United states
   Environmental Protection
   k Agency
Climate Change Indicators
in the United States, 2012

                    I—^
                  W
             ^^*
                     2ND EDITION

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EPA's updated climate change web-
site now features a more user-friendly
interface and downloadable images and
figures. To view the latest information
about EPA's climate change indicators,
along with the corresponding technical
documentation, or to suggest new
indicators for future reports, please
visit EPA's website at: www.epa.gov/
climatechange/indicators or send a
message to: climateindicators@epa


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Contents
Acknowledgments	2
Introduction	3

Summary of Key Points	6
Greenhouse Gases	10
   U.S. Greenhouse Gas Emissions	 2
   Global Greenhouse Gas Emissions	 4
   Atmospheric Concentrations of Greenhouse Gases	 6
   Climate  Forcing	20

Weather and Climate	22
   U.S. and  Global Temperature	24
   High and Low Temperatures	26
   U.S. and  Global Precipitation	28
   Heavy Precipitation	30
   Drought	32
   Tropical  Cyclone Activity	34

Oceans	36
   Ocean Heat	38
   Sea Surface Temperature	40
   Sea Level	42
   Ocean Acidity	44

Snow and  Ice	46
   Arctic Sea Ice	48
   Glaciers	50
   Lake Ice	52
   Snowfall	54
   Snow Cover	56
   Snowpack	58

Society and Ecosystems	60
   Streamflow	62
   Ragweed Pollen Season	64
   Length of Growing Season	66
   Leaf and Bloom Dates	68
   Bird Wintering Ranges	70
   Heat-Related Deaths	72

Climate Change Indicators and Human Health	74

Climate Change Resources	75

Endnotes...                                                                          .. 77

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Acknowledgments
This report was developed by EPA's Office of Atmospheric Programs, Climate Change Division, with support from the Office Re-
search and Development and the Office of Water. It also reflects the contributions and collaboration of many other individuals. EPA
received essential support from scientists and communications experts at a number of federal agencies, universities, nongovernmental
organizations, and international institutions.

Data Contributors and Indicator Reviewers
 U.S. Federal Agencies
 Centers for Disease Control and Prevention
 National Aeronautics and Space Administration
 National Oceanic and Atmospheric Administration
 National Snow and Ice Data Center
 U.S. Department of Agriculture
 U.S. Geological Survey
George Luber
Joey Comiso
Climate Prediction Center: Gerry Bell
Earth System Research Laboratory: Steve Montzka
National Climatic Data Center:
Deke Arndt, Karin Gleason, Boyin Huang
National Oceanographic Data Center: Sydney Levitus
National Ocean Service: Chris Zervas
Office of Oceanic and Atmospheric Research: Libby Jewett
Pacific Marine Environmental Laboratory:
Richard Feely, Chris Sabine
Walt Meier
Lewis Ziska
Alaska Science Center: Shad O'Neel
Maine Water Science Center: Robert Dudley, Glenn Hodgkins
New York Water Science Center: Mike McHale
Washington Water Science Center: Bill Bidlake, Mark Savoca
 Universities, Nongovernmental Urganizations, and International Institutions
 Bermuda Institute of Ocean Sciences
 California Department of Public Health
 Commonwealth Scientific and Industrial Research Organisation
 Georgia Institute of Technology
 Japan Agency for Marine-Earth Science and Technology
 Massachusetts Institute of Technology
 North Carolina State University
 Rutgers  University Global Snow Lab
 University of Nebraska-Lincoln
 Universidad de las Palmas de Gran Canaria
 University of Wisconsin-Madison
 University of Wisconsin-Milwaukee
 USA National Phenology Network
 Woods Hole Oceanographic Institution
 World Glacier Monitoring Service
 World Resources Institute
Nick Bates
Paul English
John Church, Catia Domingues, Neil White
Ray Wang
Masayoshi Ishii
Kerry Emanuel
Ken Kunkel
David Robinson
Song Feng
Melchor Gonzalez-Davila
Corinna Gries
Mark Schwartz
Jake Weltzin
Sarah Cooley
Michael Zemp
Tom Damassa

Peer Review
The report included an external peer review consisting of 12 expert reviewers: Michael C. MacCracken, Tanja Srebotnjak, Dan Tunstall, Paul Kir-
shen, Thomas R. Knutson, Gerald Meehl, Steven Nerem, W. Tad Pfeffer, Michael J. Prather, David Schimeljoel D. Schwartz, and Claudia Tebaldi.
Report Production and Design
Support for the report's production and design was provided by Eastern Research Group, Inc. (ERG).

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Introduction
    The Earth's climate is changing. Scientists are
    confident that many of the observed changes
    in the climate can be linked to the increase in
greenhouse gases in the atmosphere, caused largely
by people burning fossil fuels to generate electric-
ity, heat and cool buildings, and power vehicles (see
"The  Greenhouse Effect" below to learn about how
these  gases trap heat). Current and future emissions
will continue to increase the levels of these gases in
our atmosphere for the foreseeable future.

One way to track and communicate the causes and
effects of climate change is through the use of indica-
tors. An indicator, such as a record of Arctic sea ice
extent, represents the state or trend of certain envi-
ronmental conditions over a given area and a specified
period of time. Scientists, analysts, decision-makers, and
others use environmental indicators, including those re-
lated to climate, to help monitor environmental trends
over time, track key factors that influence the environ-
ment, and identify effects on ecosystems and society.

The climate change indicators in this report pres-
ent compelling evidence that the composition of the
  What Is Climate Change?
  Climate change refers to any significant change in measures of
  climate (such as temperature or precipitation) lasting for an ex-
  tended period (decades or longer). Climate change may result
  from natural factors and processes or from human activities.
  Global warming is a term often used interchangeably with the
  term "climate change," but they are not 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, albeit a very important one.
atmosphere and many fundamental measures of climate
in the United States are changing. Temperatures are
rising, snow and rainfall patterns are shifting, and more
extreme climate events—like heavy rainstorms and
record high temperatures—are taking place. Similar
changes are occurring around the world.

These observed changes affect people and the environ-
ment 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 fun-
                  damental disruptions in ecosystems,
                  affecting plant and animal popula-
                  tions, communities, and biodiversity.
                  Such changes can also affect society,
                  including where people  can live,
                  what kinds of crops farmers can grow,
                  and what kinds of businesses can
                  thrive in certain areas.

                  Indicators of climate change are  ex-
                  pected to become even  more numer-
                  ous and depict even clearer trends in
                  the future. Looking ahead, the U.S.
                  Environmental Protection Agency
                  (EPA) will continue to  work in part-
                  nership with other agencies, orga-
                  nizations,  and individuals to collect
                  and communicate useful data and to
                  inform policies and programs based
                  on this knowledge.

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About This Report
Climate Change Indicators in the United States, 2012,
presents 26 indicators to help readers better un-
derstand observed trends related to the causes and
effects of climate change. This document updates a
report published by EPA in 2010.

Various government agencies, academic institutions,
and other organizations contributed data critical to
the development of this  report. EPA also received
feedback from a diverse group of scientists, research-
ers, and communications experts in the public and
private sectors. This feedback helped to inform the
content and new features of this 2012 report. All
of the indicators in this report are  based on data
that have been collected and compiled according
to protocols accepted by the scientific community.
The indicators were chosen using a standard set of
criteria that considered usefulness, objectivity, data
quality, transparency, ability to meaningfully com-
municate, and relevance to climate change. In addi-
tion, the  report was peer-reviewed by independent
technical experts.
    EPA's Greenhouse Gas

    Reporting Program
    EPA is now collecting facility-level data on U.S. green-
    house gas emissions and other relevant information
    under the Greenhouse Gas Reporting Program. This
    program requires annual reporting of greenhouse gas
    data from large emissions sources across a range of
    industry sectors, as well as suppliers of products that
    would emit greenhouse gases if released or combust-
    ed. This new information will help inform the annual
    Inventory of U.S. Greenhouse Gas Emissions and Sinks,
    which currently serves as the data source for the U.S.
    Greenhouse Gas Emissions indicator.
    For more information, see: www.epa.gov/
    climatechange/emissions/ghgdata.
Who Is This Report  For?
Climate Change Indicators in the United States, 2012,
is written with the primary goal of informing read-
ers' understanding of climate change. In addition to
presenting climate change observations and trends
in the United States and globally, this report high-
lights the far-reaching significance of these changes
and their possible consequences for people, the envi-
ronment, and society.

This report is also designed to be useful for sci-
entists, analysts, decision-makers, educators, and
others who can use climate change indicators as a
tool for:

•  Assessing trends in environmental quality, factors
   that influence the environment, and effects on
   ecosystems and society.

•  Effectively supporting science-based decision-
   making and communication.

•  Evaluating existing and future climate-related
   policies and programs.
                                                  What's  New?
The 2012 report reflects the following new features
and changes:

•  Three new indicators: Snowfall, Streamflow, and
   Ragweed Pollen Season. These additions provide
   further evidence of climate change and its effects
   that are being felt by different kinds of ecosys-
   tems, as well as by society.

•  Expanded  indicators: Arctic Sea Ice was ex-
   panded to show changes in the age of ice and
   Snow Cover was expanded to show changes in
   snow cover for particular seasons. Several decades
   of historical data were added to Drought, and
   the 2010 Heat Waves indicator was converted to
   High and Low Temperatures.

•  Updated indicators: Nearly all indicators have
   been updated with additional years of data that
   have become available since the last report.

•  Regional perspectives: Several indicators include
   maps that show how trends vary by region.

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A Roadmap to the Report
Most of the indicators in this report focus on the Unit-
ed States, but some include global trends to provide
context or a basis for comparison, while others have a
regional focus. Geographic coverage depends on data
availability and the nature of what is being measured.
For example, greenhouse gas concentrations in the
atmosphere are studied on a global scale. The indica-
tors span a range of time periods, depending on data
availability. 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 cal-
   culating drought).
•  Key points about what the graphics show.
•  Background on how the indicator relates to climate
   change.
•  Information about how the indicator was developed.
•  Factors that influence the potential to draw valid
   conclusions from the indicator.

The indicators are divided into five chapters:

                 Greenhouse Gases: Greenhouse gases
                 from human activities are responsible
                 for the largest share of climate
                 change since the mid-ZO111 century.
                 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.

                 Weather and Climate: Rising global
                 average temperature is linked to
                 certain widespread changes in
                 weather patterns, which in turn
                 lead to changes in the Earth's
                 climate (the average weather over
time). This chapter focuses on indicators related to
weather and climate, including temperature, precipita-
tion, storms, and droughts.

                 Oceans: 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 certain proper-
ties of the ocean. This chapter examines trends in
ocean characteristics that relate to  climate change,
such as heat storage, temperature, and sea level.
                 SHOW and Ice: Climate change can
                 alter the Earth's snow- and ice-cov-
                 ered areas. These changes, in turn,
                 can affect air temperatures, sea levels,
                 ocean currents, and storm patterns.
                 This chapter focuses on trends in
glaciers and sea ice, snowfall, extent and depth of snow
cover, and the freezing and thawing of oceans and lakes.

                 Society and Ecosystems: Changes
                 in the Earth's climate can affect
                 public health, agriculture, water
                 supplies, energy production and use,
                 land use and development, and
                 recreation. Climate change can also
disrupt the functioning of ecosystems and increase the
risk of harm or even extinction for some species. This
chapter looks at some of the ways that climate change
is affecting society and ecosystems, including changes
in allergy seasons, heat-related deaths, streamflows, and
bird migration patterns.

The report concludes with a discussion on climate
change indicators and health, as further development
of human health indicators is of increasing importance.
Climate change impacts associated with human health
include expected increases in heat-related illness and
death, worsening air quality, and likely increases in the
frequency  and strength of certain extreme events such
as floods, droughts, and storms. Climate change may
also allow  some diseases to spread more easily. People
most vulnerable to health impacts include the poor,
the elderly, those already in poor health, the disabled,
and indigenous populations. EPA plans to explore
opportunities to work with climate and health experts
to develop indicators that communicate the effects of
climate change on health  and society more broadly.

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 uncertainty. 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. 75).
Looking Ahead
As new and more comprehensive indicator data be-
come available, EPA plans to continue to periodically
update the indicators presented in this report to docu-
ment climate change and its effects.

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Summary  of  Key Points
Human activities have substantially increased the amount of greenhouse gases in the atmosphere, leading to
warming of the climate and many other changes around the world—effects that will persist over a long time.
             U.S. Greenhouse Gas Emissions. In the United States, greenhouse gas emissions caused by hu-
             man activities increased by 10 percent from 1990 to 2010. Carbon dioxide accounts for most
             of the nation's emissions and most of this increase. 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, emissions of greenhouse gases from human
             activities increased by 26 percent from 1990 to 2005. Emissions of carbon dioxide, which ac-
             count for nearly three-fourths of total emissions, increased by 31 percent over this period. As
             with the United States, the majority of the world's emissions result from 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.1 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 or "radiative" forcing is the measurement of how substances such
             as greenhouse gases affect the amount of energy absorbed by the atmosphere. An increase in
             radiative forcing means a heating effect, which leads to warming, while a decrease in forcing
             produces cooling. From 1990 to 2011, the total radiative forcing from greenhouse gases added
             by humans to the Earth's atmosphere increased by 30 percent. Carbon dioxide has accounted
             for approximately 80 percent of this increase.

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Variations in weather and climate cause changes in temperature, precipitation, and extreme event patterns,
which can directly or indirectly affect many aspects of society.


              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 record have occurred since 1990. Recent compilations of the change in
              global average temperatures show a similar trend, and 2001-2010 was the warmest decade on
              record worldwide. Within the United States, temperatures in parts of the North, the West,
              and Alaska have increased the most.

              High and LOW Temperatures. Since the 1970s, unusually hot summer temperatures have
              become more common in the United States, and heat waves have become more frequent. In
              contrast, extremely cold winter temperatures have become less common. The most recent
              decade has had twice as many record high temperatures as record lows. The most severe heat
              waves in U.S. history remain those that occurred during the "Dust Bowl" in the 1930s.

              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
              nearly 6 percent per century in the contiguous 48 states and more than 2 percent per century
              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, eight of the top 10 years for
              extreme one-day precipitation events have occurred since 1990. The occurrence of abnor-
              mally 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 2011, roughly 30 to 60 percent of the
              U.S. land area experienced drought conditions at any given time.


              Tropical Cyclone Activity. Tropical storm activity in the Atlantic Ocean, Caribbean, and Gulf
              of Mexico has increased during the past 20 years. This increase is closely related to variations
              in sea surface temperature in the tropical Atlantic. However, changes in observation meth-
              ods over time make it difficult to know for sure whether a long-term increase has occurred.
              Records collected since the late 1800s suggest that the actual number of hurricanes per year
              has not increased.

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Changes in ocean temperature, sea level, and seawater chemistry have implications for coastal communities
and could substantially alter the biodiversity and productivity of ocean ecosystems.
              Ocean Heat. Several studies have shown 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.


              §63 Surface Temperature. Ocean surface temperatures increased around the world over the
              20th century. Even with some year-to-year variation, the overall increase is statistically sig-
              nificant, 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 seven-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 height
              of the land vary widely because the land itself moves. Along the U.S.  coastline, sea level has
              risen the most relative to the land along the Mid-Atlantic coast and parts of the Gulf Coast.
              Sea level has decreased relative to the land in parts of Alaska and the Northwest.

              Ocean Acidity. The ocean has become more acidic over the past 20 years because of increased
              levels of atmospheric carbon dioxide, which in turn dissolves in the water. Higher acidity
              has led to decreased availability of minerals such as aragonite, which is an important form of
              calcium carbonate that many marine animals use to build their skeletons and shells.
Climate change can dramatically alter the Earth's snow- and ice-covered areas, affecting vegetation and
wildlife, water supplies and transportation, and communities in Arctic regions.

              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 decreased 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. Lakes in the northern United States generally appear to be freezing later and thaw-
              ing earlier than they did in the 1800s and early 1900s. The length of time that lakes stay
              frozen has decreased at an average rate of one to two days per decade.
              Snowfall. Total snowfall 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.

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              SnOW Cover. The portion of North America covered by snow has decreased somewhat since
              1972, based on weekly measurements taken throughout the year. However, there has been
              much year-to-year variability. During the years 2002-2011, the average area covered by snow
              was 3 percent (roughly 100,000 square miles) smaller than the average extent of snow cover
              during the first 10 years of measurement (1972-1981).
                       . The depth of snow on the ground (snowpack) in early spring decreased at most
              measurement sites — some by more than 75 percent — between 1950 and 2000. However, a few
              locations in the western United States and Canada saw an increase in spring snowpack.
Climate change could require adaptation on larger and faster scales than in the past, presenting challenges to
human well-being, the economy, and natural ecosystems.

              StreamflOW. Changes in temperature, precipitation, snowpack, and glaciers can affect the
              amount of water carried by rivers and streams and the timing of peak flow. Over the last 70
              years, minimum and maximum flows have changed in many parts of the country—some
              higher, some lower. Three-fifths of the rivers and streams measured show peak winter-spring
              runoff happening at least five days earlier than it did in the past.

              Ragweed Pollen Season. Warmer temperatures and later fall frosts allow ragweed plants to produce
              pollen later into the year, potentially prolonging allergy season for millions of people. The length
              of ragweed pollen season has increased at eight out of 10 locations studied in the central United
              States and Canada since 1995. The change becomes more pronounced from south to north.

              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 particular-
              ly 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.

              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 growth is now occurring a few days earlier than it did
              in the early 1900s. Lilac and honeysuckle bloom dates vary greatly from year to year, which
              makes it difficult to determine whether a statistically meaningful  change has taken place.

              Bird Wintering BangeS. 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 35 miles
              since 1966, with a few species shifting by several hundred miles. On average, bird species have also
              moved their wintering grounds farther from the coast, consistent with rising inland temperatures.

              Heat-Belated Deaths. Over the past three decades, more than 7,000 Americans were reported
              to have died as a direct result of heat-related illnesses, such as heat stroke. The annual death
              rate rises 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 num-
              ber of deaths classified  as "heat-related."

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 Major Greenhouse Gases Associated with Human Activities
 Greenhouse gas   How it's produc
 Carbon dioxide
  Methane
  Nitrous oxide
                                                verage life
                                               in the atmosphere   warming potential
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 atmo-
sphere, 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 prac-
tices and from the anaerobic decay of organic
waste in  municipal solid waste landfills.
Emitted during agricultural and industrial activi-
ties, as well as during combustion of fossil fuels
and solid waste.
see below*
12 years
114 years
21
310
  Fluorinated
  gases
A group of gases that includes hydrofluorocar-
bons, perfluorocarbons, and sulfur hexafluoride,
among other chemicals. These gases are emitted
from a variety of industrial processes and com-
mercial and household uses, and do not occur
naturally. Sometimes used as substitutes for
ozone-depleting substances such as chlorofluoro-
carbons (CFCs).
A few weeks to
thousands of years
Varies (the highest
is sulfur hexafluo-
ride at 23,900)
This 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. EPA uses global warming potentials from the Intergovernmental Panel on Climate Change's (IPCC's) Second Assessment Report,1
as countries have agreed to do undercurrent international guidelines within the United Nations Framework Convention on Climate Change
(UNFCCC). Lifetimes come from the IPCC's Fourth Assessment Report.2

* Carbon dioxide's lifetime is poorly defined 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.
INDICATORS  IN  THIS CHAPTER
                                                                         reenhouse

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      Energy from the sun drives the
      Earth's weather and climate.
      The Earth absorbs some of the
  energy it receives from the sun
  and radiates the rest back toward
  space. However, certain gases in
  the atmosphere, called greenhouse
  gases, absorb some of the energy
  radiated from the Earth and trap
  it in the atmosphere. These gases
  essentially act as a blanket, mak-
  ing the Earth's surface warmer
  than it otherwise  would be.
  While this "greenhouse effect"
  occurs naturally, making life
  as we know it possible, human
  activities in the past century
  have substantially increased the
  amount of greenhouse gases in
  the atmosphere, causing the at-
  mosphere to trap  more heat and
  leading to changes in  the Earth's
  climate.

  What is happening?
  The major greenhouse gases emit-
  ted into the atmosphere through
  human activities are carbon
  dioxide, methane, nitrous oxide,
  and fluorinated gases (see Major
  Greenhouse Gases Associated
  With Human Activities on p.10).
  Some of these gases are produced
  almost entirely by human activi-
  ties; others come from a combina-
  tion of natural sources and human
  activities.
Many of the major greenhouse
gases can remain in the atmo-
sphere for tens to hundreds of
years after being released. They
become globally mixed in the
lower atmosphere,  reflecting con-
tributions from emissions sources
worldwide.
                  r
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
atmosphere. A second factor is
each gas's unique ability to absorb
energy. By considering both of
these factors, scientists calculate
a gas's global warming potential,
as compared to an equivalent
mass of carbon dioxide (which
is defined by a global warming
potential equal to 1).
                                   Why does it matter?
As greenhouse gas emissions
from human activities increase,
they contribute to more warming
of the climate, leading to many
other changes around the world—
in the atmosphere, on land, and
in the oceans. These changes will
have both positive and negative
effects on people, plants, and ani-
mals. Because many of the major
greenhouse gases can stay in the
atmosphere for tens to hundreds
of years after being released, their
warming effects on the climate will
persist over a long time.
  Gases and Substances Not Included in This Report
  This report addresses most of the major, well-mixed greenhouse gases that
  contribute to warming of the climate. The report does not address trends in
  emissions or concentrations of substances with shorter atmospheric lifetimes
  (i.e., less than a year) that are also relevant to climate change, such as ozone in
  the lower atmosphere, pollutants that lead to ozone formation, water vapor,
  and aerosols (atmospheric particles) such as black carbon and sulfates. These
  substances may be considered for future editions of this report.
For detailed information about data used in these indicators, see the online technical documentation at: www.epa.gov/climatechange/indicators.

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                 U.S.  Greenhouse   Gas   Emissio
                  This indicator describes emissions of greenhouse gases in the United States.
    Background
    A number of factors influence the quan-
    tities of greenhouse gases released into
    the atmosphere, including economic ac-
    tivity, population, consumption patterns,
    energy prices, land use, and technology.
    There are several ways to track these
    emissions. In addition to tracking overall
    emissions and emissions from specific
    industrial sectors in absolute terms,
    many countries  also track emissions per
    capita.


    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 under 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 Inventory of
    U.S.  Greenhouse  Gas Emissions and Sinks:
    I990-20I0.3 This indicator is restricted
    to emissions associated with human
    activities.
    This indicator reports emissions  of
    greenhouse gases according to their
    100-year global warming potential, a
    measure of how much a given amount
    of the greenhouse gas is estimated to
    contribute to global warming over a
    period of 100 years after being emitted
    (see table on p. 10). For purposes of
    comparison, global warming potential
    values are calculated in relation to
    carbon dioxide  and are expressed in
    terms of carbon dioxide equivalents.
    For additional perspective, this indica-
    tor also shows greenhouse gas emis-
    sions in relation to economic activity
    and population.
Figure 1. U.S. Greenhouse Gas Emissions by Gas, 1990-2010
This figure shows emissions of carbon dioxide, methane, nitrous oxide, and several fluorinated
gases in the United States from 1990 to 2010. For consistency, emissions are expressed in million
metric tons of carbon dioxide equivalents.
                           Methane
•S 5,000
u

| 4,000

I
.S 3,000


B 2,000
 |   1,000
 in
                                  Carbon dioxide
 ~   1990    1992    1994    1996    1998    2000    2002    2004   2006   2008   2010
                                     Year
        * MFCs are hydrofluorocarbons, PFCs are perfluorocarbons, and SF6 is sulfur hexafluoride.

        Data source: U.S. EPA, 201V
 Figure 2. U.S. Greenhouse Gas Emissions and Sinks by Economic
 Sector, 1990-2010
 This figure shows greenhouse gas sinks (negative values) and emissions by source in the United
 States from 1990 to 2010. For consistency, emissions are expressed in million metric tons of
 carbon dioxide equivalents. Totals do not match Figure  I exactly because the economic sectors
 shown here do not include emissions from U.S. territories.
                                                  -2,000
                                                                    Land use, land use change,
                                                                      and forestry (sinks)
                                               •=   1990
                                                           1992
                                                                 1994
                                                                       1996
                                                                             1998
12]
                                                                                   2000
                                                                                   Year
                                                                                         2002    2004
                                                                                                     2006
                                                                                                           2008
                                                                                                                 2010
        Data source: U.S. EPA, 20125

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   Figure 3. U.S. Greenhouse Gas Emissions per Capita and per Dollar of
   GDR1990-2010
   This figure shows trends in greenhouse gas emissions from 1990 to 2010 per capita (heavy or-
   ange line), based on the total U.S. population (thin orange line). It also shows trends in emissions
   compared with the real GDP (heavy blue line). Real GDP is the value of all goods and services
   produced in the country during a given year, adjusted for inflation (thin blue line). All data are in-
   dexed to 1990 as the base year, which is assigned a value of 100. For instance, a real GDP value
   of 163 in the year 2010 would represent a 63 percent increase since  1990.
        1990    1992    1994    1996    1998    2000    2002    2004    2006    2008    2010

         Data source: U.S. EPA, 2012'
Indicator Notes
While this indicator addresses the major
greenhouse gases emitted by human activi-
ties, 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. 20
for more details). For example, this indica-
tor excludes ozone-depleting substances
such as chlorofluorocarbons (CFCs) and
hydrochlorofluorocarbons (HCFCs), which
have high global warming potentials, as
these gases are being phased out under an
international agreement called the Mon-
treal Protocol. There are also many natural
greenhouse gas emission sources; however,
this indicator includes only emissions that
are associated with human activities.


Data  Sources
Data for this indicator came from EPA's
Inventory of U.S. Greenhouse Gas Emissions
and Sinks: 1990-2010. This report is avail-
able online at: www.epa.gov/climatechange/
ghgemissions/usinventoryreport.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
   In April 2010, U.S. greenhouse gas emissions totaled 6,822 million metric tons of carbon
   dioxide equivalents, a 10 percent increase from 1990 (see Figure  I).
   For the United States, during the period from 1990 to 2010 (see Figure  I):

   o  Emissions of carbon dioxide, the primary greenhouse gas emitted by human activities,
      increased by 12 percent.
   o  Methane emissions remained roughly the same, as higher emissions from activities
      such as livestock production and natural gas systems were largely offset by reduced
      emissions from landfills and coal mines.7
   o  Nitrous oxide emissions, largely derived from vehicle emissions and agricultural soil
      management practices, such as the use of nitrogen as a fertilizer, declined by 3 percent.
   o  Emissions of fluorinated gases (hydrofluorocarbons, perfluorocarbons, and sulfur
      hexafluoride), released  as a result of commercial, industrial, and household uses,
      increased by 58 percent.
   Electricity generation is the largest U.S. emissions source, accounting for about 32 percent of
   total U.S. greenhouse gas emissions since  1990. Transportation is the second-largest source
   of greenhouse gas emissions, accounting for 27 percent of emissions since 1990 (see Figure 2).

   Emissions sinks, the opposite of emissions sources, absorb and store emissions. In 2010,
   16 percent of U.S. greenhouse gas emissions 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 carbon from the atmosphere. Other carbon sinks are associated with how people
   use the land, including the practice of depositing yard trimmings and food scraps in landfills.
   Emissions increased at about the same rate as the population from 1990 to 2007, which caused
   emissions 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 have increased since 2009 as the U.S. economy has begun to grow again.8
   From 1990 to 2010, greenhouse gas emissions per dollar of U.S. gross domestic product
   (GDP) declined by 32 percent (see Figure 3). This change may reflect a combination of
   increased energy efficiency and structural changes in the economy.

-------
                      lobal   Greenhouse
                      as   Emissions
                  This indicator describes emissions of greenhouse gases worldwide.
    Background
    Since preindustrial times, increasing
    emissions of greenhouse gases due to
    human activities worldwide have led to
    a noticeable increase in atmospheric
    concentrations of long-lived and other
    greenhouse gases (see the Atmospheric
    Concentrations of Greenhouse Gases
    indicator on p. 16). Every country
    around the world emits greenhouse
    gases into the atmosphere, meaning
    the root causes of climate change are
    truly global. 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 coun-
    try'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 (p. 12), this indicator focuses
    on emissions of gases covered under the
    United Nations Framework Convention
    on Climate Change: carbon dioxide,
    methane, nitrous oxide, and several fluo-
    rinated gases. These are all important
    greenhouse gases that are influenced by
    human activities, and the Convention re-
    quires participating countries to develop
    and periodically submit an inventory of
    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 green-
    house gas inventories developed by
    EPA and other government agencies
    worldwide. Global estimates for carbon
    dioxide are published annually, but esti-
    mates for other gases, such as methane
    and nitrous oxide, are available only
    every fifth year.
Figure 1. Global Greenhouse Gas Emissions by Gas, 1990-2005
This figure shows worldwide emissions of carbon dioxide, methane, nitrous oxide, and several fluo-
rinated gases from  1990 to 2005. For consistency, emissions are expressed in million metric tons of
carbon dioxide equivalents. These totals do not include emissions due to land-use change or forestry.
"a> 40,000
.£ 35,000
S 30,000
 3
                                                MFCs, PFCs, and SF6*
                                                Nitrous oxide
                                                Methane
                                                Carbon dioxide
                             1995
                                           2000
                                                          2005
                                    Year
       * MFCs are hydrofluorocarbons, PFCs are perfluorocarbons, and SF6 is sulfur hexafluoride.
       Data source: World Resources Institute, 20I29


 Figure 2. Global Greenhouse Gas Emissions by Sector, 1990-2005
 This figure shows worldwide greenhouse gas emissions by sector from 1990 to 2005* For consis-
 tency, emissions are expressed in million metric tons of carbon dioxide equivalents. These totals do
 not include emissions due to land-use change or forestry.
   40,000
                                               Waste
                                               Industrial processes
                                               Agriculture
                                               Internationaltransport
                                               Energy
 E
- 5,000
             1990
                            1995
                                           2000
                                                          2005
                                    Year
14]
       * Note that the sectors shown here are different from the economic sectors used in U.S. emissions account-
       ing (see the U.S. Greenhouse Gas Emissions indicator). Emissions from international transport (aviation and
       marine) are separate from the energy sector because they are not part of individual countries' emissions
       inventories. The energy sector includes all other transportation activities.
       Data source: World Resources Institute, 2012'°

-------
Key Points
   In 2005, estimated worldwide emissions totaled nearly 39 billion metric tons of green-
   house gases, expressed as carbon dioxide equivalents. This represents a 26 percent
   increase from 1990 (see Figures  I and 2).
   Between 1990 and 2005, global emissions of all major greenhouse gases increased (see
   Figure I). Emissions of carbon dioxide increased by 31 percent, which is particularly
   important because carbon dioxide accounts for nearly three-fourths of total global emis-
   sions. Methane emissions increased the least—10 percent—while emissions of nitrous
   oxide increased by 14 percent. Emissions of fluorinated gases more than doubled.
   Energy production and use (including energy used by vehicles) represent the largest
   source of greenhouse gas emissions worldwide (about 73 percent of the total), followed
   by agriculture (16 to  17 percent) (see Figure 2).
   Carbon dioxide emissions are increasing faster in some parts of the world than in others
   (see Figure 3).
   Figure 3. Global Carbon Dioxide Emissions by Region, 1990-2008
   This figure shows carbon dioxide emissions from 1990 to 2008 for different regions of the world.
   These totals do not include emissions due to land-use change or forestry.
      35,000
      30,000
      25,000
      20,000
      15,000
   •i 10,000
      5,000
         1990     1992     1994    1996     1998    2000     2002    2004     2006    2008
                                           Year
This indicator tracks emissions of green-
house gases according to their 100-year
global warming potential, a measure of how
much a given amount of the greenhouse
gas is estimated to contribute to global
warming over a period of 100 years after
being emitted. For purposes of compari-
son, global warming potential values are
calculated in relation to carbon dioxide and
are expressed in terms of carbon dioxide
equivalents.


Indicator  Notes
Like the U.S. Greenhouse Gas Emissions
indicator (p. 12), this indicator does not
include emissions of a number of gases that
affect climate but are not covered under
the United Nations Framework Convention
on Climate Change.  For example, this indi-
cator excludes ozone-depleting substances
such as chlorofluorocarbons (CFCs) and
hydrochlorofluorocarbons (HCFCs),
which have high global warming potentials,
because these gases  are being phased out
under an international agreement called
the Montreal Protocol. This indicator is
restricted to emissions associated with hu-
man activities, but it does not account for
emissions associated with land-use change
or forestry. There are also various emis-
sions of greenhouse  gases of natural origin,
which this indicator  does not cover.
Global emissions inventories for gases other
than carbon dioxide  are limited to five-year
intervals. The United Nations Framework
Convention on Climate Change database
has more comprehensive data; however,
these data are available mainly for a group
of mostly developed  countries that account
for only about half of global greenhouse gas
emissions.  Thus, to provide a more repre-
sentative measure of global greenhouse gas
emissions,  this indicator uses the broader
CAIT database.


Data  Sources
Data for this indicator came  from the
World Resources Institute's CAIT
database, which is accessible online at:
http://cait.wri.org. CAIT compiles data
that were  originally  collected by organiza-
tions including the International Energy
Agency, EPA, the U.S. Carbon Dioxide
Information Analysis Center, and the
European Commission.
           Data source: World Resources Institute, 2012"

-------
              Atmospheric  Concentrations
              of  Greenhouse  Gases
                This indicator describes how the levels of major greenhouse gases in the atmosphere have changed over time.
    Background
    Since the Industrial Revolution began in
    the late 1700s, people have added a sig-
    nificant amount of greenhouse 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. 12-15). When greenhouse gases are
    emitted into the atmosphere, many re-
    main there for long time periods ranging
    from a decade to many millennia. Over
    time, these gases are removed from the
    atmosphere by emissions sinks, such as
    oceans, vegetation, or chemical reac-
    tions. Emissions sinks are the opposite
    of emissions sources, and they absorb
    and store emissions or cause the gases
    to break down. However, if these gases
    enter the atmosphere more quickly than
    they can be removed, their concentra-
    tions increase.
    Many greenhouse gases remain in the
    atmosphere for decades or longer.
    The greenhouse gases being reported
    here become well mixed throughout
    the entire global atmosphere because
    of their long lifetimes and because of
    transport by winds. Concentrations of
    other greenhouse gases such as tropo-
    spheric ozone, which has an atmospheric
    lifetime of hours to days, often vary
    regionally and are not included in this
    indicator.
    Concentrations of 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 I ppb for a given gas
    means there is one part of that gas in
    I billion parts of a given amount of air.
    For some greenhouse gases, even chang-
    es as small as a few parts per trillion can
    make a difference in global climate.


    About the  Indicator
    This indicator describes concentrations
    of greenhouse gases in the atmosphere.
    It focuses on the major greenhouse
    gases that result from human activities.
    These include carbon dioxide, methane,
                                       647,426 BC to 201 IAD
Figure 1. Global Atmospheric
Concentrations of Carbon
Dioxide Over lime
This figure shows concentrations of carbon
dioxide in the atmosphere from hundreds
of thousands of years ago through 2011.
The data come from a variety of historical
ice core studies and recent air monitoring
sites around the world. Each line repre-
sents a different data source.
Figure 2. Global Atmospheric
Concentrations of Methane
Over lime
This figure shows concentrations of meth-
ane in the atmosphere from hundreds of
thousands of years ago through 2011. 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.
                               -700,000  -500,000   -300,000   -100,000 0
                                     Year [negative values = BC]
                                 Data source: Various studies12
                                      646,729 BC to 201 IAD
  2,000
.5- 1,500
  1,000
§
   500
         h.rt.
                           I
VM
                               -700,000   -500,000  -300,000  -100,000 0
                                    Year [negative values = BC]
                                 Data source: Various studies13
16]

-------
                                                                                                    Key  Points
               10,000 BC to 201 IAD
                                                           1950 AD to 201 IAD
 g
•8
400

350

300

250

200

150

100

 50
   400

1.350

 g 300

£ 250

 1 200
 u
S 150
 x
_e
^ 100

"1  50
    -10,000-8000 -6000  -4000 -2000   0   2000
            Year [negative values = BC]
                                                1950 1960  1970  1980 1990 2000  2010  2020
                                                                  Year
                10,000 BC to 201 IAD
                                                            1950 AD to 201 IAD
   2,000
S-1,500
^
9
5- 1,500
!
5 1,000
a
| 500
U
n

^



^



x~


^-



-^



	 .^"
—







      -10,000-8000 -6000  -4000 -2000   0   2000
             Year [negative values = BC]
                                                  1950 1960 1970 1980 1990 2000 2010 2020
                                                                   Year
Global atmospheric concentrations
of carbon dioxide, methane, nitrous
oxide, and certain manufactured
greenhouse gases have all risen over
the last few hundred years (see
Figures I, 2, 3, and 4).
Before the industrial era began in the
late 1700s, carbon dioxide concen-
trations measured approximately
280 ppm. Concentrations have risen
steadily since then, reaching 391
ppm in 2011—a 40 percent increase.
Almost all of this increase is due to
human activities.14
The concentration of methane in the
atmosphere has more than  doubled
since preindustrial times, reaching
about 1,818 ppb in 2011. It is very
likely that this increase is predomi-
nantly due to agriculture and fossil
fuel use.15
Historical measurements show
that the current global atmospheric
concentrations of carbon dioxide
and methane are unprecedented
compared with the past 650,000
years  (see Figures I and 2).
Over  the past 100,000 years, con-
centrations of nitrous oxide in the
atmosphere have rarely exceeded
280 ppb.  Levels have risen since
the 1920s, however, reaching a new
high of 324 ppb  in 2011 (see Figure
3). This increase is primarily due to
agriculture. l6
Concentrations of many of the
halogenated gases shown in Figure 4
(gases that contain chlorine, fluorine,
or bromine) were essentially zero a
few decades ago but have increased
rapidly as they have been incorpo-
rated  into industrial products and
processes. Some of these chemicals
are now being phased out of use
because they are ozone-depletin;
substances, meaning they also cause
harm to the Earth's ozone layer.  As
a result, concentrations of some
ozone-depleting gases have begun
to stabilize or decline (see Figure 4,
left panel). Concentrations  of other
halogenated gases have continued to
rise, however, especially where the
gases  have emerged as substitutes
for ozone-depleting chemicals (see
Figure 4, right panel). Some of these
halogenated gases are considered
major greenhouse gases due to their
very high global warming potentials
and long atmospheric lifetimes (see
table on p. 10).

-------
                     Atmospheric Concentrations of Greenhouse  Gases [continued]
     nitrous oxide, and certain manufactured
     gases known as halogenated gases. This
     indicator shows concentrations of green-
     house gases over thousands of years. Recent
     measurements come from monitoring
     stations around the world, while older mea-
     surements 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.


     Indicator  Notes
     This indicator includes several of the most
     important halogenated gases, but some oth-
     ers are not shown. Many other halogenated
     gases are also greenhouse gases, but Figure
     4 is limited to a set of common examples
     that represent most  of the major types of
     these gases. The indicator also does not
     address certain other pollutants that can af-
     fect 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 measurements made in the North-
     ern Hemisphere only, where concentrations
     are expected to be slightly higher than the
     global average.


     Data Sources
     Global atmospheric concentration measure-
     ments for carbon dioxide (Figure I), methane
     (Figure 2), and nitrous oxide (Figure 3) come
     from a variety of monitoring programs and
     studies published in peer-reviewed literature.
     References for the underlying data are in-
     cluded in the corresponding exhibits. Global
     atmospheric concentration data for selected
     halogenated gases (Figure 4) were compiled
     by the Advanced Global Atmospheric Gases
     Experiment,17 the National Oceanic and
     Atmospheric Administration,18 and two stud-
     ies on nitrogen trifluoride.19-20 An  older figure
     with many of these gases appeared in the In-
     tergovernmental Panel on Climate Change's
     Fourth Assessment Report.21
Figure 3. Global Atmospheric
Concentrations of Nitrous
Oxide Over lime
This figure shows concentrations of
nitrous oxide in the atmosphere from
100,000 years ago through 2011. The
data come from a variety of historical ice
core studies and recent air monitoring
sites around the world. Each line repre-
sents a different data source.
                104,301 BC to 201 IAD
Figure 4. Global Atmospheric
Concentrations of Selected
Halogenated Gases, 1978-2011
This figure shows concentrations of
several halogenated gases (which contain
fluorine, chlorine, or bromine) in the
atmosphere. The data come from moni-
toring sites around the world. Note that
the scale is logarithmic, which means
it increases by powers of 10. This is
because the concentrations of different
halogenated gases can vary by a few
orders of magnitude. The numbers follow-
ing the name of each gas (e.g., HCFC-22)
are used to denote specific types of those
particular gases.
    350

 1.300
 __i

 | 25°
 ec
 g 200
 u

 ? 150
                                      v>
                                         100
                                         50
                                         -120,000      -80,000      -40,000
                                                 Year [negative values = BC1
                                            Data source: Various studies23
            Ozone-depleting substances
   1,000
1
    0.1
      1975
1985     1995
        Year
2005
2015
                                                                                                 Data sources: AGAGE, 2011;23 Arnold et al., 20I2;M
                                                                                                 NOAA, 2011 ;25 Weiss et al., 20082'
18]

-------
              10,000 BC to 201 IAD
               1950 AD to 201 IAD
  350
  300
  250
  200
«
  100
   50

^






1
\






— <•





A
/ v
S











\. , '













   -10,000-8000  -6000  -4000 -2000   0   2000

           Year [negative values = DC]
   350


^SOO

B
e 250
1
g 200
u

I150
•8  '
g ioo


•i  50
     1950  1960  1970  1980 1990 2000 2010  2020

                      Year
             Other halogenated gases
   1,000
       1975    1985    1995    2005    2015

                      Year
                                                        Water Vapor as a Greenhouse Gas
                                                        Water vapor is the most abundant greenhouse gas in the atmosphere.
                                                        Human activities have only a small direct influence on atmospheric
                                                        concentrations of water vapor, primarily through irrigation and defor-
                                                        estation, 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 a warmer
                                                        climate increases evaporation. This creates a positive "feedback loop"
                                                        where warming leads to more warming.

                                                                                                                               [19

-------
                         limate   Forcing
                    This indicator measures the "radiative forcing" or heating effect caused by greenhouse gases in the atmosphere.
    Background
    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.
    If this energy balance is altered, the
    Earth's average temperature will become
    warmer or cooler, leading to a variety of
    other changes in global climate.
    A number of natural and human-influ-
    enced mechanisms can affect the global
    energy balance and force changes in the
    Earth's climate. Changes in greenhouse
    gas concentrations are one such mecha-
    nism. Greenhouse gases in the atmo-
    sphere absorb and re-emit most of the
    energy that radiates upward from the
    Earth's surface, adding the heat back to
    the lower atmosphere and warming the
    Earth's surface. Because elevated con-
    centrations of many of the greenhouse
    gases emitted by human activities can
    remain in the atmosphere for decades,
    centuries, or longer, their associated
    warming effects persist over a long time.
    Factors that influence the Earth's energy
    balance can be quantified in terms  of
    "radiative climate forcing." Positive
    radiative forcing indicates a warming
    influence (for example, by decreasing the
    amount of energy that escapes to space),
    while negative forcing is associated
    with a cooling influence. The balance
    between positive and negative forcing is
    what drives the actual change in surface
    temperature.


    About the Indicator
    This indicator measures the average
    total radiative forcing of 20 greenhouse
    gases, including carbon dioxide, meth-
    ane, 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 pres-
    ent around 1750, before the Industrial
    Revolution began. Because each gas
    has a different capacity to absorb and
    emit heat energy, this indicator con-
    verts the changes in greenhouse gas
20]
Figure 1. Radiative Forcing Caused by Major Greenhouse Gases,
1979-2011
This figure shows the amount of radiative forcing caused by various greenhouse gases, based
on the concentrations present in the Earth's 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.
                                                     Carbon dioxide
                                                     Methane
                                                     Nitrous oxide
                                                     CFC-12
                                                  • CFC-11
                                                  • 15 other gases
      1980
               1985
                                                               2010
     Data source: NOAA, 20122

-------
Key Points
   In 2011, the Annual Greenhouse Gas Index was 1.30, an increase in radiative forcing of
   30 percent since 1990 (see Figure I).

   Of the greenhouse gases shown in Figure I, carbon dioxide accounts for by far the larg-
   est amount of radiative forcing, and its contribution continues to grow at a steady rate.
   By 2011, radiative forcing due to carbon dioxide was 40 percent higher than in 1990.
   Carbon dioxide accounts for approximately 80 percent of the overall increase in radia-
   tive forcing since  1990.

   Although the overall Annual Greenhouse Gas Index continues to rise, the rate of in-
   crease has slowed somewhat overtime. This  change has occurred in large part because
   methane concentrations have remained relatively steady since 1990 (although they have
   recently begun to rise again), and chlorofluorocarbon (CFC) concentrations have been
   declining because the production of these gases has been banned globally due to the
   harm they cause to the ozone layer (see  Figure I).
concentrations into a measure of the to-
tal radiative forcing (energy absorption)
caused by each gas. Radiative forcing is
calculated in watts per square meter,
which represents the rate of energy
transfer over a particular area.
The National Oceanic and Atmospheric
Administration also translates the total
radiative forcing of these measured gases
into an index value called the Annual
Greenhouse Gas Index.  This number
represents the ratio of the radiative
forcing for a particular year compared
with the radiative forcing in 1990, which
is a common baseline year for global
efforts to measure greenhouse gas
concentrations. This indicator does not
consider all substances that contribute
to climate change (see Indicator  Notes).


Indicator Notes
This indicator does not consider certain
other substances that  contribute to
climate forcing. For example, the indica-
tor does not measure  reflective  aerosol
particles in the atmosphere, which can
reduce radiative forcing, nor ground-
level ozone or black carbon (soot),
which can increase it. One gas shown  in
this indicator (methane)  can also have
an indirect influence on  radiative forcing
through its effects on water vapor and
ozone formation; these indirect  effects
are not shown.


Data  Sources
Data for this indicator were provided  by
the National Oceanic and Atmospheric
Administration. This figure and other
information are available at: www.esrl.
noaa.gov/gmd/aggi.

-------
U.S. and Global
Temperature
High and Low
Temperatures
T
U.S. and Global
Precipitation

-------
                                i
        Weather is the state of the atmo-
        sphere at any given time and
        place. Most of the weather that
 affects people, agriculture, and ecosystems
 takes place in the lower layer of the atmo-
 sphere, the troposphere (see diagram of
 the Earth's atmosphere at right). Familiar
 aspects of weather include temperature,
 precipitation, clouds, and wind. 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. Weather can vary widely,
 and extreme events occur naturally, but
 average conditions tend to remain stable
 unless the Earth experiences a force
 that can shift the climate. At various
 times in the Earth's history, the climate
 has changed in response to forces such
 as large volcanic eruptions, changes in
 greenhouse gas concentrations, and shifts
 in the Earth's orbit around the sun.

 What is happening?
 The average temperature at the surface of
 the Earth has been increasing over the past
 century, primarily because human activities
 are adding large quantities of heat-trapping
 greenhouse gases to the atmosphere.
Unusually warm days and nights have also
become more common in some places.
Generally, warmer surface tempera-
tures lead to an increase in evaporation
from the oceans and land, leading to an
increase in globally averaged precipita-
tion. However, while some regions can get
more precipitation, shifting storm patterns
and increased evaporation can cause some
areas to experience more severe droughts
than they have in the past. Scientific
studies also indicate that extreme weather
events such as storms, floods, and hur-
ricanes are likely to become more  intense.
However, because these extremes already
vary naturally, it may be difficult over
short time periods to distinguish whether
changes in their intensity and frequency
can be attributed to larger climate trends
caused by human influences.

Why does it matter?
Climate variations can directly or indi-
rectly affect many  aspects of society—in
both positive and disruptive ways. For
example, warmer average tempera-
tures reduce heating costs and improve
conditions for growing some crops; yet
extreme heat can increase illnesses and
deaths among vulnerable populations
and damage some crops. Precipitation
can replenish water supplies and sup-
port agriculture, but intense storms can
damage property, cause loss of life and
population displacement, and tempo-
rarily disrupt essential services such as
transportation, telecommunications,
energy, and water supplies.
Earth's Atmosphere
                                                                                    Source: NOAA, 2009'
For detailed information about data used in these indicators, see the online technical documentation at: www.epa.gov/climatechange/indicators.
  Heavy
  Precipitation
 Drought

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                   U.S.   and   Global  Temperature
                    This indicator describes trends in average surface temperature for the United States and the world.
24]
    Background
    Temperature is a fundamental measure-
    ment 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 temperature can lead to more
    intense heat waves, which can cause
    illness and death, especially in vulner-
    able 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 green-
    house gases are increasing in the Earth's
    atmosphere (see the Atmospheric
    Concentrations of Greenhouse Gases
    indicator on p.  16). 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 experience more
    warming than others, and some might
    experience cooling.


    About the Indicator
    This indicator examines U.S. and global
    surface temperature patterns from 1901
    to the present. U.S. surface measure-
    ments come from weather stations on
    land, while global surface measurements
    also incorporate observations  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

                     (Continued on page 25)
Figure 1. Temperatures in the Contiguous 48 States, 1901-2011
This figure shows how annual average temperatures 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 (see diagram on p. 23).
"UAH" and "RSS" represent two different methods of analyzing the original satellite measure-
ments. This graph uses the 1901 to 2000 average as a baseline for depicting change. Choosing a
different baseline period would not change the shape of the data over time.
 3
—  1
                              I
                                JL,
                                                 Lowertroposphere
                                  Earth s surface      (measured by satellite)
                                                	UAH   	RSS
 -3
 1900  1910   1920   1930  1940   1950   1960  1970   1980   1990  2000   2010   2020

  Data source: NOAA, 20122              Y63r


Figure 2. Temperatures Worldwide, 1901-2011

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 measure-
ments. Satellite measurements cover the lower troposphere, which is the lowest level of the Earth's at-
mosphere (see diagram on p. 23). "UAH" and "RSS" represent two different methods of analyzing the
original satellite measurements. This graph uses the 1901 to 2000 average as a baseline for depicting
change. Choosing a different baseline period would not change the shape of the data over time.
                                                 Lowertroposphere
                                                (measured by satellite)
                                                   UAH   	RSS
 -3
 1900  1910   1920  1930   1940   1950  1960   1970   1980  1990  2000   2010  2020

  Data source: NOAA, 20I23              Year

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   Figure 3. Rate of Temperature Change in the United States, 1901-2011
   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 contiguous 48 states, 1905 for
   Hawaii, and 1918 for Alaska).
                       Rate of temperature change t°F per century]:
baseline for comparison. Annual anomalies
are calculated for each weather station,
starting from daily and monthly average
temperatures. Anomalies for broader re-
gions have been determined by dividing the
country (or the world) into a grid, averag-
ing the data for all weather stations within
each cell of the grid, and then averaging the
grid cells together (for Figures  I 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.


Indicator Notes
Data from the early 20th century are some-
what less precise than more recent data be-
cause 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 station
moves, development (e.g.,  urbanization)
near the station, changes in instruments and
times of measurement, and other changes.


Data  Sources
The data for this indicator were provided by
the National Oceanic and Atmospheric Ad-
ministration's National Climatic Data Center,
which maintains a large collection of climate
data online at: www.ncdc.noaa.gov/oa/ncdc.
html. Surface temperature anomalies were
calculated based on monthly values from a
network of long-term monitoring stations.
Satellite data were analyzed by two inde-
pendent 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.
                -4-3-2-101234
  Datasource: NOAA.20I2"            Gray interval:-0.1 to 0.1°F
Key Points
   Since 1901, the average surface temperature across the contiguous 48 states has risen at
   an average rate of O.I3°F per decade (I.3°F per century) (see Figure I). Average tempera-
   tures have risen more quickly since the late 1970s (0.31 to 0.4S°F per decade). Seven of the
   top 10 warmest years on record for the contiguous 48 states have occurred since 1990.

   Worldwide, 2001 -2010 was the warmest decade on record since thermometer-based
   observations began. Global average surface temperature has risen at an average rate of
   O.I4°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.

   Some parts of the United States have experienced more warming than others (see Figure
   3). The North, the West, and Alaska have seen temperatures increase the most, while
   some parts of the Southeast have experienced little change. However, not all of these
   regional trends are statistically  significant.

-------


                   High  and   Low   Temperatures
                    This indicator describes trends in unusually hot and cold temperatures across the United States.
26]
Background
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 ill-
ness and death, particularly among older
adults, the very young, and other vulner-
able groups (see the Heat-Related Deaths
indicator on p. 72). People can also die
from exposure to extreme cold (hypo-
thermia). 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
demands 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.5-6  Higher heat index values
(which combine temperature and humid-
ity to describe perceived temperature)
are expected to increase discomfort and
could aggravate  health issues. Converse-
ly, cold spells are expected to decrease.
In most locations, scientists expect daily
minimum temperatures throughout the
year to become warmer at a faster rate
than daily maximum temperatures.7


About the Indicator
Trends in extreme temperatures can be
examined in a variety of ways. This indica-
tor covers several approaches by looking
at prolonged heat wave events as well as
unusually hot or  cold daily highs and lows.
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 I  shows the U.S. Annual Heat
Wave Index, which tracks the occur-
rence of heat wave conditions across the
contiguous 48 states from 1895 to 2011.
While there is no universal definition
of a heat wave, this index defines a heat
wave as a four-day period with an average
temperature that would  only be expected
to occur once every 10 years, based on
the historical record. The index value

                 (Continued on page 2 7)
                                                   Figure 1. U.S. Annual Heat Wave Index, 1895-2011
                                                   This figure shows the annual values of the U.S. Heat Wave Index from 1895 to 2011.
                                                   These data cover the contiguous 48 states.
                                                 1890  1900  1910  1920  1930  1940  1950  1960  1970  1980  1990  2000  2010  2020
                                                   Data source: Kunkel, 20128
                                                                                 Year
                                                   Figure 2. Area of the Contiguous 48 States With Unusually Hot
                                                   Summer Temperatures, 1910-2012
                                                   This graph shows the percentage of the land area of the contiguous 48 states with un-
                                                   usually 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" is based on the long-term average conditions at each location.
                                               50


                                                                                	Hotdaily highs
                                                                                ^— Hotdaily highs (smoothed)
                                                                                    Hot daily lows
                                                                                ^— Hotdaily lows (smoothed)
                                                1910   1920  1930   1940   1950   1960   1970   1980   1990   2000   2010   2020
                                                                               Year
                                                  Data source: NOAA, 2012'

-------
      Figure 3. Area of the Contiguous 48 States With Unusually
      Cold Winter Temperatures, 1911-2012
      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 individual years, while the thick lines show a nine-
      year weighted average. Blue lines represent daily highs, while purple lines represent daily
      lows. The term "unusual" is based on the long-term average conditions at each location.
                                                          Cold daily highs
                                                          Colddaily highs (smoothed)
                                                     	Cold daily lows
                                                     ^— Colddaily lows (smoothed)
    1910   1920    1930   1940

     Data source: NOAA, 2012'"
                                                               2000    2010
                                                                            2020
      Figure 4. Record Daily High and Low Temperatures in the
      Contiguous 48 States, 1950-2009
      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).
    1950s        1960s


Data source: Meehl et al., 2009"
                                   1970s        1980s
                                        Decade
                                                           1990s
                                                                       2000s
Key Points
   Heat waves were frequent and widespread in the 1930s, and these remain the most
   severe heat waves in the U.S. historical record (see Figure I). Poor land use practices
   and many years of intense drought (the "Dust Bowl") contributed to these heat waves
   by depleting soil moisture and reducing the moderating effects of evaporation.12
   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.
   The 20th century saw many winters with widespread patterns  of unusually low tem-
   peratures, including a particularly large spike in the late  1970s (see Figure 3). Since the
   1980s, though, unusually cold winter temperatures have become less common—par-
   ticularly very cold  nights (lows).
   If the climate were completely stable, one might expect to see highs and lows each account-
   ing for about SO 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 4). The most recent decade had twice as many record highs as record lows.
for a given year depends on how often
heat waves occur and how widespread
they are. For example, an index value of
0.2 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.

Figures 2 and 3 show trends in the per-
centage 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 temperatures, which usually oc-
cur during the day, and daily minimum tem-
peratures, which usually occur at night. At
each station, the recorded highs and lows
are compared with the full set of historical
records. After averaging over a particular
month or season of interest, the coldest
10 percent of years are considered "unusu-
ally cold" and the warmest 10 percent are
"unusually hot." For  example, if last year's
summer highs were the 10th 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 2012 for
summer (June through August) and from
1911  to 2012 for winter (December of the
previous year through February).
Many people are familiar with record daily
high and low temperatures, which are fre-
quently mentioned in weather reports. Fig-
ure 4 looks at 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 from 1950 through 2009.


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 funda-
mental 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 I
uses data from the National Weather
Service Cooperative Observer Net-
work; these data are available online at:
www.nws.noaa.gov/os/coop/what-is-
coop.html. Figures 2 and 3 are based on
the U.S. Climate Extremes Index; for
data and a description  of the index, see:
www.ncdc.noaa.gov/extremes/cei.html.
Figure 4 uses National Weather Service
data processed by Meehl et al. (2009).13   |27

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                   U.S.  and   Global   Precipitation
                   This indicator describes trends in average precipitation for the United States and the world.
    Background
    Precipitation can have wide-ranging ef-
    fects on human well-being and ecosys-
    tems. 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 (includ-
    ing crops) can survive in a particular
    place. Changes in precipitation can dis-
    rupt 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. 24), more
    evaporation occurs, which, in turn,
    increases overall precipitation. There-
    fore, a warming climate is expected to
    increase precipitation in many areas.
    However, just as precipitation patterns
    vary across the world, so will the 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 precipi-
    tation will not necessarily increase the
    amount of water available for drinking,
    irrigation, and industry  (see the Drought
    indicator on p. 32).


    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 anoma-
    lies, 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 I and 2)
Figure 1. Precipitation in the Contiguous 48 States, 1901-2011
This figure shows how the total annual amount of precipitation in the contiguous 48 states has
changed since 1901. This graph uses the 1901 to 2000 average as a baseline for depicting
change. Choosing a different baseline period would not change the shape of the data over time.
 CB  0
                                     iLilh
   -20
    1900   1910   1920   1930   1940  1950   1960  1970   1980  1990   2000  2010   2020
     Data source: NOAA, 20I2M
                                 Year
 Figure 2. Precipitation Worldwide, 1901-2011
 This figure shows how the total annual amount of precipitation over land worldwide has changed
 since 1901. This graph uses the 1901 to 2000 average as a baseline for depicting change. Choos-
 ing a different baseline period would not change the shape of the data over time.
15
10
c
0
-10
_1C
-•?n



r






pfli






*v






-I • •
r






L






L^J






k..
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ih^j






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n






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    1900   1910   1920   1930   1940   1950   1960   1970   1980  1990   2000  2010   2020

                                 Year
     Data source: NOAA, 20I215
28]

-------
   Figure 3. Rate of Precipitation Change in the United States, 1901-2011
   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).
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.


Indicator  Notes
Data from the early 20th century are
somewhat 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 station moves, changes
in measurement instruments, and other
changes.


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. Global,  U.S., and regional
precipitation anomalies were calculated
based on monthly values from a network of
long-term  monitoring stations.
                     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%
   Data source: NOAA, 2012"
Key Points
   On average, total annual precipitation has increased over land areas in the United States
   and worldwide (see Figures  I and 2). Since 1901, global precipitation has increased at an
   average rate of 2.3 percent per century, while precipitation in the contiguous 48 states
   has increased at a rate of 5.9 percent per century.

   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 de-
   crease in precipitation (see Figure 3).

-------




                    Heavy   Precipitation
                    This indicator tracks the frequency of heavy precipitation events in the United States.
    Background
    "Heavy precipitation" refers to instances
    during which the amount of precipitation
    experienced in a location substantially
    exceeds what is normal. What consti-
    tutes 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.17 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 necessar-
    ily mean the total amount of precipita-
    tion 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 precipitation in
    a certain period.
    One way to track heavy precipitation
    is by calculating what percentage of a
    particular location's total precipitation
    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
    I  of this indicator looks at the preva-
    lence of extreme single-day precipitation
    events over time.
Figure 1. Extreme One-Day Precipitation Events in the Contiguous 48
States, 1910-2011
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 pre-
cipitation events. The bars represent individual years, while the line is a nine-year weighted average.
    1910   1920   1930   1940   1950    1960   1970   1980   1990   2000   2010   2020
     Data source: NOAA, 20I218
                                   Year
Figure 2. Unusually High Annual Precipitation in the Contiguous 48
States, 1895-2011
This figure shows the percentage of the land area of the contiguous 48 states that experienced
much greater than normal precipitation in any given year, which means it scored 2.0 or above
on the annual Standardized Precipitation Index (SPI). The thicker orange line shows a nine-year
weighted average that smoothes out some of the year-to-year fluctuations.
                                                  1890   1900  1910  1920  1930  1940  1950  1960  1970   1980   1990  2000  2010  2020
                                                                                 Year
30]
                                                   Data source: NOAA, 2012"

-------
Key Points
   In recent years, a larger percentage of precipitation has come in the form of intense
   single-day events. Eight of the top 10 years for extreme one-day precipitation events
   have occurred since  1990 (see Figure  I).
   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  2011, the portion of the country experiencing extreme single-day
   precipitation events  increased at a rate of about half a percentage point per decade (5
   percentage points per century) (see Figure I).
   The percentage of land area experiencing much greater than normal yearly precipita-
   tion totals increased between 1895 and 2011. 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  precipitation 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.20
   Figures  I 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.21 For more information on trends in overall precipitation levels, see the
   U.S. and Global Precipitation indicator on p.  28.
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 precipi-
tation totals with the range of precipitation
totals that one would typically expect at a
specific location, based on historical 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 nor-
mal 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.


Indicator  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 I  is based
on Step #4 of the National Oceanic and
Atmospheric Administration's  U.S. Climate
Extremes Index; for data and a descrip-
tion of the index, see: www.ncdc.noaa.gov/
extremes/cei.html. 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/
climate/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.


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                       This indicator measures drought conditions of U.S. lands.
32]
Background
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 ecologi-
cal system. Drought can also be thought
of as an extended  imbalance between
precipitation and evaporation.
As average temperatures have risen be-
cause of climate change, the Earth's wa-
ter 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 contrib-
utes 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. 28) and
increased risk of flooding (see the Heavy
Precipitation indicator  on p. 30),  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).22

Drought conditions can negatively af-
fect agriculture, water supplies, energy
production, and many other aspects of
society. The impacts vary depending on
the type, location,  intensity, and duration
ofthe 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. Lower stream-
flow and ground water  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.23 Figure I shows annual values
ofthe most widely used index, the Palmer

                  (Continued on page 33)
                                                       Key Points
                                                          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  I).

                                                          Over the period from 2000 through 2011, roughly 30 to 60 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 2007 were relatively high drought years, while 2001, 2005,
                                                          2009, and 2010 were relatively low drought years. In 2011,  the overall extent of  drought
                                                          was relatively low, but the droughts that did occur were more severe than at any other
                                                          time since 2000.

                                                          According to the Drought Monitor, more than 64 percent  of the contiguous U.S. land
                                                          area was covered by moderate or greater drought by the end of September 2012. In
                                                          many portions ofthe country, 2012 has been among the driest years on record.24
                                                     Figure 1. Average Drought Conditions in the Contiguous 48 States, 1895-2011
                                                     This chart shows annual values ofthe Palmer Drought Severity Index, averaged over the entire area ofthe
                                                     contiguous 48 states. Positive values represent wetter-than-average conditions, while negative values repre-
                                                     sent 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.
                                                         1890  1900   1910  1920  1930   1940  1950   1960  1970  1980   1990  2000   2010   2020

                                                          Data source: NOAA, 20I225

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Figure 2. U.S. Lands Under Drought Conditions, 2000-2011
This chart shows the percentage of U.S. lands classified under drought conditions from 2000
through 2011. This figure uses the U.S. Drought Monitor classification system, which is described
in the table below. The data cover all 50 states plus Puerto Rico.
   100
    90
    70

                                  DO  Abnormallydry
                                  D1  Moderate drought
                                  D2  Severe drought
                                  D3  Extreme drought
                                  D4  Exceptional drought
        2000   2001   2002   2003   2004   2005   2006   2007   2008   2009   2010   2011
                                         Year
       Data source: National Drought Mitigation Center, 20I22'
   Categories of Drought Severity
    Category
     DO
Description
Abnormally dry
     Dl
     D2
Moderate drought
Severe drought
                        Extreme drought
Possible Impacts
Going into drought: short-term dry-
ness slowing planting or growth of
crops or pastures. Coming out of
drought: some lingering water deficits;
pastures or crops not fully recovered.
Some damage to crops or pastures;
streams, reservoirs, or wells low; some
water shortages developing or immi-
nent; voluntary water use restrictions
requested.
Crop or pasture losses likely; water
shortages common; water restrictions
imposed.
                     Major crop/pasture losses; widespread
                     water shortages or restrictions
                     Exceptional and widespread crop/
                     pasture losses; shortages of water in
                     reservoirs, streams, and wells, creating
                     water emergencies.
   Experts update the U.S. Drought Monitor weekly and produce maps that illustrate
   current conditions as well as short- and long-term trends. Major participants include
   the National Oceanic and Atmospheric Administration, the U.S. Department of Agri-
   culture, and the National  Drought Mitigation Center. For a map of current drought
   conditions, visit the Drought Monitor website at: http://droughtmonitor.unl.edu.
Drought Severity Index, which is calculated
from precipitation and temperature measure-
ments at weather stations. An index value of
zero represents average moisture conditions
for a given location, based on many years of
observations. A positive value means condi-
tions are wetter than average, while a nega-
tive value is drier than average. Index values
from locations across the contiguous 48
states have been averaged together to pro-
duce the national values shown in Figure I.
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
content, ground water levels, reservoir
storage, pasture/range conditions, and other
impacts. The Drought Monitor uses codes
from DO to D4 (see table  below) to classify
drought severity. This part of the indicator
covers all SO states and Puerto Rico.


Indicator Notes
Because this indicator focuses on national
trends, it does not  show how drought con-
ditions vary by region. For example, even
if half of the country suffered from severe
drought, Figure I could show an average
index value close to zero if the rest of the
country was wetter than average. Thus, Fig-
ure I might understate the degree to which
droughts are becoming more severe in  some
areas while other places receive more rain
as a result of climate change.
The U.S. Drought Monitor  (Figure 2) offers a
closer look at the percentage of the country
that is affected by drought.  However, this
index is relatively new and thus too short-lived
to be used for assessing long-term climate
trends. With several decades of data collec-
tion, future versions of this indicator should
be able to paint a more complete picture of
trends over time.
Overall, this indicator gives a broad over-
view of drought conditions in the United
States. It is not intended to replace local or
state information that might describe condi-
tions more precisely for a particular region.


Data  Sources
Data for Figure I were obtained from the
National Oceanic and Atmospheric Admin-
istration'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 Mitigation Center.
Historical  data in table form are available at:
http://droughtmonitor.unl.edu/archive.html.
Maps and current drought information  can
be found on the main Drought Monitor site
at: http://droughtmonitor.unl.edu.
                                                                                                                                        |33

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                   Tropical   Cyclone   Activity
                    This indicator examines the frequency, intensity, and duration of hurricanes and other tropical storms in the
                    Atlantic Ocean, Caribbean, and Gulf of Mexico.
34]
    Background
    Hurricanes, tropical storms, and other in-
    tense rotating storms fall into a general cat-
    egory called cyclones. There are two main
    types of cyclones: tropical and extratropical
    (those that form outside the tropics). Tropi-
    cal 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 cyclones 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 tempera-
    tures, a key factor that influences cyclone
    formation and behavior. According to the
    U.S. Global Change Research Program, it is
    very likely that increased levels of green-
    house gases have contributed to an increase
    in sea surface temperatures in areas where
    hurricanes form.27 The U.S. Global Change
    Research Program and the Intergovernmen-
    tal Panel on Climate Change project that
    tropical cyclones will likely become more
    intense over the 21st century, with higher
    wind speeds and heavier rains.28-29


    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 Atmo-
    spheric 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

                        (Continued on page 35)
Figure 1. Number of Hurricanes in the North Atlantic, 1878-2011
This graph shows the number of hurricanes that formed in the North Atlantic Ocean each year
from 1878 to 2011, 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 (2007-2011) is plotted at 2009.
   12
   10
• Total hurricanes (adjusted)
' Total hurricanes (unadjusted)
 Hurricanes reaching the United States
        1880      1900      1920


     Data source: Knutson, 20I230
    1940
    Year
                                           I960
                                                    1980
                                                             2000
Figure 2. North Atlantic Tropical Cyclone Activity According to the
Accumulated Cyclone Energy Index, 1950-2011
This figure shows total annual Accumulated Cyclone Energy (ACE) Index values from  1950
through 2011. 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.
                                                                  Above
                                                                  normal
                                                                  Near
                                                                  normal
                                                                  Below
                                                                  normal
     1950      1960      1970


      Data source: NOAA, 201231
 1980
   Year
                                        1990
                                                 2000
                                                          2010

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Key  Points
   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 hurricanes
   (particularly after being adjusted for improvements in observation methods) and the num-
   ber reaching the United States do not indicate a clear overall trend since 1878 (see Figure I).

   According to the total annual ACE Index, cyclone intensity has risen noticeably over the
   past 20 years, and six of the 10 most active years have occurred since the mid-1990s
   (see Figure 2). Relatively high levels of cyclone activity were also seen during the  1950s
   and  1960s.

   The PDI (see Figure 3) shows fluctuating cyclone intensity for most of the mid- to late
   20th century, followed by a noticeable  increase since  1995  (similar to the ACE Index).
   These trends are associated with variations in sea surface temperature in the tropical
   Atlantic (see Figure 2).

   Despite the apparent increases in tropical  cyclone activity in Figures 2 and 3, changes in
   observation  methods over time make  it difficult to know for certain whether tropical
   storm activity has actually shown a long-term increase.32
Figure 3. North Atlantic Tropical Cyclone Activity According to the
Power Dissipation Index, 1949-2011
This figure presents annual values of the Power Dissipation Index (PDI). 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 (2007-2011) is plotted at 2009.
                                                                              83.2
                    Sea surface >• \  '
                    temperature
      1950        1960        1970


     Data source: Emanuel. 2012"
1980
Year
                                                 1990
                                                            2000
                                                                      2010
                                                                              81.2
missed before the start of aircraft and satel-
lite observation, so scientists have used other
evidence to estimate the actual number of hur-
ricanes 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 dura-
tion of tropical cyclones based on wind speed
measurements.

Every cyclone has an ACE Index value, which is
a number based on the maximum wind speed
measured 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 Administra-
tion 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, dura-
tion, and frequency. For this indicator, the index
has been converted to  a scale where  100 equals
the median value (the midpoint) over a base pe-
riod 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

Overtime, data collection methods have changed
as technology has improved. For example, wind
speed collection methods have evolved sub-
stantially over the past  60 years, while aircraft
reconnaissance began in 1944 and satellite
tracking around  1966. Figure I shows  how older
hurricane counts have been  adjusted to attempt
to account for the  lack  of aircraft and  satellite
observations. Changes  in data gathering tech-
nologies could substantially  influence the overall
patterns in Figures 2  and 3. The effects of these
changes on data consistency over the  life of the
indicator would benefit from additional research.


Data Sources
Hurricane counts are reported on several Na-
tional Oceanic and Atmospheric Administration
websites and were compiled using methods de-
scribed in Knutson et al. (20IO).34The 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/
background_information.shtml. Values for the
PDI have been calculated by Kerry Emanuel at
the Massachusetts  Institute of Technology.  Both
indices are based on wind speed measurements
compiled by the National Oceanic and Atmo-
spheric Administration,                       i.

-------
                                                             •
INDICATORS IN THIS CHAPTER
            Ocean Heat
 ea Surface
Temperature

-------
                            The oceans and at-
                            mosphere interact
                            constantly—both
                        physically and chemi-
                        cally—exchanging energy,
                        water, gases, and particles.
                        This relationship influ-
                        ences the Earth's climate
                        on regional and global
                        scales. It also affects the
                        state of the oceans.

                        Covering about
                        70 percent of the
                        Earth's surface, the
                        oceans store vast
                        amounts of energy
                        absorbed from the
                        sun and move this
                        energy around the
                        globe through cur-
                        rents. The  oceans
                        are also a key com-
                        ponent of the Earth's
                        carbon cycle. Oceans
                        store a large amount of
                        carbon, either in dissolved
                        form or within plants and
                        animals (living or dead).

                        What is happening?
                        As greenhouse gases trap
                        more energy from the sun,
                        the oceans are absorbing
                        more heat, resulting in
                        an increase in sea surface
                        temperatures and ris-
                        ing sea level. Although
                        the oceans help reduce
climate change by storing
one-fifth to one-third of
the carbon dioxide that
human activities emit into
the atmosphere,1 increas-
ing levels of dissolved
carbon are changing the
chemistry of seawater and
making it more acidic.
          r ~
Why does it matter?
Changes in ocean tempera-
tures and currents brought
about by climate change
will lead to alterations in
climate patterns around the
world. For example, warm-
er waters may promote the
development of stronger
storms in the tropics, which
can cause property damage
and loss of life. Other im-
pacts come from increased
ocean acidity, which
reduces the availability of
some types of minerals,
thus making it harder for
certain organisms, such as
corals and shellfish, to build
their skeletons and shells.
These effects, in turn, could
substantially alter the bio-
diversity and productivity
of ocean ecosystems.

    Changes in ocean
    systems generally oc-
    cur over much longer
    time periods than
    in the atmosphere,
    where storms can
    form and dissipate in
    a single day. Interac-
    tions between the
    oceans and atmo-
    sphere occur slowly
    over many years, and
    so does the move-
ment of water within the
oceans, including the
mixing of deep and shal-
low waters. Thus, trends
can persist for decades,
centuries, or longer. For
this reason, even if green-
house gas emissions are
stabilized tomorrow, it will
take many more years—
decades to centuries—for
the oceans to adjust to
changes in the atmosphere
and the climate that have
already occurred.
For detailed information about data used in these indicators, see the online technical documentation at: www.epa.gov/climatechange/indicators.

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     Background
     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 tempera-
     ture 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. 40)—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,
     and the oceans are currently absorbing
     80 to 90 percent of this  extra heat—
     much more than the amount absorbed
     by the atmosphere.2 If not for the large
     heat-storage capacity provided by the
     oceans, the atmosphere  would grow
     warmer more rapidly.3 Increased heat
     absorption also changes  ocean currents
     because many currents are driven by
     differences in temperature, which causes
     differences in density. These currents
     influence climate patterns and sustain
     ecosystems—for example, coastal
     fishing grounds depend on upwelling cur-
     rents to bring nutrients to the surface.
     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. 42).
Figure 1. Ocean Heat Content, 1955-2011
This figure shows changes in ocean heat content between 1955 and 2011. Ocean heat con-
tent 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 calculated independently by three agencies: the
National Oceanic and Atmospheric Administration, Australia's Commonwealth Scientific and
Industrial Research Organisation (CSIRO), and Japan's Agency for Marine-Earth Science and
Technology (JAMSTEC).
     15
38]
       Data sources: CSIRO, 2012;" JAMSTEC, 20l2;b NOAA, 2012

-------
                                                                                       About the  Indicator
Key Points
   In three different data analyses, the long-term trend shows that the oceans have become
   warmer since 1955 (see Figure I).

   Although concentrations of greenhouse gases have risen at a relatively steady rate over
   the past few decades (see the Atmospheric Concentrations of Greenhouse Gases indica-
   tor on p. 16), the rate of change  in ocean heat content can vary from year to year (see
   Figure I). Year-to-year changes are influenced by events such as volcanic eruptions and
   recurring ocean-atmosphere patterns such as El Nino.
This indicator shows trends in global ocean
heat content from 1955 to 2011. 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 a unit 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
collection programs. Figure I shows three
independent interpretations of essentially
the same underlying data.


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 dif-
ferent versions 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 Admin-
istration and other organizations around the
world. The data were analyzed independent-
ly by researchers at the National Oceanic
and Atmospheric Administration, Australia's
Commonwealth Scientific and Industrial
Research Organisation, and Japan's Agency
for Marine-Earth Science and Technology.
                                                                                                                               |39

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                  Sea  Surface  Temperature
                   This indicator describes global trends in sea surface temperature.
    Background
    Sea surface temperature—the tem-
    perature of the water at the ocean sur-
    face—is an important physical attribute
    of the world's oceans. The surface tem-
    perature 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 and animals are present in a
    location, alter migration and breeding
    patterns, threaten sensitive ocean life
    such as corals, and change the fre-
    quency and intensity of harmful algal
    blooms.7 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 can  lead to declines
    in fish populations, which in turn could
    affect people who depend on fishing for
    food or jobs.8
    Because the oceans continuously inter-
    act with the atmosphere, sea surface
    temperature can also have profound
    effects on global climate. Based on
    increases in sea surface temperature,
    the amount of atmospheric water vapor
    over the oceans is estimated to have
    increased by about 5 percent during the
    20th century.9 This water vapor feeds
    weather systems that produce precipi-
    tation, increasing the risk of heavy rain
    and snow (see the Heavy Precipitation
    and Tropical Cyclone Intensity indica-
    tors on pp. 30 and 34, respectively).
    Changes in sea surface temperature
    can also shift storm tracks, potentially
    contributing to droughts in some areas.
Figure 1. Average Global Sea Surface Temperature, 1880-2011
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 preci-
sion of the methods used.

   2.0
   -1.5
   -2.0
    1880 1890  1900  1910  1920 1930 1940  1950  1960  1970 1980 1990  2000  2010  2020
                                 Year
      Data source: NOAA, 2012'"
40]

-------
Key Points
   Sea surface temperature increased over the 20th century and continues to rise. From 1901
   through 2011, temperatures rose at an average rate of O.I3°F per decade (see Figure I).

   Sea surface temperatures have been higher during the past three decades than at any
   other time since reliable observations began in I860 (see Figure I).

   Increases in sea surface temperature have largely occurred over two key periods: be-
   tween 1910 and 1940, and from 1970 to the present. Sea surface temperatures appear to
   have  cooled between 1880 and 1910 (see Figure I).
   Example: Average Sea Surface Temperature in 2011
   This map shows annual average sea surface temperatures around the world during the year
   2011. It is based on a combination of direct measurements and satellite measurements.
                          Average sea surface temperature t°F):
                 40
   Data source: UK Met Office, 2012"
                              50
                                           60
                                                       70
About the Indicator
This indicator tracks average global sea
surface temperature from I860 through
2011 using data compiled by the National
Oceanic and Atmospheric Administration.
Techniques for measuring sea surface tem-
perature have evolved since the 1800s. For
instance, the earliest data were collected by
inserting a thermometer into a water sam-
ple 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 for this indicator 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.


Indicator Notes
Because this indicator tracks sea surface
temperature at a global scale, the data
shown in Figure I  do not necessarily reflect
local or  regional trends.
Due to denser sampling and improve-
ments 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 I.


Data Sources
Data for this indicator were provided by the
National Oceanic and Atmospheric Admin-
istration'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.

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                      Sea   Level
                       This indicator describes how sea level has changed over time. The indicator describes two types of sea level
                       changes: absolute and relative.
   Background
   As the temperature of the Earth changes,
   so does sea level. Temperature and sea
   level are linked for two main reasons:
   I.  Changes in the volume of water and
      ice on land (namely glaciers and ice
      sheets) can increase or decrease the
      volume of water in the ocean  (seethe
      Glaciers indicator on p. SO).
   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. 38).
   Changing sea levels can affect human
   activities in  coastal areas. For example,
   rising sea levels can lead to increased
   coastal flooding and erosion, which is
   a particular concern in low-lying areas.
   Higher sea level also makes coastal
   infrastructure more vulnerable to damage
   from storms. Sea level rise can  alter eco-
   systems, transforming marshes and other
   wetlands into open water and freshwater
   systems into salt water systems.
   The sea level changes that affect coastal
   systems involve more  than just  expand-
   ing oceans, however, because the  Earth's
   continents can also rise and fall  relative
   to the oceans. Land can rise through pro-
   cesses such as sediment accumulation  (the
   process that built the  Mississippi Delta)
   and geological uplift (for example, as gla-
   ciers 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 engi-
   neering 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  contrast, 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, 1880-2011
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 shaded band shows the likely range of values,  based on
the number of measurements collected and the precision of the methods  used.
    12
    10
.M  8
 CB  4
 Trend based on tide gauges
> Satellite measurements
    -2
     1880  1890  1900  1910  1920  1930  1940  1950  1960  1970  1980  1990  2000  2010  2020
                                        Year
     Data sources: CSIRO, 20I2;'2NOAA, 20I2'3
  Key Points
     After a period of approximately 2,000 years of little change (not shown here), global aver-
     age sea level rose throughout the 20th century, and the rate of change has accelerated in re-
     cent years.14 When averaged over all the world's oceans, absolute sea level increased at an
     average rate of 0.07 inches per year from 1880 to 2011 (see Figure I). From 1993 to 2011,
     however, average sea level rose at a rate of O.I I to 0.13 inches per year—roughly twice as
     fast as the long-term trend.
     Relative sea level rose along much of the U.S. coastline between I960 and 2011, particu-
     larly 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.
     While absolute sea level has increased steadily overall, particularly in recent decades, re-
     gional trends vary, and absolute sea level has decreased in some places.15 Relative sea level
     also has not risen uniformly because of regional and local changes in land movement and
     long-term changes  in coastal circulation patterns.
42]

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Figure 2. Relative Sea Level Change Along U.S. Coasts, 1960-2011
This map shows cumulative changes in relative sea level from I960 to 2011 at tide gauge stations
along U.S. coasts. Relative sea level reflects changes in sea level as well as land elevation.

iih"S
..tf
f
t
                                  II1
           Alaska
  *    I

                                   Hawaii and
                                 Pacific Islands
                          Relative sea level change [inches]:
£-8
1
-7.99
to -6
I
-5.99
to -4
I
-3.99
to -2
1
-1.99
toO
I
t
0.01
to 2
t
2.01
to 4
t
4.01
to 6
t
6.01
to 8
1
>8
Data source: NOAA, 2012"
About the  Indicator
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 instru-
ments measure absolute sea level change
over nearly the entire ocean surface. Many
tide gauges have collected data for more
than 100 years, while satellites have col-
lected data since the early  1990s.
Figure I  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
calibration with recent satellite data.  Figure
2 shows trends at a  more local scale, high-
lighting the 1960 to  2011 change in relative
sea level at 68 tide gauges along the Atlantic,
Pacific, and Gulf coasts of the United States.


Indicator Notes
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  different  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 temperatures, salt content, wind
patterns, atmospheric pressure (and thus
storm tracks), and currents. Obtaining a
reliable trend can require many years of
data, which is why the satellite record in
Figure I  has been supplemented with a
longer-term reconstruction based on tide
gauge measurements.


Data  Sources
Absolute sea level trends were provided  by
Australia's  Commonwealth Scientific and
Industrial Research  Organisation and the
National Oceanic and Atmospheric Admin-
istration. 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  interac-
tive online  map (http://tidesandcurrents.
noaa.gov/sltrends/sltrends.shtml) with links
to detailed data for  each tide gauge.
                                                                                                                                 |43

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                       This indicator shows changes in the chemistry of the ocean, which relate to the amount of carbon dissolved in the water.
     Background
     The ocean plays an important role in
     regulating the amount of carbon diox-
     ide in the atmosphere. As atmospheric
     concentrations of carbon dioxide rise
     (see the Atmospheric Concentrations of
     Greenhouse Gases indicator on p. 16),
     the ocean absorbs more carbon dioxide.
     Because of the  slow mixing time between
     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  approxi-
     mately 40 percent of the carbon dioxide
     produced by human activities.17

     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 marine life. Carbon
     dioxide reacts with sea water to produce
     carbonic acid. The resulting increase in acid-
     ity (measured by lower pH values) reduces
     the availability of minerals such as aragonite,
     which is a form of calcium carbonate that
     corals, some types of plankton, and other
     creatures rely on to produce their hard skel-
     etons and shells. Declining pH and reduced
     availability of minerals 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.18

     While changes in ocean pH and mineral
     availability caused by the uptake of atmo-
     spheric carbon  dioxide generally occur over
     many decades, these properties can fluctuate
     over shorter periods, especially in coastal
     and surface waters. For example, increased
     photosynthesis  and respiration 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 I shows pH values and levels of dis-
     solved carbon dioxide at three locations that
     have collected measurements consistently
     over the last few decades. These data have
     been  either measured directly or calculated
     from  related measurements such as dissolved
Key Points
   Measurements made over the last few decades have demonstrated that ocean carbon diox-
   ide 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 I).

   Historical modeling suggests that since the 1880s, increased carbon dioxide has led to
   lower aragonite saturation levels (less availability of minerals) in the oceans around the
   world (see Figure 2).

   The largest decreases in aragonite saturation have occurred in tropical waters (see Figure
   2). However, decreases in cold areas may be of greater concern because colder waters typi-
   cally have lower aragonite levels to begin with."
    Figure 1. Ocean Carbon Dioxide Levels and Acidity 1983-2011
    This figure shows the relationship between changes in ocean carbon dioxide levels (measured in
    the left column as a partial pressure—a common way of measuring the amount of a gas) and
    acidity (measured as pH in the right column). The data come from two observation stations in
    the North Atlantic Ocean (Canary Islands and Bermuda) and one in the Pacific (Hawaii). The
    up-and-down pattern shows the influence of seasonal variations.
     1980 1985  1990 1995 2000 2005 2010 2015
                                             7.95
                                               1980  1985 1990  1995 2000 2005 2010 2015
   500

   450

   400

   350

   300
e

.2
CB

O9
H 250
inn
                          Canary Islands
.5   1980 1985  1990 1995 2000 2005 2010 2015
                                             7.95
                                               1980  1985 1990  1995 2000 2005 2010 2015
   250
                                             7.95
     1980 1985  1990 1995 2000 2005 2010 2015
                                               1980  1985 1990  1995 2000 2005 2010 2015
                                         Year
     Data sources: Bates et al., 20l2;2°Gonzalez-Davila, 20I2;21 University of Hawaii, 20I222
44]

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  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.
        Increasing
         Acidity
         Neutral
        Increasing
        Alkalinity
Battery Acid
Lemon Juice
Vinegar
                                                         Acid Rain
                            Milk
             Adult Fish Die
          Fish Reproduction Affected

          Normal Range of Precipitation pH

         Normal Range of Stream pH
                            Baking Soda
                            Sea Water
Milk of Magnesia

Ammonia
Lye
  Source: Environment Canada, 200823


  Figure 2. Changes in Aragonite Saturation of the World's Oceans, 1880-2012
  This map shows changes in the amount ofaragonite dissolved in ocean surface waters between the
  1880s and the most recent decade (2003-2012). Aragonite is a form of calcium carbonate that
  many marine animals use to build their skeletons and shells. Aragonite saturation is a ratio that
  compares the amount ofaragonite that is actually present with the total amount ofaragonite that
  the water could hold if it were completely saturated. The more negative the change in aragonite
  saturation, the larger the  decrease in aragonite available in the water, and the harder it is for
  marine creatures to produce their skeletons and shells.
                   Change in aragonite saturation at the ocean surface (Q,r):
-0.8         -0.7        -0.6

 Data source: Feely et al., 2009M
                                -0.5
                                          -0.4
                                                     -0.3
                                                               -0.2
                                                                          -0.1
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
overtime in the amount ofaragonite
dissolved in ocean water, which is called
aragonite saturation. This map was created
by comparing average conditions during the
1880s with  average conditions during the
most recent 10 years (2003-2012). Aragonite
saturation has only been measured at se-
lected locations during the last few decades,
but it can be calculated  reliably for different
times and locations based on the relation-
ships scientists have observed among arago-
nite 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
indirectly based on actual measurements.


Indicator  Notes

This indicator focuses on surface waters,
which absorb carbon dioxide from the at-
mosphere within a few  months.25 It can take
much longer for changes in pH and mineral
saturation to  spread to deeper waters, so
the full effect of increased atmospheric car-
bon dioxide concentrations on ocean acidity
may not be seen for many decades, if not
centuries. Studies suggest that the impacts of
ocean acidification may be greater at depth,
because the availability  of minerals like ara-
gonite is naturally lower in  deeper waters.26

Ocean chemistry is not uniform around the
world, so local conditions can cause pH or
aragonite saturation measurements to differ
from the global average. For example,  car-
bon dioxide dissolves more readily in cold
water than  in warm water, so colder regions
could experience greater impacts  from acid-
ity than warmer regions.


Data Sources
Data for Figure I came from three studies:
the Bermuda Atlantic Time-Series Study,
the European  Station for Time-Series in
the Ocean (Canary Islands), and the Hawaii
Ocean Time-Series. Bermuda data were
analyzed by Bates et al. (2012)27 and are
available at: http://bats.bios.edu. Canary
Islands data were analyzed by Gonzalez-
Davila et al. (20IO)28 and are available at:
www.eurosites.info/estoc/data.php. Hawaii
data were analyzed by Dore et al.  (2009)29
and are available at: http://hahana.soest.
hawaii.edu/hot/products/products.html.

The map in Figure 2 was created by the Na-
tional Oceanic and Atmospheric Adminis-
tration and the Woods Hole Oceanographic
Institution using Community Earth System
Model data. Related information can be
found at: http://sos.noaa.gov/Datasets/list.
php?category=Ocean.

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INDICATORS IN THIS CHAPTER
                                    Glaciers
Lake Ice

-------
    Tie Earth's surface contains many
    orms of snow and ice, including
    ;ea ice, lake and river ice, snow
cover, glaciers, ice caps and sheets, and
frozen ground. Together, these fea-
tures are sometimes referred to as the
"cryosphere," a term for all parts of the
Earth where water exists in solid form.

Snow and ice are an important part
of the global climate system. Because
snow and ice are highly reflective,
much of the sunlight that hits these
surfaces is reflected back into space
instead of warming the Earth. The
presence  or absence of snow and
ice affects heating and cooling over
the Earth's surface, influencing the
planet's energy balance.

Climate change can dramatically alter
the Earth's snow- and ice-covered
areas. Unlike other substances found
on the Earth, snow and ice exist at
temperatures close to their melting
point and can thus change between
solid and liquid states in response to
  Important Concepts
  in This Chapter
  Snowfall refers to the amount of snow
  that falls in a particular location.
  Snow cover refers to the area of land that
  is covered by snow at any given time.
  Snowpack refers to the thickness of
  snow that accumulates on the ground.
relatively minor changes in tempera-
ture. As a result, prolonged warming
or cooling trends can result in signifi-
cant changes across the landscape as
snow and ice masses shrink or grow
over time.

What is  happening?
Some regions that usually receive
snow are receiving less snowfall and
do not have as much snow on the
ground. Glaciers in the United States
and around the world have generally
shrunk, and the rate at which they are
melting appears to have accelerated
over the  last decade. Additionally, the
amount of ice in the Arctic Ocean has
decreased, and many lakes are freezing
later in the fall and melting earlier in
the spring.

Why does it matter?
Reduced snowfall and less snow cover
on the ground could diminish the
beneficial insulating effects of snow for
vegetation and wildlife, while also af-
fecting water supplies, transportation,
cultural practices, travel, and recre-
ation for millions of people. For com-
munities in Arctic regions, reduced
sea ice could increase  coastal erosion
and exposure to storms, threatening
homes and property, while thawing
ground could damage roads and build-
ings and  accelerate erosion.
Such changing climate conditions can
have worldwide implications because
                                                                                    The Cryosphere
                                                                                                       Snow
                                                                                                       Sea ice
                                                                                                     • H Ice sheets
                                                                                                       Glaciers and
                                                                                                       ice caps
                                                                                                       Permafrost
                                                                                                       [continuous!
Source: UNEP, 2007'

snow and ice influence air tempera-
tures, sea level, ocean currents, and
storm patterns. For example, melting
ice sheets on Greenland and Antarc-
tica add fresh water to the ocean, in-
creasing sea level and possibly chang-
ing ocean circulation that is driven by
differences in temperature and salin-
ity. Because of their light color, snow
and ice also reflect more sunlight than
open water or bare ground, so a reduc-
tion in snow cover and ice causes the
Earth's surface to absorb more energy
from the sun.

Thawing of frozen ground and reduced
sea ice in the Arctic could affect
biodiversity on local and global scales,
leading to harmful effects not only
on polar bears and seals, but also on
migratory species that breed or feed
in these areas. These changes could
affect people by compromising their
livelihoods and traditional means of
gathering food, particularly Arctic
indigenous populations. Conversely,
reduced snow and ice could present
commercial opportunities for others,
including ice-free shipping lanes and
increased access to natural resources.
          For detailed information about data used in these indicators, see the online technical documentation at: www.epa.gov/climatechange/indicators.

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48]
     Background
     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 tempera-
     tures 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  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 en-
     ergy balance and keeping polar regions
     cool. (For more information on the ef-
     fects of surface color on reflecting sun-
     light, see the Snow Cover indicator on
     p. 56.) Sea ice also keeps the air cool by
     forming a barrier between the 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 reduced, 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
     indicator of Arctic conditions because
     older ice is generally thicker and stron-
     ger 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 bears and walruses rely
     on the presence of sea ice to preserve
     their hunting, breeding, and migrating
     habits. These animals face the threat of
     declining birth rates and restricted ac-
     cess 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, Inupiat, 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.
Dwindling Arctic Sea Ice
      ^H
       September
Source: NASA, 20I22
Key  Points
   September 2012 had the lowest sea ice extent on record, 49 percent below the 1979-
   2000 average for that month.

   The September 2012 record low sea ice extent was 1.3 million square miles (an area five
   times the size of Texas) less than the historical  1979-2000 average (see Figure I).

   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 larg-
   est decreases have occurred in the summer and fall.3-4

   Evidence of the age of Arctic sea ice suggests an overall loss of multi-year ice. The pro-
   portion of sea ice five years or older has declined dramatically over the recorded time
   period, from more than 30 percent of September ice in the 1980s to 4 percent in 2012.
   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.

-------
Figure 1. September Monthly Average Arctic Sea Ice Extent, 1979-2012
This figure shows Arctic sea ice extent from 1979 through 2012 using data from September of
each year, which is when the minimum extent typically occurs.
     1975      1980      1985

      Data source: NSIDC.20I25
                                1990
                                         1995
                                                  2000
                                                           2005
                                                                    2010
                                                                              2015
Figure 2. Age of Arctic Sea Ice at Minimum September Week, 1983-2012"
This figure shows the distribution of Arctic sea ice extent by age group during the peak melting
week in September of each year.
         1985
                      1990
                                   1995
                                               2000
                                                           2005
                                                                        2010
                                       Year
      *The total extent in Figure 2 differs from Figure I because Figure I shows a monthly average, while
      Figure 2 shows conditions during a single week.

      Data source: NSIDC, 20I26
About the  Indicator
Figure I  presents trends in Arctic sea ice
extent from 1979, when extensive measure-
ments started, to 2012. Sea ice extent is
defined as the area of ocean where at least
IS percent of the surface is frozen. This
threshold was chosen because scientists
have found that it gives the best approxima-
tion of the edge of the ice. Data are collect-
ed 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 distri-
bution until the ice  has been tracked for at
least five years.


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 an-
nual and decadal variability, also affect the
extent of sea ice.  Determining 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 Center.
Data for Figure I  are also available online at:
http://nsidc.org/data/seaice_index/archives/
index.html, while  Figure 2 is based on an
analysis by the University of Colorado and
a graph published at: http://nsidc.org/arctic-
seaicenews/2012/10/poles-apart-a-record-
breaking-summer-and-winter. The National
Snow and Ice Data Center produces a
variety of reports and a seasonal newsletter
analyzing Arctic sea ice data.
                                                                                                                               |49

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                        his indicator examines the balance between snow accumulation and melting in glaciers, and it describes how
                       glaciers around the world have changed overtime.
50]
Background
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 Cas-
cades, and throughout Alaska. A glacier
flows naturally like a river, only much
more slowly. At  higher elevations, gla-
ciers 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 accumula-
tion, a glacier is in equilibrium and its
mass will neither increase nor decrease.
In many areas, glaciers provide communi-
ties and ecosystems with a reliable source
of streamflow and drinking water, particu-
larly 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. 42). The
same kinds of changes occur on a much
larger scale within the giant ice sheets that
cover Greenland and Antarctica, poten-
tially leading to even bigger implications for
sea level. Small glaciers tend to respond
more quickly to climate change than the
giant ice sheets, however, and they have
added more water to the oceans than the
ice sheets have in recent decades.7


About the  Indicator
This  indicator is  based on  long-term
monitoring data collected  at selected gla-
ciers around the world. Scientists collect
detailed measurements to determine gla-
cier mass balance, which is the net gain
or loss of snow and ice over the course
of the year. A negative mass balance indi-
cates that a glacier has lost ice or snow.
The cumulative mass balance overtime
reveals long-term trends. For example, if
cumulative mass balance becomes  more
negative over time, it means glaciers are
losing mass more quickly than they can
accumulate new snow.

                   (Continued on page 51)
                                                       Photographs of McCall Glacier. Alaska, 1958 and 2003
                                                       Sources: Post, I958;8 Nolan, 2003'
                                                      Key  Points
On average, glaciers worldwide have been losing mass since at least the 1970s (see
Figure I), which in turn has contributed to observed changes in sea level (see the Sea
Level indicator on p. 42). Measurements from a smaller number of glaciers suggest that
they have been shrinking since the  1940s. The rate at which glaciers are losing mass ap-
pears to have accelerated over roughly the last  decade.

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 measurements indicate a
loss of glacier mass over time.

Trends for the three benchmark glaciers are consistent with the retreat of glaciers
observed throughout the western United States, Alaska, and other parts of the world.10
Observations 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 indica-
tor on p. 24).

-------
Figure 1. Average Cumulative Mass Balance of "Reference" Glaciers

Worldwide, 1945-2010
This 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 in later years indicate a net loss of ice and snow compared with the base year of 1945.
For consistency, measurements are in meters of water equivalent, which represent changes in the aver-
age 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 2010, hence the smaller number of sites.
                1950
                           1960
                                     1970
                                               1980
                                                         1990
                                                                   2000
                                                                             2010
  E
     40
     20
  jg  0

      1940       1950       1960       1970       1980       1990       2000       2010
        Data source: WGMS, 2011," 20I2'2
                                         Year
 Figure 2. Cumulative Mass Balance of Three U.S. Glaciers,  1958-2010
 This figure shows the cumulative mass balance of the three U.S. Geological Survey "benchmark"
 glaciers since measurements began in the 1950s or 1960s. For each glacier, the mass balance
 is set at zero for the base year of 1965. Negative values in later years 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. The dashed line in the
 lower right corner represents a preliminary number for 2010.
              1960          1970

        Data source: USGS, 201213
1980
  Year
                                                    1990
                                                                 2000
                                                                              2010
                                                                                           Glaciers Shown in Figure 2
                                                                                                n I/
                                                                                                    Gulkana Glacier
                                                                                                    o
                                                                                                o
                                                                                                Wolverine Glacier
                   South Cascade Glacier
                   o
                   WA


Figure I  shows trends in mass balance for a
set of 37 reference glaciers around the world
that have been measured consistently since
the 1970s, including a few that have been
measured since the 1940s. Data from these
reference glaciers have been averaged to-
gether to depict changes over time. Figure 2
shows trends for three "benchmark" glaciers:
South Cascade Glacier in Washington state,
Wolverine Glacier near Alaska's south-
ern coast, and Gulkana Glacier in Alaska's
interior. These three glaciers were chosen
because they have been studied extensively
by the U.S. Geological Survey for many years
and because they are thought to be represen-
tative of other glaciers nearby.

This indicator describes the change in
glacier mass balance, which is measured as
the average change in thickness across  the
surface of a glacier. The change in ice or
snow has been converted to the equivalent
amount of liquid water.


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 combina-
tion of global and local climate variations.
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 nearly two decades of satel-
lite 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 I, based on measure-
ments 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 peri-
odic reports and measurements of the bench-
mark glaciers, are available on the program's
website at: http://ak.water.usgs.gov/glaciology.
[51

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                      This indicator measures the amount of time that ice is present on lakes in the United States.
52]
     Background
     The formation of ice cover on lakes in
     the winter and its disappearance the fol-
     lowing spring depends on climate factors
     such as air temperature, cloud cover,
     and wind. Conditions such as heavy rains
     or snowmelt in locations upstream or
     elsewhere in the watershed also affect
     lake ice duration. 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 mois-
     ture 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 sunlight 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, whereas an ice- and snow-covered
     lake will  reflect the sun's energy rather
     than absorb it. (For more information
     on ice and snow  reflecting sunlight, see
     the Snow Cover  indicator on  p. 56.)
     The timing and duration of ice cover on
     lakes and other bodies of water can also
     affect society—particularly shipping and
     transportation, hydroelectric power
     generation, and fishing. The impacts can
     be either positive or negative. For ex-
     ample, reduced ice cover on a large lake
     could extend the open-water shipping
     season but require vessels to  reduce
     their cargo capacity, as increased evapo-
     ration leads  to lower water levels.


     About the Indicator
     This indicator analyzes the  dates at
     which lakes freeze and thaw. Freeze
     dates are when a continuous and im-
     mobile ice cover forms over a body of
     water. Thaw dates are when the ice

                      (Continued on page 53)
 Key Points
    The time that lakes stay frozen has generally decreased since the mid-1800s. For most
    of the lakes in this indicator, the duration of ice cover has decreased at an average rate
    of one to two days per decade (see Figure I).

    The lakes covered by this indicator are generally freezing later than they did in the past.
    Freeze dates have grown later at a rate of roughly half a day to one day per decade (see
    Figure 2).

    Thaw dates for most of these lakes show a general trend toward earlier  ice breakup in
    the spring (see Figure 3).

    The changes in freeze and thaw dates shown here are consistent with other studies. For ex-
    ample, a broad study of lakes and rivers throughout the Northern Hemisphere found that
    since the mid-1800s, freeze dates have occurred later, at an average rate of 5.8 days per 100
    years, and thaw dates have occurred earlier, at an average rate of 6.5 days per 100 years.15
Detroit Lake
  MN       oShell Lake
                 wi
                          Mirror Lake o
oGrand Traverse Bay     LakeGeorgeo
                                   Ml
                                NY   oQtsegoLake
    Lake Mendota o0 Lake Monona
 Figure 1. Duration of Ice Cover for Selected U.S. Lakes, 1850-2010
 This figure displays the duration (in days) of ice cover for eight U.S. lakes. The data are
 available from approximately 1850 to 2010, depending on the lake, and have been smoothed
 using a nine-year moving average.
    1840     1860     1880     1900     1920    1940

                                       Year

                  Detroit Lake     — Lake Mendota
                  Lake George     — Lake Michigan
                                 (Grand Traverse Bay)

      Data source: NSIDC, 2011"
                                                   1960     1980     2000     2020
                   • Lake Monona      Otsego Lake
                   •Mirror Lake    —Shell Lake

-------
Figure 2. Date of First Freeze for Selected U.S. Lakes, 1850-2010
This figure shows the "ice-on" date, or date of first freeze, for eight U.S. lakes. The data
are available from approximately 1850 to 2010, depending on the lake, and have been
smoothed using a nine-year moving average.
   November 1
   December 1
 a)  January 1
    February 1
      Marchl
          1840    1860     1880    1900    1920    1940    1960    1980    2000    2020
                                           Year
Detroit Lake     — LakeMendota      — LakeMonona
Lake George
                                                                    OtsegoLake
                                  Lake Michigan       — Mirror Lake     — Shell Lake
                                  (Grand Traverse Bay)
           Data source: NSIDC, 201 1"
Figure 3. Date of Ice Thaw for Selected U.S. Lakes, 1850-2010
This figure shows the "ice-off" date, or date of ice thawing and breakup, for eight U.S.
takes. The data are available from approximately 1850 to 2010, depending on the lake,
and have been smoothed using a nine-year moving average.
   March 1
cover breaks up and open water becomes
extensive.
Freeze and thaw dates have been recorded
through human visual observations for more
than ISO 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 fo-
cuses on eight lakes within the United States
that have the longest and most complete
historical records. The lakes of interest are
located in Minnesota, Wisconsin, Michigan,
and New York.


Indicator Notes
Although there  is a lengthy historical record
of freeze and thaw  dates for a much larger
set of lakes and  rivers, some records are
incomplete, with breaks ranging from brief
lapses to large gaps in data. This  indicator
is limited to eight lakes with fairly complete
historical records.
Data used in this indicator are all based on
visual observations. Records based on visual
observations by individuals are open to
some interpretation and can differ from one
individual to the next. In addition, historical
observations for lakes have typically been
made from the shore, which might not
be representative of lakes as a whole or
comparable to more recent satellite-based
observations.


Data Sources
Data were obtained from the Global Lake
and River Ice Phenology Database, which
is maintained by the National Snow and Ice
Data Center. These data are available at:
http://nsidc.org/data/lake_river_ice.
    Mayl
        1840    1860     1880    1900     1920     1940    1960     1980     2000    2020

                                          Year

                — Detroit Lake     —LakeMendota       —LakeMonona      OtsegoLake
                  LakeGeorge     — Lake Michigan       —MirrorLake     —Shell Lake
                                  (Grand Traverse Bay)

        Data source: NSIDC, 201118

-------
                       This indicator uses two different measures to show how snowfall has changed in the contiguous 48 states.
54]
     Background
     Snowfall is an important aspect of win-
     ter in many parts of the United States.
     People depend on snow to provide
     water when it melts in the spring, and
     many communities rely on snow for win-
     ter 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. 56
     and the Snowpack indicator on  p. 58).
     Warmer temperatures cause more
     water to evaporate from the land and
     oceans, which leads to larger storms
     and more precipitation. 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 people who depend
     on these activities to make a living.


     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 ex-
     ample, a storm that deposits 10 inches
     of snow). Figure I  shows how snowfall
     accumulation totals changed between

                      (Continued on page 55)
 Figure 1. Change in Total Snowfall in the Contiguous 48 States,
 1930-2007
 This figure shows the average rate of change in total snowfall from 1930 to 2007 at 419
 weather stations in the contiguous 48 states. Blue circles represent increased snowfall; red
 circles represent a decrease.

                          Rate of change (percent per year):
<-1.2
  -0.9
to -1.2
  -0.6
to -0.9
  -0.3
to -0.6
  -0.1
to -0.3
 -0.1
to 0.1
  0.1
to 0.3
                                                     0.3
                                                   to 0.6
  0.6
to 0.9
  0.9
to 1.2
            Less snowfall
Data source: Kunkel et al., 200915
                                                          More snowfall
    Key Points
       Total snowfall has decreased in most parts of the country since widespread
       observations became available in 1930, with 57 percent of stations showing a
       decline (see Figure I).

       In addition to changing the overall rate of precipitation, climate change can also
       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 stations across the contiguous 48
       states have experienced a decrease  in the proportion of precipitation falling as
       snow (see Figure 2).

       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-
       precipitation 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

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Figure 2. Change in Snow-to-Precipitation Ratio in the Contiguous
48 States, 1949-2011
This figure shows the percentage change in winter snow-to-precipitation ratio from 1949 to
2011 at 289 weather stations in the contiguous 48 states. This ratio measures what percent-
age 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.
                                  Percent change:
 <-40
          -30
         to-40
 -20     -10      -2      -2      2      10
to -30   to -20    to -10    to 2    to 10    to 20
 20
to 30
 30
to 40
                                                                              >40
         Lower percentage of snow
                       Filled circles represent statistically significant trends.
                   Open circles represent trends that are not statistically significant.
                                    Higher percentage of snow
1930 and 2007 at more than 400 weather
stations. These stations were selected be-
cause they had high-quality data for the full
timespan of this indicator.
Figure 2 of this indicator 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). For this comparison, snow has been
converted to the equivalent amount of
liquid water. These data are available from
1949 to 2011.


Indicator  Notes
Several factors make it difficult to measure
snowfall precisely. The snow accumulations
shown in Figure I 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 sur-
rounding environment (such as tall trees).
Similarly, snow gauges for Figure 2 may
catch slightly 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.21 As a result,
some parts of the country have a higher sta-
tion density than others.
Both figures are limited to the winter
season. Figure  I 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.


Data Sources
This indicator shows trends based on two
sets of weather records collected and
maintained by the National Oceanic and
Atmospheric Administration. Figure I was
adapted from an analysis by Kunkel et al.
(2009)22 based on records from Coopera-
tive Observer Program weather  stations.
Figure 2 is an updated version  of an analysis
by Feng and Hu (2007)23 using data from the
U.S. Historical Climatology Network. Ad-
ditional information  about the Cooperative
Observer Program is available online at:
www.nws.noaa.gov/om/coop. Information
about the U.S. Historical  Climatology Net-
work can be found at: www.ncdc.noaa.gov/
oa/climate/research/ushcn.

-------
     Background
     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 reflects much of the sunlight
     that hits it. In contrast, darker surfaces
     such as open water absorb  more light
     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, while less
     snow cover means the Earth will absorb
     more heat and become warmer.
     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 tempera-
     tures. 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 Atmospheric
     Administration. The indicator was cre-
     ated 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 calcu-
     lated for each season: spring (defined as
     March-May), summer (June-August),
     fall (September-November), and winter
     (December-February). All maps were
     recently reanalyzed using the most pre-
     cise  methods available, making this the
     best available data set for assessing snow
     cover on a continental scale.
Figure 1. Snow-Covered Area in North America, 1972-2011
This graph shows the average area covered by snow in a given calendar year, based on an analysis of
weekly maps. The area is measured in square miles. These data cover all of North America (not includ-
ing Greenland).
  4.0
    1970     1975    1980     1985     1990     1995    2000     2005     2010     2015

                                       Year
     Data source: Rutgers University Global Snow Lab, 20I224
K|


-------
Figure 2. Snow-Covered Area in North America by Season, 1972-2011
This graph shows the average area covered by snow during spring (March-May), summer (June-August),
fall (September-November), 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).
Key Points
   1970     1975     1980     1985     1990     1995
                                      Year
                                                  2000     2005     2010     2015
   Overall, during the period from 1972 to
   2011, snow covered an average of
   3.23 million square miles of North America
   (see Figure I).
   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 mil-
   lion square miles, with the minimum value
   occurring in 1998 and the maximum in 1978
   (see Figure I).
   Looking at averages by decade suggests that
   the extent of North America covered by
   snow has decreased somewhat overtime.
   The average extent for the most recent
   decade (2002-2011) was 3.21 million square
   miles, which is 3 percent (100,000 square
   miles) smaller than the average extent
   during the first 10 years of measurement
   (1972-1981) (see Figure I).
   The largest decreases in  snow cover have
   occurred in spring and summer, whereas
   fall snow cover has remained fairly steady
   and winter cover appears to have increased
   slightly  in recent years (see Figure 2). Sprin;
   and summer snow cover can have a particu-
   larly important influence on water supplies.
     Data source: Rutgers University Global Snow Lab, 20I22'
                                                                                         Indicator  Notes
                                                                                         Although satellite-based snow cover maps
                                                                                         are available starting in the mid-1960s, some
                                                                                         of the early years are missing data from sev-
                                                                                         eral 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 pro-
                                                                                         vided  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.
                                                                                                                                 [57.

-------
     Background
     Temperature and precipitation are key
     factors affecting snowpack, which is
     the amount or thickness of snow that
     accumulates on the ground. In a warm-
     ing 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 moun-
     tain snowpack for power, irrigation, and
     drinking water. In most western  river
     basins, snowpack is a larger component
     of water storage than  human-construct-
     ed reservoirs.26
     Changes in  mountain snowpack can af-
     fect 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 snow-
     pack 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 snow-
     pack since the 1930s. In the early years
     of data collection, researchers measured
     snow water equivalent manually, but since
     I960, measurements at some locations
     have been collected with automated in-
     struments. This indicator is based on data
     from approximately 800 permanent re-
     search sites in the western United States
Figure 1. Trends in April Snowpack in the Western United States and
Canada, 1950-2000
This map shows trends in April snowpack in the western United States and part of Canada,
measured in terms of snow water equivalent. Blue circles represent increased snowpack; red
circles represent a decrease.
                                   • ••1
                                 Percent change:
  <-80
            -60
           to-80
 -40
to-60
 -20
to-40
  0
to-20
 0
to 20
 20
to 40
 40
to 60
 60
to 80
                                                                          >80
 Data source: Mote, 20092'
58]
                      (Continued on page 59)

-------
Key Points
   From 1950 to 2000, April snowpack declined at most of the measurement sites
   (see Figure I), with some relative losses exceeding 75 percent.

   In general, the largest decreases were observed in western Washington, west-
   ern Oregon, and northern California. April snowpack decreased to a lesser
   extent in the northern Rockies.

   A few areas have seen increases in snowpack, primarily in the southern Sierra
   Nevada of California and in the Southwest.
                                                                         V

                                                                           I
and Canada. 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.


Indicator Notes
Natural changes in the Earth's climate could
affect snowpack in such a way that trends
might slightly differ if measured over a dif-
ferent time period. The 1950s registered
some of the highest snowpack measure-
ments of the 20th century in the Northwest.
While these values could be magnifying the
extent of the snowpack decline depicted in
Figure I, the general direction of the trend
is the same regardless 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. 24)
snowfall trends may be  partially influenced
by nonclimatic factors such as observa-
tion methods, land-use  changes, and forest
canopy changes.


Data  Sources
Data for this indicator came from the U.S.
Department of Agriculture's Natural Re-
sources Conservation Service Water and Cli-
mate Center. The map was constructed using
methods described in Mote et al. (2005).28
The U.S. Department of Agriculture data are
available at: www.wcc.nrcs.usda.gov.

-------
Growing
Season

-------

          The indicators in this re-
          port show that changes are
          occurring throughout the
       Earth's climate system, includ-
       ing increases in air and ocean
       temperatures, more extreme
       weather events, a rise in sea
       level, widespread melting of gla-
       ciers, and longer ice-free periods
       on lakes and rivers. Changes
       such as these can present  a wide
       range of challenges to human
       well-being, the economy,  and
       natural ecosystems.

       What is happening?
       For society, human health
       effects from increases in
       temperature are likely to
       include increases in heat-
       related  illnesses and deaths,
       especially in urban areas.
       Changes in precipitation
       patterns and timing affect
       streamflow and water avail-
       ability, while more severe
       storms and floods damage
       property and infrastructure
       (such as roads, bridges, and
       utilities) and cause loss of
       life. Warming temperatures are
       also affecting the length of the
       growing season in the United
       States as the timing between
       the last (spring) and first (fall)
       frost has expanded by nearly two
       weeks over the last 100 years.
While species have adapted to environmental
change for millions of years, climate change
could require adaptation on larger and faster
scales than current species have successfully
achieved in the  past.
Changes in climate can af-
fect ecosystems by influencing
animal behavior, such as nest-
ing and migration patterns, as
well as the timing and extent of
natural processes such as flower
blooms and the length of pollen
seasons in many areas.
Why does it matter?
Ecosystems provide humans
with food, clean water, and a
variety of other services that can
be affected by climate change.
While species 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 success-
fully achieved in the past, thus
increasing the risk of extinction
   for some species.

   The more the climate
   changes, the greater the
   potential effects on so-
   ciety and ecosystems.
   The nature and extent of
   climate change effects, and
   whether these effects will
   be harmful or beneficial,
   will vary regionally and
   over time. The extent to
   which climate change will
   affect different ecosystems,
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.
For detailed information about data used in these indicators, see the online technical documentation at: www.epa.gov/climatechange/indicators.
  Bloom
  MM. H*W
                                      Related
                                      Deaths

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     Background
     Streamflow is a measure of the amount
     of water 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 sustained flow when
     snow melts in the spring. The amount of
     streamflow is important because very
     high flows can cause erosion and damag-
     ing floods, while very low flows can
     diminish water quality, harm fish, and
     reduce the amount of water available for
     people to use. The timing of peak flow
     is important because it affects the ability
     of reservoir managers to store water
     to meet people's 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. 58) can
     alter the size and timing of peak stream-
     flows. More precipitation is expected to
     cause higher average streamflow in some
     places, while  heavier storms (see the
     Heavy Precipitation indicator on p. 30)
     could lead to larger peak flows. More
     frequent or severe droughts will reduce
     streamflow in certain areas.


     About  the  Indicator
     The U.S. Geological Survey measures
     streamflow in rivers and streams across
     the United States using continuous mon-
     itoring devices called stream gauges. This
     indicator is based on 211 stream gauges
     located in areas where trends will not be
     artificially influenced by dams, reservoir
     management, wastewater treatment
     facilities, or other activities.
62]
Figure 1. Volume of Seven-Day Low Streamflows in the United States, 1940-2009
This map shows percentage changes in the minimum amount of water carried by rivers and streams
across the country, based on the long-term rate of change from  1940 to 2009. Minimum streamflow is
based on the seven-day period with the lowest average flow during a given year.
            ff
                             A
                  T        T        „         A        A
      Data source:
      i jcr c 9 n 17'   More than    2m to 50%  20% decrease to  20% to 50%    More than
      UO"OO, ZUIZ   cnn/. i-«    ->nn/. in».«->i-«    in».«->i-«   cnn/. i».u«-u-«
                50% decrease
                           decrease
                                   20% increase
                                              increase
                                                      50% increase
Figure 2. Volume of Three-Day High Streamflows in the United States, 1940-2009
This map shows percentage changes in the maximum amount of water carried by rivers and streams
across the country, based on the long-term rate of change from 1940 to 2009. Maximum streamflow is
based on the three-day period with the highest average flow during a given year.
                                  T   A

                                       *»;
                                             "T
                                             _    A
                                                         o  no
                                                       I  o  o c
                                       ^      A
                                            A A 8
                                         OA°  A
                              •
      Data source:
      USGS, 20I22
 More than
50% decrease
20% to 50%
 decrease
20% decrease to
 20% increase
20% to 50%
 increase
 More than
50% increase

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    Key Points
        Over the past 70 years, seven-day low flows have generally increased in the
        Northeast and Midwest (in other words, on the driest days, streams are carrying
        more water than before). Low flows have generally decreased (that is, streams
        are carrying less water than before) in parts of the Southeast and the Pacific
        Northwest. Overall, more sites have seen increases than decreases (see Figure I).

        Three-day high-flow trends vary from region to region across the country. For
        example, streams in the Northeast 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 (see Figure 2). Overall, more sites
        have seen increases than decreases.

        Sixty percent of the streams measured show winter-spring runoff happening
        more than five days earlier than it did in the past. The most dramatic change has
        occurred in the Northeast (see Figure 3).
Figure 3. liming of Winter-Spring Runoff in the United States, 1940-2009
This map shows changes in the timing of peak spring flow carried by rivers and streams, based on
the long-term rate of change from 1940 to 2009. This analysis focuses on parts of the country where
streamflow is strongly influenced by snowmelt. It is based on the winter-spring center of volume, which
is the date when half of the streamflow between January I and May 31 of each year has passed.
           T
        More than
       10 days earlier
 5 to 10
days earlier
  2to5
days earlier
2 days earlier
to 2 days later
 2 to 5
days later
 5 to 10
days later
 More than
10 days later
       Data source: USGS, 20I23
                                                                           This indicator examines three important
                                                                           measures of streamflow conditions that
                                                                           occur over the course of a year. Figure I
                                                                           looks  at the driest conditions each year,
                                                                           which are commonly calculated by averag-
                                                                           ing the lowest seven consecutive days of
                                                                           streamflow over the year. This method
                                                                           captures the year's most severe, sustained
                                                                           dry spell. Figure 2 examines high flow condi-
                                                                           tions,  which are commonly calculated as the
                                                                           highest average flow over three consecutive
                                                                           days. Based on typical weather patterns,
                                                                           three  days is an  optimal length of time to
                                                                           capture runoff associated with large storms
                                                                           and peak snowmelt.
                                                                           Figure 3 shows changes in the timing of
                                                                           spring runoff over time. This measure is lim-
                                                                           ited to 55 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 I and  May 31 and then determine
                                                                           the date when exactly half of the water has
                                                                           gone by. This date is called the winter-spring
                                                                           center of volume. A long-term trend toward
                                                                           an earlier date suggests that  spring  snow-
                                                                           melt is happening earlier.


                                                                           Indicator Notes
                                                                           Measurements were taken in areas  where
                                                                           streamflow is not highly affected by human
                                                                           influences, including changes in land cover.
                                                                           However, changes in land cover and land use
                                                                           overtime 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 watersheds with  minimal human
                                                                           impacts, which have been classified  as refer-
                                                                           ence gauges.4 Daily average streamflow data
                                                                           are stored in the National Water Informa-
                                                                           tion System and are publicly available at:
                                                                           http://waterdata.usgs.gov/nwis.

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     Background
     More than half of Americans have at
     least one allergy.5 Allergies are a major
     public health concern, with hay fever
     (congestion, runny nose, itchy eyes)
     accounting for more than 13 million
     visits to physicians' offices and other
     medical facilities every year.6 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.7
     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 rag-
     weed plant can produce up to a billion
     pollen grains in one season, and these
     grains can be carried  long distances by
     the wind.8
     Climate change can affect pollen  aller-
     gies  in several ways. Warmer spring
     temperatures cause some plants to start
     producing pollen earlier (see the Leaf
     and Bloom Dates indicator on p. 68),
     while warmer fall temperatures extend
     the growing season for other plants  such
     as ragweed (see the Length of Grow-
     ing Season indicator on  p. 66). Warmer
     temperatures and increased carbon
     dioxide concentrations also enable rag-
     weed and other plants to produce more
     pollen.9 This 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
     10 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 eleva-
     tion  but across a range  of latitudes from
     south to north. At each  location, air
Figure 1. Change in Ragweed Pollen Season, 1995-2011
This figure shows how the length of ragweed pollen season changed at 10 locations in the central
United States and Canada between 1995 and 2011. Red circles represent a longer pollen season;
blue circles represent a shorter season. Larger circles indicate larger changes.
                        +26 days
                                          k+18days
                                            +24 days
                                                   ^+21 days
                                                       +14 days
                                                      P ^+13 days
                                            1+12 days
                                                 ,+12 days
                                       .-may
                                      .-may
     Change in length of pollen season:
        • Decrease  • Increase
Data source: Ziska et al., 2012'"
64]

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Key Points
   Since 1995, ragweed pollen season has grown longer at eight of the 10 locations
   studied (see Figure I).
   The increase in ragweed season length becomes more pronounced from south
   to north. Ragweed season increased by 24 days in Fargo, North Dakota, and
   26 days in Saskatoon, Saskatchewan (see Figure I). This trend is consistent with
   many other observations showing that climate is changing more rapidly at higher
   latitudes."
   The trends in Figure I 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. Meanwhile, some southern stations have
   experienced only a modest change in frost-free season length since I995.12

samples have been collected and examined
for at least 16 years as part of a national
allergy monitoring network. Pollen spores
are counted and identified using micro-
scopes.
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 2011, which was de-
termined 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 lim-
ited 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
variations in temperature and precipita-
tion, 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 Na-
tional 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 2009.13

-------
     Background
     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 differ-
     ent 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 grow-
     ing season can have both positive and
     negative effects. Moderate warming can
     benefit crop and pasture yields  in mid-
     to high-latitude regions, yet even slight
     warming decreases yields in seasonally
     dry and low-latitude regions.14 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 irriga-
     tion. 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 grow-
     ing 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.
     Trends in the growing season were cal-
     culated using temperature data from 750
     weather stations throughout the contigu-
     ous 48 states. These data were obtained
     from the National Oceanic and Atmo-
     spheric Administration's National Climatic
     Data Center. Growing season  length and
     the timing of spring and fall frosts were
     averaged across the nation, then com-
     pared with long-term average  numbers
     (1895-2011) to determine how each year
     differed from the long-term average.
Figure 1. Length of Growing Season in the Contiguous 48 States, 1895-2011
This figure shows the length of the growing season in the contiguous 48 states compared with a long-
term average. For each year, the line represents the number of days shorter or longer than average.
The line was smoothed using an  11-year moving average. Choosing a different long-term average for
comparison would not change the shape of the data over time.
   -15
     1890  1900   1910  1920  1930   1940  1950   1960  1970   1980  1990   2000  2010  2020
      Data source: Kunkel, 20I215



Figure 2. Length of Growing Season in the Contiguous 48 States,
1895-2011: West Versus East
This figure shows the length of the growing season in the western and eastern United States
compared with a long-term average. For each year, the line represents the number of days shorter
or longer than average. The lines were smoothed using an 11-year moving average. Choosing a
different long-term average for comparison would not change the shape of the data over time.
     1890  1900   1910  1920  1930   1940  1950   1960  1970   1980  1990   2000  2010  2020
                                        Year
      Data source: Kunkel, 2012"
66|

-------
Key  Points
   The average length of the growing season in the contiguous 48 states has in-
   creased by nearly two weeks since the beginning of the 20th century. A particu-
   larly large and steady increase occurred over the last 30 years (see Figure I).

   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 aver-
   age rate of about 22 days per century since 1895, compared with a rate of about
   eight days per century in the East (see Figure 2).

   The final spring frost is now occurring earlier than at any point since 1895, and
   the first fall frosts are arriving later. Since I960, the last spring frost has oc-
   curred an average of about three days earlier than the long-term average, and
   the first fall frost has occurred about two days later (see Figure 3).
Figure 3. liming of Last Spring Frost and First Fall  Frost in the

Contiguous 48 States, 1895-2011

This figure shows the timing of the last spring frost and the first fall frost in the contiguous
48 states compared with a long-term average. Positive values indicate that the frost oc-
curred later in the year, and negative values indicate that the frost occurred earlier in the
year. The lines were smoothed using an  11-year moving average. Choosing a different long-
term average for comparison would not  change the shape of the data over time.
    1890  1900   1910  1920   1930  1940   1950  1960   1970  1980  1990  2000  2010   2020
                                       Year
     Data source: Kunkel, 2012"
Indicator Notes

Changes in measurement techniques and in-
struments over time can affect trends. This
indicator only includes data from weather
stations with a consistent record of data
points for the time period.


Data  Sources
All three figures are based on temperature
data compiled by the National Oceanic and
Atmospheric Administration's National
Climatic Data Center, and these data are
available online at: www.ncdc.noaa.gov/oa/
ncdc.html. Frost timing and growing season
length were analyzed by Kunkel  (2012).l8

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                         eaf   and    Bloom
                      .his indicator examines the timing of leaf growth and flower blooms for selected plants in the United States.
     Background
     The timing of natural events, such as
     flower blooms and animal migration, is
     influenced by changes in climate. Phe-
     nology is the study of such important
     seasonal events. Phenological events are
     influenced by a  combination of climate
     factors, including light, temperature,
     rainfall, and humidity. Different plant and
     animal species respond to different cues.
     Scientists have very high confidence
     that the earlier arrival of spring events
     is linked to recent warming trends in
     global climate.19 Disruptions in the tim-
     ing of these events can have a variety of
     impacts on ecosystems and human soci-
     ety. For example, an earlier spring might
     lead to longer growing seasons (see the
     Length of Growing Season indicator on
     p. 66), more abundant invasive species
     and pests, and earlier and longer allergy
     seasons.
     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. Some phenological
     indicators cover broad trends, such
     as overall "leaf-on" dates (when trees
     grow new leaves in the spring), using a
     combination of satellite data and ground
     observations. Others rely on ground
     observations that look at specific types
     or species of plants or animals. Two par-
     ticularly useful indicators of the timing of
     spring events are the first leaf dates and
     the first bloom dates of lilacs and honey-
     suckles, which have an easily monitored
     flowering season, relatively high survival
     rate, and large geographic distribution.
     The first leaf date in these plants relates
     to the timing of "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.20


     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
 Figure 1. First Leaf Dates in the Contiguous 48 States, 1900-2010
 This figure shows modeled trends in lilac and honeysuckle first leaf dates across the contiguous
 48 states, using the 1981 to 2010 average as a baseline. Positive values indicate that leaf growth
 began later in the year, and negative values indicate that leafing occurred earlier. The thicker line
 was smoothed using a nine-year weighted average. Choosing a different long-term average for
 comparison would not change the shape of the data over time.
   -15
     1900   1910   1920   1930   1940   1950    1960    1970   1980   1990   2000   2010
      Data source: Schwartz, 20112
Figure 2. First Bloom Dates in the Contiguous 48 States, 1900-2010
This figure shows modeled trends in lilac and honeysuckle first bloom dates across the contigu-
ous 48 states, using the 1981 to 2010 average as a baseline. Positive values indicate that
blooming began later in the year, and negative values indicate that blooming occurred earlier.
The  thicker line was smoothed using a nine-year weighted average. Choosing a different long-
term average for comparison would not change the shape of the data over time.

    15
    1900   1910    1920   1930   1940   1950   1960   1970   1980    1990    2000   2010
                                      Year
68]
                                                       Data source: Schwartz, 201I22

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Key Points
   First leaf growth in lilacs and honeysuckles in the contiguous 48 states is now
   occurring a few days earlier than it did in the early 1900s. Although the data
   show a great deal of year-to-year variability, a noticeable change toward earlier
   dates seems to have begun around the 1980s (see Figure I).

   Lilac and honeysuckle bloom dates show a high degree of year-to-year vari-
   ability, which makes it difficult to determine whether a statistically meaningful
   change has taken place (see Figure 2).

   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.23
gaps in time or space, computer models have
been used to provide a more complete un-
derstanding of long-term trends nationwide.
The models for this indicator were devel-
oped using data from the USA National
Phenology Network, which collects ground
observations from a network of federal
agencies, field stations, educational institu-
tions, 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 hon-
eysuckles. 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 leaf and bloom dates
and climate factors (particularly tempera-
tures), 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 normal." This indica-
tor presents the average deviation across all
stations.


Indicator  Notes
Plant phenological events are studied using
several data collection  methods, including
satellite images, models, and direct obser-
vations. The use of varying data collection
methods  in addition to different phenologi-
cal 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 genetic influences
by relying on cloned plant species (that is,
plants with no genetic differences).


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 Clima-
tology Network and are available at: www.
ncdc.noaa.gov/oa/climate/research/ushcn.
Data for this indicator were analyzed using
methods described by McCabe et al. (2011).24
                                                                                                                                  |69

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                    Bird   Wintering
                     This indicator examines changes in the winter ranges of North American birds.
    Background
    Changes in climate can affect ecosystems by
    influencing animal behavior and distribution.
    Birds are a particularly good indicator of
    environmental change for several reasons:
    •  Each species of bird has adapted to cer-
       tain 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 pat-
       tern, 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 migratory and nonmigratory
       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 distribution 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 habitats or habits occurs
       across a range of bird types, it suggests
       that a common force might be contribut-
       ing to that change.
    Temperature and precipitation patterns are
    changing across the United States (see the
    U.S. and Global Temperature indicator on
    p. 24 and the  U.S. and Global Precipitation
    indicator on p. 28). Some bird  species can
    adapt to generally warmer temperatures by
    changing where they live—for  example, by
    migrating further north in the summer but
    not as far south in the winter, or by shift-
    ing inland as winter temperature extremes
    grow less severe. Nonmigratory 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 tempera-
    ture cues, such as migration and breeding
    (especially egg-laying).
70j
Figure 1. Change in Latitude of Bird Center of Abundance, 1966-2005
This figure shows annual change in latitude of bird center of abundance for 305 widespread bird
species in North America from 1966 to 2005. Each winter is represented by the year in which it be-
gan (for example, winter 2005-2006 is shown as 2005). The shaded band shows the likely range
of values, based on the number of measurements collected and the precision of the methods used.
   -20
    1965
            1970
                     1975
                             1980
                                     1985
                                     Year
                                             1990
                                                      1995
                                                              2000
                                                                      2005
      Data source: National Audubon Society, 20092-

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  Key Points
     Among 305 widespread North American bird species, the average mid-
     December to early January center of abundance moved northward between
     1966 and 2005. The average species shifted northward by 35 miles during this
     period (see Figure I). Trends in center of abundance are closely related to
     winter temperatures.26

     On average, bird species have also moved their wintering grounds farther from
     the coast since the 1960s (see Figure 2). This shift also relates to changes in
     winter temperatures.27

     Some species have moved farther than others. Of the 305 species studied, 177
     (58 percent)  have shifted their wintering grounds significantly northward since
     the 1960s, but some others have not moved at all. A few species have moved
     northward by as much as 200 to 400 miles.28
Figure 2. Change in Distance to Coast of Bird Center of Abundance,
1966-2005
This figure shows annual change in distance to the coast of bird center of abundance for 305
widespread bird species in North America from 1966 to 2005. Each winter is represented by the
year in which it began (for example, winter 2005-2006 is shown as 2005). The shaded band
shows the likely range of values, based on the number of measurements collected and the preci-
sion of the methods used.
             1970
                      1975
                               1980
                                       1985
                                       Year
                                                1990
                                                         1995
                                                                  2000
                                                                           2005
      Data source: National Audubon Society, 200929
About the Indicator
This indicator looks at the "center of abun-
dance" of 305 widespread North American
bird species over a 40-year period. The center
of abundance is a point on the map that repre-
sents 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 Soci-
ety'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 Nation-
al Audubon Society, Bird Studies Canada, local
Audubon chapters, and other bird clubs to iden-
tify  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.


Indicator Notes
Many factors can influence bird ranges, includ-
ing 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 Audu-
bon Society analysis found large  differences
among coastal birds, grassland birds,  and birds
adapted to feeders, which all have varying abili-
ties to adapt to temperature changes.30
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. Data
for this indicator were analyzed by the Nation-
al Audubon  Society in 200931 and are available
at: www.audubon.org/bird/bacc/index.html.
                                                                                                                                  III

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72]
     Background
When people are exposed to extreme heat,
they can suffer from potentially deadly heat-
related illnesses 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 Exces-
sive Heat Events Guidebook at: www.epa.gov/
heatisland/about/pdf/EH Eguide_final.pdf)

Unusually hot summer temperatures have
become more frequent across the contiguous 48
states in recent decades32 (see the High and Low
Temperatures indicator on p. 26), and extreme
heat events (heat waves) are expected to be-
come longer, more  frequent, and more intense in
the future.33 As a result, the risk of heat-related
deaths and illness is also expected to increase.34
Increases in summertime temperature variabil-
ity may increase the risk of heat-related death
for the elderly and  other vulnerable popula-
tions.35 Older adults carry 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 as the baby boomer
generation  ages. People with certain diseases,
such as cardiovascular and respiratory illnesses,
are especially vulnerable to  excessive heat ex-
posure, 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 warm-
er, while others do not expect the number to
change at all.36-37 Any decrease in cold-related
deaths, however, will not be enough to  out-
weigh the increase  in heat-related deaths.38-39


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 certifi-
cate, 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.
Multiplying the annual number of deaths per
U.S.  population that year by one million will
result in the death  rate shown in Figure I.

This  indicator shows heat-related deaths using
two methodologies. One method shows deaths
for which excessive natural heat was stated as
the underlying cause of death from  1979 to 2009.
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
                       (Continued on page 73)
Figure 1. Deaths Classified as "Heat-Related" in the United States,
1979-2009
This figure shows the annual rates for deaths classified as "heat-related" by medical profession-
als 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 contributing cause of death during the months from May to Septem-
ber, based on a broader set of data that became available in 1999.
                                                                                  Underlying and contributing
                                                                                  causes of death (May-Sept)
                                                                                  Underlying cause of death
                                                                                  (all year)
                                                            0.0
                                                             1975
                                                                        1980
                                                                                   1985
                                                                                             1990
                                                                                                        1995
                                                                                                                  2000
                                                                                                                             2005
                                                                                                                                       2010
                                                                                                  Year
                                                           * Between 1998 and 1999, the World Health Organization 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, 20I240-41
                                                            Key  Points
      During the 31 years of data collection (1979-2009), 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 I). Overall, a total of
      over 7,000 Americans suffered heat-related deaths since 1979.

      For years where the two records overlap (1999-2009), accounting for those ad-
      ditional 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-related deaths.

      The indicator shows a peak in heat-related deaths in 2006, a year that was associ-
      ated with widespread heat waves and was the second-hottest year on record in
      the contiguous 48 states (see the U.S. and  Global Temperature indicator on p. 24).

      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 classified as "heat-related" overtime.
      Dramatic increases in heat-related deaths are closely associated with both the
      occurrence of hot temperatures 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. 73) suggest that there were hundreds
      more deaths than were actually reported as "heat-related" on death certificates.

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 Example: Examining Heat-Related Deaths During the 1995 Chicago

 Heat Wave*
 Many factors can influence the nature, extent, and timing of health consequences associated with
 extreme heat events.42 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.43 However, studies that compared the total
 number of deaths during this heat wave (regardless of the recorded cause of death) with the long-
 term average of daily deaths found that the heat wave led to about 700 more deaths than would
 otherwise have been expected.44 Differences in estimated heat-related deaths that result from
 different methods may be even larger when considering the entire nation and longer time periods.
              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
                                                                             110
                   A
                 Average
               daily deaths,
                1990-2000
                                                                              50
    6/1
6/11
8/20
8/30
*This graph shows data for the Chicago Standard Metropolitan Statistical Area.

Data sources: CDC, 20\2;K NOAA, 20I2""
back to 1999. For example, in a case where
cardiovascular disease 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 1999-2009
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
certificate. In many cases, the medical examiner
might classify the cause of death as a cardiovas-
cular 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.47 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.48
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 individual's vulner-
ability to heat. Other important factors, such
as the overall vulnerability of the population,
the extent to which people have adapted 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 condi-
tioning, health care, public education, cool-
ing centers during heat waves, infrastructure
standards, and air quality  management can also
make a big difference in death rates. For exam-
ple, after a  1995 heat wave, the city of Milwau-
kee developed a plan for responding to  extreme
heat conditions in the future; during the 1999
heat wave, heat-related deaths were roughly
half of what would have been  expected.49

Future development related to this indica-
tor 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 Preven-
tion (CDC). The 1979-2009 underlying cause
data are publicly available through the CDC
WONDER database at: http://wonder.cdc.gov/
mortSQL.html. The 1999-2009 analysis was
developed by CDC's Environmental Public
Health Tracking Program, which provides
a summary at: www.cdc.gov/nceh/tracking.    |73

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                                         •ill
Climate Change Indicators and Human Health
  Phis report provides several environmental and
   ecological indicators of observed change related to
  I climate. Although climate change can affect human
health in a number of direct and indirect ways, well-
defined, consensus-based "health" indicators are limited.

Many societal and environmental factors can influence
how climate change will affect health in any given
community. These factors include the extent, frequen-
cy, and severity of climate change impacts; the ability
of communities to prepare for and respond to the risks
posed by climate change; and the vulnerability of the
different populations living in the community.

Because the impacts of climate change on health are com-
plex, often indirect, and dependent on multiple societal
and environmental factors, the development of appropri-
ate climate-related health indicators is challenging and
still emerging. To ensure that response measures are effec-
tive and adverse health effects are avoided, it is important
for climate-related health indicators to be clear, measur-
able, timely, and closely linked to changes in climate.1'2

Climate-related health indicators will be instrumental
not only in tracking and measuring health impacts of
climate change but also, more importantly, in iden-
tifying areas where the protection of public health is
needed most. EPA plans to explore opportunities to
  In 2011, The Centers for Disease Control and Prevention
  (CDC) introduced a suite of indicators to track the effects of
  climate change on human health through the Environmental
  Public Health Tracking Network (EPHTN). The network links
  standardized metrics from local, state, and national databases
  on environmental hazards and human health effects with
  climate information.3 EPHTN is an emerging effort from which
  useful indicators may be leveraged to track potential direct
  and indirect health effects due to climate change. For more
  information about EPHTN, go to: www.cdc.gov/nceh/tracking.
work with climate and health experts to develop indi-
cators that communicate the effects of climate change
on health and society more broadly.

Key human health impacts and vulnerabilities associ-
ated with climate change include:

•  A warmer climate will increase the risk of heat-
   related illness and death. A warmer climate is also
   expected to decrease the risk of cold-related illness
   and death.
•  Climate change is expected to worsen conditions
   for air quality, including exposure to ground-level
   ozone, which can aggravate lung diseases and lead
   to premature death.
•  Climate change will likely increase the frequency
   and strength of certain extreme events (such as
   floods, droughts, and storms) that threaten human
   safety and health.
•  Changes in temperature and precipitation can
   spread or shift the geographic range of certain
   diseases and alter the seasons for pollen, affecting
   human exposure to infection, asthma, and other
   respiratory diseases.
•  Vulnerable populations including the poor,  the
   elderly, those already in poor health, the disabled,
   and indigenous populations are most at risk.

For more information about climate change impacts
and human health, visit EPA's website: www.epa.gov/
climatechange/impacts-adaptation/health.html.

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Climate  Change Resources
EPA's Climate Change website (www.epa.gov/
climatechange) provides a good starting point for fur-
ther 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 accompa-
   nying 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 Publication Tool.
•  Learn about EPA's regulatory initiatives and partner-
   ship 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 Assessment.
                                                        •  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 Sci-
   ence Museum (https://koshland-science-museum.org)
   provides an interactive online Earth Lab where people
   can learn more about these issues.
•  The National Oceanic and Atmospheric Admin-
   istration (NOAA) is charged with helping society
   understand, plan for, and respond to climate variabil-
   ity and change. Find out more about NOAAs climate
   indicators and other activities at: www.climate.gov.
•  NOAAs 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 re-
   lationship 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/climate_landuse) 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/pagel.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 informa-
   tion 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.

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Endnotes
Introduction
 I.   IPCC (Intergovernmental Panel on Climate Change). 2007. Climate change 2007:
     Synthesis report (Fourth Assessment Report). Cambridge, United Kingdom:
     Cambridge University Press.
Summary of Key Points
 I.   IPCC (Intergovernmental Panel on Climate Change). 2007. Climate change 2007:
     Synthesis report (Fourth Assessment Report). Cambridge, United Kingdom:
     Cambridge University Press.
Greenhouse Gases
 I.   IPCC (Intergovernmental Panel on Climate Change). 1995. Climate change  1995:
     The science of climate change (Second Assessment Report). Cambridge, United
     Kingdom: Cambridge University Press.
 2.   IPCC (Intergovernmental Panel on Climate Change). 2007. Climate change 2007:
     The physical science basis (Fourth Assessment Report). Cambridge, United
     Kingdom: Cambridge University Press.                                           13.
 3.   U.S. EPA (U.S. Environmental Protection Agency). 2012. Inventory of U.S.
     greenhouse gas emissions and sinks: 1990-2010. USEPA #EPA 430-R-I2-OOI.
     www.epa.gov/climatechange/ghgemissions/usinventoryreport.html.
 4.   ibid.
 5.   ibid.
 6.   ibid.
 7.   ibid.
 8.   ibid.
 9.   World Resources Institute. 2012. Climate Analysis Indicators Tool (CAIT).
     Version 9.0. Accessed May 2012. http://cait.wri.org.
 10.   ibid.
 II.   ibid.
 12.   EP/CA Dome C, Antarctica: approximately 647,426 BC to 4 / 1,548 BC
     Siegenthaler, U., T. F. Stocker, E. Monnin, D. Liithi, J. Schwander, B. Stauffer,
     D. Raynaud, J.M. Barnola, H. Fischer, V. Masson-Delmotte, and J. Jouzel. 2005.
     Stable carbon cycle-climate relationship during the late Pleistocene. Science
     310(5752): 1313-13 17. Accessed May 15, 2007. ftp://ftp.ncdc.noaa.gov/pub/data/
     paleo/icecore/antarctica/epica_domec/edc-co2-650k-390k.txt.
     Vbstok Station, Antarctica: approximately 415,157 BC to 339 BC
     Barnola, J.M., D. Raynaud, C. Lorius, and N.I. Barkov. 2003. Historical CO2 record
     from the Vostok ice core. In: Trends: A compendium of data on global change.
     Oak Ridge, TN: U.S. Department of Energy. Accessed September 14, 2005.
     http://cdiac.ornl.gov/trends/co2/vostok.html.
     EP/CA Dome C, Antarctica: approximately 9002 EC to 1515 AD
     Fluckiger, J., E. Monnin, B. Stauffer, J. Schwander, T.F. Stocker, J. Chappellaz, D.
     Raynaud, and J.M. Barnola. 2002. High resolution Holocene N2O ice core record
     and its relationship with CH4 and CO2. Global Biogeochem. Cycles  16(1):10-1 I.
     Accessed April 30, 2007. ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/antarctica/
     epica_domec/ readme_flueckiger2002.txt.
     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 CO2 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, 2005. http://cdiac.ornl.
     gov/trends/co2/lawdome.html.
     Siple Station, Antarctica: approximately 1744 AD to  1953 AD
     Neftel, A., H.  Friedli, E. Moor, H. Lotscher, H. Oeschger, U. Siegenthaler, and  B.
     Stauffer. 1994. Historical CO2 record from the Siple Station ice core. In: Trends:
     A compendium of data on global change. Oak Ridge, TN: U.S. Department of
     Energy. Accessed September 14, 2005. http://cdiac.ornl.gov/trends/co2/siple.html.
     Mauna Loa, Hawaii: 1959 AD to 2011 AD
     NOAA (National Oceanic and Atmospheric Administration). 2012. Annual
     mean CO2 concentrations for Mauna Loa, Hawaii. Accessed May 10, 2012.
     ftp://ftp.cmdl.noaa.gov/ccg/co2/trends/co2_annmean_mlo.txt.
     Barrow, Alaska: 1974 AD to 2011 AD
     Cape Matatula, American Samoa: 1976 AD to 2011 AD
     South Pole, Antarctica: 1976 AD to 2011 AD
     NOAA (National Oceanic and Atmospheric Administration). 2012. Monthly mean
     CO2 concentrations for Barrow, Alaska; Cape Matatula, American Samoa; and the
     South Pole. Accessed May 10, 2012. ftp://ftp.cmdl.noaa.gov/ccg/co2/in-situ.
     Cape Grim, Australia: 1992 AD to 2006 AD
     Shetland islands, Scotland: 1993 AD to 2002 AD                                     lb"
     Steele, L.P., P.B. Krummel, and R.L. Langenfelds. 2007. Atmospheric CO2
     concentrations (ppmv) derived from flask air samples collected at Cape Grim,
     Australia, and Shetland Islands, Scotland. Commonwealth Scientific and Industrial        16.
     Research Organisation. Accessed January 20,  2009. http://cdiac.esd.ornl.gov/ftp/         17
     trends/co2/csiro.
14.
Lampedusa Island, Italy. 1993 AD to 2000 AD
Chamard, P., L. Ciattaglia, A. di Sarra, and F. Monteleone. 2001. Atmospheric CO2
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, 2005. http://cdiac.ornl.gov/trends/co2/lampis.html.
EP/CA Dome C, Antarctica: approximately 646,729 BC to 1888 AD
Spahni, R., J. Chappellaz, T.F. Stocker, L. Loulergue, G. Hausammann, K.
Kawamura, J. Fluckiger, J. Schwander, D. Raynaud, V. Masson-Delmotte, and J.
Jouzel. 2005. Atmospheric methane and nitrous oxide of the late Pleistocene from
Antarctic ice cores. Science 310(5752): 1317-1321. Accessed May 15, 2007. ftp://
ftp. ncdc. noaa.gov/pu b/data/paleo/icecore/antarctica/epica_domec/edc-ch4-2 005-
650ktxt.
Vbstok Station, Antarctica: approximately 415,172 BC to 346 BC
Petit, J.R..J. Jouzel, D. Raynaud, N.I. Barkov, J.M. Barnola, I. Basile, M. Bender, J.
Chappellaz, M. Davis, G. Delaygue, M. Delmotte, V.M. Kotlyakov, M. Legrand,
V. Lipenkov, C. Lorius, L. Pepin, C. Ritz, E. Saltzman, and M. Stievenard.  1999.
Climate and atmospheric history of the past 420,000 years from the Vostok  ice
core, Antarctica. Nature 399:429-436. Accessed April 24, 2007. ftp://ftp.ncdc.
noaa.gov/pub/data/paleo/icecore/antarctica/vostok/ch4nat.txt.
Green/and G/SP2 ice core: approximately 87,798 BC to 8187 BC
Byrd Station, Antarctica: approximately 85,929 BC to 6748 BC
Green/and GRIP ice core: approximately 46,933 BC to 8/29 BC
Blunier, T., and E.J. Brook. 2001. Timing of millennial-scale climate change in
Antarctica and Greenland during the last glacial period. Science 291:109-1 12.
Accessed September 13, 2005. ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/
greenland/summit/grip/synchronization/readme_blunier2001 .txt.
EP/CA Dome C, Antarctica: approximately 8945 BC to / 760 AD
Fluckiger, J., E. Monnin, B. Stauffer, J. Schwander, T.F. Stocker, J. Chappellaz, D.
Raynaud, and J.M. Barnola. 2002. High resolution Holocene N2O ice core record
and its relationship with CH4 and CO2. Global Biogeochem. Cycles  16(1):10-1 I.
Accessed April 24, 2007. ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/antarctica/
epica_domec/ readme_flueckiger2002.txt.
Law Dome, Antarctica: approximately 1008 AD to 1980 AD
Various Greenland locations: approximately 1075 AD to 1885 AD
Etheridge, D.M., L.P. Steele,  R.J. Francey, and R.L. Langenfelds. 2002. Historical
CH4 records since about 1000 AD from ice core data. In: Trends: A compendium
of data on global change. Oak Ridge, TN: U.S. Department of Energy. Accessed
September 13, 2005. http://cdiac.ornl.gov/trends/atm_meth/lawdome_meth.html.
Green/and S'rteJ: approximately 1598 AD to  1951 AD
WDCGG (World Data Centre for Greenhouse Gases). 2005. Atmospheric
CH4 concentrations for Greenland Site J. Accessed September I, 2005.
http://ds.data.jma.go.jp/gmd/wdcgg.
Cape Grim, Australia:  1984 AD to 2010 AD
NOAA (National Oceanic and Atmospheric Administration). 201 I.  Monthly  mean
CH4 concentrations for Cape Grim, Australia. Accessed October 27, 201 I.
ftp ://ftp.cmdl.noaa.gov/ccg/ch4/flask/month/ch4_cgo_surface-flask_ I _ccgg_month.txt.
Mauna Loa, Hawaii: 1987 AD to 2011 AD
NOAA (National Oceanic and Atmospheric Administration). 201 I.  Monthly  mean
CH4 concentrations for Mauna Loa, Hawaii. Accessed May 10, 2012. ftp://ftp.cmdl.
noaa.gov/ccg/ch4/in-situ/mlo/ch4_mlo_surface-insitu_ I _ccgg_month.txt.
Shetland Islands, Scotland: 1993 AD to 2001 AD
Steele, L.P., P.B. Krummel, and R.L. Langenfelds. 2002. Atmospheric CH4
concentrations from sites in the CSIRO Atmospheric Research GASLAB air
sampling network (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/csiro-shetlandch4.html.
IPCC (Intergovernmental Panel on Climate Change). 2007. Climate change 2007:
The physical science basis (Fourth Assessment Report). Cambridge, United
Kingdom:  Cambridge University Press.
IPCC (Intergovernmental Panel on Climate Change). 2007. Climate change 2007:
Synthesis report (Fourth Assessment Report). Cambridge, United Kingdom:
Cambridge University Press.
ibid.
AGAGE (Advanced Global Atmospheric Gases Experiment). 201 I. ALE/GAGE/
AGAGE data base. Accessed November 201 I. http://agage.eas.gatech.edu/data.htm.

-------
18.  NOAA (National Oceanic and Atmospheric Administration). 201 I.
     Halocarbons and other atmospheric trace species. Accessed October 201 I.
     www.esrl.noaa.gov/gmd/hats.
19.  Arnold, T., J. MUhle, P.K. Salameh, C-M. Harth, D.J. Ivy, and R.F. Weiss. 2012.
     Automated measurement of nitrogen trifluoride in ambient air. Analytical
     Chemistry 84(1 I ):4798-^804.
20.  Weiss, R.F.J. MUhle, P.K. Salameh, and CM. Harth. 2008. Nitrogen trifluoride in
     the global atmosphere. Geophys. Res. Lett.  35:120821.
21.  IPCC (Intergovernmental Panel on Climate  Change). 2007. Climate change 2007:
     Synthesis report (Fourth Assessment Report). Cambridge, United Kingdom:
     Cambridge University Press.
22.  Greenland GISP2 ice core: approximately 104,301 EC to 1871 AD
     Taylor Dome, Antarctica: approximately 30,697 E>C to 497 E>C
     Sowers, T., R.B. Alley, and J. Jubenville. 2003. Ice core records  of atmospheric
     N2O covering the last 106,000 years. Science 301 (5635):945-948. Accessed
     September 14, 2005. www.ncdc.noaa.gov/pub/data/paleo/icecore/antarctica/
     taylo r/tayl o r_n 2o .txt.
     EPICA Dome C, Antarctica: approximately 9000 BC to / 780 AD
     Fliickiger, J., E. Monnin,  B. Stauffer, J. Schwander, T.F. Stocker, J. Chappellaz, D.
     Raynaud, and J.M. Barnola. 2002. High resolution Holocene N2O ice core record
     and its relationship with CH4 and CO2. Global Biogeochem. Cycles 16(1):10-1 I.
     Accessed September 14, 2005. ftp://ftp.ncdc.noaa.gov/pub/data/paleo/icecore/
     antarctica/epica_domec/ read me_flueckiger2002.txt.
     Antarctica: approximately 1756 AD to 1964 AD
     Machida, T., T. Nakazawa, Y. Fujii, S. Aoki, and O.  Watanabe. 1995. Increase in
     the atmospheric nitrous oxide concentration during the last 250 years. Geophys.
     Res. Lett. 22(2l):292l-2924. Accessed Septembers, 2005. ftp://daac.ornl.gov/
     data/global_climate/global_N_cycle/data/global_N_pe rturbations.txt.
     Antarctica: approximately 1903 AD to 1976 AD
     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 383:23 1-235.
     Accessed September 8, 2005. ftp://daac.ornl.gov/data/global_climate/global_N_
     eye le/data/glo bal_N_perturbations.txt.
     Cape Grim, Australia: 1979 AD to 2010 AD
     AGAGE (Advanced Global Atmospheric Gases Experiment). 201 I. Monthly
     mean N2O concentrations for Cape Grim, Australia. Accessed May  10, 2012.
     http://ds.data.jma.go.jp/gmd/wdcgg.
     South Pole, Antarctica: 1998 AD to 2011 AD
     Barrow, Alaska: 1999 AD to 2011  AD
     Mauna Loa, Hawaii: 2000 AD to 2011 AD
     NOAA (National Oceanic and Atmospheric Administration). 2012. Monthly mean
     N2O concentrations for Barrow, Alaska; Mauna Loa, Hawaii; and the South Pole.
     Accessed May 10, 2012. www.esrl.noaa.gov/gmd/hats/insitu/cats/cats_conc.html.
23.   AGAGE (Advanced Global Atmospheric Gases Experiment). 201 I. ALE/GAGE/
     AGAGE data base. Accessed November 201 I. http://agage.eas.gatech.edu/data.htm.
24.   Arnold, T., J. MUhle, P.K. Salameh, C.M. Harth, D.J. Ivy, and R.F. Weiss. 2012.
     Automated measurement of nitrogen trifluoride in ambient air. Analytical
     Chemistry 84(1 I ):4798-4804.
25.   NOAA (National Oceanic and Atmospheric Administration). 201 I.
     Halocarbons and other atmospheric trace species. Accessed October  201 I.
     www.esrl.noaa.gov/gmd/hats.
26.   Weiss, R.F.,J. MUhle, P.K. Salameh, and C.M. Harth. 2008. Nitrogen  trifluoride in
     the global atmosphere. Geophys. Res. Lett. 35:L20821.
27.   NOAA (National  Oceanic and Atmospheric Administration). 2012. The NOAA
     Annual Greenhouse Gas Index. Accessed October 2012. www.esrl.noaa.gov/
     gmd/aggi.
Weather and Climate
 I.   NOAA (National Oceanic and Atmospheric Administration). 2009. Layers of the
     atmosphere, www.srh.noaa.gov/jetstream/atmos/layers.htm.
 2.   NOAA (National Oceanic and Atmospheric Administration). 2012. National
     Climatic Data Center. Accessed April 2012. www.ncdc.noaa.gov/oa/ncdc.html.
 3.   ibid.
 4.   ibid.
 5.   Karl, T.R., J.M. Melillo, and T.C. Peterson (eds.). 2009. Global climate change
     impacts in the United States. New York, NY: Cambridge University Press.
 6.   National Research Council. 201 I. Climate stabilization targets: Emissions,
     concentrations, and  impacts over decades to millennia. Washington, DC: National
     Academies Press.
 7.   Meehl, G.A., T.F. Stocker, W.D. Collins, P. Friedlingstein, AT. Gaye, J.M.
     Gregory, A. Kitoh, R. Knutti, J.M. Murphy, A. Noda, S.C.B. Raper, I.G. Watterson,
     A.J. Weaver, and Z.-C. Zhao. 2007. Global climate projections. In: Climate change
     2007: The physical science basis (Fourth Assessment Report). Cambridge, United
     Kingdom:  Cambridge University Press.
 8.   Kunkel, K. 2012. Updated version of a figure that originally appeared in U.S.
     Climate Change Science Program's 2008 report: Synthesis and Assessment
     Product 3.3: Weather and climate extremes in a changing climate, www.
     climatescience.gov/Library/sap/sap3-3/final-report/sap3-3-final-Chapter2.pdf.
 9.   NOAA (National Oceanic and Atmospheric Administration). 2012. U.S. Climate
     Extremes  Index. Accessed October 2012. www.ncdc.noaa.gov/oa/climate/
     research/cei/cei.html.
 10.  NOAA (National Oceanic and Atmospheric Administration). 2012. U.S. Climate
     Extremes  Index. Accessed March 2012. www.ncdc.noaa.gov/oa/climate/research/
     cei/cei.html.
 I I.  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:L2370I.
 12.  CCSP (U.S. Climate  Change Science Program). 2008. Synthesis and Assessment
     Product 3.3: Weather and climate extremes in a changing climate.
     www.climatescience.gov/Library/sap/sap3-3/final-report/sap3-3-final-Chapter2.pdf.
 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:L2370I.
 14.  NOAA (National Oceanic and Atmospheric Administration). 2012. National
     Climatic Data Center. Accessed April 2012. www.ncdc.noaa.gov/oa/ncdc.html.
 15.  ibid.
 16.  ibid.
 17.  Tebaldi, C., K. Hayhoe, J.M. Arblaster, and G.A. Meehl. 2006. Going to the
     extremes: An intercomparison  of model-simulated historical and future changes
     in extreme events. Climatic Change 79:185-21 I.
 18.  NOAA (National Oceanic and Atmospheric Administration). 2012. U.S. Climate
     Extremes  Index. Accessed March 2012. www.ncdc.noaa.gov/oa/climate/research/
     cei/cei.html.
19.  NOAA (National Oceanic and Atmospheric Administration). 2012. Standardized
     Precipitation Index data files. Accessed March 2012. ftp://ftp.ncdc.noaa.gov/pub/
     data/cirs.
20.  NOAA (National Oceanic and Atmospheric Administration). 2012. National
     Climatic Data Center. Personal communication: Analysis by Derek Arndt, April
     2012.
21.  CCSP (U.S. Climate Change Science Program). 2008. Synthesis and Assessment
     Product 3.3: Weather and climate extremes in a changing climate.
     www.climatescience.gov/Library/sap/sap3-3/final-report/sap3-3-final-Chapter2.pdf.
22.  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.
23.  Heim, R.R. 2002. A review of twentieth-century drought indices used in the
     United States. Bull. Amer. Meteor. Soc. 83(8): I 149-1 165.
24.  NOAA (National Oceanic and Atmospheric Administration). 2012. State of the
     climate: Drought: September 2012. Accessed November 2012. www.ncdc.noaa.
     gov/sotc/drought/2012/9.
25.  NOAA (National Oceanic and Atmospheric Administration). 2012. National
     Climatic Data Center. Accessed January 2012. www.ncdc.noaa.gov/oa/ncdc.html.
26.  National Drought Mitigation Center. 2012. Drought Monitor archives. Accessed
     January 2012. http://droughtmonitor.unl.edu/archive.html.
27.  CCSP (U.S. Climate Change Science Program). 2008. Synthesis and Assessment
     Product 3.3: Weather and climate extremes in a changing climate.
     www.climatescience.gov/Library/sap/sap3-3/final-report/sap3-3-final-Chapter2.pdf.
28.  ibid.
29.  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.
30.  Knutson, T.R. 2012 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.
31.  NOAA (National Oceanic and Atmospheric Administration). 2012. The 201 I
     North  Atlantic hurricane season: A climate perspective, www.cpc.ncep.noaa.gov/
     products/expert_assessment/hurrsummary_20l l.pdf.
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.  Emanuel, K.A. 2012 update to data originally published in: Emanuel, K.A. 2007.
     Environmental factors affecting tropical cyclone power dissipation. J. Climate
     20(22):5497-5509.
34.  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
 I.   Khatiwala, S., F. Primeau, and T. Hall. 2009. Reconstruction of the history of
     anthropogenic CO2 concentrations in the ocean. Nature 462:346-349.
 2.   Levitus, S., J. Antonov, and T. Boyer. 2005. Warming of the world ocean,
     1955-2003. Geophys. Res. Lett. 32102604.
 3.   ibid.
 4.   CSIRO (Commonwealth Scientific and Industrial Research Organisation). 2012.
     Data downloads: Sea level and ocean heat content. Accessed February 2012.
     www.cmar.csiro.au/sealevel/sl_data_cmar.html.
 5.   JAMSTEC (japan Agency for Marine-Earth Science and Technology). 2012 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.]. Oceanogr. 65:287-299.
 6.   NOAA  (National  Oceanic and Atmospheric Administration). 2012. Global ocean
     heat and salt content. Accessed February 2012. www.nodc.noaa.gov/OC5/3M_
     HEAT_CONTENT.
 7.   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 community:
     The effects of coral bleaching and physical disturbance. Proc. Natl. Acad. Sci.
     97(IO):5297-5302.
 8.   Pratchett, M.S., S.K. Wilson, M.L.  Berumen, and  M.I. McCormick. 2004. Sublethal
     effects of coral bleaching on an  obligate coral feeding butterflyfish. Coral Reefs
     23(3):352-356.
 9.   Trenberth, K.E., P.D. Jones, P. Ambenje, R. Bojariu, D. Easterling, A. Klein Tank
     D. Parker, F. Rahimzadeh, J.A. Renwick, M. Rusticucci, B. Soden, and P. Zhai.
     2007. Observations: Surface and atmospheric climate change. In: Climate change
     2007: The physical science basis (Fourth Assessment Report). Cambridge,  United
     Kingdom: Cambridge University Press.
 10.   NOAA  (National  Oceanic and Atmospheric Administration). 2012. Extended
     reconstructed sea surface temperature (ERSST.v3b). National Climatic Data
     Center. Accessed April 2012. www.ncdc.noaa.gov/ersst.
 I I.   UK Met Office. 2012. Hadley Centre,  HadlSST I.I: Global sea ice coverage
     and sea surface temperature (1870-present). NCAS British Atmospheric Data
     Centre. Accessed May 2012. http://badc.nerc.ac.uk/view/badc.nerc.ac.uk	
     ATOM_
     dataent_hadisst.
 12.   CSIRO (Commonwealth Scientific and Industrial Research Organisation). 2012
     update to data originally published in: Church, J.A., and N.J. White. 201 I. Sea-
     level rise from the late  19th to the early 21a century. Surv. Geophys. 32:585-602.
 13.   NOAA  (National  Oceanic and Atmospheric Administration). 2012. Laboratory
     for Satellite Altimetry: Sea level rise. Accessed May 2012. http://ibis.grdl.noaa.gov/
     SAT/SeaLevelRise/LSA_SLR_timeseries_global.php.
Snow and Ice
I.    UNEP (United Nations Environment Programme). 2007. Global outlook
     for ice and snow. Cartographer: Hugo Ahlenius, UNEP/GRID-Arendal.
     www.unep.org/geo/geo_ice.
     Map based on the following data sources:
     Armstrong, R.L., and M.J. Brodzik 2005. Northern Hemisphere EASE-Grid weekly
     snow cover and sea ice extent version 3. National Snow and Ice Data Center.
     Armstrong, R.L., M.J. Brodzik K. Knowles, and M. Savoie. 2005. Global monthly
     EASE-Grid snow water equivalent climatology. National Snow and  Ice Data Center.
     Brown, J., O.J. Ferrians, Jr., J.A. Heginbottom, and E.S. Melnikov. 2001. Circum-
     Arctic map of permafrost and ground-ice conditions. National Snow and Ice Data
     Center/World Data Center for Glaciology.
     National Geospatial-lntelligence Agency. 2000. Vector map level 0.
     http://geoengine.nima.mil/ftpdir/archive/vpf_data/vOsoa.tar.gz.
     Stroeve, J., and W. Meier. 2005. Sea ice trends and climatologies from SMMR
     and SSM/I. National Snow and Ice Data Center. http://nsidc.org/data/smmr_ssmi_
     ancillary/month ly_means.html.
2.    NASA (National Aeronautics and Space Administration). 2012. Sea  ice yearly
     minimum 1979-201 I. NASA/Goddard Space Flight Center Scientific Visualization
     Studio. Supplemented by personal communication with NASA in  November 2012
     to obtain a draft image for 2012. http://svs.gsfc.nasa.gov/vis/aOOOOOO/a003800/
     a003893.
3.    NSIDC (National Snow and Ice Data Center). 2012. Arctic sea ice news and
     analysis, http://nsidc.org/icelights/arctic-sea-ice.
4.    Comiso, J. 2012. Large decadal decline of the Arctic multiyear ice cover.
     J. Climate 25(4): I 176-1 193.
5.    NSIDC (National Snow and Ice Data Center). 2012. Archived monthly sea ice
     data and images. Accessed October 2012. http://nsidc.org/data/seaice_index/
     archives/index.html.
                                                                                   14.  Titus, J.G., EX. 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 region.
                                                                                       U.S. Climate Change Science Program and the Subcommittee on Global Change
                                                                                       Research. www.climatescience.gov/Library/sap/sap4- l/final-report/default.htm.
                                                                                   15.  University of Colorado at Boulder. 2012. Sea level change: 2012 release #2.
                                                                                       Accessed May 2012. http://sealevel.colorado.edu.
                                                                                   16.  NOAA (National Oceanic and Atmospheric Administration). 2012 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://tidesandcurrents.
                                                                                       noaa.gov/publications/techrpt36.pdf.
                                                                                   17.  IPCC (Intergovernmental Panel on Climate Change). 2007. Climate change 2007:
                                                                                       The physical science basis (Fourth Assessment Report). Cambridge, United
                                                                                       Kingdom: Cambridge University Press.
                                                                                   18.  Wootton, J.T., C.A. Pfister, and J.D. Forester. 2008. Dynamic patterns and
                                                                                       ecological impacts of declining ocean pH in a high-resolution multi-year dataset.
                                                                                       Proc. Natl. Acad. Sci. I05(48):I8848-I8853.
                                                                                   19.  Feely, R.A., S.C. Doney, and S.R. Cooley. 2009. Ocean acidification: Present
                                                                                       conditions and future changes in a high-CO2 world. Oceanography 22(4):36-47.
                                                                                  20.  Bates, N.R., M.H.P. Best, K. Neely, R. Garley, A.G. Dickson, and R.J. Johnson.
                                                                                       2012. Detecting anthropogenic carbon dioxide uptake and ocean acidification in
                                                                                       the North Atlantic Ocean. Biogeosciences Discuss. 9:989-1019.
                                                                                  21.  Gonzalez-Davila, M. 2012 update to data originally published in: Gonzalez-Davila,
                                                                                       M., J.M. Santana-Casiano, M.J.  Rueda, and O. Llinas. 2010. The water column
                                                                                       distribution of carbonate system variables  at the ESTOC site from 1995 to 2004.
                                                                                       Biogeosciences Discuss. 7:1995-2032.
                                                                                  22.  University of Hawaii. 2012. Hawaii Ocean Time-Series. Accessed June 2012.
                                                                                       http://hahana.soest.hawaii.edu/hot/products/HOT_surface_CO2.txt.
                                                                                  23.  Recreated from Environment  Canada. 2008. The pH scale.
                                                                                       www.ec.gc.ca/eau-water/default.asp?lang=En&n=FDF30C 16-1.
                                                                                  24.  Feely, R.A., S.C. Doney, and S.R. Cooley. 2009. Ocean acidification: Present
                                                                                       conditions and future changes in a high-CO2 world. Oceanography 22(4):36-47.
                                                                                  25.  ibid.
                                                                                  26.  ibid.
                                                                                  27.  Bates, N.R., M.H.P. Best, K. Neely, R. Garley, A.G. Dickson, and R.J. Johnson.
                                                                                       2012. Detecting anthropogenic carbon dioxide uptake and ocean acidification in
                                                                                       the North Atlantic Ocean. Biogeosciences Discuss. 9:989-1019.
                                                                                  28.  Gonzalez-Davila, M., J.M. Santana-Casiano, M.J. Rueda, and O. Llinas. 2010. The
                                                                                       water column distribution of carbonate system variables at the ESTOC site from
                                                                                        1995 to 2004. Biogeosciences Discuss. 7:1995-2032.
                                                                                  29.  Dore.J.E., R.  Lukas, D.W. Sadler, M.J. Church, and D.M. Karl. 2009. Physical  and
                                                                                       biogeochemical modulation of ocean acidification in  the central North Pacific.
                                                                                       Proc. Natl. Acad. Sci. USA 106:12235-12240.
                                                                                   6.   NSIDC (National Snow and Ice Data Center). 2012. Arctic sea ice news and
                                                                                        analysis. October 2, 2012. http://nsidc.org/arcticseaicenews/20l2/IO/poles-apart-
                                                                                        a-record-breaking-summer-and-winter.
                                                                                   7.   National Research Council. 201 I. Climate stabilization targets: Emissions,
                                                                                        concentrations, and impacts over decades to millennia. Washington, DC: National
                                                                                        Academies Press.
                                                                                   8.   Post, A. 1958. McCall Glacier. Glacier photograph collection. Boulder, Colorado:
                                                                                        National Snow and Ice Data Center/World Data Center for Glaciology.
                                                                                        http://nsidc.org/data/g00472.html.
                                                                                   9.   Nolan, M.  2003. McCall Glacier. Glacier photograph collection. Boulder,
                                                                                        Colorado: National Snow and Ice Data Center/World Data Center for Glaciology.
                                                                                        http://nsidc.org/data/g00472.html.
                                                                                   10.   Lemke, P., J.  Ren, R.B. Alley, I. Allison, J. Carrasco, G. Flato, Y. Fujii, G. Kaser,
                                                                                        P. Mote, R.H. Thomas, and T. Zhang. 2007. Observations: Changes in snow, ice
                                                                                        and frozen ground. In: Climate change 2007: The physical science  basis (Fourth
                                                                                        Assessment  Report). Cambridge, United Kingdom: Cambridge University Press.
                                                                                   I I.   WGMS (World Glacier Monitoring Service). 201  I. Glacier mass balance bulletin
                                                                                        no.  I I (2008-2009). Zemp, M., S.U. Nussbaumer, I. Gartner-Roer, M. Hoelzle, F.
                                                                                        Paul, and W. Haeberli (eds.). ICSU(WDS)/IUGG(IACS)/UNEP/UNESCO/WMO.
                                                                                        Zurich, Switzerland: World Glacier Monitoring Service, www.wgms.ch/mbb/
                                                                                        mbb I I /wgms_201 I _gmbb I I .pdf.
                                                                                   12.   WGMS (World Glacier Monitoring Service). 2012. Preliminary glacier mass
                                                                                        balance data 2009/2010. www.wgms.ch/mbb/sumlO.html.
                                                                                   13.   USGS (U.S. Geological Survey). 2012. Water resources of Alaska—
                                                                                        glacier and snow program,  benchmark glaciers. Accessed June 2012.
                                                                                        http://ak.water.usgs.gov/glaciology.
                                                                                   14.   GCRP (U.S.  Global Change Research Program). 2009. Global climate change
                                                                                        impacts in the United States, www.globalchange.gov/what-we-do/assessment/
                                                                                        previous-assessments/global-climate-change-impacts-in-the-us-2009.

-------
15.   Magnuson.J.J., D.M. Robertson, B.J. Benson, R.H. Wynne, D.M. Livingstone, T.
     Arai, R.A. Assel, R.G. Barry, V. Card, E. Kuusisto, N.G. Granin, T.D. Prowse, K.M.
     Stewart, and V.S. Vuglinski. 2000. Historical trends in lake and river ice cover in the
     Northern Hemisphere. Science 289:1743-1746. Errata 2001. Science 291:254.
16.   NSIDC (National Snow and Ice Data Center). 201 I. Global lake and river ice
     phenology. Internal development version accessed by NSIDC staff, December
     201 I. http://nsidc.org/data/lake_river_ice.
17.   ibid.
18.   ibid.
19.   Kunkel, K.E., M. Palecki, L Ensor, K.G. Hubbard, D. Robinson, K. Redmond, and
     D. Easterling. 2009. Trends in twentieth-century U.S. snowfall using a quality-
     controlled dataset. Journal of Atmospheric and Oceanic Technology 26:33—44.
20.   Feng, S. 2012 update to data originally published in: Feng, S., and Q. Hu. 2007.
     Changes  in winter snowfall/precipitation ratio in the contiguous  United States.
     Journal of Geophysical Research I I2:DI5I09.
21.   Kunkel, K.E., M. Palecki, L. Ensor, K.G. Hubbard, D. Robinson, K. Redmond, and
     D. Easterling. 2009. Trends in twentieth-century U.S. snowfall using a quality-
     controlled dataset. Journal of Atmospheric and Oceanic Technology 26:33—44.
22.  ibid.
23.  Feng, S., and Q. Hu. 2007. Changes in winter snowfall/precipitation ratio in the
     contiguous United States. Journal of Geophysical Research I I2:DI5I09.
24.  Rutgers University Global Snow Lab. 2012. Area of extent data: North America
     (no Greenland). Accessed January 2012. http://climate.rutgers.edu/snowcover.
25.  Rutgers University Global Snow Lab. 2012. Area of extent data: North America
     (no Greenland). Accessed February 2012. http://climate.rutgers.edu/snowcover.
26.  Mote, P.W., A.F. Hamlet, M.P. Clark and D.P. Lettenmaier. 2005. Declining
     mountain snowpack in Western North America. Bull. Amer. Meteor. Soc.
27.  Mote, P.W. 2009 update to data originally published in: Mote, P.W., A.F. Hamlet,
     M.P. Clark, and D.P. Lettenmaier. 2005. Declining mountain snowpack in
     Western North America. Bull. Amer. Meteor. Soc. 86(l):39-49.
28.  Mote, P.W., A.F. Hamlet, M.P. Clark and D.P. Lettenmaier. 2005. Declining
     mountain snowpack in Western North America. Bull. Amer. Meteor. Soc.
     86(l):39-49.
Society and Ecosystems
 I.   USGS (U.S. Geological Survey). 2012. Analysis of data from the National Water
     Information System.
 2.   ibid.
 3.   ibid.
 4.   Falcone, J.A., D.M. Carlisle, D.M. Wolock and M.R. Meador. 2010. GAGES: A
     stream gage database for evaluating natural and altered flow conditions in the
     conterminous United States. Ecology 9 I (2):621.
 5.   Arbes, S.J.Jr., P.J. Gergen, L. Elliott, and D.C. Zeldin. 2005. Prevalences of
     positive skin test responses to 10 common allergens  in the U.S. population:
     Results from the third National Health and Nutrition Examination Survey. J.
     Allergy Clin. Immunol. I  !6(2):377-383.
 6.   Schappert, S.M., and E.A. Rechtsteiner. 201 I. Ambulatory medical care utilization
     estimates for 2007. National  Center for Health Statistics. Vital Health Stat
     13(169). www.cdc.gov/nchs/data/series/sr_ 13/sr13_169.pdf.
 7.   Arbes, S.J.Jr., P.J. Gergen, L. Elliott, and D.C. Zeldin. 2005. Prevalences of
     positive skin test responses to 10 common allergens  in the U.S. population:
     Results from the third National Health and Nutrition Examination Survey. J.
     Allergy Clin. Immunol. I  !6(2):377-383.
 8.   National Institute of Allergy and Infectious Diseases. 201 I. Pollen allergy.
     www.niaid.nih.gov/topics/allergicDiseases/understanding/pollenallergy/Pages/
     default.aspx.
 9.   Wayne, P., S. Foster, J. Connolly, F. Bazzaz, and P. Epstein. 2002. Production of
     allergenic pollen by ragweed  (Ambrosia artemisiifolia L.) is increased in CO2-
     enriched atmospheres. Ann. Allergy. Asthma Im. 88:279-282.
 10.   Ziska, L., K. Knowlton, C. Rogers, D. Dalan, N. Tierney, M. Elder, W. Filley, J.
     Shropshire, L.B. Ford, C. Hedberg, P. Fleetwood, K.T.  Hovanky, T. Kavanaugh, G.
     Fulford, R.F. Vrtis, J.A. Patz,J.  Portnoy, F. Coates, L. Bielory, and D. Frenz. 2012
     update to data originally published in: Ziska, L, K. Knowlton, C. Rogers, D. Dalan,
     N. Tierney, M. Elder, W. Filley, J. Shropshire, LB. Ford, C. Hedberg, P. Fleetwood,
     K.T. Hovanky, T. Kavanaugh, G. Fulford, R.F. Vrtis, J.A. Patz, J. Portnoy, F. Coates, L.
     Bielory, and D. Frenz. 201 I. Recent warming by latitude associated with increased
     length of ragweed pollen season in central North America. PNAS 108:4248—4251.
 I I.   IPCC (Intergovernmental Panel on Climate Change).  2007. Climate change 2007:
     Synthesis report (Fourth Assessment Report). www.ipcc.ch/publications_and_
     data/ar4/syr/en/contents.html.
 12.   Ziska, L, K. Knowlton, C. Rogers, D. Dalan, N. Tierney, M. Elder, W. Filley, J.
     Shropshire, L.B. Ford, C. Hedberg, P. Fleetwood, K.T. Hovanky, T. Kavanaugh, G.
     Fulford, R.F. Vrtis, J.A. Patz, J. Portnoy, F. Coates,  L. Bielory, and D. Frenz. 201 I.
     Recent warming by latitude associated with increased length of ragweed pollen
     season in central North  America. PNAS 108:4248^251.
 13.   ibid.
 14.   Easterling, W.E., P.K. Aggarwal, P. Batima,  K.M. Brander, L. Erda, S.M. Howden,
     A. Kirilenko, J. Morton, J.-F. Soussana, J. Schmidhuber, and F.N. Tubiello. 2007.
     Food, fibre and forest products. In: Climate change 2007: Impacts, adaptation
     and vulnerability (Fourth Assessment Report). Cambridge, United Kingdom:
     Cambridge University Press.
 15.   Kunkel, K.E. 2012 update to data originally published  in:  Kunkel, K.E.,  D.R.
     Easterling, K. Hubbard, and K. Redmond. 2004. Temporal variations in frost-free
     season in the United States: 1895-2000. Geophys. Res.  Lett. 31103201.
 16.   ibid.
 17.   ibid.
 18.   ibid.
 19.   Rosenzweig, C., G. Casassa, D.J. Karoly, A. Imeson, C. Liu, A. Menzel, S. Rawlins,
     T.L. Root, B. Seguin, and P. Tryjanowski. 2007. Assessment of observed changes
     and responses in natural and  managed systems. In: Climate change 2007: Impacts,
     adaptation, and vulnerability (Fourth Assessment Report). Cambridge, United
     Kingdom: Cambridge University Press.
20.  Schwartz, M.D., R. Ahas, and A. Aasa. 2006. Onset of spring starting earlier
     across the Northern Hemisphere. Glob. Chang. Biol. 12:343-351.
21.  Schwartz, M.D. 201 I  update to data originally published in: Schwartz, M.D., R.
     Ahas, and A. Aasa. 2006. Onset of spring starting earlier across the Northern
     Hemisphere. Glob. Chang. Biol. 12:343-351.
22.  ibid.
23.  For example, see: Schwartz, M.D., R. Ahas, and A. Aasa. 2006. Onset of spring
     starting earlier across the Northern Hemisphere. Glob. Chang. Biol. 12:343-351.
24.  McCabe, G.J., T.R. Ault, B.I. Cook J.L Betancourt, and M.D. Schwartz.  201 I.
     Influences of the El Nino Southern Oscillation  and the Pacific Decadal Oscillation
     on the timing of the North American spring. Int. J. Climatol. (online).
25.  National Audubon Society. 2009. Northward shifts in the abundance of North
     American birds in early winter: A response to  warmer winter temperatures?
     www.audubon.org/bird/bacc/techreport.html.
26.  ibid.
27.  ibid.
28.  ibid.
29.  ibid.
30.  ibid.
31.  ibid.
32.  Hansen, J., M. Sato, and R. Ruedy. 2012. Perception of climate change. PNAS.
     Published online: August 6, 2012.
33.  GCRP (U.S. Global Change Research Program). 2009. Global climate change
     impacts in the United States, www.globalchange.gov/usimpacts.
34.  IPCC (Intergovernmental Panel on Climate Change). 2007. Climate change 2007:
     Impacts, adaptation, and vulnerability (Fourth Assessment Report). Cambridge,
     United Kingdom: Cambridge University Press.
35.  Zanobetti, A., M.S. O'Neill, C.J. Gronlund, and J.D. Schwartz. 2012. Summer
     temperature variability and long-term survival among elderly people with chronic
     disease. PNAS I09(I7):6608-66I3.
36.  IPCC (Intergovernmental Panel on Climate Change). 2007. Climate change 2007:
     Impacts, adaptation, and vulnerability (Fourth Assessment Report). Cambridge,
     United Kingdom: Cambridge University Press.
37.  Medina-Ramon, M., and J. Schwartz. 2007. Temperature, temperature extremes,
     and mortality: A study of acclimatization and effect modification in 50 U.S. cities.
     Occup. Envi. Med. 64(l2):827-833.
38.  ibid.
39.  IPCC (Intergovernmental Panel on Climate Change). 2007. Climate change 2007:
     Impacts, adaptation, and vulnerability (Fourth Assessment Report). Cambridge,
     United Kingdom: Cambridge University Press.
40.  CDC (U.S. Centers for Disease Control and Prevention). 2012. CDC WONDER
     database. Accessed August 2012. http://wonder.cdc.gov/mortSQL.html.
41.  CDC (U.S. Centers for Disease Control and Prevention). 2012. Indicator: Heat-
     related mortality. National Center for Health Statistics. Annual national totals
     provided by National Center for Environmental Health staff in August 2012.
     http://ephtracking.cdc.gov/showlndicatorPages.action.
42.  Anderson, G.B., and M.L. Bell. 201 I. Heat waves  in the United States: Mortality
     risk during heat waves and effect modification  by heat wave characteristics in 43
     U.S. communities. Environ. Health Perspect. I !9(2):2IO-2 18.
43.  CDC (U.S. Centers for Disease Control and Prevention). 1995. Heat-related
     mortality - Chicago, July 1995. Morbidity and Mortality Weekly Report
     44(3l):577-579.
44.  NRC (National Research Council). 201 I. Climate stabilization targets: Emissions,
     concentrations, and  impacts over decades to millennia. Washington, DC: National
     Academies Press.

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45.   CDC (U.S. Centers for Disease Control and Prevention). 2012. CDC WONDER     48.
     database. Accessed August 2012. http://wonder.cdc.gov/mortSQL.html.
46.   NOAA (National Oceanic and Atmospheric Administration). 2012. National
     Climatic Data Center. Accessed August 2012. wwwl.ncdc.noaa.gov/pub/            49.
     orders/72774.dat.
47.   Medina-Ramon, M., and J. Schwartz. 2007. Temperature, temperature extremes,
     and mortality: A study of acclimatization and effect modification in 50 U.S. cities.
     Occup. Envi. Med. 64(l2):827-833.
Kaiser, R., A. Le Tertre, J. Schwartz, C.A. Gotway, W.R. Daley, and C.H. Rubin.
2007. The effect of the 1995 heat wave in Chicago on all-cause and cause-specific
mortality. Am. J. Public Health 97(Supplement I ):S 158-S162.
Weisskopf, M.G., H.A. Anderson, S. Foldy, L.P. Hanrahan, K. Blair, T.J. Torok, and
P.D. Rumm. 2002. Heat wave morbidity and mortality, Milwaukee, Wis., 1999 vs.
1995: An improved response? Am. J. Public Health 92:830-833.
Climate Change Indicators and Human Health
 I.   English, P.B., A. H. Sinclair, Z. Ross, H. Anderson, V. Boothe, C. Davis,  K. Ebi, B.
     Kagey, K. Malecki, R. Shultz, and E. Simms. 2009. Environmental health  indicators
     of climate change for the United States: Findings from the State Environmental
     Health Indicator Collaborative. Environ. Health Perspect. I 17(1 I):I673-I68I.
 2.   Portier, C.J., T.K. Thigpen, S.R. Carter, C.H. Dilworth, A.E. Grambsch, J. Gohlke,
     J. Hess, S.N. Howard, G. Luber.J.T. Lutz, T. Maslak, N. Prudent, M. RadtkeJ.P.
     Rosenthal, T. Rowles, P.A. Sandifer, J. Scheraga, P.J. Schramm, D. Strickman, J.M.
     Trtanj, and  P.-Y. Whung. 2010. A human health perspective on climate  change:
A report outlining the research needs on the human health effects of climate
change. Research Triangle Park, NC: Environmental Health Perspectives/National
Institute of Environmental Health Sciences.
GCRP (U.S. Global Change Research Program). 201 I. Societal indicators for
the National Climate Assessment. NCA Report Series, Volume 5c.
http://library.globalchange.gov/national-climate-assessment-societal-indicators-
works hop-report.

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