level data were provided by the Canadian Hydrographic Service and the National
Oceanic and Atmospheric Administration's Center for Operational Oceanographic Products
and Services, and can be downloaded from: www.glerl.noaa.gov/data/now/wlevels/levels.html.
Surface water temperature data were provided by the National Oceanic and Atmospheric
Administration's Great Lakes Environmental Research Laboratory (satellite data at:
http://coastwatch.glerl.noaa.gov).
90
-------
Figure 1. Water Levels of the Great Lakes,
1860-2013
Figure 2. Surface Water Temperatures of the
Great Lakes, 1995-2013
Lake Superior
IB
E
o
c
IQ
1
i
1860 1880 1900 1920 1940 1960 1980 2000 2020
1860 1880 1900 1920 1940 1960 1980 2000 2020
1880 1900 1920 1940 1960 1980 2000 2020
1860 1880 1900 1920 1940 1960 1980 2000 2020
Year
Annual average surface water temperature (°F)
80
70
60
50
40
30
19
80
70
60
50
40
30
19
80
70
60
50
40
30
19
80
70
60
50
40
30
19
80
70
60
50
40
30
19
^^-
95 20
LakeS
jperior
00 2005 2010 20
Lake Michigan
95 2000 2005 2010 20
±f
Lake Huron
^^-J^
95 2000 2005 2010 20
Lake Erie
95 2000 2005 2010 20
Lake Ontario
95 2000 2005 20
10 20
Lake Superior
Year
80
70
60
50
40
30
80
70
60
50
40
30
J FMAMJ JASOND
Lake Michigan
J FMAMJ JASOND
Lake Huron
J FMAMJ JASOND
Lake Erie
80
70
60
50
40
30
J FMAMJ JASOND
Lake Ontario
J FMAMJ JASOND
Month
— 1995-2004
— 2005-2013
This figure displays how water levels in each of the Great Lakes have changed since
1860. For each year, the shaded band shows the range of monthly average water levels,
and the line in the middle shows the annual average. The graph uses the 1981 to 2010
average as a baseline for depicting the change. Choosing a different baseline period
would not change the shape of the data over time. Lakes Michigan and Huron are
shown together because they are connected at the same water level.
Data source: NOAA, 201428
This figure shows the average surface water temperatures in each of the Great
Lakes, as measured by satellites. The graphs on the left show annual averages,
while the graphs on the right show how average daily temperatures have changed
between two time periods. The full time period has been divided approximately in
half for comparison: 2005-2013 (nine years) versus 1995-2004 (10 years).
Data source: NOAA, 201429
91
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I
Bird Wintering Ranges
This indicator examines changes in the winter ranges of North American birds.
KEY POINTS
5 Among 305 widespread North
American bird species, the average
mid-December to early January cen-
ter of abundance moved northward
by more than 40 miles between
1966 and 2013 (see Figure 1). Trends
in center of abundance moving
northward can be closely related to
increasing winter temperatures.30
5 On average, bird species have also
moved their wintering grounds far-
ther from the coast since the 1960s
(see Figure 2). A shift away from the
coasts can also relate to changes in
winter temperatures. Inland areas
tend to experience more extreme
cold than coastal areas, but those
extremes are becoming less severe as
the climate warms overall.31
3 Some species have moved farther
than others. A total of 48 species
have moved northward by more than
200 miles. Of the 305 species studied,
186 (61 percent) have shifted their
wintering grounds northward since
the 1960s, while 82 (27 percent) have
shifted southward. Some others have
not moved at all.
Changes in climate can affect ecosystems by influencing animal behavior and ranges. Birds are a particularly
good indicator of environmental change for several reasons:
• Each species of bird has adapted to or evolved to favor certain habitat types, food sources, and tem-
perature ranges. In addition, the timing of certain events in their life cycles—such as migration and
reproduction—is driven by cues from the environment. For example, many North American birds follow
a regular seasonal migration pattern, moving north to feed and breed in the summer, then moving south
to spend the winter in warmer areas. Changing conditions can influence the distribution of both migra-
tory and non-migratory birds as well as the timing of important life cycle events.
• Birds are easy to identify and count, and thus there is a wealth of scientific knowledge about their distri-
bution and abundance. People have kept detailed records of bird observations for more than a century.
• There are many different species of birds living in a variety of habitats, including water birds, coastal
birds, and land birds. If a change in behavior or range occurs across a range of bird types, it suggests
that a common external factor might be the cause.
Temperature and precipitation patterns are changing across the United States (see the U.S. and Global Tem-
perature indicator on p. 28 and the U.S. and Global Precipitation indicator on p. 34). Some bird species can
adapt to generally warmer temperatures by changing where they live—for example, by migrating farther north
in the summer but not as far south in the winter, or by shifting inland as winter temperature extremes grow
less severe. Non-migratory species might shift as well, expanding into newly suitable habitats while moving
out of areas that become less suitable. Other types of birds might not adapt to changing conditions and could
experience a population decline as a result. Climate change can also alter the timing of events that are based
on temperature cues, such as migration and breeding (especially egg-laying).
ABOUT THE INDICATOR
This indicator looks collectively at the "center of abundance" of hundreds of widespread North American bird
species over a 48-year period. The center of abundance is a point on the map that represents the middle of
each species' distribution. If a whole population of birds were to shift generally northward, one would see the
center of abundance shift northward as well.
For year-to-year consistency, this indicator uses observations from the National Audubon Society's Christmas
Bird Count, which takes place every year in early winter. The Christmas Bird Count is a long-running citizen
science program in which individuals are organized by the National Audubon Society, Bird Studies Canada,
local Audubon chapters, and other bird clubs to identify and count bird species. The data presented in this
indicator were collected from more than 2,000 locations throughout the United States and parts of Canada. At
each location, skilled observers follow a standard counting procedure to estimate the number of birds within a
15-mile diameter "count circle" over a 24-hour period. Study methods remain generally consistent from year to
year. Data produced by the Christmas Bird Count go through several levels of review before Audubon scientists
analyze the final data, which have been used to support a wide variety of peer-reviewed studies.
92
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Figure 1. Change in Latitude of Bird Center of Abundance,
1966-2013
-20
1965
1970
1975
1980
1985
1990
Year
1995 2000 2005
2010
2015
This figure shows annual change in latitude of bird center of abundance for 305 widespread bird species in
North America from 1966 to 20 13, Each winter is represented by the year in which it began (for example, winter
2013-2014 is shown as 2013). The shaded band shows the likely range of values, based on the number of mea-
surements collected and the precision of the methods used.
Data source: National Audubon Society, 201432
Figure 2. Change in Distance to Coast of Bird Center of
Abundance, 1966-201 3
INDICATOR NOTES
Many factors can influence bird ranges,
including food availability, habitat alteration,
and interactions with other species. As a result,
some of the birds included in this indicator
might have moved north for reasons other
than changing temperatures. This indicator
does not show how responses to climate
change vary among different types of birds. For
example, a more detailed National Audubon
Society analysis found large differences among
coastal birds, grassland birds, and birds adapt-
ed to feeders, which all have different abilities
to adapt to temperature changes.34
Some data variations can be caused by differ-
ences among count circles, such as inconsis-
tent level of effort by volunteer observers, but
these differences are carefully corrected in
Audubon's statistical analysis.
DATA SOURCES
Bird center of abundance data were collected
by the annual Christmas Bird Count organized
by the National Audubon Society and Bird
Studies Canada. Recent and historical Christ-
mas Bird Count data are available at: http://
birds.audubon.org/christmas-bird-count. The Na-
tional Audubon Society published a previous
version of this analysis in 2009;35 it is available
atwww.audubon.org/bird/bacc/index.html.
11
£ u
25
20
15
1- 10
I/I
0 -
II -5
-10
-15
1965 1970 1975 1980 1985
1990
Year
1995 2000 2005 2010 2015
This figure shows annual change in distance to the coast of bird center of abundance for 272 widespread bird species in
North America from 1966 to 2013. This figure covers 272 species instead of the 305 species shown in Figure 1 because
33 of the species in Figure 1 need access to salt water, which means they cannot move inland. Each winter is represent-
ed by the year in which it began (for example, winter 2013-2014 is shown as 2013). The shaded band shows the likely
range of values, based on the number of measurements collected and the precision of the methods used.
Data source: National Audubon Society, 201433
93
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Leaf and Bloom Dates
This indicator examines the timing of leaf growth and
in the United States.
ridely distributed plants
KEY POINTS
5 First leaf and bloom dates In lilacs
and honeysuckles In the contigu-
ous 48 states show a great deal of
year-to-year variability, which makes
it difficult to determine whether a
statistically meaningful change has
taken place. However, earlier dates
appear more prevalent in the last few
decades (see Figure 1).
5 Leaf and bloom events are generally
happening earlier throughout the
North and West but later in much of
the South (see Figures 2 and 3). This
observation is generally consistent
with regional differences in tempera-
ture change (see the U.S. and Global
Temperature indicator on p. 28).
3 Other studies have looked at trends
in leaf and bloom dates across all
of North America and the entire
Northern Hemisphere. These studies
have also found a trend toward
earlier spring events—some more
pronounced than the trends seen in
just the contiguous 48 states.36
The timing of natural events, such as flower blooms and animal migration, can be influenced by changes
in climate. Phenology is the study of such important seasonal events. Phenological events are influenced
by a combination of environmental factors, including temperature, light, rainfall, and humidity. Different
plant and animal species respond to different cues.
Scientists have high confidence that the earlier arrival of spring events is linked to recent warming trends in
global climate.37 Disruptions in the timing of these events can have a variety of impacts on ecosystems and hu-
man society. For example, an earlier spring might lead to longer growing seasons (see the Length of Growing
Season indicator on p. 80), more abundant invasive species and pests, and earlier and longer allergy seasons.
Unusually warm weather in late winter can create a "false spring" that triggers the new growth of plants to
begin too early, leaving them vulnerable to any subsequent frosts.
Because of their close connection with climate, the timing of phenological events can be used as an indicator
of the sensitivity of ecological processes to climate change. Two particularly useful indicators of the timing
of spring events are the first leaf dates and the first bloom dates of lilacs and honeysuckles, which have an
easily monitored flowering season, a relatively high survival rate, and a large geographic distribution. The first
leaf date in these plants relates to the timing of events that occur in early spring, while the first bloom date is
consistent with the timing of later spring events, such as the start of growth in forest vegetation.38
ABOUT THE INDICATOR
This indicator shows trends in the timing of first leaf dates and first bloom dates in lilacs and honeysuckles
across the contiguous 48 states. Because many of the phenological observation records in the United States
are less than 40 years long, and because these records may have gaps in time or space, computer models have
been used to provide a more complete understanding of long-term trends nationwide.
The models for this indicator were developed using data from the USA National Phenology Network, which
collects ground observations from a network of federal agencies, field stations, educational institutions, and
citizens who have been trained to log observations of leaf and bloom dates. For consistency, observations
were limited to a few specific types of lilacs and honeysuckles. Next, models were created to relate actual leaf
and bloom observations with records from nearby weather stations. Once scientists were able to determine
the relationship between climate factors (particularly temperatures) and leaf and bloom dates, they used this
knowledge to estimate leaf and bloom dates for earlier years based on historical weather records. They also
used the models to estimate how leaf and bloom dates would have changed in a few areas (mostly in the far
South) where lilacs and honeysuckles are not widespread.
This indicator uses data from several hundred weather stations throughout the contiguous 48 states. The
exact number of stations varies from year to year. For each year, the timing of first leaf and first bloom at each
station was compared with the 1981 to 2010 average to determine the number of days' "deviation from nor-
mal." This indicator presents the average deviation across all stations, along with maps that compare the most
recent 10-year period (2004-2013) with a mid-20*-century baseline (1951-1960) at individual stations. These
time periods were chosen to match published studies.39
INDICATOR NOTES
Plant phenological events are studied using several data collection methods, including satellite images, models,
and direct observations. Locational differences, the use of varying data collection methods, and different phe-
nological indicators (such as leaf or bloom dates for different types of plants) can lead to a range of estimates
of the arrival of spring.
Climate is not the only factor that can affect phenology. Observed variations can also reflect plant genetics,
changes in the surrounding ecosystem, and other factors. This indicator minimizes the influence of genetic
variations by relying on cloned plant species (that is, plants with no genetic differences).
94
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Figure 1. First Leaf and Bloom Dates in the Contiguous 48 States, 1900-2013
T? 15
OS
o
7
CO
E
o
*i-
c
o
'S
•*•.*•*.
«
/.
N.Tr
*>•
Change in first bloom date:
• • ° • •
More than -4 to -1 to Within 1 to 4 to More than
-8 days -8 days -4 days 1 day 4 days 8 days 8 days
Earlier
Later
This figure shows modeled trends in lilac and honeysuckle first bloom dates at weather
stations across the contiguous 48 states. This map compares the average first bloom
date during two 10-year periods.
Data source: Schwartz, 201342
DATA SOURCES
Leaf and bloom observations were compiled by the USA National Phenology Network and are available at: www.usanpn.org. This indicator is also
based on climate data that were provided by the U.S. Historical Climatology Network and are available at: www.ncdc.noaa.gov/oa/climate/research/ushcn.
Data for this indicator were analyzed using methods described by Schwartz et a I. (2013).43
95
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Community Connection
Cherry Blossom Bloom Dates
in Washington, D.C.
KEY POINTS
3 Based on the entire 94 years of data
in Figure 1, Washington's blossoms
reach their peak on April 4 in an av-
erage year. By comparison, the peak
bloom date in 2014 was April 10.
5 Peak bloom date for the cherry trees
is occurring earlier than it did in the
past. Since 1921, peak bloom dates
have shifted earlier by approximately
five days.
O While the length of the National
Cherry Blossom Festival has continued
to expand, the Yoshino cherry trees
have bloomed near the beginning of
the festival in recent years. During
some years, the festival missed the
peak bloom date entirely.
In Washington, D.C., the arrival of spring brings a splash of color as the city's iconic cherry trees burst into
bloom. The city has enjoyed cherry blossoms each year dating back to 1912, when Japan gave 3,020 cherry
trees to the United States as a gift of friendship. There are currently almost 3,800 of these trees around
Washington's Tidal Basin, and the beautiful blooms set against the backdrop of the national monuments bring
more than 1.5 million visitors to the area every year during the National Cherry Blossom Festival. Not surpris-
ingly, the Festival is planned to coincide with the peak bloom of the cherry trees every year.
The exact timing of peak bloom varies from year to year, and it is largely driven by local temperatures during
the winter and early spring. As the Leaf and Bloom Dates indicator (p. 94) explains, scientists have very high
confidence that recent warming trends in global climate are causing spring events such as leaf growth and
flower blooms to happen earlier.44 In the case of Washington's cherry blossoms, earlier bloom dates could affect
tourism and the local economy.
The peak bloom date for the most common type of cherry tree around Washington's Tidal Basin—the Yoshino
variety—has been carefully estimated and recorded since 1921 by the National Park Service. The peak bloom
date is defined as the day when 70 percent of the blossoms are in full bloom.
Figure 1 shows how the peak bloom date of the Yoshino cherry trees has changed since 1921. It also shows
the dates of the National Cherry Blossom Festival, which has grown to several weeks as its popularity has
expanded. As Figure 1 shows, there is considerable variability in the peak bloom date, which makes predicting
the exact timing difficult. Each year, meteorologists, city planners, the National Park Service, and more than one
million tourists speculate about the timing of peak bloom.
96
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Figure 1. Peak Bloom Date for Cherry Trees Around Washington, D.C.'s
Tidal Basin, 1921-2014
March 1
March 11
National Cherry Blossom Festival
April 30
1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020
Year
This figure shows the peak bloom date each year for the main type of cherry tree around the Tidal Basin in Washington,
D. C The peak bloom date occurs when 70 percent of the blossoms are in full bloom. The shaded band shows the timing
of the annual National Cherry Blossom Festival. The festival began in 1934 but was not held during World War II.
Data source: National Park Service, 201445
NOTES
In addition to winter and early spring temperatures, the timing of the peak bloom for cherry trees can
be affected by other weather, climate, and location factors. For example, extended growing periods and
warmer autumns could affect bloom dates by altering other stages of cherry tree growth.46
DATA SOURCES
Peak bloom dates and festival dates were provided by the National Park Service. The data shown here
and other information about Washington's cherry trees can be found online at: www.nps.gov/cherry.
97
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Climate Change Resources
EPA's Climate Change website (www.epa.gov/climatechange) provides a good starting point for further exploration of this topic.
From this site, you can:
• View the latest information about EPA's climate change indicators (www.epa.gov/climatechange/indicators) and download figures as
well as accompanying technical documentation.
• Learn more about greenhouse gases and the science of climate change, discover the potential impacts of climate change on human
health and ecosystems, read about how people can adapt to changes, and get up-to-date news.
• Read about greenhouse gas emissions, look through EPA's greenhouse gas inventories, and explore EPA's Greenhouse Gas Data Publica-
tion Tool.
• Learn about EPA's regulatory initiatives and partnership programs.
• Search EPA's database of frequently asked questions about climate change and ask your own questions. Explore a glossary of terms
related to climate change, including many terms that appear in this report.
• Find out what you can do at home, on the road, at work, and at school to help reduce greenhouse gas emissions.
• Explore U.S. climate policy and climate economics.
• Find resources for educators and students.
Many other government and nongovernment websites also provide information about climate change. Here are some examples:
• The Intergovernmental Panel on Climate Change (IPCC) is the international authority on climate change science. The IPCC website
(www.ipcc.ch/index.htm) summarizes the current state of scientific knowledge about climate change.
• The U.S. Global Change Research Program (www.globalchange.gov) is a multi-agency effort focused on improving our understanding
of the science of climate change and its potential impacts on the United States through reports such as the National Climate Assess-
ment.
• The National Academy of Sciences (http://nas-sites.org/americasclimatechoices) has developed many independent scientific reports
on the causes of climate change, its impacts, and potential solutions. The National Academy's Koshland Science Museum (https://kosh-
land-science-museum.org) provides an interactive online Earth Lab where people can learn more about these issues.
• The National Oceanic and Atmospheric Administration (NOAA) is charged with helping society understand, plan for, and respond to
climate variability and change. Find out more about NOAA's climate indicators and other activities at: www.climate.gov.
• NOAA's National Climatic Data Center website (www.ncdc.noaa.gov/oa/ncdc.html) provides access to data that demonstrate the
effects of climate change on weather, climate, and the oceans.
• The Centers for Disease Control and Prevention (CDC) provides extensive information about the relationship between climate change
and public health at: www.cdc.gov/climateandhealth/default.htm.
• The U.S. Geological Survey's Climate and Land Use Change website (www.usgs.gov/climatejanduse) looks at the relationships
between natural processes on the surface of the earth, ecological systems, and human activities.
• The National Aeronautics and Space Administration (NASA) maintains its own set of climate change indicators (http://climate.nasa.
gov). Another NASA site (http://earthobservatory.nasa.gov/Features/EnergyBalance/page1 .php) discusses the Earth's energy budget
and how it relates to greenhouse gas emissions and climate change.
• The National Snow and Ice Data Center's website (http://nsidc.org/cryosphere) provides more information about ice and snow and how
they influence and are influenced by climate change.
• The Woods Hole Oceanographic Institution's website (www.whoi.edu/main/climate-ocean) explains how climate change affects the
oceans and how scientists measure these effects.
For more indicators of environmental condition, visit EPA's Report on the Environment (www.epa.gov/roe). This resource presents a
wide range of indicators of national conditions and trends in air, water, land, human health, and ecological systems.
98
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Endnotes
INTRODUCTION
1. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013:The physical science basis. Working Group I contribution to the IPCC Fifth
Assessment Report. Cambridge, United Kingdom: Cambridge University Press. www.ipcc.ch/report/arB/wg1.
UNDERSTANDING GREENHOUSE GASES
1. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013:The physical science basis. Working Group I contribution to the IPCC Fifth
Assessment Report. Cambridge, United Kingdom: Cambridge University Press. WWW.ipCC.ch/report/ar5/wg1.
GREENHOUSE GASES
2.
IPCC (Intergovernmental Panel on Climate Change). 2013. Climate
change 2013:The physical science basis. Working Group I contribution
to the IPCC Fifth Assessment Report. Cambridge, United Kingdom:
Cambridge University Press. www.ipcc.ch/report/arB/wg1.
ibid.
3. U.S. EPA (U.S. Environmental Protection Agency). 2014. Inventory of U.S.
greenhouse gas emissions and sinks: 1 990-201 2. EPA 430-R-1 4-003.
www.epa.gov/climatechange/ghgemissions/usinventoryreport.html.
4. ibid.
B. ibid.
6. ibid.
7. ibid.
8. ibid.
9. IPCC (Intergovernmental Panel on Climate Change). 2014. Climate
change 2014: Mitigation of climate change. Working Group III contribu-
tion to the IPCC Fifth Assessment Report. Cambridge, United Kingdom:
Cambridge University Press. www.ipcc.ch/report/arB/wg3.
1 0. WRI (World Resources Institute). 2014. Climate Analysis Indicators Tool
(CAIT) 2.0:WRI's climate data explorer.Accessed May 2014. http://cait.
wri.org.
11. FAO (Food and Agriculture Organization). 201 4. FAOSTAT: Emissions —
land use.Accessed May 2014. http://faostat3.fao.org/faostat-gateway/go/
to/download/G2/*/E.
12. WRI (World Resources Institute). 2014. Climate Analysis Indicators Tool
(CAIT) 2.0:WRI's climate data explorer.Accessed May 2014. http://cait.
wri.org.
13. FAO (Food and Agriculture Organization). 201 4. FAOSTAT: Emissions —
land use.Accessed May 2014. .
1 4. WRI (World Resources Institute). 2014. Climate Analysis Indicators Tool
(CAIT) 2.0:WRI's climate data explorer.Accessed May 2014. http://cait.
wri.org.
1B. IPCC (Intergovernmental Panel on Climate Change). 2014. Climate
change 2014: Mitigation of climate change. Working Group III contribu-
tion to the IPCC Fifth Assessment Report. Cambridge, United Kingdom:
Cambridge University Press. www.ipcc.ch/report/arB/wg3.
1 6. EPICA Dome C and Vostok Station, Antarctica: approximately 796,562
BC to 1813 AD
Luthi, D., M. Le Floch, B. Bereiter.T. Blunier, J.-M. Barnola, U. Siegenthal-
er, D. Raynaud, J. Jouzel, H. Fischer, K. Kawamura, and T.F. Stocker. 2008.
High-resolution carbon dioxide concentration record 6BO,000-800,000
years before present. Nature 4B3:379-382. www.ncdc.noaa.gov/paleo/
pubs/luethi2008/luethi2008.html.
Law Dome, Antarctica, 75-year smoothed: approximately 1010 AD to
1975 AD
Etheridge, D.M., L.P. Steele, R.L Langenfelds, R.J. Francey, J.M. Barnola,
and V.I. Morgan. 1998. Historical C02 records from the Law Dome DE08,
DE08-2, and DSS ice cores. In: Trends: A compendium of data on global
change. Oak Ridge.TN: U.S. Department of Energy. Accessed September
14, 200B. http://cdiac.ornl.gov/trends/co2/lawdome.html.
Siple Station, Antarctica: approximately 1744AD to 1953AD
Neftel, A., H. Friedli, E. Moor, H. Lotscher, H. Oeschger, U. Siegenthaler,
and B. Stauffer. 1994. Historical carbon dioxide record from the Siple
Station ice core. ln:Trends:A compendium of data on global change.
Oak Ridge.TN: U.S. Department of Energy. Accessed September 14,
200B. http://cdiac.ornl.gov/trends/co2/siple.html.
Mauna Loa, Hawaii: 1959AD to 2013AD
NOAA (National Oceanic and AtmosphericAdministration). 2014.
Annual mean carbon dioxide concentrations for Mauna Loa, Hawaii.
Accessed April 7, 2014. ftp://ftp.cmdl.noaa.gov/products/trends/co2/
co2_annmean_mlo.txt.
Barrow, Alaska: 1974AD to 2012 AD
Cape Matatula, American Samoa: 1976AD to 2012AD
South Pole, Antarctica: 1976AD to 2012 AD
NOAA (National Oceanic and AtmosphericAdministration). 2014.
Monthly mean carbon dioxide concentrations for Barrow, Alaska; Cape
Matatula, American Samoa; and the South Pole. Accessed April 7, 2014.
ftp://ftp.cmdl.noaa.gov/data/trace_gases/co2/in-situ.
Cape Grim, Australia: 1992 AD to 2006AD
Shetland Islands, Scotland: 1993 AD to 2002 AD
Steele, L.P, P.B. Krummel, and R.L. Langenfelds. 2007. Atmospheric
C02 concentrations (ppmv) derived from flask air samples collected at
Cape Grim, Australia, and Shetland Islands, Scotland. Commonwealth
Scientific and Industrial Research Organisation. Accessed January 20,
2009. http://cdiac.esd.ornl.gov/ftp/trends/co2/csiro.
Lampedusa Island, Italy: 1993 AD to 2000 AD
Chamard, P., L. Ciattaglia,A. di Sarra, and F. Monteleone. 2001 .Atmo-
spheric carbon dioxide record from flask measurements at Lampedusa
Island. In: Trends: A compendium of data on global change. Oak Ridge,
TN: U.S. Department of Energy. Accessed September 14, 200B. http://
cdiac.ornl.gov/trends/co2/lampis.html.
99
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17. EPICA Dome C, Antarctica: approximately 797,446 BC to 1937AD
Loulergue, L.,A. Schilt, R. Spahni.V. Masson-Delmotte, T. Blunier, B. Lemieux,
J.-M. Barnola, D. Raynaud.T.F. Stocker, and J. Chappellaz. 2008. Orbital and
millennial-scale features of atmospheric CH4 over the past 800,000 years.
Nature 453:383-386. www.ncdc.noaa.gov/paleo/pubs/loulergue2008/louler-
gue2008.html.
Law Dome, Antarctica: approximately 1008AD to 1980 AD
Etheridge, D.M., L.P. Steele, R.J. Francey, and R.L. Langenfelds. 2002. Historic
CH4 records from Antarctic and Greenland ice cores, Antarctic firn data, and
archived air samples from Cape Grim,Tasmania. ln:Trends:A compendi-
um of data on global change. Oak Ridge, TN: U.S. Department of Energy.
Accessed September 13, 2005. http://cdiac.ornl.gov/trends/atm_meth/law-
dome_meth.html.
Cape Grim, Australia: 1984AD to2013AD
NOAA (National Oceanic and AtmosphericAdministration). 2014. Monthly
mean CH4 concentrations for Cape Grim, Australia. Accessed April 8, 2014.
ftp://ftp.cmdl.noaa.goV/data/t race jases/ch4/flask/surface/ch4_cgo_sur-
face-flask_1_ccgg_month.txt.
Mauna Loa, Hawaii: 1987AD to 2013 AD
NOAA (National Oceanic and AtmosphericAdministration). 2014. Monthly
mean CH4 concentrations for Mauna Loa, Hawaii. Accessed April 8,2014.
ftp://ftp.cmdl.noaa.gov/data/trace4j ases/ch4/in-situ/surface/mlo/ch4_mlo_sur-
face-insitu_1 _ccgg_month .txt.
Shetland Islands, Scotland: 1993 AD to 2001 AD
Steele, L.P., P.B. Krummel, and R.L. Langenfelds. 2002.Atmospheric methane
record from Shetland Islands, Scotland (October 2002 version). In: Trends:
A compendium of data on global change. Oak Ridge,TN: U.S. Department
of Energy. Accessed September 13, 2005. http://cdiac.esd.ornl.gov/trends/
atm_meth/csiro/csi ro-shetlandch4.htm I.
18. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change
2013:The physical science basis.Working Group I contribution to the IPCC
Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University
Press, www.ipcc.ch/report/ar5/wg1.
19. ibid.
20. ibid.
21. EPICA Dome C, Antarctica: approximately 796,475 BC to 1937AD
Schilt, A., M. Baumgartner,T. Blunier, J. Schwander, R. Spahni, H. Fischer, and
T.F. Stocker. 2010. Glacial-interglacial and millennial scale variations in the
atmospheric nitrous oxide concentration during the last 800,000 years. Qua-
ternary Sci. Rev. 29:182-192. ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/
antarctica/epica_domec/edc-n2o-2010-800k.txt.
Antarctica: approximately 1903 AD to 1976AD
Battle, M., M. Bender,T. Sowers, P.Tans, J. Butler, J. Elkins, J. Ellis,T. Conway,
N.Zhang, P. Lang, and A. Clarke. 1996. Atmospheric gas concentrations
over the past century measured in air from firn at the South Pole. Nature
383:231 -235. ftp://daac.ornl.gov/data/global_dimate/global_N_cyde/data/
global_N_perturbations.txt.
Cape Grim, Australia: 1979 AD to 2012 AD
AGAGE (Advanced Global Atmospheric Gases Experiment). 2014. Monthly
mean N20 concentrations for Cape Grim, Australia. Accessed April 8, 2014.
http://ds.data.jma.go.jp/gmd/wdcgg/cgi-bin/wdcgg/catalogue.cgi.
South Pole, Antarctica: 1998AD to 2013 AD
Barrow, Alaska: 1999 AD to 2013 AD
Mauna Loa, Hawaii:2000AD to 2013AD
NOAA (National Oceanic and AtmosphericAdministration). 2014. Monthly
mean N20 concentrations for Barrow, Alaska; Mauna Loa, Hawaii; and the
South Pole. Accessed April 8, 2014.www.esrl.noaa.gov/gmd/hats/insitu/cats/
cats_conc.html.
22. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change
2013:The physical science basis.Working Group I contribution to the IPCC
Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University
Press, www.ipcc.ch/report/ar5/wg1.
23. AGAGE (Advanced Global Atmospheric Gases Experiment). 2014. ALE/GAGE/
AGAGE data base. Accessed May 2014. http://agage.eas.gatech.edu/data.htm.
24. Arnold,! 2013 update to data originally published in: Arnold,T., C.M. Harth,
J. Muhle, A.J. Manning, P.K. Salameh, J. Kim, D.J. Ivy, L.P. Steele, V.V. Petrenko,
J.P. Severinghaus, D. Baggenstos, and R.F.Weiss. 2013. Nitrogen trifluoride
global emissions estimated from updated atmospheric measurements. P.
Natl. Acad. Sci. USA 110(6):2029-2034. Data updated May 2013.
25. NOAA (National Oceanic and Atmospheric Administration). 2013. Halo-
carbons and Other Atmospheric Trace Species group (HATS). Accessed July
2013. www.esrl.noaa.gov/gmd/hats.
26. NASA (National Aeronautics and Space Administration). 2013. Data—TOMS/
SBUVTOR data products. Accessed November 2013. http://science.larc.nasa.
gov/TOR/data.html.
27. NASA (National Aeronautics and Space Administration). 2014. SBUV merged
ozone data set (MOD). Version 8.6. Pre-online release provided by NASA
staff, May 2014. http://acdb-ext.gsfc.nasa.gov/Data_services/merged/index.
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28. NASA (National Aeronautics and Space Administration). 2014.Tropospheric
ozone data from AURA OMI/MLS. Accessed May 2014. http://acdb-ext.gsfc.
nasa.gov/Data_services/doud_slice/new_data.html.
29. NOAA (National Oceanic and AtmosphericAdministration). 2014.The NOAA
Annual Greenhouse Gas Index. Accessed May2014.www.esrl.noaa.gov/gmd/
aggi.
30. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change
2013:The physical science basis.Working Group I contribution to the IPCC
Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University
Press, www.ipcc.ch/report/ar5/wg1.
WEATHER AND CLIMATE
1. NOAA (National Oceanic and AtmosphericAdministration). 2014. National
Climatic Data Center. Accessed May2014.www.ncdc.noaa.gov/oa/ncdc.html.
2. ibid.
3. NOAA (National Oceanic and Atmospheric Administration). 2013. National
Climatic Data Center. Accessed April 2013.www.ncdc.noaa.gov/oa/ncdc.html.
4. CCSP (U.S. Climate Change Science Program). 2008. Synthesis and Assess-
ment Product 3.3:Weather and climate extremes in a changing climate.
www.globalchange.gov/browse/reports/sap-33-weather-and-dimate-extremes-
changing-climate.
5. Melillo, J.M..T.C. Richmond, and G.W. Yohe (eds.). 2014. Climate change
impacts in the United States:The third National Climate Assessment. U.S.
Global Change Research Program, http://nca2014.globalchange.gov.
National Research Council. 2011. Climate stabilization targets: Emissions,
concentrations, and impacts over decades to millennia. Washington, DC:
National Academies Press.
IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change
2013:The physical science basis.Working Group I contribution to the IPCC
Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University
Press, www.ipcc.ch/report/ar5/wg1.
Kunkel, K. 2014. Updated version of Figure 2.3 in: CCSP (U.S. Climate Change
Science Program). 2008. Synthesis and Assessment Product 3.3:Weather
and climate extremes in a changing climate.www.globalchange.gov/browse/
reports/sap-33-weather-and-climate-extremes-changing-climate.
NOAA (National Oceanic and AtmosphericAdministration). 2014. U.S. Cli-
mate Extremes Index. Accessed April 2014.www.ncdc.noaa.gov/extremes/cei.
100
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10. ibid.
11. NOAA (National Oceanic and Atmospheric Administration). 2014. National
Climatic Data Center. Accessed April 2014.www.ncdc.noaa.gov/oa/ncdc.html.
12. ibid.
13. Meehl, G.A., C.Tebaldi, G.Walton, D. Easterling, and L McDaniel. 2009.
Relative increase of record high maximum temperatures compared to record
low minimum temperatures in the U.S. Geophys. Res. Lett. 36:123701.
14. ibid.
15. NOAA (National Oceanic and Atmospheric Administration). 2013. National
Climatic Data Center. Accessed April 2013.www.ncdc.noaa.gov/oa/ncdc.html.
16. ibid.
17. ibid.
18. NOAA (National Oceanic and Atmospheric Administration). 2012. National
Climatic Data Center. Personal communication:Analysis by DerekArndt,
April 2012.
19. CCSP (U.S. Climate Change Science Program). 2008. Synthesis and Assess-
ment Product 3.3:Weather and climate extremes in a changing climate.
www.globalchange.gov/browse/reports/sap-33-weather-and-climate-extremes-
changing-climate.
20. Melillo, J.M..T.C. Richmond, and G.W. Yohe (eds.). 2014. Climate change
impacts in the United States:The third National Climate Assessment. U.S.
Global Change Research Program, http://nca2014.globalchange.gov.
21. NOAA (National Oceanic and Atmospheric Administration). 2014. U.S. Cli-
mate Extremes Index. Accessed March 2014.www.ncdc.noaa.gov/extremes/
cei.
22. NOAA (National Oceanic and AtmosphericAdministration). 2014. Standard-
ized Precipitation Index data files.Accessed March 2014. ftp://ftp.ncdc.noaa.
gov/pub/data/cirs.
23. NOAA (National Oceanic and Atmospheric Administration). 2013. State of
the climate: Drought: December 2012. Accessed July 2013. www.ncdc.noaa.
gov/sotc/drought/2012/12.
24. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change
2013:The physical science basis.Working Group I contribution to the IPCC
Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University
Press. www.ipcc.ch/report/arB/wg1.
25. Heim, R.R. 2002. A review of twentieth-century drought indices used in the
United States. B. Am. Meteorol. Soc. 83(8):1149-1165.
26. NOAA (National Oceanic and AtmosphericAdministration). 2014. National
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27. National Drought Mitigation Center. 2014. Maps and data. Accessed March
2014. http://droughtmonitor.unl.edu/MapsAndData.aspx.
28. MacDonald, G.M. 2010. Water, climate change, and sustainability in the
Southwest. P. Natl. Acad. Sci. USA 107(50):21256-21262.
29. NOAA (National Oceanic and AtmosphericAdministration). 2014. National
Climatic Data Center. Accessed March 2014.www.ncdc.noaa.gov/oa/ncdc.
html.
30. National Drought Mitigation Center. 2014. Maps and data. Accessed January
2014. http://droughtmonitor.unl.edu/MapsAndData.aspx.
31. NOAA (National Oceanic and AtmosphericAdministration). 2014. National
Climatic Data Center. Accessed March 2014.www.ncdc.noaa.gov/oa/ncdc.
html.
32. IPCC (Intergovernmental Panel on Climate Change). 2012. Managing the
risks of extreme events and disasters to advance climate change adaptation.
Cambridge, United Kingdom: Cambridge University Press. http://ipcc-wg2.
gov/SREX.
33. Melillo, J.M..T.C. Richmond, and G.W. Yohe (eds.). 2014. Climate change
impacts in the United States: The third National Climate Assessment. U.S.
Global Change Research Program, http://nca2014.globalchange.gov.
34. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change
2013:The physical science basis.Working Group I contribution to the IPCC
Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University
Press, www.ipcc.ch/report/ar5/wg1.
35. Knutson, T.R. 2014 update to data originally published in: Knutson, T.R., J.L.
McBride, J. Chan, K. Emanuel, G. Holland, C. Landsea, I. Held, J.P. Kossin, A.K.
Srivastava, and M. Sugi. 2010.Tropical cyclones and climate change. Nature
Geosci.3:157-163.
36. NOAA (National Oceanic and AtmosphericAdministration). 2014.The Atlan-
tic Hurricane Database Re-analysis Project.www.aoml.noaa.gov/hrd/hurdat/
comparison_table.html.
37. Emanuel, K.A. 2014 update to data originally published in: Emanuel, K.A.
2007. Environmental factors affecting tropical cyclone power dissipation. J.
Climate 20(22):5497-5509.
38. Knutson,T.R., J.L. McBride, J. Chan, K. Emanuel, G. Holland, C. Landsea, I.
Held, J.P. Kossin, A.K. Srivastava, and M. Sugi. 2010.Tropical cyclones and
climate change. Nature Geosci. 3:157-163.
OCEANS
1. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change
2013:The physical science basis.Working Group I contribution to the IPCC
Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University
Press, www.ipcc.ch/report/ar5/wg1.
2. Levitus,S.J.I.Antonov.T.P. Boyer,O.K. Baranova, H.E.Garcia, R.A. Locarnini,
A.V. Mishonov, J.R. Reagan, D. Seidov, E.S. Yarosh, and M.M.Zweng. 2012.
World ocean heat content and thermosteric sea level change (0-2000 m),
1955-2010. Geophys. Res. Lett. 39:L10603.
3. ibid.
4. Based on a total global energy supply of 13,113 million tons of oil equiv-
alents in the year 2011, which equates to 5.5 x 1020 joules. Source: IEA
(International Energy Agency). 2013. Key world energy statistics. http://www.
iea.org/publications/freepublications/publication/KeyWorld2013.pdf.
5. CSIRO (Commonwealth Scientific and Industrial Research Organisation).
2014. Data downloads: Global mean thermosteric sea level (GThSL) and
global ocean heat content (GOHC) timeseries for the upper 700m. Accessed
April 2014. www.cmar.csiro.au/sealevel/thermal_expansion_ocean_heat_
timeseries.html.
6. MRI/JMA (Meteorological Research Institute/Japan Meteorological Agency).
2014 update to data originally published in: Ishii, M., and M. Kimoto. 2009.
Reevaluation of historical ocean heat content variations with time-varying
XBT and MBT depth bias corrections. J. Oceanogr. 65:287-299.
7. NOAA (National Oceanic and AtmosphericAdministration). 2014. Global
ocean heat and salt content. Accessed April 2014.www.nodc.noaa.gov/
OC5/3M_HEAT_CONTENT.
8. For example, see: Ostrander, G.K., K.M.Armstrong, E.T. Knobbe, D. Gerace,
and E.P. Scully. 2000. Rapid transition in the structure of a coral reef commu-
nity: The effects of coral bleaching and physical disturbance. P. Natl.Acad.
Sci. USA. 97(10):5297-5302.
9. Pratchett, M.S., S.K.Wilson, M.L. Berumen, and M.I. McCormick. 2004. Sub-
lethal effects of coral bleaching on an obligate coral feeding butterflyfish.
Coral Reefs 23(3):352-356.
10. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change
2013:The physical science basis.Working Group I contribution to the IPCC
Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University
Press, www.ipcc.ch/report/ar5/wg1.
11. ibid.
12. NOAA (National Oceanic and Atmospheric Administration). 2014. Extended
reconstructed sea surface temperature (ERSST.v3b). National Climatic Data
Center. Accessed March 2014.www.ncdc.noaa.gov/ersst.
13. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change
2013:The physical science basis.Working Group I contribution to the IPCC
Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University
Press, www.ipcc.ch/report/ar5/wg1.
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14. Titus, J.G., E.K.Anderson, D.R. Cahoon, S. Gill, R.E.Thieler, and J.S.Williams.
2009. Coastal sensitivity to sea-level rise: A focus on the Mid-Atlantic re-
gion. U.S. Climate Change Science Program and the Subcommittee on Global
Change Research, http://library.globalchange.gov/products/assessments/sap-4-
1-coastal-sensitivity-to-sea-level-rise-a-focus-on-the-mid-atlantic-region.
15. University of Colorado at Boulder. 2014. Sea level change: 2014 release #3.
Accessed May 2014. http://sealevel.colorado.edu.
16. CSIRO (Commonwealth Scientific and Industrial Research Organisation).
2013 update to data originally published in: Church, J.A., and NJ. White.
2011. Sea-level rise from the late 19th to the early 21st century. Surv. Geo-
phys. 32:585-602.
17. NOAA (National Oceanic and AtmosphericAdministration). 2014. Laboratory
for Satellite Altimetry: Sea level rise. Accessed April 2014. http://ibis.grdl.
noaa.gov/SAT/SeaLevel Rise/LSA_SLR_timeseries_global.php.
18. NOAA (National Oceanic and Atmospheric Administration). 2014 update to
data originally published in: NOAA. 2001. Sea level variations of the United
States 1854-1999. NOAA Technical Report NOS CO-OPS 36. http://tidesand-
currents.noaa.gov/publications/techrpt36doc.pdf.
19. Crowell, M., K. Coulton, C.Johnson, J.Westcott, D. Bellomo, S. Edelman, and
E. Hirsch. 2010. An estimate of the U.S. population living in 100-year coastal
flood hazard areas. J. Coastal Res. 26(2):201-211.
20. NOAA (National Oceanic and Atmospheric Administration). 2013. Coastal
Change Analysis Program. Accessed December2013.www.csc.noaa.gov/
digitalcoast/data/ccapregional.
21. ibid.
22. Titus, J.G., E.K.Anderson, D.R. Cahoon, S. Gill, R.E.Thieler, and J.S.Williams.
2009. Coastal sensitivity to sea-level rise: A focus on the Mid-Atlantic re-
gion. U.S. Climate Change Science Program and the Subcommittee on Global
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12. USDA (U.S. Department of Agriculture) Forest Service. 2014.1991-1997
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14. USDA (U.S. Department of Agriculture) Forest Service. 2014.1991-1997
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16. ibid.
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19. ibid.
20. ibid.
21. ibid.
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27. Quinn, F.H. 1985. Temporal effects of St. Clair River dredging on Lakes St.
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28. NOAA (National Oceanic and AtmosphericAdministration). 2014. Great
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31.
32.
33.
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ibid.
34. National Audubon Society. 2009. Northward shifts in the abundance of
North American birds in early winter: A response to warmer winter tempera-
tures? www.audubon.org/bird/bacc/techreport.html.
35. ibid.
36. For example, see: Schwartz, M.D., R.Ahas, and A.Aasa. 2006. Onset of
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37. IPCC (Intergovernmental Panel on Climate Change). 2014. Climate change
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bridge University Press, www.ipcc.ch/report/ar5/wg2.
38. Schwartz, M.D., R.Ahas, and A.Aasa. 2006. Onset of spring starting earlier
across the Northern Hemisphere. Glob. Chang. Biol. 12:343-351.
39. Schwartz, M.D., T.R.Ault, and J.L. Betancourt. 2013. Spring onset variations
and trends in the continental United States: Past and regional assessment
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40. Schwartz, M.D. 2013 update to data originally published in: Schwartz, M.D.,
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continental United States: Past and regional assessment using tempera-
ture-based indices. Int. J. Climatol. 33:2917-2922.
41. ibid.
42. ibid.
43. Schwartz, M.D., T.R.Ault, and J.L. Betancourt. 2013. Spring onset variations
and trends in the continental United States: Past and regional assessment
using temperature-based indices. Int. J. Climatol. 33:2917-2922.
44. IPCC (Intergovernmental Panel on Climate Change). 2014. Climate change
2014: Impacts, adaptation, and vulnerability. Working Group II contribution
to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cam-
bridge University Press, www.ipcc.ch/report/ar5/wg2.
45. National Park Service. 2014. Bloom schedule. Accessed April 18, 2014. www.
nps.gov/cherry/cherry-blossom-bloom.htm.
46. Chung, U., L. Mack, J.I. Yun, and S. Kim. 2011. Predicting the timing of cherry
blossoms in Washington, D.C. and Mid-Atlantic states in response to climate
change. PLoS ONE 6(11):e27439.
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Cover photograph: Portage Glacier, Alaska
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