CLIMATE CHANGE
INDICATORS
           2016
IN THE UNITED STATES
   United States
   Environmental Protection
   Agency
           FOURTH EDITION
        ft

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Find Us Online
Please visit EPA's website at: www.epa.gov/climate-indicators.
There you can:
•  View the latest information about EPA's climate change indicators
•  View additional graphs, maps, and background information
•  Access corresponding technical documentation
•  Download images and figures
•  Suggest new indicators for future reports
You can also send an email to: dimateindicators(5)epa.qov
Suggested Citation
U.S. Environmental Protection Agency. 2016.
Climate change indicators in the United States, 2016. Fourth edition.
EPA 430-R-16-004. www.epa.gov/climate-indicators

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    Contents
   Acknowledgments	
   Introduction	
   Understanding Greenhouse Gases.
   Summary of Key Points	
  Greenhouse Gases	12
  U.S. Greenhouse Gas Emissions	13
  Global Greenhouse Gas Emissions	14
  Atmospheric Concentrations of Greenhouse Gases..15
  Climate Forcing	16
^Weather and Climate	17
  U.S. and Global Temperature	18
  High and Low Temperatures	20
  U.S. and Global Precipitation	22
  Heavy Precipitation	24
  Tropical Cyclone Activity	25
  River Flooding	26
  Drought	28
    A Closer Look: Drought in the Southwest	30
  Oceans	31
  Ocean Heat	32
  Sea Surface Temperature	33
  Sea Level	34
    A Closer Look:
    Land Loss Along the Atlantic Coast	35
  Coastal Flooding	36
  Ocean Acidity	38
                                                                                              .2
                                                                                              .3
                                                                                              .6
                                                                                              ..7
                                                    Snow and Ice	
                                                    Arctic Sea Ice	
                                                    Collecting Snow and Ice Data	
                                                    Antarctic Sea Ice	
                                                    Glaciers	
                                                    Lake Ice	
                                                      Community Connection:
                                                      Ice Breakup in Two Alaskan Rivers.
                                                    Snowfall	
                                                    Snow Cover	
                                                    Snowpack	
.39
..40
..42
..43
..44
,..46
 ,.47
 .48
 ..50
 ..52
                                                    Understanding the Connections Between
                                                    Climate Change and Human Health	55
                                                    Health and Society	59
                                                    Heat-Related Deaths	60
                                                    Heat-Related Illnesses	62
                                                    Heating and Cooling Degree Days	64
                                                    Lyme Disease	65
                                                    West Nile Virus	66
                                                    Length of Growing Season	68
                                                    Ragweed Pollen Season	70

                                                    Ecosystems	71
                                                    Wildfires	72
                                                    Streamflow	74
                                                    Stream Temperature	76
                                                      Tribal Connection: Trends in Stream Temperature
                                                      in the Snake River	77
                                                    Great Lakes Water Levels	78
                                                    Bird Wintering Ranges	79
                                                    Marine Species Distribution	80
                                                    Leaf and Bloom  Dates	82
                                                      Community Connection: Cherry Blossom Bloom
                                                      Dates in Washington, D.C	83
Climate Change Resources
Endnotes
Photo Credits...
                                                                                                84
                                                                                                86
                                                                                               .92

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  Acknowledgments
DATA CONTRIBUTORS AND INDICATOR REVIEWERS
EPA wishes to thank the federal government agencies, nongovernmental organizations, and other institutions that partic-
ipate in the ongoing EPA Indicators project. Their commitment, contributions, and collaboration have helped to make this
report possible.
U.S. Governmental Organizations
• Centers for Disease Control and Prevention: C. Ben
  Beard, Lars Eisen, Micah Hahn, George Luber, Ambarish
  Vaidyanathan
• National Aeronautics and Space Administration: Joey
  Comiso, Stacey Frith, Thorsten Markus, Walt Meier
• National Oceanic and Atmospheric Administration
   -  Climate Prediction Center: Gerry Bell
   -  Earth System Research Laboratory: Ed Dlugokencky,
      Steve Montzka
   -  National Marine Fisheries Service: Roger Griffis
   -  National Centers for Environmental Information: Deke
      Arndt, Karin Gleason, Boyin Huang
   -  National Ocean Service: John Marra, William Sweet,
      Chris Zervas
• U.S. Department of Agriculture
   -  Agricultural Research Service: Lewis Ziska
   -  Forest Service: Jennifer Lecker, Karen Short
• U.S. Geological Survey
   -  Alaska Science Center:  Shad O'Neel, Louis Sass
   -  Maine Water Science Center: Robert Dudley, Glenn
      Hodgkins
   -  New York Water Science Center: Michael McHale
   -  Virginia Water Science Center: John Jastram, Karen Rice
   -  Washington Water Science Center: Matt Bachmann
Universities, Nongovernmental Organizations,
and International Institutions
•  Bermuda Institute of Ocean Sciences: Nick Bates
•  Commonwealth Scientific and Industrial Research
  Organisation: John Church, Catia Domingues, Didier
  Monselesan, Neil White
•  Georgia Institute of Technology: Ray Wang
•  Japan Agency for Marine-Earth Science and Technology:
  Masayoshi Ishii
•  Massachusetts Institute of Technology: Kerry Emanuel
•  National Audubon Society: Justin Schuetz, Candan Soykan
•  National Physical Laboratory/University of Edinburgh: Tim
  Arnold
•  North Carolina State University: Ken Kunkel
•  Oregon State University, The Oregon Climate Change
  Research Institute: Philip Mote, Darrin Sharp
•  Rutgers University, Global  Snow Lab: David Robinson: Marine
  and Coastal Sciences: Malin Pinsky
•  Scripps Institution of Oceanography, University of California
  San Diego: Jens Muhle
•  Universidad de las Palmas de Gran Canaria: Melchor
  Gonzalez-Davila
•  University of Bristol: Matthew Rigby
•  University of Colorado: Mark Tschudi
•  University of Iowa: Iman Mallakpour, Louise Slater, Gabriele
  Villarini
•  University of Montana: John Dore
•  University of Nebraska-Lincoln: Song Feng
•  University of Wisconsin-Madison: Corinna Cries
•  University of Wisconsin-Milwaukee: Mark Schwartz
•  Woods Hole Oceanographic Institution: Ivan Lima
•  World Glacier Monitoring Service: Michael Zemp
•  World Resources Institute: Tom Damassa
PEER REVIEW
The report was peer reviewed by 11 external, independent experts: Christopher M. Barker, Kristie L. Ebi, Andrew J. Elmore,
Anthony Janetos, Paul Kirshen, Noah Molotch, Bart Ostro, Charles H. Peterson, Kenneth W. Potter, Mark Serreze, and Tanja
Srebotnjak.
REPORT DEVELOPMENT AND PRODUCTION
Support for content development, data analysis, and report design and production was provided by Eastern Research
Group, Inc. (ERG). Abt Associates also provided analytical and content development support.

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The Earth's climate is changing. Temperatures are rising, snow and rainfall patterns
are shifting, and more extreme climate events—like heavy rainstorms and record-high
temperatures—are already taking place. Scientists are highly confident that many
of these observed changes can be linked to the levels of carbon dioxide and other
greenhouse gases in our atmosphere, which have increased because of human
activities.

HOW IS THE CLIMATE CHANGING?
Since the Industrial Revolution began in the 1700s, people have added a significant
amount of greenhouse gases into the atmosphere, largely by burning fossil fuels to
generate electricity, heat and cool buildings, and power vehicles—as well as by clear-
ing forests. The major greenhouse gases that people have added to the atmosphere
are carbon dioxide, methane, nitrous oxide, and fluorinated gases. When these gases
are emitted into the atmosphere, many remain there for long time periods, ranging
from a decade to thousands of years. Past emissions affect our atmosphere in the
present day; current and future emissions will continue to increase the levels of these
gases in our atmosphere for the foreseeable future.
"Greenhouse gases" got their name because they trap heat (energy) like a green-
house in the lower part of the atmosphere (see "The Greenhouse Effect" below). As
more of these gases are added to the atmosphere, more heat is trapped. This extra
heat leads to higher air temperatures near the Earth's surface, alters weather pat-
terns, and raises the temperature of the oceans.
These observed changes affect people and the environment in important ways. For
example, sea levels are rising, glaciers are melting, and plant and animal life cycles
are changing. These types of changes can  bring about fundamental  disruptions in
ecosystems, affecting plant and  animal populations, communities, and biodiversity.
Such changes can also affect people's health and quality of life,  including where peo-
ple can live, what kinds of crops are most viable, what kinds of businesses can thrive
in certain areas, and the condition of buildings and infrastructure. Some of these
changes may be beneficial to certain people and places, as indicators like Length of
Growing Season point out. Over time, though, many more of these changes will  have
negative consequences for people and society.1
                                        What Is Climate Change?
                                        Climate change refers to any
                                        substantial change in measures
                                        of climate (such as temperature
                                        or precipitation) lasting for an ex-
                                        tended period (decades or longer).
                                        Natural factors have caused the
                                        climate to change during previous
                                        periods of the Earth's history, but
                                        human activities are the primary
                                        cause of the changes that are
                                        being observed now.
                                        Global warming is a term often
                                        used interchangeably with the
                                        term "climate change," but they are
                                        not entirely the same thing. Global
                                        warming refers to an average
                                        increase  in the temperature of
                                        the atmosphere near the Earth's
                                        surface. Global warming is just one
                                        aspect of global climate change,
                                        though a very important one.

                                        Why Use Indicators?
                                        One important way to track
                                        and communicate the causes
                                        and effects of climate change is
                                        through the use of indicators. An
                                        indicator represents the state or
                                        trend of certain environmental or
                                        societal conditions over a given
                                        area and a specified period of time.
                                        For example, long-term  measure-
                                        ments of temperature in the  United
                                        States and globally are used  as
                                        an indicator to track and better
                                        understand the effects of changes
                                        in the Earth's climate.
                        The  Greenhouse  Effect
                        Some solar radiation
                         is reflected by the
                          Earth and the
                        I  atmosphere.
     Most radiation is absorbed
     by the Earth's surface
     and warms it.
Some of the infrared radiation
passes through the atmosphere.
Some is absorbed and re-emitted
in all directions by greenhouse
gas molecules. The effect of this
is to warm the Earth's surface
and the lower atmosphere.
              Infrared radiation
              is emitted by the
              Earth's surface.
                               Earth's surface
How Do the Indicators Relate
to Climate Change?
All of the indicators in this report
relate to either the causes or
effects of climate change. Some
indicators show trends that can be
more directly linked to human-in-
duced climate change than others.
Collectively, the trends depicted in
these indicators provide important
evidence of "what climate change
looks like."

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ABOUT THIS REPORT
The U.S. Environmental Protection Agency (EPA) publishes this report to communicate information about the science and
impacts of climate change, assess trends in environmental quality, and inform decision-making. Climate Change Indicators in the
United States, 2016, is the fourth edition of a report first published by EPA in 2010. This report presents 37 indicators to help read-
ers understand changes observed from long-term records related to the causes and effects of climate change, the significance
of these changes, and their possible consequences for people, the environment, and society. While the indicators presented in
this report do not cover all possible measures of the causes and effects of climate change, as might be found in the full body of
scientific literature, they represent a wide-ranging set of indicators that show observed changes in the Earth's climate system and
several climate-relevant impacts.

About EPA's Indicators
Each of EPA's 37  indicators covers a specific climate-related topic, such as U.S. Greenhouse Gas Emissions. Some indicators
present a single measure or variable; others have multiple measures, reflecting different data sources or different ways to group,
characterize, or zoom in on the data. EPA follows an established framework to identify data sets, select indicators, obtain  inde-
pendent expert review, and publish this report.
Data sources: All of EPA's indicators are based on peer-reviewed, publicly available data from government agencies, academic
institutions, and other organizations. In addition  to being published here, these data sets have been published in the scientific
literature and in other government or academic reports.  EPA also received input from scientists, researchers, and communica-
tions experts in nongovernmental and private sectors during the compilation of this report.
Indicator selection: EPA carefully screened and selected each indicator using a standard set of criteria that consider usefulness,
data quality, and  relevance to climate change. This process ensures that all indicators are based on credible data. For more
information about EPA's indicator criteria and selection process, see the technical support document available at:
www.epa.gov/climate-indicators.
Expert review: This report, along with all of EPA's climate change indicators and supporting documentation, was peer-reviewed
by independent technical experts.
Publication: This report,  the corresponding website, and the accompanying detailed technical documentation have been
designed to ensure that the indicators are presented and documented clearly and transparently.
All of EPA's climate change indicators relate to either the causes or effects of climate change. Some indicators are more directly
influenced by climate than others (e.g., indicators related to health outcomes), yet they all have met  EPA's criteria and have a
scientifically based relationship to climate.  This report does not attempt to identify the extent to which climate change is causing
a trend in an observed indicator. Connections between human activities, climate change, and observed indicators are explored in
more detail elsewhere  in  the scientific  literature.
EPA's indicators generally cover broad geographic scales and many years of data, as this is the most appropriate way to view
trends relevant to climate change. After all, the Earth is a complex system, and there will always be natural variations from one
year to the next—for example, a very warm year followed by  a colder year. The Earth's climate also goes through other natural
cycles that can play out over a period of several years or even decades. Thus, EPA's indicators present trends for as many years as
the underlying data allow.
For more information, see "Frequently Asked Questions  About EPA's Climate Change Indicators," available at: www.epa.gov/
climate-indicators/frequent-questions.

How Is This Report Useful?
Climate Change Indicators in the United States, 2016, is written with the primary
goal of informing readers' understanding of climate change.  It is also designed to
be useful for the  public, scientists, analysts, decision-makers, educators, and others
who can use climate change indicators as a tool  for:
•  Effectively communicating relevant climate science information in a sound,
   transparent, and easy-to-understand way.
•  Assessing trends in environmental  quality, factors that  influence the environ-
   ment, and effects on ecosystems and society.
•  Informing science-based decision-making.

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A Roadmap to the Report
The indicators are grouped into six chapters: Greenhouse Gases, Weather
and Climate, Oceans, Snow and Ice, Health and Society, and Ecosystems.
Some chapters also include a "Community Connection," "Tribal Connection,"
or "A Closer Look" feature that highlights a specific region, data record, or
area of interest. Several indicators highlight the important ways in which the
observed changes can have implications for human health.
Each indicator in this report fills one or two pages, and contains:
•  One or more graphics depicting changes over time.
•  Background on how the indicator relates to climate change.
•  What's Happening: Key points about what the indicator shows.
•  About the Indicator: A description of the data source and how the
   indicator was developed.
Additional resources that can provide readers with more information appear
at the end of the report (see Climate Change Resources on p. 84).
EPA's website provides a more complete version of each indicator, including
more  background information, additional graphs or maps in some cases,
downloadable data, interactive maps and animations for selected indicators,
and important notes to help readers interpret the data. EPA also compiles
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. All of this information is available on EPA's website
at: www.epa.gov/climate-indicators.

WHAT'S NEW IN 2016?
The 2016 report reflects previously reported indicators and has added the
following new indicators and features:
•  Seven new  indicators: River Flooding,  Coastal Flooding, Antarctic
   Sea Ice, Heat-Related Illnesses, West Nile Virus, Stream Temperature,
   and Marine Species Distribution.
•  Three expanded indicators: Arctic Sea Ice was expanded to look at
   changes in the length of the melt season. Similarly, Snow Cover now
   examines changes in the length of the snow cover season. Heat-Related
   Deaths has a new graph that focuses on heat-related cardiovascular
   disease deaths, including trends for specific at-risk groups.
•  Updated  indicators: Nearly all indicators have been updated with
   additional years  of data that have become available since the last report.
•  Tribal connection: The report  includes an example of stream tempera-
   ture trends in the Pacific Northwest and highlights how changes may
   affect salmon, a  tribally important resource.

LOOKING AHEAD
Indicators of climate change are expected to become even more numerous
and to depict even clearer trends in the future. EPA will continue to work
in partnership with coordinating bodies, such as the U.S. Global Change
Research Program, and with other agencies, organizations, and individuals
to collect and communicate useful data and to inform policies and programs
based on this knowledge. As new and more comprehensive indicator data
become available, EPA will continually update the indicators presented in
this report.
Understanding the Connections
Between Climate Change and
Human Health
It can be tempting to think of climate
change as something that affects other
places, other people, or something in the
distant future. However, climate change
already poses a very real threat to the
American people. One of the biggest
concerns is its effect on human health.
Scientists' understanding of how climate
change increases risks to human health
has advanced significantly in recent
years. In April 2016, the U.S. Global
Change Research Program (USGCRP)
published the largest-ever assessment of
the state of the science.2 Its conclusion:
Every American is vulnerable to the
health impacts associated with climate
change.
As the impacts increase, and as we learn
more about them and how best to track
them over time, government agencies
and communities are also finding new
ways to respond to climate-related
threats. In recognition of the growing
body of evidence about the health risks
of climate change, this edition of EPA's
climate change indicators report includes
new indicators on several health-related
topics, along with a special section on the
connections between climate change and
health (beginning on p. 53). In addition,
several indicators include boxes that
highlight topics related to human health.
These topics are highlighted with
the following icon:

Note, however, that improved under-
standing of human health risks due to
climate change does not necessarily
correspond to increased long-term data
for trend assessment. The USGCRP's
Climate and Health Assessment identifies
the importance of long-term environ-
mental health data and monitoring.
       'Y    it*
                           A*,

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     Understanding  Greenhouse  Gases
MAJOR GREENHOUSE GASES ASSOCIATED WITH HUMAN ACTIVITIES
The major greenhouse gases emitted into the atmosphere are carbon dioxide, methane, ni-
trous oxide, and fluorinated gases (see the table below). Some of these gases are produced
almost entirely by human activities; others come from a combination of natural sources and
human activities.
Many of the major greenhouse gases can remain in the atmosphere for tens to thousands of
years after being released. They become globally mixed in the lower part of the atmosphere,
called the troposphere (the first several miles above the Earth's surface), reflecting the
combined contributions of emissions sources worldwide from the past and present. Due to
this global mixing, the impact of emissions of these gases does not depend on where in the
world they are emitted. Also, concentrations of these gases are similar regardless of where
they are measured, as long as the measurement  is far from any large sources or sinks of that
gas.
Some other substances have much shorter atmospheric lifetimes (i.e., less than a year) but
are still relevant to climate change. Important short-lived substances that affect the climate
include water vapor, ozone in the troposphere, pollutants that lead to ozone formation, and
aerosols (atmospheric particles) such as black carbon and sulfates. Water vapor, tropospher-
ic ozone, and black carbon contribute to warming, while other aerosols produce a cooling
effect. Because these substances are short-lived, their climate impact can be influenced by
the location of their emissions, with concentrations varying greatly from place to place.
Several factors determine how strongly a particular greenhouse gas affects 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 warm-
ing potential, which measures how much a given amount of the greenhouse gas is estimated to contribute to global warming
over a specific period of time (for example, 100 years) after being emitted. For purposes of comparison, global warming potential
values are calculated in relation to carbon dioxide, which is assigned a global warming potential equal to 1. The table below
describes sources, lifetimes, and global warming potentials for several important long-lived greenhouse gases.

Major Long-Lived Greenhouse Gases and Their Characteristics
Gases and Substances
Included in This Report
This report focuses on most of
the major, well-mixed greenhouse
gases that contribute to the
vast majority of warming of the
climate. It also includes certain
substances with shorter atmo-
spheric lifetimes (i.e., less than a
year) that are relevant to climate
change. In addition to several
long-lived greenhouse gases, the
online version of the Atmospher-
ic Concentrations of Greenhouse
Gases indicator tracks concen-
trations of ozone in the layers of
the Earth's atmosphere, while
Figure 2 of the Climate Forcing
indicator on EPA's website shows
the influence of a variety of
short-lived substances.
Greenhouse
gas
Carbon
dioxide
Methane
Nitrous oxide
Fluorinated
gases
How it's produced
Emitted primarily through the burning of fossil fuels (oil, natural gas, and coal),
solid waste, and trees and wood products. Changes in land use also play a role.
Deforestation and soil degradation add carbon dioxide to the atmosphere, while
forest regrowth takes it out of the atmosphere.
Emitted during the production and transport of oil and natural gas as well as coal.
Methane emissions also result from livestock and agricultural practices and from
the anaerobic decay of organic waste in municipal solid waste landfills.
Emitted during agricultural and industrial activities, as well as during combustion
of fossil fuels and solid waste.
A group of gases that contain fluorine, including hydrofluorocarbons, perfluoro-
carbons, and sulfur hexafluoride, among other chemicals. These gases are emitted
from a variety of industrial processes and commercial and household uses and do
not occur naturally. Sometimes used as substitutes for ozone-depleting substanc-
es such as chlorofluorocarbons (CFCs).
Average lifetime in
the atmosphere
see below*
12. 4 years
121 years
A few weeks to
thousands of years
100-year
global warming
potential
1
28-36
265-298
Varies (the highest
is sulfur hexafluoride
at 23,500)
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.
Global warming potentials and lifetimes come from the Intergovernmental Panel on Climate Change's Fifth Assessment Report.'

* Carbon dioxide's lifetime cannot be represented with a single value because the gas is not destroyed over time, but instead moves among different parts of the
ocean-atmosphere-land system. Some of the excess carbon dioxide is absorbed quickly (for example, by the ocean surface), but some will remain in the atmo-
sphere for thousands of years, due in part to the very slow process by which carbon is transferred to ocean sediments.

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Summary of  Key  Points
                    U.S. Greenhouse Gas Emissions. In the United States, greenhouse gas emissions caused by
                    human activities increased by 7 percent from 1990 to 2014. Since 2005, however, total U.S.
                    greenhouse gas emissions have decreased by 7 percent. Electricity generation is the largest
                    source of greenhouse gas emissions in the United States, followed by transportation.

                    Global Greenhouse Gas Emissions. Worldwide, net emissions of greenhouse gases from hu-
                    man activities increased by 35 percent from 1990 to 2010. Emissions of carbon dioxide, which
                    account for about three-fourths of total emissions, increased by 42 percent over this period.


                    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 the current global atmospheric concentrations of carbon dioxide are unprecedented
                    compared with the past 800,000 years, even after accounting for natural fluctuations.

                    Climate Forcing. Climate  forcing  refers to a change in the Earth's energy balance, leading to
                    either a warming or cooling effect over time. An increase in the atmospheric  concentrations of
                    greenhouse gases produces a positive climate forcing, or warming effect. From 1990 to 2015,
                    the total warming effect from greenhouse gases added by humans to the Earth's atmosphere
                    increased by 37 percent. The warming effect associated with carbon dioxide  alone increased
                    by 30 percent.
                    U.S. and Global Temperature. Average temperatures have risen across the contiguous 48
                    states since 1901. Average global temperatures show a similar trend, and all of the top 10
                    warmest years on record worldwide have occurred since 1998. Within the United States,
                    temperatures in parts of the North, the West, and Alaska have increased the most.

                    High and Low Temperatures. Nationwide, unusually hot summer days (highs) have become
                    more common over the last few decades. Unusually hot summer nights (lows) have become
                    more common at an even faster rate. This trend indicates less "cooling off" at night. Although
                    the United States has experienced many winters with unusually low temperatures, unusually
                    cold winter temperatures have become less common—particularly very cold nights (lows).
                    U.S. and Global Precipitation. Total annual precipitation has increased over land areas in the
                    United States and worldwide. Since 1901, precipitation has increased at an average rate of 0.08
                    inches per decade over land areas worldwide. However, shifting weather patterns have caused
                    certain areas, such as 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. The prevalence of extreme single-day pre-
                    cipitation events remained fairly steady between 1910 and the 1980s but has risen substantially
                    since then. Nationwide, nine of the top 10 years for extreme one-day precipitation events have
                    occurred since 1990.

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                       ;
Tropical Cyclone Activity. Tropical storm activity in the Atlantic Ocean, the Caribbean, and the
Gulf of Mexico has increased during the past 20 years. Storm intensity is closely related to varia-
tions in sea surface temperature in the tropical Atlantic. However, changes in observation methods
over time make it difficult to know for sure whether a longer-term increase in storm activity has
occurred.
                          River Flooding. Increases and decreases in the frequency and magnitude of river flood events
                          vary by region. Floods have generally become larger across parts of the Northeast and Midwest
                          and smaller in the West, southern Appalachia, and northern Michigan. Large floods have become
                          more frequent across the Northeast, Pacific Northwest, and parts of the northern Great Plains, and
                          less frequent in the Southwest and the Rockies.
                          Drought. Over the period from 2000 through 2015, roughly 20 to 70 percent of the U.S. land area
                          experienced conditions that were at least abnormally dry at any given time. However, this index
                          has not been in use for long enough to compare with historical drought patterns.
                                A Closer Look: Temperature and Drought in the Southwest. The southwestern United
                                States is particularly sensitive to changes in temperature and thus vulnerable to drought,
                                as even a small decrease in water availability in this already arid region can stress natural
                                systems and further threaten water supplies.
                          Ocean Heat. Three independent analyses show that the amount of heat stored in the ocean has
                          increased substantially since the 1950s. Ocean heat content not only determines sea surface
                          temperature, but also affects sea level and currents.
                          Sea Surface Temperature. Ocean surface temperatures increased around the world during the
                          20th century. Even with some year-to-year variation, the overall increase is clear, and sea surface
                          temperatures have been consistently 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 of the world's oceans, sea level has risen at a rate of roughly
                          six-tenths of an inch per decade since 1880. The rate of increase has accelerated in recent years to
                          more than an inch per decade. Changes in sea level relative to the land vary by region. Along the
                          U.S. coastline, sea level has risen the most along the Mid-Atlantic coast and parts of the Gulf coast,
                          where some stations registered increases of more than 8 inches between 1960 and 2015. Sea level
                          has decreased relative to the land in parts of Alaska and the Pacific Northwest.
                      P
      A Closer Look: Land Loss Along the Atlantic Coast. As sea level rises, dry land and
      wetlands can turn into open water. Along many parts of the Atlantic coast, this problem is
      made worse by low elevations and land that is already sinking. Between 1996 and 2011, the
      coastline from Florida to New York lost more land than  it gained.
             Ar
            -I
Coastal Flooding. Flooding is becoming more frequent along the U.S. coastline as sea level rises.
Nearly every site measured has experienced an increase in coastal flooding since the 1950s. The
rate is accelerating in many locations along the East and Gulf coasts. The Mid-Atlantic region
suffers the highest number of coastal flood days and has also experienced the largest increases in
flooding.

Ocean Acidity. The ocean has become more acidic over the past few decades because of in-
creased levels of atmospheric carbon dioxide, which dissolves in the water. Higher acidity affects
the balance of minerals in the water, which can  make it more difficult for certain marine animals to
build their protective skeletons or shells.
8

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    Arctic Sea Ice. Part of the Arctic Ocean is covered by ice year-round. The area covered by ice is
    typically smallest in September, after the summer melting season. The annual minimum extent of
    Arctic sea ice has decreased over time, and in September 2012 it was the smallest ever recorded.
    The length of the melt season for Arctic ice has grown, and the ice has also become thinner,
    which makes it more vulnerable to further melting.
    Antarctic Sea Ice. Antarctic sea ice extent in September and February has increased somewhat
    over time. The September maximum extent reached the highest level on record in 2014—about
    7 percent larger than the 1981-2010 average. Slight increases in Antarctic sea ice are outweighed
    by the loss of sea ice in the Arctic during the same time period, however.

    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 are thawing earlier in spring compared with the
    early 1900s. All 14 lakes studied were found to be thawing earlier in the year, with thaw dates
    shifting earlier by up to 24 days over the past 110 years.
P
Community Connection: Ice Breakup in Two Alaskan Rivers. Regions in the far north are
warming more quickly than other parts of the world. Two long-running contests on the
Tanana and Yukon rivers in Alaska—where people guess the date when the river ice will
break up in the spring—provide a century's worth of evidence revealing that the ice on
these rivers is generally breaking up earlier in the spring than it once did.
    Snowfall. Total snowfall—the amount of snow that falls in a particular location—has decreased
    in most parts of the country since widespread records began in 1930. One reason for this decline
    is that nearly 80 percent of the locations studied have seen more winter precipitation fall in the
    form of rain instead of snow.

    Snow Cover. Snow cover refers to the area of land that is covered by snow at any given time.
    Between 1972 and 2015, the average portion of North America  covered by snow decreased at
    a rate of about 3,300 square miles per year, based on weekly measurements taken throughout
    the year. There has been much year-to-year variability, however. The length of time when snow
    covers the ground has become shorter by nearly two weeks since 1972, on average.

    Snowpack. The depth of snow on the ground (snowpack) in early spring decreased at more than
    90 percent of measurement sites in the western United States  between 1955 and 2016. Across all
    sites, snowpack depth declined by an average of 23 percent during this time period.

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                             Heat-Related Deaths. Since 1979, more than 9,000 Americans were reported to have died as a
                             direct result of heat-related illnesses such as heat stroke. The annual death rate is higher when
                             accounting for deaths in which heat was reported as a contributing factor, including the inter-
                             action of heat and cardiovascular disease. People aged 65+ are a particular concern: a growing
                             demographic group that is several times more likely to die from heat-related cardiovascular
                             disease than the general population. Considerable year-to-year variability and  certain limitations
                             of the underlying data for this indicator make it difficult  to determine whether  the United States
                             has experienced long-term trends in the number of deaths classified as "heat-related."

                             Heat-Related Illnesses. From 2001 to 2010, a total of about 28,000 heat-related hospitaliza-
                             tions were recorded across 20 states. Annual heat-related hospitalization rates ranged from
                             fewer than one case per 100,000 people in some states  to nearly four cases per 100,000 in
                             others. People aged 65+ accounted for more heat-related hospitalizations than any other age
                             group from 2001 to 2010, and males were hospitalized for heat-related illnesses more than twice
                             as often as females.
                             Heating and Cooling Degree Days. Heating and cooling degree days measure the difference
                             between outdoor temperatures and the temperatures that people find comfortable indoors. As
                             the U.S. climate has warmed in recent years, heating degree days have decreased and cooling
                             degree days have increased overall, suggesting that Americans need to use less energy for
                             heating and more energy for air conditioning.

                             Lyme Disease. Lyme disease is a bacterial illness spread by ticks that bite humans. Tick habitat
                             and populations are influenced by many factors, including climate. Nationwide, the rate of
                             reported cases of Lyme disease has approximately doubled since 1991. The number and distribu-
                             tion of reported cases of Lyme disease have increased in the Northeast and upper Midwest over
                             time, driven by multiple factors.
                             West Nile Virus. West Nile virus is spread by mosquitoes, whose habitat and populations are
                             influenced by temperature and water availability. The incidence of West Nile virus neuroinvasive
                             disease in the United States has varied widely from year to year and among geographic regions
                             since tracking began in 2002. Variation in disease incidence is affected by climate and many
                             other factors, and no obvious long-term trend can be detected yet through this limited data set.
                             Length of Growing Season. The length of the growing season for crops has increased in almost
                             every state. States in the Southwest (e.g., Arizona and California) have seen the most dramatic
                             increase. In contrast, the growing season  has actually become shorter in a few southeastern
                             states. The observed changes reflect earlier spring warming as well as later arrival of fall frosts.

                             Ragweed Pollen Season. Warmer temperatures and later fall frosts allow ragweed plants
                             to produce pollen later into the year, potentially prolonging the allergy season for millions of
                             people. The length of ragweed pollen season has increased at 10 out of 11 locations studied in
                             the central United States and Canada since 1995. The change becomes more pronounced from
                             south to north.
10

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     Wildfires. Of the 10 years with the largest acreage burned since 1983, nine have occurred since
     2000. Fires burn more land in the western United States than in the East.
     Streamflow. Changes in temperature, precipitation, snowpack, and glaciers can affect the rate
     of streamflow and the timing of peak flow. Over the last 75 years, minimum, maximum, and
     average flows have changed in many parts of the country—some higher, some lower. Most of
     the rivers and streams measured show peak winter-spring runoff happening at least five days
     earlier than  it did in the mid-20th century.

     Stream Temperature. Stream  temperatures have risen throughout the Chesapeake Bay region—
     the area of focus for this indicator. From 1960 through 2014, water temperature increased at 79
     percent of the stream sites measured in the region. Temperature has risen by an average of 1.2°F
     across all sites and 2.2°F at the sites where trends were statistically significant.
          Tribal Connection: Water Temperature in the Snake River. Between 1960 and 2015,
          water temperatures increased by 1.4°F in the Snake River at a site in eastern Washington.
          Several species of salmon use the Snake River to migrate and spawn, and these salmon
          play an important role in the diet, culture, religion, and economy of the region's Native
          Americans.
     Great Lakes Water Levels. Water levels in most of the Great Lakes appear to have declined
     in the last few decades. However, the most recent levels are all within the range of historical
     variation. Water levels in lakes are influenced by water temperature, which affects evaporation
     rates and ice formation.

     Bird Wintering Ranges. Some birds shift their range or alter their migration habits to adapt to
     changes in temperature or other environmental conditions. Long-term studies have found that
     bird species in North America have shifted their wintering grounds northward by an average of
     more than 40 miles since 1966, with several species shifting by hundreds of miles.

     Marine Species Distribution. The average center of biomass for 105 marine fish  and inverte-
     brate species along U.S. coasts shifted northward by about 10 miles between 1982 and 2015.
     These species also moved an average of 20 feet deeper. Shifts have occurred among several
     economically important fish and shellfish species. For example, American lobster, black sea
     bass, and red hake in the Northeast have moved northward by an average  of 119 miles.
     Leaf and Bloom Dates. Leaf growth and flower blooms are examples of natural events whose
     timing can be influenced by climate change. Observations of lilacs and honeysuckles in the
     contiguous 48 states suggest that first leaf dates and bloom dates show a great deal of year-to-
     year variability.
P
Community Connection: Cherry Blossom Bloom Dates in Washington, D.C. Peak bloom
dates of the iconic cherry trees in Washington, D.C., recorded since the 1920s, indicate
that cherry trees are blooming slightly earlier than in the past. Bloom dates are key to
planning the Cherry Blossom Festival, one of the region's most popular spring attractions.
                                                                                                   11

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         Greenhouse  Gases
Greenhouse gases from human activities are the most significant driver of
observed climate change since the mid-20th century.1 The indicators in this
chapter characterize emissions of the major greenhouse gases resulting from
human activities, the concentrations of these gases in the atmosphere, and
how emissions and concentrations have changed over time. When comparing
emissions of different gases, these indicators use a concept called "global
warming potential" to convert amounts of other gases into carbon dioxide
equivalents.
WHY DOES IT MATTER?

As greenhouse gas emissions from human activities increase, they build up
in the atmosphere and warm the climate, leading to many other changes
around the world—in the atmosphere, on land, and in the oceans. The
indicators in other chapters of this report illustrate many of these changes.
Such changes have both positive and negative effects on people, society,
and  the environment—including plants and animals. Because many of the
major greenhouse gases stay in the atmosphere for tens to thousands of
years after being released, their warming effects on the climate persist over
a long time and can therefore affect both present and future generations.

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                      U.S.  Greenhouse  Gas  Emissions
                    This indicator describes emissions of greenhouse gases in the United States.
A      number of factors influence the quantities of greenhouse gases released into the atmosphere, including economic
      activity, population, consumption patterns, energy prices, land use, and technology. There are several ways to track
      these emissions, such as by measuring emissions directly, calculating emissions based on the amount of fuel that
people burn, and estimating other activities and their associated emissions.
   U.S. Greenhouse Gas Emissions and Sinks by Economic Sector,
   1990-2014
                                        Commercial
                                                        Residential
         -1,000
        -2,000
                           Land use, land-use change, and forestry (net sink)
             1990  1992  1994  1996  1998  2000  2002  2004  2006  2008  2010  2012  2014

                                          Year

   This figure shows greenhouse gas emissions (positive values) and sinks (negative values),
   by source, in the United States from 1990 to 2014. For consistency, emissions are expressed
   in million metric tons of carbon dioxide equivalents. All electric power emissions are
   grouped together in the "Electricity generation" sector, so other sectors such as "Resi-
   dential" and "Commercial" show only non-electric sources, such as burning oil or gas for
   heating. The economic sectors shown here do not include emissions from U.S. territories
   outside the 50 states.  Data source: U.S. EPA, 20162
In 2014, U.S. greenhouse
gas emissions totaled 6,870
million metric tons (15.1 trillion
pounds) of carbon dioxide
equivalents. This 2014 total
represents a 7-percent increase
since 1990 but a 7-percent
decrease since 2005.
Among the various sectors of
the U.S. economy, electricity
generation (power plants) ac-
counts for the largest share of
emissions—31 percent of total
greenhouse gas emissions
since 1990. Transportation
is the second-largest sector,
accounting for 26 percent of
emissions since 1990.
Emissions sinks, the oppo-
site of emissions sources,
absorb carbon dioxide from
the atmosphere. In 2014,11
percent of U.S. greenhouse gas
emissions were offset by net
sinks resulting from land use
and forestry practices. Growing
forests remove carbon from
the atmosphere, outweighing
emissions from wildfires. Other
carbon emissions and sinks
result from crop practices,
burning biofuels, or depositing
yard trimmings and food
scraps in landfills.
ABOUT THE INDICATOR
This indicator focuses on emissions of carbon dioxide, methane, nitrous oxide, and several fluorinated gases. Data and
analysis for this indicator come from  EPA's annual Inventory of U.S. Greenhouse Gas Emissions and Sinks.3 This indicator
focuses on emissions associated with human activities, though some emissions and sinks from unmanaged lands are also
included. Each greenhouse gas has a different lifetime (how long it stays in the atmosphere) and a different ability to trap
heat in our atmosphere. To allow different gases to be compared and added together, emissions are converted into carbon
dioxide equivalents using each gas's 100-year global warming potential. This analysis uses global warming potentials from
the Intergovernmental Panel on Climate Change's Fourth Assessment Report. It starts in 1990, which is a common baseline
year for global agreements to track and reduce greenhouse gas emissions. Other parts of this indicator available online
track U.S. emissions by greenhouse gas, per capita, and per dollar of gross domestic product since 1990.
                                                                                                                   13

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                        Global Greenhouse  Gas  Emissions

                      This indicator describes emissions of greenhouse gases worldwide.

     Increasing emissions of greenhouse gases due to human activities worldwide have led to a substantial increase in atmospheric
     concentrations of these gases (see the Atmospheric Concentrations of Greenhouse Gases indicator on p. 15). Every
     country around the world emits greenhouse gases into the atmosphere, meaning the root cause of climate change is truly
    global in scope. Some countries produce far more greenhouse gases than others, and several factors—such as economic activity,
    population, income level, land use, and climatic conditions—can influence a country's emissions levels. Tracking greenhouse gas
    emissions worldwide provides a global context for understanding the United States' and other nations' roles in climate change.

    ABOUT THE INDICATOR
    Data and analysis for this indicator come from the World Resources Institute's Climate Analysis Indicators Tool, which compiles
    data from peer-reviewed and internationally recognized greenhouse gas inventories developed by EPA and other government
    agencies worldwide. The Climate  Analysis Indicators Tool includes estimates of emissions and sinks associated with land use
    and forestry activities, which come from global estimates compiled by the Food  and Agriculture Organization of the United
    Nations. Each greenhouse gas has a different lifetime (how long it stays in the atmosphere) and a different ability to trap heat
    in our atmosphere. To allow different gases to be compared and added together, emissions are converted into carbon dioxide
    equivalents using each gas's global warming  potential, which measures how much a given amount of the gas is estimated to
    contribute to global warming over a period of 100 years after being emitted. This analysis uses global warming potentials from
    the Intergovernmental  Panel on Climate Change's Second Assessment Report. Other parts of this indicator available online track
    global greenhouse gas emissions  by sector and region since 1990.
       Global Greenhouse Gas Emissions by Gas, 1990-2010
             50,000
       O  C  40,000
       ** .2
       B  >
       
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                     Atmospheric Concentrations  of
                    Greenhouse  Gases
                  This indicator describes how the levels of major greenhouse gases in the atmosphere have changed over time.
     Since the Industrial Revolution began in the 1700s, people have added a substantial amount of heat-trapping green-
     house gases into the atmosphere by burning fossil fuels, cutting down forests, and conducting other activities (see
     the U.S. and Global Greenhouse Gas Emissions indicators on pp. 13 and 14). Many of these gases remain in
the atmosphere for long time periods ranging from a decade to many millennia, which allows them to become well mixed
throughout the global atmosphere. As a result of human activities, these gases are entering the atmosphere more quickly
than they are being removed by chemical reactions or by emissions sinks, such as the oceans and vegetation, which absorb
greenhouse gases from the atmosphere. Thus, their concentrations are  increasing, which contributes to global warming.
   Global Atmospheric Concentrations of Carbon Dioxide Over Time


               800,000 BCE to 2015 CE               1950 to 2015 CE
   "£ 450
    a.
   3: 400
    C
   .° 350
    (Q
   £ 300
    C
    u 250
    C
    O
    u

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   JQ
    (5
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200

150

100

 50
       -800,000 -600,000 -400,000 -200,000

             Year (negative values = BCE)
                                  1950  1960 1970 1980 1990 2000 2010 2020

                                                 Year
   This figure shows concentrations of carbon dioxide in the atmosphere from hundreds of
   thousands of years ago through 2015, measured in parts per million (ppm). The data come
   from a variety of historical ice core studies and recent air monitoring sites around the world.
   Each line represents a different data source. Data source: Compilation of 10 underlying
   datasets6
Global atmospheric
concentrations of carbon
dioxide have risen signifi-
cantly over the last few
hundred years.
 Historical measurements
show that the current
global atmospheric
concentrations of carbon
dioxide are unprecedented
compared  with the past
800,000 years.
Since the beginning of the
industrial era, concentra-
tions of carbon dioxide
have increased from an
annual average of 280 ppm
in the late 1700s to 401
ppm as measured at Mauna
Loa in 2015—a 43-percent
increase. This increase is
due to human activities.7
ABOUT THE INDICATOR
This indicator describes concentrations of greenhouse gases in the atmosphere. The graph above focuses on carbon
dioxide, which accounts for the largest share of warming associated with human activities. Recent measurements come
from monitoring stations around the world, while measurements of older air come from air bubbles trapped in layers of ice
from Antarctica and Greenland. By determining the age of the ice layers and the concentrations of gases trapped inside,
scientists can learn what the atmosphere was like thousands of years ago. Other parts of this indicator available online track
global atmospheric concentrations of methane and nitrous oxide over the past 800,000 years and global atmospheric
concentrations of selected halogenated gases and ozone over the last few decades. Ozone acts as a greenhouse gas in the
lower atmosphere.
                                                                                                           15

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                         Climate Forcing
                       This indicator measures the "radiative forcing" or heating effect caused by greenhouse gases in the atmosphere.
            When energy from the sun reaches the Earth, the planet absorbs some of this energy and radiates the rest back to
            space as heat. A variety of physical and chemical factors—some natural and some influenced by humans—can shift
            the balance between incoming and outgoing energy, which forces changes in the Earth's climate. These changes
    are measured by the amount of warming or cooling they can produce, which is called "radiative forcing." Changes that have a
    warming effect are called "positive" forcing, while those that have a cooling effect are called "negative" forcing. When positive
    and negative forces are out of balance, the result is a change in the  Earth's average surface temperature. Greenhouse gases trap
    heat in the lower atmosphere and cause positive radiative forcing.
       Radiative Forcing Caused by Major Long-Lived Greenhouse Gases,
       1979-2015
           3.0
                                                           Carbon dioxide
                                                           Methane
                                                           Nitrous oxide
                                                        • CFC-12
                                                        • CFC-11
                                                         ~~ 15 other gases
                                                                                X
                                                                                01
                                                                               _C
                                                                                
                                                                                IB
                                                                           0.8   $
3
O


1
«
                                                                                3
                                                                                C
                                                                                c
0.6
                                                                           0.4
                                                                           0.2
              1980
                       1985
                               1990
                                       1995    2000

                                          Year
                                                        2005
                                                                2010
                                                                        2015
      This figure shows the amount of radiative forcing caused by various greenhouse gases, based
      on the change in concentration of these gases in the Earth's atmosphere since 1750. Radiative
      forcing is calculated in watts per square meter, which represents the size of the energy imbal-
      ance in the atmosphere. On the right side of the graph, radiative forcing has been converted
      to the Annual Greenhouse Gas Index, which is set to a value of 1.0 for 1990. Data source:
      NOAA, 2076s
In 2015, the Annual Green-
house Gas Index was 1.37,
which represents a 37-percent
increase in radiative forcing (a
net warming influence) since
1990.
Of the greenhouse gases
shown in the figure, carbon
dioxide accounts for by far
the largest share of radiative
forcing since 1990, and its
contribution continues to
grow at a steady rate. Carbon
dioxide alone would account
for a 30-percent increase in
radiative forcing since 1990.
Although the overall Annual
Greenhouse Gas Index
continues to rise, the rate of
increase has slowed somewhat
since the baseline year 1990,
in large part because methane
concentrations have increased
at  a slower rate in recent years
and because chlorofluoro-
carbon (CFC) concentrations
have been declining as
production of CFCs has been
phased out globally.
    ABOUT THE  INDICATOR
    This indicator measures the average total radiative forcing of 20 long-lived 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 these gases in the atmosphere, compared with the concentrations that were present around 1750, before
    the Industrial Revolution began. Because each gas has a different ability to absorb and emit energy, this indicator converts the
    changes in greenhouse gas concentrations into a measure of the total radiative forcing (warming effect) caused by each gas.
    The right side of the graph shows the Annual Greenhouse Gas Index, which compares the radiative forcing for a particular year
    with the radiative forcing in 1990, which  is a common baseline year for global agreements to track and reduce greenhouse gas
    emissions.
16

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        Weather  and  Climate
Rising global average temperature is associated with widespread changes in
weather patterns. Scientific studies indicate that extreme weather events such as
heat waves and large storms are likely to become more frequent or more intense
with human-induced climate change. This chapter focuses on observed changes
in temperature, precipitation, storms, floods, and droughts.
WHY DOES IT MATTER?
Long-term changes in climate can directly or indirectly affect many aspects of
society in potentially disruptive ways. For example, warmer average temperatures
could increase air conditioning costs and affect the spread of diseases like
Lyme disease, but could also improve conditions for growing some crops. More
extreme variations in weather are also a threat to society. More frequent and
intense extreme heat events can  increase illnesses and deaths, especially among
vulnerable populations, and damage some crops. While increased precipitation
can replenish water supplies and support agriculture, intense storms can damage
property; cause loss of life and population displacement; and temporarily disrupt
essential services such as transportation, telecommunications, energy, and water
supplies.
  WEATHER AND CLIMATE
  Weather is the state of the atmosphere at any given time and place. Most of the weather that affects people,
  agriculture, and ecosystems takes place in the lower layer of the atmosphere. Familiar aspects of weather
  include temperature, precipitation, clouds, and wind that people experience throughout the course of a day.
  Severe weather conditions include hurricanes, tornadoes, blizzards, and droughts.
  Climate is the long-term average of the weather in a given place. While the weather can change in minutes
  or hours, a change in climate is something that develops over longer periods of decades to centuries. Cli-
  mate is defined not only by average temperature and precipitation but also by the type, frequency, duration,
  and intensity of weather events such as heat waves, cold spells, storms, floods, and droughts.
  While the concepts of climate and weather are often confused, it is important to understand the difference.
  For example, the eastern United States experienced a cold and snowy winter in 2014/2015, but this short-
  term regional weather phenomenon does not negate the long-term rise in national and global temperatures,
  sea level, or other climate indicators.
                                                                                     17

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                        U.S.  and Global Temperature
                     This indicator describes trends in average surface temperature for the United States and the world.
            Warmer temperatures are one of the most direct signs that the climate is changing. Concentrations of heat-trapping
            greenhouse gases are increasing in the Earth's atmosphere (see the Atmospheric Concentrations of Greenhouse Gases
            indicator on p. 15). In response, average temperatures at the Earth's surface are increasing and are expected to
   continue rising. Because climate change can shift the wind patterns and ocean currents that drive the world's climate system,
   however, some areas are warming more than others, and some have experienced cooling.

   ABOUT THE INDICATOR
   This indicator is based on daily temperature records from thousands of long-term weather monitoring stations, which have
   been compiled by the National Oceanic and Atmospheric Administration's National Centers for Environmental Information. The
   indicator was developed by calculating annual anomalies, or differences, compared with the average temperature from 1901
   to 2000. For example, an anomaly of +2.0 degrees means the average temperature was 2 degrees higher than the long-term
   average. Daily, monthly, and annual anomalies have been calculated for each weather station. Global anomalies have been deter-
   mined by dividing the world into a grid, averaging the data for each cell of the grid, and then averaging the grid cells together.
   For the map, anomalies have been averaged together and compared over time within small regions called climate divisions. The
   online version of this indicator also includes a graph of annual temperature anomalies for the contiguous 48 states since 1901.
   Hawaii and U.S. territories are not included, due to limitations in available data.
      •WHAT'S HAPPENING

       Worldwide, 2015 was the
       warmest year on record
       and 2006-2015 was the
       warmest decade on record
       since thermometer-based
       observations began. Global
       average surface tempera-
       ture has risen at an average
       rate of 0.15°F per decade
       since 1901.
Temperatures Worldwide, 1901-2015
 m
 £
 o
 m
 
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Rate of Temperature Change in the United States, 1901-2015
                 Rate of temperature change (°F per century):
           -3.5
                       -2-101

                             Gray interval:-0.1 to0.1°F
                                                            3   3.5
This figure shows how annual average air temperatures have changed in different parts of
the United States since the early 20th century (since 7907 for the contiguous 48 states and
1925 for Alaska). The data are shown for climate divisions, as defined by the National Oceanic
and Atmospheric Administration. Data source: NOAA, 20162
•WHAT'S HAPPENING-^
  • The average surface tempera-
   ture across the contiguous 48
   states has risen at an average
   rate of 0.14°F per decade
   since 1901, which is similar to
   the global rate. Since 1979,
   the contiguous 48 states have
   warmed by 0.29°F to 0.46°F
   per decade, which is faster
   than the global rate.
  • Some parts of the United
   States  have experienced more
   warming than others. The
   North,  the West, and Alaska
   have seen temperatures
   increase the most, while some
   parts of the Southeast have
   experienced little change. Not
   all of these regional trends
   are statistically significant,
   however.
                                                                                                                   19

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                        High  and Low  Temperatures
                      This indicator describes trends in unusually hot and cold temperatures across the United States.
         Unusually hot or cold temperatures can result in prolonged extreme weather events like summer heat waves or winter cold
         spells. Heat waves can lead to illness and death, particularly among older adults, young children, and other populations
         of concern (see the Heat-Related Deaths and Heat-Related Illnesses indicators on pp. 60 and 62). People can also
    die from exposure to extreme cold (hypothermia). In addition, exposure to extreme 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. As the Earth's climate warms overall, however, heat waves are expected to
    become more frequent, longer, and more intense, while cold spells are expected to decrease.3'4
    ^WHAT'S HAPPENING

      • Nationwide, unusually hot
       summer days (highs) have
       become more common over
       the last few decades. Unusu-
       ally hot summer nights (lows)
       have become more common
       at an even faster rate. This
       trend indicates less "cooling
       off" at night.
     HEALTH
     CONNECTION
     As extremely hot temperatures
     become more common, people
     may be exposed to extreme heat
     more often. This could increase the
     risk of heat-related illnesses and
     deaths—particularly as nighttime
     temperatures rise and people
     are less able to cool off at night.
     Lack of air conditioning, working
     outdoors, and other social factors
     can increase exposure among
     certain groups.6
Area of the Contiguous 48 States With Unusually Hot Summer
Temperatures, 1910-2015
   90
   80
   70
	Hot daily highs
^— Hot daily highs (smoothed)
 — Hot daily lows
^— Hot daily lows (smoothed)
                                         10
                                          o
                                          1910   1920
                                                      1930
                                                                                        1990  2000  2010   2020
This graph shows the percentage of the land area of the contiguous 48 states with unusually
hot daily high and low temperatures during the months of June, July, and August. The thin
lines represent individual years, while the thick lines show a nine-year weighted average. Red
lines represent daily highs, while orange lines represent daily lows. The term "unusual" in this
case is based on the long-term average conditions at each location.
Data source: NOAA, 2015s
20

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   Area of the Contiguous 48 States With Unusually Cold Winter
   Temperatures, 1911-2016
   IB
   c
   IB
      90
      80
      70
      60
50
       	Cold daily highs
          Cold daily highs (smoothed)
       	Cold daily lows
          Cold daily lows (smoothed)

•WHAT'S HAPPENING^

  • The 20th century had many
   winters with widespread
   patterns of unusually low
   temperatures, including a
   particularly large spike in the
   late 1970s. Since the 1980s,
   though, unusually cold winter
   temperatures have become
   less common—particularly
   very cold nights (lows).
        1910   1920
                                                              2000   2010   2020
   This graph shows the percentage of the land area of the contiguous 48 states with unusu-
   ally 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" in this case is based on the long-term average conditions at each loca-
   tion. Data source: NOAA, 20167
ABOUT THE INDICATOR

This indicator is based on temperature measurements from weather stations overseen by the National Oceanic and Atmo-
spheric Administration's National Weather Service. 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. The figures show trends in the percentage of the country's
area experiencing unusually hot temperatures in the summer and unusually cold temperatures in the winter. These graphs
are based on daily maximum temperatures, which usually occur during the day, and daily minimum temperatures, which
usually occur at night. At each station, the recorded highs and lows are compared with the full set of historical records. After
averaging over a particular month or season of interest, the coldest 10 percent of years are considered "unusually cold" and
the warmest 10 percent are "unusually hot." Additional components of this indicator are available online, including a graph
of changes in annual heat wave index values, maps showing changes in unusually hot and cold days, and a graph that tracks
record daily high and low temperatures for the contiguous 48 states.
                                                                                                                     21

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                        U.S. and  Global  Precipitation
                      This indicator describes trends in average precipitation for the United States and the world.
        Precipitation can have wide-ranging effects on human well-being and ecosystems. Rainfall, snowfall, and the timing of
        snowmelt can all affect the amount of surface water and groundwater available for drinking, irrigation, and industry. They
        also influence river flooding and can determine what types of animals and plants (including crops) can survive in a par-
    ticular place. Changes in precipitation can disrupt a wide range of natural processes, particularly if these changes occur more
    quickly than plant and animal species can adapt. As average temperatures at the Earth's surface rise (see the U.S. and Global
    Temperature indicator on p. 18), more evaporation occurs,  which in turn increases overall precipitation. Therefore, a warming
    climate is expected to increase precipitation in many areas. Just as precipitation patterns vary across the world, however, so will
    the precipitation effects of climate change. Some areas will experience decreased precipitation. Also, because higher tempera-
    tures lead to more evaporation, increased precipitation will not necessarily increase the amount of water available for drinking,
    irrigation, and industry (see the Drought indicator on p. 28).
      •WHAT'S HAPPENING
       On average, total annual pre-
       cipitation has increased over
       land areas worldwide. Since
       1901, global precipitation has
       increased at an average rate
       of 0.08 inches per decade.
Precipitation Worldwide, 1901-2015
                                      u
                                      c
                                      _>»
                                      (Q
                                      E
                                      o
                                      c
                                      (Q
                                      C
                                      o
                                          -6
                                           1900  1910  1920  1930  1940  1950  1960  1970  1980  1990  2000  2010  2020

                                                                          Year

                                      This figure shows how the total annual amount of precipitation over land worldwide has
                                      changed since 1901. This graph uses the 1901-2000 average as a baseline for depicting
                                      change.  Choosing a different baseline period would not change the shape of the data over
                                      time. Data source: Blunden andArndt8

22

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   Change in Precipitation in the United States, 1901-2015
                           Percent change in precipitation:
                  -30
                         -20
                                 -10
                                        -2  2
                                                 10
                                                        20
                                                                30
   This figure shows the rate of change in total annual precipitation in different parts of the
   United States since the early 20th century (since 7907 for the contiguous 48 states and 1925
   for Alaska). The data are shown for climate divisions, as defined by the National Oceanic and
   Atmospheric Administration. Data source: NOAA, 20169
WHAT'S HAPPENING-

 •  On average, total annual
   precipitation has increased
   over land areas in the United
   States. Some parts of the
   United States have experi-
   enced greater increases in
   precipitation than others. A
   few areas, such as the South-
   west, have seen a decrease in
   precipitation. Not all  of these
   regional trends are statistical-
   ly significant, however.
ABOUT THE INDICATOR
This indicator is based on daily precipitation records from thousands of long-term weather monitoring stations, which have
been compiled by the National Oceanic and Atmospheric Administration's National Centers for Environmental Information.
The indicator was developed by looking at total annual precipitation at each weather station, comparing annual totals with
long-term (1901-2000) averages to determine annual anomalies (differences), and examining trends in anomalies over
time. Global anomalies have been determined by dividing the world into a grid, averaging the data for each cell of the grid,
and then averaging the grid cells together. For the map, anomalies have been averaged together and  compared over time
within small regions called climate divisions. The online version of this indicator also includes a graph of annual precipitation
anomalies for the contiguous 48 states since 1901. Hawaii and U.S. territories are not included, due to limitations in available
data.
                                                                                                                    23

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                         Heavy  Precipitation
                      This indicator tracks the frequency of heavy precipitation events in the United States.
         Heavy precipitation refers to instances during which the amount of rain or snow experienced in a location substantially
         exceeds what is normal. What constitutes a period of heavy precipitation varies according to location and season. Climate
         change can affect the intensity and frequency of precipitation. Warmer oceans increase the amount of water that evapo-
    rates 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.10 The potential impacts of heavy precipitation include crop damage,
    soil erosion, and an increase in flood risk (see the River Flooding indicator on  p. 26). In addition, runoff from precipitation can
    impair water quality as pollutants deposited on land wash into water bodies.
       Extreme One-Day Precipitation Events in the Contiguous 48 States,
       1910-2015
           25
            •WHAT'S HAPPENING^
             •  In recent years, a larger
               percentage of precipitation has
               come in the form of intense
               single-day events. Nine of the
               top 10 years for extreme one-
               day precipitation events have
               occurred since 1990.
             •  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 2015, the portion of the
               country experiencing extreme
               single-day precipitation events
               increased at a rate of about
               half a percentage  point per
               decade.
                                                                 2000   2010   2020
       This figure shows the percentage of the land area of the contiguous 48 states
       where a much greater than normal portion of total annual precipitation has
       come from extreme single-day precipitation events. The bars represent indi-
       vidual years, while the line is a nine-year weighted average.
       Data source: NOAA, 2016"
    ABOUT THE INDICATOR
HEALTH CONNECTION
Heavy precipitation events
followed by extreme flooding events can lead to
injuries and even drownings. Flooding can also
damage buildings, allowing water or moisture to
enter. This could lead to mold, bacteria, or other
air quality problems that have adverse effects on
health, such as worsening of asthma.12
    This indicator is based on precipitation measurements collected at weather
    stations throughout the contiguous 48 states. These data are compiled and managed by the National Oceanic and Atmospheric
    Administration's National Centers for Environmental Information. 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 (for example, inches of rain falling in a 24-hour period). One way to
    track heavy precipitation is by calculating what percentage of a particular location's total 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,
    as shown here. The results shown here are consistent with other methods of assessing changes in heavy precipitation, which
    also show increases over time.BThe online version of this indicator also tracks unusually high annual precipitation totals in the
    contiguous 48 states since 1895 using a scale called the Standardized Precipitation Index.
24

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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.
      Hurricanes, tropical storms, and other intense rotating storms fall into a general category called cyclones. The effects of
      tropical cyclones are numerous and well known. At sea, storms disrupt and endanger shipping traffic. When cyclones
      encounter land, their intense rains and high winds can cause severe property damage, loss of life, soil erosion, and
flooding. The associated storm surge—the large volume of ocean water pushed toward shore by the cyclone's strong
winds—can cause severe flooding and destruction. Climate change is expected to affect tropical cyclones by increasing sea
surface temperatures, a key factor that influences cyclone formation and behavior. The U.S. Global Change Research Pro-
gram and the Intergovernmental Panel on Climate Change project that, more likely than not, tropical cyclones will become
more intense over the 21st century, with higher wind speeds and heavier rains.14'15
   North Atlantic Tropical Cyclone Activity According to the Power
   Dissipation Index, 1949-2015
                                                                    83.2
                                                                    82.8
          *         Power
         / \   . Dissipation Index
                 Sea surfaced
                temperature
                                                                    81.2
       1950
               1960
                        1970
                                1980
                                        1990
                                                 2000
                                                         2010
                                                                 2020
                                    Year
   This figure presents annual values of the Power Dissipation Index, which accounts for cyclone
   strength, duration, and frequency. Tropical North Atlantic sea surface temperature trends
   are provided for reference. Note that sea surface temperature is measured in different units,
   but the values have been plotted alongside the Power Dissipation Index 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 (2011-2015) is plotted at 2013.
   Data source: Emanuel, 2016'6
•WHAT'S HAPPENING-
   • The Power Dissipation Index
    shows fluctuating cyclone
    intensity for most of the
    mid- to late 20th century,
    followed by a noticeable
    increase since 1995. These
    trends are shown with
    associated variations in sea
    surface temperature in the
    tropical North Atlantic, for
    comparison.
   • Despite the apparent
    increases in tropical cyclone
    activity during recent years,
    changes in observation
    methods over time make it
    difficult to know whether
    tropical storm activity has
    actually shown a lon-
    ger-term increase.17
ABOUT THE INDICATOR
This indicator is based on data maintained by the National Oceanic and Atmospheric Administration's National Hurricane
Center in a database referred to as HURDAT (HURricane DATa). It presents an analysis of HURDAT data using the Power
Dissipation Index developed by Dr. Kerry Emanuel at the Massachusetts Institute of Technology. This index tracks the
frequency, strength, and duration of tropical cyclones, based on measurements of wind speed. Other parts of this indicator
available online track long-term trends in the number of hurricanes in the North Atlantic Ocean, as well as tropical cyclone
activity according to another index called the Accumulated Cyclone Energy Index.
                                                                                                                  25

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                         River  Flooding
                      This indicator examines changes in the size and frequency of inland river flood events in the United States.
         Rivers and streams experience flooding as a natural result of large rain storms or spring snowmelt that quickly drains into
         streams and rivers. Climate change may cause these floods to become larger or more frequent than they used to be in
         some places, yet smaller and less frequent in other places. Warmer temperatures can lead to changes in the size and
    frequency of heavy precipitation events, which may in turn affect the size and frequency of river flooding (see the Heavy Precip-
    itation indicator on p. 24).18 Changes in streamflow, the timing of snowmelt (see the Streamflow indicator on p. 74), and
    the amount of snowpack that accumulates in the winter (see the Snowpack indicator on p. 52) can also affect flood patterns.
    Although regular flooding helps to maintain the nutrient balance of soils in the flood plain, larger or more frequent floods could
    damage homes, roads, bridges, and other infrastructure; wipe out farmers' crops; harm or displace people; contaminate water
    supplies; and disrupt ecosystems by displacing aquatic life, impairing water quality, and increasing soil erosion.
        WHAT'S HAPPENING

        Floods have generally
        become larger in rivers and
        streams across large parts of
        the Northeast and Midwest.
        Flood magnitude has gener-
        ally decreased in the West,
        southern Appalachia, and
        northern Michigan.
Change in the Magnitude of River Flooding in the United States,
1965-2015

I	
                                         Significant
                                         decrease
                     Insignificant
                      decrease
Insignificant
  increase
Significant
 increase
                                        This figure shows changes in the size of flooding events in rivers and streams in the United
                                        States between 1965 and2015. Blue upward-pointing symbols show locations where floods
                                        have become larger; brown downward-pointing symbols show locations where floods have
                                        become smaller. Larger, solid-color symbols represent stations where the change was statis-
                                        tically significant. Data source: Slater and Villarini, 2016'9
26

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   Change in the Frequency of River Flooding in the United States,
   1965-2015
                                           ***A   AA*
                                             *.  VAA*
          A'
                 s
                                                       •WHAT'S HAPPENING-
                                                         • Large floods have become
                                                           more frequent across the
                                                           Northeast, Pacific Northwest,
                                                           and parts of the northern
                                                           Great Plains. Flood frequen-
                                                           cy has decreased in some
                                                           other parts of the country,
                                                           especially the Southwest and
                                                           the Rockies.
                                                         • Increases and decreases in
                                                           frequency and magnitude of
                                                           river flood events generally
                                                           coincide with increases and
                                                           decreases in the frequency
                                                           of heavy rainfall events.2122
    Significant
     decrease
Insignificant
  decrease
Insignificant
  increase
Significant
 increase
   This figure shows changes in the frequency of flooding events in rivers and streams in
   the United States between 1965 and 2015. Blue upward-pointing symbols show loca-
   tions where floods have become more frequent; brown downward-pointing symbols
   show locations where floods have become less frequent. Larger, solid-color symbols
   represent stations where the change was statistically significant. Data source: Slater
   and VillarinL 201620
ABOUT THE INDICATOR
                                                   HEALTH CONNECTION
                                                   In addition to the physical health
                                                   impacts of weather-related disasters,
                                                   like drowning, injuries, or infections,
                                                   some people exposed to floods also
                                                   experience serious mental health
                                                   consequences. Extreme flood events
                                                   that involve loss of life or homes are
                                                   associated with long-term anxiety,
                                                   depression, and post-traumatic stress
                                                   disorder.25
This indicator is based on data from stream gauges maintained by the U.S. Geo-
logical Survey. Each gauge measures water level and discharge—the amount of
water flowing past the gauge. This indicator uses peak and daily discharge data
from a subset of long-term stream gauge stations to identify when the largest
flow events have happened and how the size and frequency of large flood events have changed over time. Besides climate
change, several other types of human influences could affect the frequency and magnitude of floods—for example, dams,
floodwater management activities, agricultural practices, and changes in land use. To minimize these influences, this indica-
tor focuses on a set of sites that are not heavily influenced by human activities. The analysis was developed and updated by
researchers at the University of Iowa.
                                                                                                                    27


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                         Drought
                      This indicator measures drought conditions of U.S. lands.
          Meteorologists generally define drought as a prolonged period of dry weather caused by a lack of precipitation that results
          in a serious water shortage for some activity, population, or ecological system. Drought can also be thought of as an
          extended imbalance between precipitation and evaporation. As average temperatures have risen due to climate change,
    evaporation has increased, making more water available in the air for precipitation, but contributing to drying over some land
    areas and less moisture in the soil. Drought conditions can negatively affect agriculture, water supplies, energy production,
    and many other aspects of society. Lower streamflow and groundwater levels can also harm plants and animals, and dried-out
    vegetation increases the risk  of wildfires.


    ABOUT THE  INDICATOR
    Drought can be measured by looking at precipitation, soil moisture, streamflow, vegetation health, and other variables.24The
    most widely used method is the Palmer Drought Severity Index, which is calculated from precipitation and temperature mea-
    surements at weather stations. The Palmer Index is shown in the graph below, based on data from the National Oceanic and
    Atmospheric Administration. The second graph shows a newer index called the Drought Monitor, which is based on a combi-
    nation of drought indices (including Palmer) plus additional factors such as snow water content, groundwater levels, reservoir
    storage, pasture/range conditions, and other impacts. The Drought Monitor uses codes from DO to D4 to classify drought
    severity. Drought  Monitor data were provided by the National Drought Mitigation Center.
    ^•WHAT'S HAPPENING

      •  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.
Average Drought Conditions in the Contiguous 48 States, 1895-2015
      HEALTH CONNECTION
      Rising temperatures and
      prolonged drought pose
      unique threats to indigenous
      populations because of their
      economic and cultural de-
      pendence on  land and water
      supplies. Warming and drought
      can threaten medicinal and
      culturally important plants and
      animals, and can reduce
      water quality  and availabili-
      ty, making tribal populations
      particularly vulnerable to
      waterborne illnesses.26
                                                                      Wet
                                                                                                              Dry
    1890 1900  1910  1920 1930 1940 1950  1960  1970 1980 1990  2000  2010 2020

                                   Year

This chart shows annual values of the Palmer Drought Severity Index, averaged over the
entire area of the contiguous 48 states. Positive values represent wetter-than-average
conditions, while negative values represent drier-than-average conditions. A value be-
tween -2 and -3 indicates moderate drought, - J to -4 is severe drought, and -4 or below
indicates extreme drought. The thicker line is a nine-year weighted average.
Data source: NOAA, 201625
28

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U.S. Lands Under Drought Conditions, 2000-2015

    100

     90

     80

     70
                  -R1

             DO  Abnormally dry
             D1  Moderate drought
             D2  Severe drought
             D3  Extreme drought
             D4  Exceptional drought
        2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015

                                        Year

This chart shows the percentage of U.S. lands classified under drought conditions from
2000 through 2015. 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.
Data source: National Drought Mitigation Center, 201627
Categories of Drought Severity
•WHAT'S  HAPPENING^
     •  Over the period from
       2000 through 2015,
       roughly 20 to 70 percent
       of the U.S. land area  ex-
       perienced conditions that
       were at least abnormally
       dry at any given time.
       The years 2002-2003
       and 2012-2013 had a
       relatively large area with
       at least abnormally dry
       conditions, while 2001,
       2005, and 2009-2011 had
       substantially less area
       experiencing drought.
     •  During the latter half of
       2012, more than half of the
       U.S. land area was covered
       by moderate or greater
       drought. In several states,
       2012 was among the driest
       years on record.28 See
       Temperature and Drought
       in the Southwest on  p.
       30 for a closer look at
       recent drought conditions
       in one of the hardest-hit
       regions.
  Category   Description
                              Possible Impacts
 DO
 D1
 D2
Abnormally
dry
Moderate
drought
Severe
drought
             Extreme
             drought
             Exceptional
             drought
Going into drought: short-term dryness 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, reser-
voirs, or wells low; some water shortages developing
or imminent; voluntary water use restrictions
requested.
Crop or pasture losses likely; water shortages
common; water restrictions imposed.
             Major crop/pasture losses; widespread water shortag-
             es 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 Agriculture, and the Na-
tional Drought Mitigation Center. For a map of current drought conditions, visit the Drought
Monitor website at: htto://droucihtmonitor. unl.edu.
                                                                                                                    29

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          A CLOSER LOOK: DROUGHT IN THE SOUTHWEST
                                                                                                Q
          Much of the American Southwest (Arizona, California, Colorado, Nevada, New Mexico, and Utah) experiences
          low annual rainfall and seasonally high temperatures that contribute to its characteristic arid climate. Yet this
          landscape actually supports a vast array of plants and animals, along with millions of people who call it home.
          Water is already scarce, so even a small increase in temperature (which drives evaporation) or a decrease in
          precipitation can threaten natural systems and society. Droughts also contribute to increased pest outbreaks and
          wildfires, and they reduce the amount of water available for generating electricity. The last decade has seen the
          most persistent droughts in the Southwest since recordkeeping began in 1895.
           Drought Severity in the Southwestern United States, 1895-2015
              1890 1900 1910  1920  1930  1940 1950 1960 1970  1980  1990 2000 2010 2020
                                             Year

          This chart shows annual values of the Palmer Drought Severity Index, averaged over six states in the Southwest (Arizona,
          California, Colorado, Nevada, New Mexico, and Utah). Positive values represent wetter-than-average conditions, while negative
          values represent drier-than-average conditions. A value between -2 and -J indicates moderate drought, -3 to -4 is severe
          drought, and -4 or below indicates extreme drought. The thicker line is a nine-year weighted average.
          Data source: NOAA, 201629
30

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                    *
       - "•   i
                   : :"   : •-

            Oceans
Covering about 70 percent of the Earth's surface, the world's oceans have a
two-way relationship with weather and climate. The oceans influence the weather
on local to global scales, while changes in climate can fundamentally alter many
properties of the oceans. This chapter examines how some of these important
characteristics of the oceans have changed over time.
WHY DOES IT MATTER?
As greenhouse gases trap more energy from the sun, the oceans are absorbing
more heat, resulting in an increase in sea surface temperatures and rising sea
level. Changes in ocean temperatures and currents brought about by climate
change will lead to alterations in climate patterns around the world. For example,
warmer waters may promote the development of stronger storms in the tropics,
which can cause property damage and loss of life. The impacts associated with
sea level rise and stronger storms are especially relevant to coastal communities.

Although the oceans help reduce climate change by storing large amounts of
carbon dioxide, increasing levels of dissolved  carbon are changing the chemistry
of seawater and making it more acidic. Increased ocean acidity makes it more
difficult for certain organisms, such as corals and shellfish, to build their skeletons
and shells.  These effects, in turn, could substantially alter the biodiversity and
productivity of ocean ecosystems.

Changes in ocean systems generally occur over much longer time periods than in
the atmosphere, where storms can form and dissipate in a single day. Interactions
between the oceans and atmosphere occur slowly over many months to years,
and so does the movement of water within the oceans, including the mixing
of deep and shallow waters. Thus, trends can  persist for decades, centuries,
or longer. For this reason, even if greenhouse gas emissions were stabilized
tomorrow,  it would take many more years—decades to centuries—for the oceans
to adjust to changes in the atmosphere and the climate that have already
occurred.
                                                                                31

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                         Ocean  Heat
                      This indicator describes trends in the amount of heat stored in the world's oceans.
            When sunlight reaches the Earth's surface, the world's oceans absorb this energy as heat, which currents distribute
            around the world. Water has a much higher heat capacity than air, meaning that oceans can absorb large amounts
            of heat with only a slight increase in temperature. As a result, increasing concentrations of heat-trapping greenhouse
    gases have not caused the oceans to warm as much as the atmosphere, even though they have absorbed more than 90 percent
    of the Earth's extra heat since 1955.12 If not for the large heat-storage capacity provided by the oceans, the atmosphere would
    grow warmer more rapidly.3 Water temperature reflects the amount of heat in the water at a particular time and location, and
    it plays an important role in the Earth's climate system, because heat from ocean surface waters provides energy for storms,
    influences weather patterns, and can change ocean currents. 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. 34).
    xWHAT'S HAPPENING
       In three different data analyses,
       the long-term trend shows
       that the oceans have become
       warmer since 1955.
       Although concentrations of
       greenhouse gases have risen
       at a relatively steady rate over
       the past few decades (see the
       Atmospheric Concentrations
       of Greenhouse Gases indicator
       on p. 15), the rate of change
       in ocean heat content can vary
       from year to year. Year-to-year
       changes are influenced by
       events such as volcanic erup-
       tions and recurring ocean-atmo-
       sphere patterns such as El Nino.
Ocean Heat Content, 1955-2015
   20
                                                1960
                                                            1970
                                                                       1980
                                                                                  1990
                                                                                             2000
                                                                                                        2010
                                                                            Year
                                      This figure shows changes in ocean heat content between 1955 and2015. Ocean heat content
                                      is measured in joules, a unit of energy, and compared against the 1971-2000 average, which
                                      is set at zero for reference. Choosing a different baseline period would not change the shape
                                      of the data over time. The lines were independently calculated using different methods by
                                      government agencies in three countries. For reference, an increase of 1 unit on this graph (1
                                      x 1022 joules) is equal to approximately 18 times the total amount of energy used by all the
                                      people on Earth in a year.4 Data sources: CSIRO, 2016;5MRI/JMA, 2016;6 NOAA, 20167
    ABOUT THE INDICATOR
    This indicator measures changes in the amount of heat energy stored in the ocean, based on measurements of ocean tem-
    peratures 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 are carefully adjusted to account for differences among
    measurement techniques and data collection programs. This indicator is based on analyses conducted by government agencies
    in three countries: the National Oceanic and Atmospheric Administration, the Japan Meteorological Agency's Meteorological
    Research Institute, and Australia's Commonwealth Scientific and Industrial  Research Organisation.
32

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                    Sea  Surface Temperature
                   This indicator describes global trends in sea surface temperature.
      As the oceans absorb more heat, sea surface temperature increases and the ocean circulation patterns that transport
      warm and cold water around the globe change, affecting which species are present in marine ecosystems, altering
      migration and breeding patterns, threatening corals, and changing the frequency and intensity of harmful algal
blooms.8 Over the long term, increases in sea surface temperature could weaken the circulation patterns that bring nutrients
from the deep sea to surface waters, contributing to declines in fish populations that would affect people who depend on
fishing for food or jobs.9 Higher sea surface temperature causes an increase in the amount of atmospheric water vapor,
which increases the risk of heavy rain and snow (see the Heavy Precipitation and Tropical Cyclone Activity indicators on pp.
24 and 25).10 Changes in sea surface temperature can also shift storm tracks, potentially contributing to droughts in
some areas.11
   Average Global Sea Surface Temperature, 1880-2015
       2.0
       1.5
      -2.0
        1880
                 1900
                          1920
                                   1940      1960

                                       Year
                                                     1980
                                                              2000
                                                                       2020
   This graph shows how the average surface temperature of the world's oceans has changed
   since 1880. This graph uses the 1971 to 2000 average as a baseline for depicting change.
   Choosing a different baseline period would not change the shape of the data over time.
   The shaded band shows the range of uncertainty in the data, based on the number of
   measurements collected and the precision of the methods used.
   Data source: NOAA, 201&2
                   •WHAT'S HAPPENING•>
                      • Sea surface temperature
                       increased during the 20th
                       century and continues to
                       rise. From 1901 through
                       2015, temperature rose at an
                       average rate of 0.13°F per
                       decade.
                      • Sea surface temperature
                       has been consistently higher
                       during the past three de-
                       cades than at any other time
                       since reliable observations
                       began in 1880.
                      • Based on the historical
                       record, increases in sea
                       surface temperature have
                       largely occurred over two
                       key periods: between 1910
                       and 1940, and from about
                       1970 to the present. Sea
                       surface temperature appears
                       to have cooled between
                       1880 and 1910.
                                               v,-,
ABOUT THE INDICATOR
The global average sea surface temperature data shown here
are derived from the Extended Reconstructed Sea Surface
Temperature analysis developed by the National Oceanic and
Atmospheric Administration's (NOAA's) National Centers
for Environmental Information. Temperature measurements
are collected from ships, as well as at stationary and drifting
buoys. NOAA has carefully reconstructed and filtered the data in the figure to correct for biases in different collection tech-
niques and to minimize the effects of sampling changes over various locations and times. The data are shown as anomalies,
or differences, compared with the average sea surface temperature from 1971 to 2000. The online version of this indicator
also presents global changes in sea surface temperature in a map.
HEALTH CONNECTION
Rising sea surface temperature means that Vibrio bacteria
and blooms of harmful algae can occur in new places or at
new times of the year. Humans can be exposed to Vibrio
and algal toxins by eating contaminated seafood or through
direct contact with contaminated drinking or recreational
waters. Vibrio can cause gastrointestinal illness and blood-
stream infections; algal toxins can cause gastrointestinal
illness and neurologic symptoms; and both can cause death
in severe cases.15
                                                                                                                33

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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.
          As the temperature of the Earth changes, so does sea level. Temperature and sea level are linked for two main reasons.
          First, changes in the volume of water and ice on land (namely glaciers and ice sheets) can increase or decrease the vol-
          ume of water in the ocean (see the Glaciers indicator on p. 44). Second, 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. 32). Rising sea level inundates
    low-lying wetlands and dry land, erodes shorelines, contributes to coastal flooding, and increases the flow of salt water into
    estuaries and nearby groundwater aquifers. Higher sea level also makes coastal infrastructure more vulnerable to damage from
    storms, due to an increased likelihood of flooding from higher storm surges.
       Global Average Absolute Sea Level Change, 1880-2015

           12
        0)
        u

           10
        01   a
        C   8
        IB
       -C
       2   6
        0)

       —   4
        (Q
        0)
        (A

       .1   2
       *3
        m
^ Trend based on tide gauges
^— Satellite measurements









            1880
                      1900
                               1920
                                         1940      1960
                                             Year
                                                           1980
                                                                     2000
                                                                              2020
       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 mea-
       sured 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,  20,000years ago.) The shaded band
       shows the likely range of values, based on the number of measurements collected and the pre-
       cision of the methods used. Data sources: CSIRO, 2015;'4 NOAA, 201&5
•WHAT'S HAPPENING •
• 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
  recent years.16 When averaged
  over all of the world's oceans,
  absolute sea level has risen at
  an average rate of 0.06 inches
  per year from 1880 to 2013.
  Since 1993, however, average
  sea level has risen at a rate of
  0.11 to 0.14 inches per year—
  roughly twice as fast as the
  long-term trend.
    ABOUT THE INDICATOR
    Scientists measure sea level change in two different ways. Relative sea level change refers to 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  nearby land. This distinction matters because the land itself can rise
    or fall relative to the ocean—rising due to processes such as sediment accumulation and geological uplift, or falling because of
    erosion, sediment compaction, natural subsidence, groundwater withdrawal, or engineering projects that prevent rivers from
    naturally depositing sediments along their banks. The first graph above shows absolute sea level change averaged across the
    Earth's oceans since 1880. The long-term trend is based on tide gauges, which  measure relative sea level change but have been
    adjusted to show absolute trends through calibration with recent satellite data. The second graph shows the change in relative
    sea level based on tide gauges that have measured water levels at 67 points along the U.S. coast since at least 1960. The Na-
    tional Oceanic and Atmospheric Administration and Australia's Commonwealth Scientific and Industrial Research Organisation
    compiled the data for this indicator.
34

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"\
• Relative sea level rose along Relative Sea Level Change Along U.S. Coasts, 1960-2015
much of the U.S. coastline
between 1960 and 2015,
particularly the Mid-Atlantic
coast and parts of the Gulf
coast, where some stations
registered increases of more
than 8 inches. 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 eleva-
tion has risen more rapidly.
• 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.




















This map shows cumulative
changes in relative sea level
from 1960 to 2015 at tide gauge
stations along U.S. coasts. Relative
sea level reflects changes in sea
level as well as land elevation.
Data source: NOAA, 2016'7
<,


Ml




1






Alaska

'T'"" |

1 1



























_

tt}
A.
ItM

|

Hawaii and
Pacific Islands
'ttj



* i'1'
1
tt
# '
S
If
1 ^



























Relative sea level change (inches):
-7.99 -5.99 -3.99
<-S to -6 to -4 to -2
I 1 1 '
V




-1.99
too
0.01 2.01
to 2 to 4


T T t
4.01 6.01 > 8
to 6 to 8

J
A CLOSER LOOK: LAND LOSS ALONG THE ATLANTIC COAST
                                                                                            a
The Atlantic coast is particularly vulnerable to rising sea level because of its low elevations and sinking shorelines. The
graph below shows the amount of land lost to sea level rise along the Atlantic coast from Florida to New York, dividing
the Atlantic coast into two regions for purposes of comparison. It is based on satellite imagery from the National
Oceanic and Atmospheric Administration's Coastal Change Analysis Program. These data have been collected and ana-
lyzed at five-year intervals since 1996. Roughly 20 square miles of dry land and wetlands were converted to open water
along the Atlantic coast between 1996 and 2011. More of this loss occurred in the Southeast than in the Mid-Atlantic.
Land Loss Along the Atlantic Coast, 1996-2011
3
E
3
u
      25

      20

      15

      10

       5

       0

      -5
             '
                 Mid-Atlantic
                 Southeast
                                                                                                Mid-Atlantic
                1996-2001
                                                                                           Southeast
                                     1996-2006
                                   Time period
                                                          1996-2011
This graph shows the net amount of land converted to open water along the Atlantic coast during three time periods: 1996-2001,
1996-2006, and 1996-2011. The results are divided into two regions: the Southeast and the Mid-Atlantic. Negative numbers show
where land loss is outpaced by the accumulation of new land. Data source: NOAA, 2013'8
                                                                                                               35

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                          Coastal  Flooding
                       This indicator shows how the frequency of coastal flooding has changed over time.
          As sea level rises relative to the coast due to climate change (see the Sea Level indicator on p. 34), one of the most
          noticeable consequences is an increase in coastal flooding during high tide and during storm surges. Many coastal cities
          have defined minor or "nuisance" flooding thresholds. When water rises above this level, minor flooding typically occurs
    in some streets, many storm drains become ineffective, and a coastal flood advisory may be issued. Recurrent coastal flooding
    can cause impacts such as frequent road closures, reduced stormwater drainage capacity, and deterioration of infrastructure not
    designed to withstand frequent inundation or exposure to salt water. Rising sea level increases the likelihood of flooding at high
    tide and during storm surges, and it also inundates low-lying wetlands and dry land, erodes shorelines, and increases the flow
    of salt water into estuaries and nearby groundwater aquifers. Millions of Americans and more than $1 trillion of property and
    infrastructure are at risk of damage from coastal flooding.19
    ^WHAT'S HAPPENING

        Flooding is becoming
        more frequent along the
        U.S. coastline. Nearly
        every site measured has
        experienced an increase
        in coastal flooding since
        the 1950s. The rate is
        accelerating in many
        locations along the East
        and Gulf coasts.
        The Mid-Atlantic region
        suffers the  highest num-
        ber of coastal flood days
        and has also experienced
        the largest  increases in
        flooding. Since 2010,
        Wilmington, North Car-
        olina, has flooded  most
        often—49 days per year-
        followed by Annapolis,
        Maryland, at 46 days per
        year. Annapolis, Wilm-
        ington, and two locations
        in New Jersey (Sandy
        Hook and Atlantic City)
        have also seen some of
        the most dramatic overall
        increases in frequency:
        floods are now at least
        10 times more common
        there than they were in
        the 1950s. The Mid-At-
        lantic's subsiding land
        and higher-than-average
        relative sea level rise
        both contribute to this
        increase in flooding (see
        the Sea Level indicator on
        p. 34).
Frequency of Flooding Along U.S. Coasts, 2010-2015 Versus 1950-1959
            > Seattle, WA
                                                                       Boston, MA
                                                                      mfm
       | San Francisco, CA
            I LaJolla,CA
               New London, CT

            Kings Point, NY| ~~U^vidence, Rl

         Sandy Hook, NJ ™   »t  VjWontauk, NY
     Philadelphia, PA_j     . »\ Battery, NY

        Balti
Washington, DC

      Annapolis, MD

        Wilmington, NC


   FortPulaski.GA •—•
                                          ,    St. Petersburg, FL
                                          \— GalvestonBay.TX
                                          | Port Isabel,TX
                                                                       Sewells Point, VA
                                                                    1 Charleston, SC
                                                                  Fernandina Beach, FL
                                                                  Mayport, FL
                                                                --— Key West, FL
                Average number of flood days per year:
                                                     o
         J
                                                    1950s 2010s
This map shows the average number of days per year in which coastal waters rose above the local
threshold for minor flooding at 27 sites along U.S. coasts. Each small bar graph compares the first
decade of widespread measurements  (the 1950s in orange) with the most recent decade (the
2010s in purple). Data source: NOAA, 201620
36

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     Average Number of Coastal Flood Events per Year, 1950-2015
          Boston, MA

        Providence, Rl •

       New London, CT '

         Montauk,NY •

        Kings Point, NY

          Battery, NY

       Sandy Hook, NJ

       Atlantic City, NJ

       Philadelphia, PA

           Lewes, DE

        Baltimore, MD

        Annapolis, MD

       Washington, DC

      Sewells Point, VA

       Wilmington, NC

        Charleston, SC

       Fort Pulaski, GA

   Fernandina Beach, FL •

          Mayport,FL ^_

          Key West, FL L

      St. Petersburg, FL

      Galveston Bay, TX [

        Port Isabel, TX

          LaJolla,CA

      San Francisco, CA

          Seattle, WA

          Honolulu, HI

                   0
1950-1969
1970-1989
1990-2009
2010-2015
•WHAT'S HAPPENING^

    •  Flooding has increased less
      dramatically in places where
      the local flood threshold
      is higher (for example, the
      Northeast and locations
      on the Gulf of Mexico) or
      where relative sea level
      has not risen as quickly
      as it has elsewhere in the
      United States (for example,
      Hawaii and  the West Coast,
      as shown by the Sea Level
      indicator on p. 34).
                              10          20          30         40
                              Average number of flood events per year
                        This graph shows the
                        average number of days per
                        year in which coastal waters
                        rose above the local thresh-
                        old for minor flooding at 27
                        sites along U.S. coasts. The
                        data have been averaged
                        over multi-year periods for
                        comparison.
                        Data source: NOAA, 20162'
                                       ^ A
ABOUT THE INDICATOR
Coastal flooding trends in this indicator are based on measurements
from 27 permanent tide gauge stations along U.S. coasts where local
weather forecasting offices have defined thresholds for minor, moderate,
and major flooding and where complete data are available from 1950 to
present. The indicator tracks the number of days per year when each tide
gauge measured water that was higher than the minor flooding level. The
original tide gauge data and the analysis come from the National Oceanic
and Atmospheric Administration, which derived daily maximum water
levels from hourly data.
                                                                            50
           HEALTH CONNECTION
           Recurrent coastal flooding can increase the
           risk that drinking water and wastewater
           infrastructure will fail, putting people at
           risk of exposure to pathogens and harmful
           chemicals.22 Heavy rain during high tides can
           lead to flooding of basements and standing
           water in streets, which can also harbor
           disease-carrying vectors such as mosquitoes.
           Extreme flood events that involve loss of life
           or homes are also associated with long-term
           anxiety, depression, and post-traumatic stress
           disorder.25
                                                                                                                      37

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                        Ocean  Acidity
                       This indicator describes changes in the chemistry of the ocean that relate to the amount of carbon dioxide dissolved
                       in the water.
          As the concentration of carbon dioxide in the atmosphere increases, the ocean absorbs more of it. Over the past 250 years,
          oceans have absorbed about 28 percent of the carbon dioxide produced by human activities that burn fossil fuels.24
          Rising levels of carbon dioxide dissolved in the ocean negatively affect some marine life, because carbon dioxide reacts
    with sea water to produce carbonic acid. The increase in acidity changes the balance of minerals in the water and makes it
    more difficult for corals and plankton to produce the mineral calcium carbonate, which is the primary component of their hard
    skeletons and shells. Resulting declines in coral and  plankton populations can change marine ecosystems and ultimately affect
    fish populations and the people who depend on them.25 Signs of damage are already starting to appear in certain areas.26

    ABOUT  THE INDICATOR
    This indicator describes trends in pH and related properties of ocean water, based on a combination of direct observations, cal-
    culations, and modeling. The graph shows pH values and levels of dissolved 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 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 online version of this indicator shows a map of changes in aragonite
    saturation of the world's oceans. Aragonite is a form of  calcium carbonate that many organisms produce and use to build their
    protective skeletons or shells. Saturation state is a measure of  how easily aragonite can dissolve in the water.
    ^•WHAT'S HAPPENING-
       Measurements made over the
       last few decades have demon-
       strated 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).
Ocean Carbon Dioxide Levels and Acidity, 1983-2015
                          Bermuda
8.20
8.15
8.10
8.05
8.00
                                                                Bermuda
                                                                                7.95
                                                                     2020
                                                                                   1980   1990  2000   2010   2020
                                          8.20
                                          8.15
                                          8.10
                                          8.05
                                          8.00
                                                                                                   Canary Islands
                                                                                7.95
                                                                     2020
                                                                                   1980   1990  2000   2010   2020
            1990
                                                          I   I   I   I   I
                                                         2000   2010   2020
                                                         Year
                                                                                8.20
                                                                                8.15
                                                                                8.10
                                                                                8.05
                                                                                8.00
                                                                  Hawaii
                                          7.95
                                             1980   1990
                                                         2000
                                                         Year
                                                               2010   2020
                                      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 three observation
                                      stations. The up-and-down pattern shows the influence of seasonal variations.
                                      Data sources: Bates, 2016;27Gonzalez-Davila, 2012;28 Dore, 201529
38

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          Snow  and  Ice
The Earth's surface contains many forms of snow and ice, including sea,
lake, and river ice; snow cover; glaciers, ice caps, and ice sheets; and frozen
ground. Climate change can dramatically alter the Earth's snow- and ice-
covered areas because snow and ice can easily change between solid and
liquid states in response to relatively minor changes in temperature. This
chapter focuses on trends in snow, glaciers, and the freezing and thawing of
oceans and lakes.
WHY DOES IT MATTER?
Reduced snowfall and less snow cover on the ground could diminish the
beneficial insulating effects of snow for vegetation and wildlife, while also
affecting water supplies, transportation, cultural practices, travel, and
recreation for millions of people. For communities 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
buildings and accelerate erosion. Conversely, reduced snow and ice could
present commercial opportunities for others, including ice-free shipping lanes
and increased access to natural resources.

Such changing climate conditions can have worldwide implications because
snow and ice influence air temperatures, sea level, ocean currents,  and storm
patterns. For example, melting ice sheets on Greenland and Antarctica add
fresh water to the ocean, increasing sea level and possibly changing ocean
circulation that is driven by differences in temperature and salinity. Because
of their light color, snow and ice also reflect more sunlight than open water or
bare ground, so a reduction in snow cover and ice causes the Earth's surface
to absorb more energy from the sun and become warmer.
                                                                             39

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                        Arctic Sea  Ice
                      This indicator tracks the extent, age, and melt season of sea ice in the Arctic Ocean.
        Sea ice is an integral part of the Arctic Ocean. Each year some of this ice melts during the summer because of warmer tem-
        peratures and sunlight, typically reaching its minimum thickness and extent in mid-September. The ice freezes and begins
        expanding again in the fall. Sea ice extent is an important indicator of global climate change because warmer air and water
    temperatures are reducing the amount of sea ice present. Sea ice reflects sunlight, which helps to keep polar regions cool. Sea
    ice is also important because it provides habitat for animals such as polar bears and walruses, and because wildlife and ice travel
    are vital to the traditional subsistence lifestyle of indigenous Arctic communities.
    s-WHAT'S HAPPENING
      • September 2012 had the
        lowest sea ice extent ever
        recorded, 44 percent below
        the 1981-2010 average for that
        month.
      • The September 2015 sea
        ice extent was more than
        700,000 square miles less
        than the historical 1981-2010
        average for that month—a
        difference more than two
        and a half times the size of
        Texas. March sea ice extent
        reached the lowest extent on
        record in 2015 and hit roughly
        the same low again in 2016—
        about 7 percent less than the
        1981-2010 average.
Dwindling Arctic Sea Ice
 ^^^^^^^m
 September 1979
                                                                                 Source: NASA, 2016'
                                     March and September Monthly Average Arctic Sea Ice Extent, 1979-2016
                                           1975    1980    1985    1990   1995   2000   2005   2010   2015   2020

                                                                          Year
                                     This figure shows Arctic sea ice extent for the months of September and March of each year
                                     from 1979 through September 2015 and March 2016. September and March are when the mini-
                                     mum and maximum extent typically occur each year. Data source: NSIDC, 20162
40

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    Age of Arctic Sea Ice at Minimum September Week, 1983-2015
               1985
                                                               2010
                                                                        2015
    This figure shows the distribution of Arctic sea ice extent by age group during the week in
    September with the smallest extent of ice for each year. Total extent differs between this
    figure and the one on p. 40 that one shows a monthly average, while this one shows condi-
    tions during a single week. Data source: NSIDC, 2075J
    Arctic Sea Ice Melt Season, 1979-2015

       Mayl
                                   Start of melt season
       May 31                          ^    T
                                                                         Length
                                                                         of melt
                                                                         season
       Sep28
       Oct28
•WHAT'S HAPPENING-

    • Evidence of the age of
      Arctic sea ice suggests
      that fewer patches of ice
      are persisting for multiple
      years (i.e., generally thick
      ice that has survived one
      or more melt seasons).
      The proportion 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 9
      percent in 2015. A growing
      percentage of Arctic sea
      ice is only one or two years
      old. Less old multi-year ice
      implies that the ice cover
      is thinning, which makes it
      more vulnerable to further
      melting.
    • Since 1979, the length of
      the melt season  for Arctic
      sea ice has grown by 37
      days. Arctic sea ice now
      starts melting 11  days ear-
      lier and it starts refreezing
      26 days later than it used
      to, on average.
            1980
                    1985
                            1990
                                   End of melt sea son
                                    1995     2000

                                        Year
                                                    2005
                                                            2010
                                                                    2015
    This figure shows the timing of each year's Arctic sea ice melt season. The shaded band spans
    from the date when ice begins to melt consistently until the date when it begins to refreeze.
    Data source: NASA, 20164
ABOUT THE INDICATOR
This indicator shows the extent of sea ice in the Arctic region, which is defined as the area of ocean where at least 15 percent
of the surface is frozen. It also examines the age distribution of sea ice and the start and end dates of each year's Arctic sea
ice melt season. This indicator is based on routine monitoring of sea ice conditions from satellite measurements, which be-
gan in 1979. Here, the melt season start date is defined as the date when satellites detect consistent wetness on the surface
of the ice and snow; the end date is when the surface air temperature stays consistently at or below the freezing point and
ice begins to grow in the open ocean. Data for this indicator were gathered by the  National Snow and Ice Data Center using
satellite imaging technology and data processing methods developed by the National Aeronautics and Space Administra-
tion and the University of Colorado, Boulder.
                                                                                                                      41

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                                   Collecting
                 SNOW  AND   ICE  DATA
             From "low-tech" backyard observations made by citizens in their own neighborhood
             to "high-tech" global satellite images of some of the most remote places in the
             world, scientists use a variety of techniques to track climate change. Below are some
        examples of the data collection methods used to create the indicators in this chapter.

         Satellites provide an efficient way to collect the kind of
         data that would be difficult to measure in person, such
         as measurements that need to be made at regular inter-
         vals over large areas or in remote locations. For example,
         the Arctic Sea Ice indicator (p. 40) is derived from
         data-rich images taken by satellites that orbit the Earth
         every day, using instruments that can tell the difference
         between sea ice and open water.
         You don't need a Ph.D. to be a scientist. Many citizen
         scientists have helped to create high-quality datasets
         of climate indicators that date back further than some
         modern climate monitoring programs. The Lake Ice
         indicator (p. 46) includes local observations made
         from the same vantage point throughout the year. Some
         lakes have multiple observers, such as residents on both
         sides of a lake who can compare notes.
Sometimes going out in the field and measuring by hand
is the best way to collect precise information and main-
tain a long-running dataset. For the Glaciers indicator (p.
44), scientists visit the same glaciers twice a year at
locations marked with a network of stakes, where they
measure snow depth and density.
Automated observation stations make it possible to
collect data continuously from places that may be
difficult or expensive to reach. For example, scientists
once had to travel to remote snowpack measurement
sites by ski, snowshoe, snowmobile, or helicopter. Now,
the Snowpack indicator (p. 52) uses hundreds of
snow telemetry stations that automatically record and
relay data back to a central computer every 15 minutes.
42

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                     Antarctic  Sea  Ice
                   This indicator tracks the extent of sea ice around Antarctica.
     The Southern Ocean around Antarctica freezes to form sea ice every year. This sea ice reaches its maximum extent in
     September or early October and melts in the summer months (December to February). Like Arctic sea ice (see the
     Arctic Sea Ice indicator on p. 40), Antarctic sea ice affects global climate, regional climate, and ecosystems. Unlike
the Arctic, where a large area of sea ice lasts year-round, the sea ice around Antarctica is thinner, and nearly all of it melts
in a typical summer. Warmer air and ocean temperatures are generally expected to reduce the amount of sea ice present
worldwide. While warming has already driven a noticeable decline in sea ice in the Arctic, extent for the Antarctic as a
whole has not declined (and has actually increased slightly), which may reflect influences of wind patterns, ocean currents,
and precipitation around the continent.5
                                                                              •WHAT'S HAPPENING^
   February and September Monthly Average Antarctic Sea Ice Extent,
   1979-2016
    VI
        •
    2   7
    a

    I  6
    c
    O   5
    c
    Si
    x
    0)
    v
   A
September
                     February
                       V  '
4

3

2

1

0
1975  1980   1985   1990   1995   2000  2005   2010  2015   2020

                            Year
   This figure shows Antarctic sea ice extent for the months of February and September
   of each year from 1979 through September 2015 and February 2016. February and
   September typically have the minimum and maximum extent each year. Data source:
   NSIDC, 201&
Antarctic sea ice extent in
September and February has
increased somewhat over time,
although the most recent year
was below average. The Septem-
ber maximum extent reached
the highest level on record in
2014—about 7  percent larger
than the 1981-2010 average-but
in 2015 it was slightly below
the 1981-2010 average. As for
February extent, 2013, 2014,  and
2015 were three of the six largest
years on record, but extent in
2016 was about 9 percent below
the 1981-2010 average.
Slight increases in Antarctic  sea
ice are outweighed by the loss
of sea ice in the Arctic during
the same time  period (see the
Arctic Sea Ice indicator on p.
40). Overall, the Earth has
lost sea ice at an average rate
of 13,500 square miles per year
since 1979—equivalent to losing
an area larger than the state
of Maryland  every year.7 This
decrease affects the Earth's
energy balance.
ABOUT THE INDICATOR
This indicator examines the extent of sea ice in the Southern Ocean, which is defined as the area of ocean where at least 15
percent of the surface is frozen. It is based on routine monitoring of sea ice conditions from satellite measurements, which
began in 1979. Monthly average sea ice extent data for this indicator were gathered by the National Snow and Ice Data
Center using satellite imaging technology and data processing methods developed by the National Aeronautics and Space
Administration. Data are collected throughout the year, but for comparison, this indicator focuses on the months when sea
ice typically reaches its minimum and maximum extent.
                                                                                                                 43

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                         Glaciers
                        This indicator examines the balance between snow accumulation and melting in glaciers, and it describes how
                        glaciers in the United States and around the world have changed over time.
A          glacier is a large mass of snow and ice that has accumulated over many years and is present year-round. In many areas,
          glaciers provide communities and ecosystems with a reliable source of streamflow and drinking water, particularly in
          times of extended drought and late in the summer, when seasonal snowpack has melted away. Glaciers are important as
    an indicator of climate change because physical changes in glaciers—whether they are growing or shrinking, advancing or  re-
    ceding—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. 34).
    ^•WHAT'S  HAPPENING-
       •  On average, glaciers world-
        wide have been losing mass
        since at least the 1970s, which
        in turn has contributed to
        observed changes in sea level
        (see the Sea Level indicator on
        p. 34). A longer measure-
        ment record from a smaller
        number of glaciers suggests
        that they have been shrinking
        since the 1940s. The rate at
        which glaciers are losing mass
        appears to have accelerated
        over roughly the last decade.
Average Cumulative Mass Balance of "Reference" Glaciers Worldwide,
1945-2015
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       -30
         1940

    0)
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 •7  *
 •*• 'u   0
   £    1940
                                                        1950     1960     1970    1980    1990    2000    2010     2020

                                                       *~*

                                                        1950
                                                                1960     1970    1980

                                                                               Year
                                                                                       1990
                                                                                               2000
                                                                                                       2010     2020
                                       This figure shows the cumulative change in mass balance of a set of "reference" glaciers
                                       worldwide beginning in 1945. The line on the upper graph represents the average of all the
                                       glaciers that were measured. Negative values indicate a net loss of ice and snow compared
                                       with the base year of 1945. For consistency, measurements are in meters of water equivalent,
                                       which represent changes in the average thickness of a glacier. The small chart below shows
                                       how many glaciers were measured in each year. Some glacier measurements have not yet
                                       been finalized for the last few years, hence the smaller number of sites.
                                       Data source: WGMS, 2076s
44

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   Cumulative Mass Balance of Three U.S. Glaciers, 1958-2014
         -30
         -35
           1955  1960  1965  1970  1975  1980  1985  1990  1995  2000  2005  2010  2015

                                          Year

   This figure shows the cumulative mass balance of the three U.S. Geological Survey "bench-
   mark" 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 indicate a net loss of ice and
   snow compared with the base year. For consistency, measurements are in meters of water
   equivalent, which represent changes in the average thickness of a glacier. Data sources:
   O'Neel et a/., 2014;9 USGS, 2019°
       •WHAT'S HAPPENING >

            •  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.
              Year-to-year trends
              vary, with some glaciers
              gaining mass in certain
              years (for example,
              Wolverine Glacier during
              the 1980s), but the
              measurements clearly
              indicate a loss of glacier
              mass overtime.
            •  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.11
ABOUT THE INDICATOR
This indicator examines changes in glacier mass balance, which is the net
gain or loss of snow and ice over the course of the year. It can also be
thought of as the average change in thickness across the surface of a glacier.
The change in ice or snow has been converted to an equivalent amount of
liquid  water. If cumulative mass balance becomes more negative over time, it
means glaciers are losing mass more quickly than they can accumulate new
snow. The first graph above shows the average change across 40 reference
glaciers around the world that have been measured consistently for many
decades. The  World Glacier Monitoring Service compiled these  data, based
on measurements collected by a variety of organizations around the world.
Data for the second graph come from the U.S. Geological  Survey Benchmark
Glacier Program, which has studied three U.S. "benchmark" glaciers exten-
sively for many years. These three glaciers are thought to  be  representative
of other glaciers nearby.
AK
     Gulkana Glacier
     o
Wolverine Glacier
  South Cascade Glacier
               o
              WA
                                                                                                                    45

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                    f    Lake  Ice
                       This indicator measures the amount of time that ice is present on lakes in the United States.
        Lake 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. Reduced ice cover leads to increased
    evaporation and lower water levels, as well as an increase in water temperature and sunlight penetration, which in turn can
    affect plants and animals. The timing and duration of ice cover on lakes and other bodies of water can also affect society—par-
    ticularly in relation to shipping and transportation, hydroelectric power generation, and fishing.
    rWHAT'S  HAPPENING-
       • All of the lakes shown here
        were found to be thawing
        earlier in the year. Spring thaw
        dates have grown earlier by
        up to 24 days  in the past 110
        years.
Change in Ice Thaw Dates for Selected U.S. Lakes, 1905-2015
                                         Detroit Lake
                                           -2 days
LakeOsakis    Shell Lake
 -1 day       -6 days
             Lake Superior at Bayfield
                 " -24 days
             Moosehead Lake
                -6 days

     Cobbosseecontee Lake Damariscotta Lake
                                                                                            -10 days
 Mirror Lake
  -6 days "

Lake George
  -4 days
                                                    Jf*
                                                   ^
                                                                                                          "9days
                                                                                                     Sebago Lake
                                                                                                       -15 days
                                               Lake Mendota
                                                 -5 days
                    Lake Monona
                      -8 days
                                                          Geneva Lake
                                                            -4 days
                                                                                        Otsego Lake
                                                                                         -4 days
                                                                 Change in ice thaw date:

                                                                         •  Earlier

                                       This figure shows the change in the "ice-off" date, or date of ice thawing and breakup, for 14
                                       U.S. lakes during the period from 1905 to 2015. All of the lakes have red circles with negative
                                       numbers, which represent earlier thaw dates. Larger circles indicate larger changes.
                                       Data source: Various organizations'2
    ABOUT THE INDICATOR

    This indicator analyzes thaw dates, which occur when the ice cover on a lake
    breaks up, and open water becomes extensive. Thaw dates have been recorded
    through human visual observations for more than 100 years. Historical records
    from many lakes have been compiled in the Global Lake and River Ice Phenology
    Database, which was developed by the University of Wisconsin-Madison. Data for
    other lakes have been maintained by local officials or published in local news-
    papers. This indicator focuses on 14 lakes within the United States that have the
    longest and most complete historical records. The online version of this indicator
    tracks thaw dates for a smaller set of lakes dating back to 1840, and it also tracks
    changes in freeze dates, which occur when a continuous and immobile ice cover
    forms on the lake. Freeze dates come from visual observations recorded by the
    same sources as the thaw dates shown here.

46

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                                                                I
                                                               I
 COMMUNITY CONNECTION:
 ICE BREAKUP IN TWO ALASKAN RIVERS
           Q
 The Tanana and Yukon rivers in Alaska provide a particularly
 noteworthy record of northern climate because, for a century
 or more, local citizens have recorded the date when the ice
 on these rivers starts to move or break up each spring. In fact,
 the towns of Nenana, Alaska, and Dawson City, just over the
 border in Canada, hold annual competitions to guess when ice
 breakup will occur. To measure the exact time of ice breakup,
 residents place a tripod on the ice in the center of the river.
 This tripod is attached by a cable to a clock on the shore, so
 that when the ice under the tripod breaks or starts to move,
 the tripod will move and pull the cable, stopping the clock with
 the exact date and time of the river ice breakup. The Tanana
 and Yukon rivers both demonstrate long-term  trends toward
 earlier ice breakup in the spring.  Ice breakup dates for both
 rivers have shifted earlier by approximately seven days over their respective periods of record, and
 earliest breakup on record at Dawson City. However, other recent breakup dates for both rivers are
 range of historical variation.
            Dawson City,
              Yukon

            2016 had the
            within the
Ice Breakup Dates for Two Alaskan Rivers, 1896-2016

    April 1
               > Tanana River
               • Yukon River
   April 16
Q.
J   Mayl
as

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                  '
                         Snowfall
                      This indicator uses two different measures to show how snowfall has changed in the contiguous 48 states.
         Snowfall is an important aspect of winter in much of the United States. Warmer temperatures cause more water to evapo-
         rate from the land and oceans, which leads to more precipitation, larger storms, and more variation in precipitation in some
         areas. In  general, a warmer climate will cause more of this precipitation to fall in the form of rain instead of snow. Some
    places could see more snowfall, however, if temperatures rise but still remain below the freezing point, or if storm tracks change.
    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
    communities that rely on these activities.
    r
WHAT'S HAPPENING-
        Total snowfall has decreased
        in many parts of the country
        since widespread observations
        became available in 1930, with
        57 percent of stations showing
        a decline. Among all of the
        stations shown, the average
        change is a decrease of 0.19
        percent per year.
                                Change in Total Snowfall in the Contiguous 48 States, 1930-2007
                                                             Rate of change (percent per year):

                                                •     •••••••
                                                -0.9     -0.6     -0.3    -0.1   -0.1    0.1     0.3     0.6      0.9
                                               to-1.2   to-0.9   to-0.6  to-0.3  to 0.1   to 0.3  to 0.6   to 0.9    to 1.2
                                                  Less snowfall
                                                                                            More snowfall
                                      This figure shows the average rate of change in total snowfall from 79JO to 2007 at 419 weath-
                                      er stations in the contiguous 48 states. Blue circles represent increased snowfall; red circles
                                      represent a decrease. Data source: Kunkel et a/., 2009'5
48

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   Change in Snow-to-Precipitation Ratio in the Contiguous 48 States,
   1949-2016
          .••<** 2
         o
        s<
       <>•
              "0°
                      <
                    *
                   «9°  o o,
                                  Percent change:
   <-40
            -30
           to-40
 -20
to-30
 -10
to-20
 -2
to-10
 -2
to 2
 2
to 10
 10
to 20
 20
to 30
 30
to 40
                                                                             >40
            Lower percentage of snow
                                  Higher percentage of snow
                        Filled circles represent statistically significant trends.
                    Open circles represent trends that are not statistically significant.

    This figure shows the percentage change in winter snow-to-precipitation ratio from 1949 to
    2016 at 246 weatherstations 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. Solid-color circles repre-
    sent stations where the trend was statistically significant. Data source: NOAA, 2016'6
•WHAT'S HAPPENING >

   •  In addition to changing the
     overall rate of precipitation,
     climate change can lead
     to changes in the type of
     precipitation. One reason
     for the decline in total
     snowfall is because more
     winter precipitation is
     falling in the form of rain
     instead of snow. Nearly
     80 percent of the stations
     across the contiguous 48
     states have experienced a
     decrease in the proportion
     of precipitation falling as
     snow.
   •  Snowfall trends vary by
     region. The Pacific  North-
     west 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 in the past.
ABOUT THE INDICATOR
The graph on p. 48 shows changes in total snowfall, which is determined by the height of snow that accumulates each
day. This analysis was adapted from a study by Kunkel et al. (2009).17 The graph above shows trends in the proportion of
total precipitation that falls in the form of snow during each winter season. This is called the "snow-to-precipitation" ratio,
and it is based on comparing the amount of snowfall with the total amount of precipitation (snow plus rain) in each year.
Both graphs are based on daily records from hundreds of weather stations. These data have been collected and maintained
by the National Oceanic and Atmospheric Administration. Stations were selected for this indicator because they had
high-quality data for the entire time period of interest.
                                                                                                                         49

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                        Snow Cover
                      This indicator measures the amount of land in North America that is covered by snow.
        Snow cover refers to the amount of land covered by snow at any given time, which is influenced by the amount of precipi-
        tation that falls as snow. 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, however; it also exerts an influence on climate. More snow
    means more energy reflects back to space, resulting in cooling, while less snow cover means more energy is absorbed at the
    Earth's surface, resulting in warming. Some plants and animals may depend on snow to insulate them from sub-freezing winter
    temperatures, and humans and  ecosystems also rely on snowmelt to provide soil moisture and replenish streams and ground-
    water.
    rWHAT'S HAPPENING-
      •  When averaged over the entire
        year, snow covered an average
        of 3.24 million square miles
        of North America during the
        period from 1972 to 2015.
      •  The extent of snow cover has
        varied from year to year. The
        average area covered by snow
        has ranged from 3.0 million to
        3.6 million square miles, with
        the minimum value occurring
        in 1998 and the maximum  in
        1978.
      •  Between 1972 and 2015, the
        average extent of North
        American snow cover de-
        creased at a rate of about
        3,300 square miles per year.
        The average area covered  by
        snow during the most recent
        decade (2006-2015) was 3.21
        million square miles, which is
        about 4 percent smaller than
        the average extent during  the
        first 10 years of measurement
        (1972-1981)-a difference
        of 122,000 square miles, or
        approximately an area the size
        of New Mexico.
Snow-Covered Area in North America, 1972-2015
      4.0
        1970
              1975   1980   1985   1990   1995   2000   2005   2010

                                      Year
                                                               2015   2020
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 including Greenland). Data source: Rutgers University Global Snow Lab, 2016'8
50
        s

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                                                                            •WHAT'S HAPPENING
Snow Cover Season in the United States, 1972-2013

     SepSO

     OctSO

     Nov29

     Dec 29
Start of snow cover
     J
                                                                     Length
                                                                     of snow
                                                                     cover
                                                                     season
                                                        2007
                                                               2012
This figure shows the timing of each year's snow cover season in the contiguous 48 states
and Alaska, based on an average of all parts of the country that receive snow every year. The
shaded band spans from the first date of snow cover until the last date of snow cover.
Data source: NOAA, 2015'9
Since 1972, the U.S. snow
cover season has become
shorter by nearly two
weeks, on average. By far
the largest change has
taken place in the spring,
with the last day of snow
shifting earlier by 19 days
since 1972. In contrast, the
first date of snow cover
in the fall has remained
relatively unchanged.
          ABOUT THE INDICATOR
          This indicator tracks the area covered by snow since 1972, based on maps generated by analyzing satellite images collectec
          by the National Oceanic and Atmospheric Administration. The first graph was created by analyzing each weekly map to
          determine the extent of snow cover, then averaging the weekly observations together to get a value for each year. This par
          of the analysis covers all of North America, not including Greenland. The second graph focuses on the contiguous 48 state:
          plus Alaska. It shows the average date when snow first starts to cover the ground in the fall, the average last date of snow
          cover in the spring, and the length of time between them. These snow cover season dates have been averaged over all parl
          of the country that regularly receive snow. The online version of this indicator also shows changes in snow-covered area by
          season over this same time period.
                                                                         ~
                                                                                 51

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                        Snowpack
                      This indicator measures trends in mountain snowpack in the western United States.
        Temperature and precipitation are key factors affecting snowpack, which is the amount or thickness of snow that accumu-
        lates on the ground. Mountain snowpack plays a key role in the water cycle in western North America, storing water in the
        winter when snow falls and then releasing it as runoff in spring and summer when the snow melts. Millions of people in the
    West depend on the melting of mountain snowpack for power, irrigation, and drinking water. Changes in mountain snowpack
    can affect agriculture, winter recreation, and tourism in some areas, as well as plants and wildlife. In a warming climate, more
    precipitation is expected to fall as rain rather than snow in most areas—reducing the extent and depth of snowpack. Higher
    temperatures in the spring can cause snow to melt earlier.

    ABOUT THE INDICATOR
    This indicator examines more than a half-century of snowpack measurements from the United States Department of Agri-
    culture's Natural Resources Conservation Service and the California Department of Water Resources. Snowpack is commonly
    measured in snow water equivalents, which can be thought of as the depth of water that would result if the entire snowpack
    were to melt. Snowpack data have been collected over the years using a combination of manual measurements and automated
    instruments. This indicator shows long-term rates of change for April 1, the most frequent observation date, because it could
    reflect changes in snowfall and it is extensively used for spring streamflow forecasting.
    ^WHAT'S HAPPENING-
      •  From 1955 to 2016, April snow-
        pack declined at more than 90
        percent of the sites measured.
        The average change across
        all sites amounts to about a
        23-percent decline.
      •  Large and consistent de-
        creases have been observed
        throughout the western
        United States. Decreases have
        been especially prominent in
        Washington, Oregon, and the
        northern  Rockies.
      •  While some stations have
        seen increases in snowpack,
        all 11 states included  in this
        indicator  have experienced
        a decrease in snowpack on
        average over the time period.
        In the Northwest (Idaho,
        Oregon, Washington), all but
        three stations saw decreases
        in snowpack over the period
        of record.
Trends in April Snowpack in the Western United States, 1955-2016

                                                       Percent change:

                                                           >80

                                                           60 to 80

                                                           40 to 60

                                                           20 to 40

                                                           Or to 20

                                                           Oto-20

                                                           -20 to -40

                                                           -40 to -60

                                                           -60 to -80

                                                           <-80
                                      This map shows trends in April snowpack in the western United States, measured in terms
                                      of snow water equivalent. Blue circles represent increased snowpack; red circles represent a
                                      decrease. Data source: Mote and Sharp, 201620
52

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Understanding the Connections Between
CLIMATE   CHANGE
     HUMAN  HEALTH

      Climate change poses many threats to the health and well-being of
      Americans, from increasing the risk of extreme heat events and heavy
      storms to increasing the risk of asthma attacks and changing the spread
  of certain diseases carried by ticks and mosquitoes. Some of these health
  impacts are already happening in the United States.
  Scientists' understanding of the multiple ways that climate change increases
  risks to human health has advanced significantly in recent years. This section
  highlights key concepts from one of the latest climate and health assessments
  (see the box below) to help illustrate how climate change can affect human
  health in the United States. In addition, this section demonstrates how EPA's
  climate change indicators advance the dialogue in connecting climate change
  and human health.
   ACCESS USGCRP'S CLIMATE AND HEALTH
   ASSESSMENT ONLINE
   The U.S. Global Change Research Program (USGCRP) report, The
   Impacts of Climate Change on Human Health in the United States: A
   Scientific Assessment, was published in April 2016.
   This comprehensive report captures the state of scientific knowledge
   about observed and projected impacts of climate change on human  j^UMAN™
   health in the United States. The report is available online at: https://
   health2016.qlobalchanqe.gov.

                                .
i


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How Does Climate Change Affect Human Health?
Climate change can exacerbate existing health threats or create new public health challenges through a variety of path-
ways. Figure 1 summarizes these connections by linking climate impacts to changes in exposure, which can then lead to
negative effects on health (health outcomes). This figure also shows how other factors—such as where people live and
their age, health, income, or ability to access health care resources—can positively or negatively influence people's vulner-
ability to human health effects. For example, a family's income, the quality of their housing, or their community's emergen-
cy management plan can all affect that family's exposure to extreme heat, the degree to which their health is affected by
this threat, and their ability to adapt to impacts of extreme heat (for more examples, see Figure 4).

 Figure 1. Climate Change and Health Pathway
       ENVIRONMENTAL AND
      INSTITUTIONAL CONTEXT
      • Land-use change
      • Ecosystem change
      • Infrastructure condition
      • Geography
      • Agricultural production
       and livestock use
                                            CLIMATE IMPACTS
                                         Increased temperatures
                                         Precipitation extremes
                                         Extreme weather events
                                         Sea level rise
       EXPOSURE
Extreme heat
Poor air quality
Reduced food and
water quality
Changes in infectious
agents
Population displacement
                                           HEALTH OUTCOMES
                                         Heat-related illness
                                         Cardiopulmonary illness
                                         Food-, water-, and
                                         vector-borne disease
                                         Mental health
                                         consequences and stress
        SOCIAL AND
   BEHAVIORAL CONTEXT
Age and gender
Race and ethnicity
Poverty
Housing and infrastructure
Education
Discrimination
Access to care and
community health
infrastructure
Preexisting health
conditions
Figure 7 shows how climate change can affect people by changing their exposure to health threats (moving from top to bottom) and by
influencing the environmental, institutional, social, and behavioral factors that affect a person's or community's health (moving through
the boxes on the sides).
What Can Indicators Tell Us About Climate Change and Human Health?
As shown in Figure 1, the impacts of climate change on health are complex, often indirect, and dependent on multiple soci-
etal and environmental factors. Tracking changes in climate impacts and exposures improves understanding of changes in
health risk, however, even if the actual health outcome is difficult to quantify. For example, the flooding pathway in Figure
2 shows how indicators of certain climate impacts like Sea Level Rise, Heavy Precipitation, and Coastal Flooding could be
used by state and local health officials to better understand changes in human exposure to contaminated waters (a health
risk). By recognizing changing risks, these officials can better understand how climate change affects the number of
people who get sick with gastrointestinal illnesses (a health outcome). Thus, even where health data or long-term records
are unavailable or where the links between climate and health outcomes are complex, indicators play an important role in
understanding climate-related health impacts.


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Figure 2. Connecting Climate Change Indicators to Health Pathways
The following three examples show how climate impacts can affect health. The numbered circles identify where climate change indica-
tors provide key information on changes occurring at different points along the pathways. Other factors can play a role in determining a
person's vulnerability to climate-related health outcomes; see Figure 1 and Figure 4.
      CLIMATE IMPACTS
         EXPOSURE
                             Extreme Heat
                      More frequent,
                       severe, and
                     prolonged heat
                         waves

                        Elevated
                      daytime and
                       nighttime
                     temperatures
                                           Flooding
    Rising sea levels,
        heavier
    precipitation, and
      more intense
    tropical cyclones
I
 Contaminated
water and debris
 from flooding

                      Tick-borne
                       Disease

                      Changes in
                       extreme
                     temperatures,
                     precipitation,
                     and seasonal
                       weather
                       patterns
I
                                                                       Disease-
                                                                    carrying ticks
                                                                       earlier in
                                                                    season and in
                                                                     new parts of
                                                                       country
                                                                                                     m
                                                                                                     CD
                                                                                                     X
     HEALTH OUTCOMES
                            Heat-related
                            deaths and
                              illnesses
                                        Gastrointestinal
                                      illnesses and injuries
                           Lyme disease
            Where EPA's climate change indicators fit into each example exposure pathway:

                                         Q Sea Level (p. 34)

                                         Q Heavy Precipitation (p. 24)
  High and Low
' Temperatures (p. 20)
         U.S. and Global
         Temperature (p. 18)

         Heat-Related Deaths
         (p. 60)

         Heat-Related Illnesses
         (p. 62)
                                     Tropical Cyclone Activity
                                     (p. 25)

                                     River Flooding (p. 26)

                                     Coastal Flooding (p. 36)
                                         High and Low
                                         Temperatures (p. 20)
                                         ©U.S. and Global
                                         Precipitation (p. 22)

                                     ©  Lyme Disease (p. 65)
                                                                                            \

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                                                      MAKE THE HEALTH CONNECTION
                                                      Look for this symbol throughout the report
                                                      to identify climate change indicators with
                                                      important health connections.
Who's at Risk?
Every American faces a risk of health impacts associated
with climate change. Some people, however, face higher
risks than others because of differences in the hazards to
which they are exposed, their sensitivity to these hazards,
and their ability to adapt (see Figure 3). Thus, it is import-
ant to be able to identify "populations of concern," which
include groups  representing people of all ages, living in
different places, who interact with their environment in different ways. Figure 4 shows some examples of how certain
populations are more vulnerable to health impacts because of differences in their exposure, sensitivity, or ability to adapt
to climate-related stresses.
It is important to remember that the different health impacts identified here do not occur in isolation; people can face
multiple threats at the same time, at different stages in their lives, or accumulating over the course of their lives. Risks
may increase as people are exposed to multiple health threats. For example, extremely hot days can lead to heat-related
illness as well as poor air quality, by  increasing the chemical reactions that produce smog. In addition, many of the
factors that influence whether a person is exposed to health threats or whether they become ill, such as an individual's
personal habits, living conditions, and access to medical care (see Figure 1), can also change over time.
  The effects of climate change also affect people's mental health. In particular, climate- or weather-related disasters
  can increase the risk of adverse mental health consequences, especially if they result in damage to homes and
  livelihoods or loss of loved ones. The mental health impacts of these events can range from minimal stress and
  distress symptoms to clinical disorders, such as anxiety, depression, and post-traumatic stress.
Figure 3. Determinants of Vulnerabilty
         EXPOSURE
      Exposure is contact
  between a person and one
      or more biological,
  psychosocial, chemical, or
      physical stressors,
  including stressors affected
      by climate change.
                                           SENSITIVITY
                                      Sensitivity is the degree to
                                           which people or
                                      communities are affected,
                                          either adversely or
                                         beneficially, by their
                                         exposure to climate
                                        variability or change.

                                         VULNERABILITY
                                  of Human Health to Climate Change
                                       HEALTH OUTCOMES
                           Injury, acute and chronic illness (including mental
                            health and stress-related illness), developmental
                                          issues, and death.
    ABILITY TO ADAPT
Adaptive capacity is the ability
of communities, institutions, or
 people to adjust to potential
    hazards such as climate
 change, to take advantage of
opportunities, or to respond to
        consequences.


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Figure 4. Examples of Climate Change Vulnerabilty
         EXPOSURE
       SENSITIVITY
  ABILITY TO ADAPT
Low-income populations may be
exposed to climate change threats
because of socioeconomic factors. For
example, people who cannot afford air
conditioning are more likely to suffer
from unsafe indoor air temperatures.
Pregnant women are sensitive to
health risks from extreme weather
such as hurricanes and floods. These
events can affect their mental health
and the health of their unborn babies
by contributing to low birthweight or
preterm birth.
Older adults may have limited ability
to cope with extreme weather if, for
example, they have difficulty accessing
cooling centers or other support ser-
vices during a heat wave. Heat-related
deaths are most commonly reported
among adults aged 65 and over.
Occupational groups such as first
responders and construction workers
face more frequent or longer expo-
sure to climate change threats. For
example, extreme heat and disease-
carrying insects and ticks particularly
affect outdoor workers.
People with pre-existing medical
conditions, such as asthma, are
particularly sensitive to climate change
impacts on air quality. People who
have diabetes or who take medications
that make it difficult to regulate body
temperature are sensitive to extreme
heat.
People with disabilities face challeng-
es preparing for and responding to
extreme weather events. For example,
emergency or evacuation instructions
are often not accessible to people with
learning, hearing, or visual disabilities.
M~
People in certain locations may be
exposed to climate change threats,
such as droughts, floods, or severe
storms, that are specific to where they
live. For example, people living by
the coast are at increased risk from
hurricanes, sea level rise, and storm
surge.
Children are more sensitive to respi-
ratory hazards than adults because of
their lower body weight, higher levels
of physical activity, and still-develop-
ing lungs. Longer pollen seasons may
lead to more asthma episodes.
Indigenous people who rely on
subsistence food have limited options
to adapt to climate change threats to
traditional food sources. Rising tem-
peratures and changes in the growing
season affect the safety, availability,
and nutritional value of some tradi-
tional foods and medicinal plants.
                                                                                                 \

-------
Additional Resources
Climate change threatens human health, including mental health, as well as access to clean air, safe drinking
water, nutritious food, and shelter. Understanding the threats that climate change poses to human health
can help people and communities work together to lower risks and be prepared.
The following EPA resources on how climate change affects your health can be found at:
www.epa.gov/climatechanae/impacts:
            Cllnwrto QW99,
            Environmental
Eight fact sheets on populations shown in
Figure 4 that are particularly vulnerable to
the health impacts of climate change.
A clickable map with examples of state impacts
and resources to help individuals and communi-
ties prepare and respond to climate threats.
An online 10-question quiz to challenge
you and your friends on climate and health
knowledge.
A fact sheet highlighting health impacts of
climate change at different stages of life, from
infancy through adolescence to adulthood.

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            Health  and  Society
Changes in the Earth's climate can affect public health, agriculture, water supplies,
energy production and use, land use and development, and recreation. The nature
and extent of these effects, and whether they will be harmful or beneficial, will vary
regionally and over time. This chapter looks at some of the ways that climate change is
affecting human health and society, including changes in Lyme disease, West Nile virus,
ragweed pollen season, heat-related deaths and hospitalizations, heating and cooling
needs, and the agricultural growing season across the United States.

Climate indicators provide key information on changes to environmental exposures and
health outcomes (see Understanding the Connections Between  Climate Change and
Human Health on p. 53). Because impacts on human health are complex, often indirect,
and dependent on multiple societal and environmental factors (including how people
choose to respond to these impacts), the development of appropriate health-related
climate indicators is challenging and still emerging.  Even where health data or long-
term records are unavailable or where the links between climate and health outcomes
are complex, indicators play an important role in understanding climate-related health
impacts.


WHY DOES IT MATTER?

Changes in climate affect the average weather conditions to which we are accustomed.
These changes may result in multiple threats to human health and welfare. Warmer
average temperatures will continue to lead to hotter days and more frequent and longer
heat waves, which could increase the number of heat-related illnesses and deaths.
Increases in the frequency or severity of extreme weather events, such  as storms,
increase the risk of dangerous flooding, high winds, and other direct threats to people
and property. Warmer temperatures also reduce air quality by increasing the chemical
reactions that produce smog, and, along with changes in precipitation patterns and
extreme events, could enhance the spread of some diseases.

In addition, climate change could require adaptation on larger and faster scales than
in the past, presenting challenges to human well-being and the economy. The more
extensively and more rapidly the climate changes, the larger the potential effects on
society. The extent to which climate change affects  different regions and sectors of
society depends not  only on the sensitivity of those systems to climate change, but
also on their ability to adapt to or cope with climate change. Populations of particular
concern include the poor, children, the elderly, those already in poor health, the
disabled, and indigenous populations.
                                                                                    59

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                         Heat-Related  Deaths
                       This indicator presents data on deaths classified as "heat-related" in the United States.
         Unusually hot summer temperatures have become more common across the contiguous 48 states in recent decades1 (see
         the High and Low Temperatures indicator on p. 20). When people are exposed to extreme heat, they can suffer from
         potentially deadly illnesses, such as heat exhaustion and heat stroke. Hot temperatures can also contribute to deaths from
    heart attacks, strokes, and other forms of cardiovascular disease. Heat is the leading weather-related killer in the United States,
    even though most heat-related deaths are preventable through outreach and intervention.
    Extreme heat events (heat waves) are expected to become longer, more frequent, and more intense in the future.2 As a result,
    the risk of heat-related deaths and illness is also expected to increase.3 Reductions in cold-related deaths are projected to be
    smaller than increases in heat-related deaths in most regions.4 Death rates can also change, however, as people acclimate to
    higher temperatures and as communities strengthen their heat response plans and take other steps to continue to adapt.
        Deaths Classified as "Heat-Related" in the United States, 1979-2014
           4.0
        $  3.5
        fl.
        S  3.0
        Q.
        C
           2-S
           2.0
           1.5
' Underlying and contributing
 causes of death (May-Sept)

 Underlying cause of death
 (all year)
           1.0


           0.5
             1975
                     1980
                             1985
                                      1990
                                              1995

                                             Year
                                                      2000
                                                              2005
                                                                      2010
                                                                               2015
         This figure shows the annual rates for deaths classified as "heat-related" by medical profes-
         sionals 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 September, based on a broader set of data that became available in 1999.
         Data source: CDC, 2076s6

         * 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.
     HEALTH CONNECTION
     Older adults, particularly those with preexisting health con-
     ditions, can be especially vulnerable to extreme heat. Those
     taking medications that make it difficult to regulate body
     temperature, who live alone, or who have limited mobility are
     at higher risk for heat-related illness and death.7
•WHAT'S HAPPENING^

• Between 1979 and 2014, the
  death rate as a direct result of
  exposure to heat (underlying
  cause of death) generally
  hovered around 0.5 to 1 deaths
  per million people, with spikes
  in certain years. Overall, a total
  of more than 9,000 Americans
  have died from heat-related
  causes since 1979, according to
  death certificates.
• For years in  which the two
  records overlap (1999-2014),
  accounting for those additional
  deaths in which heat was listed
  as a contributing factor results
  in a higher death rate—nearly
  double for some years-
  compared with the estimate
  that only includes deaths
  where heat was listed as the
  underlying cause.
• The indicator shows a peak in
  heat-related deaths in 2006, a
  year that was associated with
  widespread  heat waves and
  was one of the hottest years
  on record in the contiguous 48
  states (see the U.S. and Global
  Temperature indicator on
  P. 18).
60

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   Summer Deaths Due to Heat and Cardiovascular Disease in the United
   States, 1999-2014
                                                         Age 65+
                                                         Non-Hispanic blacks
                                                         General population
        1998
               2000
                       2002
                              2004
                                      2006    2008

                                         Year
                                                     2010
                                                            2012
                                                                    2014
                                                                            2016
   This figure shows rates for deaths that medical professionals have classified as being
   caused by a combination of cardiovascular disease (diseases of the circulatory system)
   and heat exposure. This graph presents summer (May to September) death rates from
   1999 to 2014 for three population groups in the 50 states and the District of Columbia.
   The purple line shows rates for the entire population, the green line shows rates for
   non-Hispanic black people, and the pink line shows rates for people aged 65 and older.
   Data source: CDC, 2076s
•WHAT'S  HAPPENING
  •  The death rate from heat-re-
    lated cardiovascular disease
    ranged from 0.08 deaths per
    million people in 2004 to 1.08
    deaths per million people in
    1999. Overall, the interaction
    of heat and cardiovascular
    disease caused about
    one-fourth of the heat-re-
    lated deaths recorded in the
    "underlying and contributing
    causes" analysis since 1999.
  •  Since 1999, people aged 65+
    have been several times more
    likely to die from heat-related
    cardiovascular disease than
    the general population, while
    non-Hispanic blacks general-
    ly have had higher-than-av-
    erage rates.
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 compiled by the U.S. Centers for Disease Control and Prevention's National Vital
Statistics System. It includes deaths for which excessive natural heat was listed on the death certificate as the main cause
of death (also known as the underlying cause), which can be tracked back to 1979. It also examines deaths for which heat
was listed as either the underlying cause or a contributing factor, based on a broader set of data that at present can only be
evaluated 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. The second graph takes a closer look at heat-related cardiovascular disease deaths, which include deaths due to
heart  attacks, strokes, and other diseases related to the circulatory system. It shows death rates for the overall population as
well as two groups with a higher risk: people aged 65 and older and non-Hispanic blacks.
   The numbers shown here do not capture the full extent of heat-related deaths. 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, partic-
   ularly if they do not occur during an identified or publicized heat event. For example, of the estimated 700 excess deaths
   during the 1995 heat wave in Chicago, only 465 were recorded and attributed to the extreme heat event (see the online
   version of this indicator). This type of undercounting is not limited to large heat events. Furthermore, deaths can occur
   from exposure to heat (either as an underlying cause or as a contributing factor) that is not classified as extreme and
   therefore is often not recorded as such. Some statistical approaches estimate that more than 1,300 deaths per year in the
   United States are due to extreme heat, compared with about 600 deaths per year in the "underlying and contributing
   causes" data set shown in Figure I.9
                                                                                                                       61

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                         Heat-Related Illnesses
                      This indicator tracks how often people are hospitalized because of exposure to heat.
         Heat-related illnesses can occur when a person is exposed to high temperatures, such that their body cannot cool itself
         sufficiently through sweating. Symptoms range from mild swelling, rashes, or cramps to potentially deadly heat exhaus-
         tion and heat stroke. Unusually hot summer temperatures have become more common across the contiguous 48 states in
    recent decades.10
    Extreme heat events (heat waves) are expected to become longer, more frequent, and more intense in the future.11 As a result,
    the risk of heat-related illness is expected to increase.12 Hospitalization rates can also change, however, as people acclimate to
    higher temperatures and as communities strengthen their heat response plans and take other steps to continue to adapt.
    ^•WHAT'S HAPPENING-

      •  From 2001 to 2010, the 20
        states covered in this figure
        recorded a total of about
        28,000 heat-related hospital-
        izations.13 The resulting annual
        rates ranged from 1.1 cases per
        100,000 people in 2004 to 2.5
        cases per 100,000 people in
        2006, with a 10-year average
        rate of 1.8 cases per 100,000
        people.
      •  The pattern in the figure shown
        here largely matches the
        pattern in heat-related deaths
        during the same period (see the
        Heat-Related Deaths indicator
        on  p. 60), including a low
        value in 2004 and a peak in
        2006. Considerable year-to-
        year variability makes it difficult
        to determine whether heat-
        related illnesses have increased
        or decreased to a meaningful
        degree since 2001.
Heat-Related Hospitalizations in 20 States, 2001-2010
     2000  2001  2002  2003   2004   2005   2006   2007

                                    Year
                                                   2008  2009  2010  2011
This graph shows the annual rate ofhospitalizations classified as "heat-related" by medical
professionals in 20 states that participate in CDC's hospitalization tracking program, from
2001 to 2010. The rate is based on hospital discharge records for May 1 to September JO of
every year. The rate has been age-adjusted to account for the effects of population change
over time—for example, if the proportion of older adults has increased.
Data source: Choudhary and Vaidyanathan, 2014'4
    ABOUT THE INDICATOR
    This indicator shows rates for hospital admissions due to "heat-related" illnesses such as heat exhaustion, heat cramps, mild
    heat edema (swelling in the legs and hands), heat syncope (fainting), and heat stroke. It is based on hospital discharge records,
    which include a diagnosis determined by a physician or other medical professional. The indicator covers a group of states across
    a wide range of regions and climate zones that have participated in a national hospital data tracking program since at least 2001.
    All of these states require hospitals to submit discharge data to a state organization, which then compiles and reports the data
    to the U.S. Centers for Disease Control and Prevention (CDC). The data for this indicator come from CDC's Environmental Public
    Health Tracking Program, which includes hospitalization rates per 100,000 people and the total number of heat-related hospital-
    izations broken out by sex and age group.
62

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Average Rate of Heat-Related Hospitalizations in 23 States, 2001-2010
            Average annual hospital admissions per 100,000 people:
                       I
I
I
                       i
                                                           No data
This map shows the 2001-2010 average rate of hospitalizations classified as "heat-related" by
medical professionals in 23 states that participate in CDC's hospitalization tracking program.
Rates are based on hospital discharge records for May 1 to September 30 of every year. Rates
have been age-adjusted to account for differences in the population distribution over time
and between states—for example, if one state has a higher proportion of older adults than
another. Data source: CDC, 2016'5
Heat-Related Hospitalizations in 20 States by Sex and Age, 2001-2010
    25,000
•WHAT'S HAPPENING^
    •  Heat-related hospitalization
      rates vary widely among
      the 23 states studied (see
      the top figure). Average
      rates from 2001 to 2010
      ranged from fewer than one
      case  per 100,000 people in
      some states to nearly four
      cases per 100,000 people
      in others. The highest
      rates occurred in Kansas,
      Louisiana, Missouri, South
      Carolina, and Tennessee.
      Relatively high hospitaliza-
      tion rates in the Southeast
      and Midwest suggest a con-
      nection between hotter and
      more humid summers and
      increased rates of heat-re-
      lated illness, compared with
      other regions.17
    •  People aged 65+accounted
      for more heat-related
      hospitalizations than any
      other age group from 2001
      to 2010, and males were
      hospitalized for heat-
      related illnesses more than
      twice as often as females
      (see the bottom figure).
      Men tend to have a higher
      risk of heat-related illness
      than  women because they
      are more likely to work in
      outdoor occupations such
      as construction.18
 ~  20,000
 £•  15,000
 0)  10,000
_Q
 IB   5,000
                   Sex
                 Age Group
              Male    Female
                                    0-4
                                            5-14     15-34    35-64     65+
This graph shows the total number of hospitalizations classified as "heat-related" by medical
professionals in 20 states that participate in CDC's hospitalization tracking program, from
2001 to 2010. Totals are broken out by sex and by age group. The graph shows 10-year totals
based on hospital discharge records for May 1 to September 30 of every year. Data source:
Choudharyand Vaidyanathan, 2014'6
                                                    This indicator does not
                                                    cover every state, and it
                                                    could overlook illnesses
                                                    that were not diagnosed as
                                                    heat-related, did not result
                                                    in a hospitalization, or were
                                                    not fully documented or
                                                    reported.  Nonetheless,  this
                                                    data set represents the best
                                                    available source of observed
                                                    data for tracking heat-related
                                                    hospitalizations across
                                                    multiple states.
                                                                                                                      63

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                        Heating  and Cooling  Degree Days
                      This indicator examines changing temperatures from the perspective of heating and cooling needs for buildings.
         As climate change contributes to an increase in average temperatures, an increase in unusually hot days, and a decrease
         in unusually cold days (see the U.S. and Global Temperature and High and Low Temperatures indicators on pp. 18
         and 20), the overall demand for heating is expected to decline, and the demand for cooling is expected to increase.
    One way to measure the influence of temperature change on energy demand is using heating and cooling degree days, which
    measure the difference between outdoor temperatures and a temperature that people generally find comfortable indoors.
    These measurements suggest how much energy people might need to use to heat and cool their homes and workplaces, thus
    providing a sense of how climate change could affect people's daily lives and finances.
    ^WHAT'S HAPPENING
        Heating degree days have
        declined in the contiguous
        United States, particularly in
        recent years, as the climate has
        warmed. This change suggests
        that heating needs have
        decreased overall.
        Overall, cooling degree days
        have increased over the past
        100 years. The increase is most
        noticeable over the past few
        decades, suggesting that air-
        conditioning energy demand
        has also been increasing
        recently.
Heating and Cooling Degree Days in the Contiguous 48 States, 1895-2015

    6,000
o»
0)
D
5,000



4,000



3,000



2,000



1,000
                         • Heating degree days (colder than 65°F)

                         • Cooling degree days (warmer than 65°F)
                                                                                             1990      2010
                                      This figure shows the average number of heating and cooling degree days per year across
                                      the contiguous 48 states. Data source: NOAA, 2016'9
    ABOUT THE INDICATOR
    This indicator uses daily temperature data from thousands of weather stations across the contiguous 48 states to calculate
    heating and cooling degree days. A "degree day" is determined by comparing the daily average outdoor temperature with a
    defined baseline temperature for indoor comfort (in this case, 65°F). For example, if the average temperature on a particular day
    is 78°F, then that day counts as 13 cooling degree days, as a building's interior would need to be cooled by 13°F to reach 65°F.
    Conversely, if the average outdoor temperature is 34°F, then that day counts as 31 heating degree days, as a building's interior
    would need to be warmed by 31°F to reach 65°F. This does not mean that all people will actually heat or cool buildings to 65°F;
    it is just a number to allow for consistent comparisons over time and across the country.
    The graph above was created by calculating the total number of heating and cooling degree days per year at each weather
    station, averaging the results from all stations within small regions called climate divisions, then calculating a national average
    weighted by the population of each climate division. This population-weighting approach produces a national  average that more
    closely reflects the conditions that the average resident would experience. Data and analyses were provided by the National
    Oceanic and  Atmospheric Administration. The online version  of this indicator also shows a map with changes in annual heating
    and cooling degree days by state.
64

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                      Lyme Disease
                    This indicator tracks the rate of reported Lyme disease cases across the United States.
    Lyme disease is a bacterial illness transmitted through the bite of certain species of ticks (commonly known as deer
    ticks). It can cause fever, fatigue, joint pain, and skin rash, as well as more serious joint and nervous system compli-
    cations. Warming temperatures are projected to expand the range of suitable tick habitat,20 increasing the potential
risk of Lyme disease. Also, because deer ticks are mostly active when temperatures are above 45 ° F, shorter winters could
extend the period when ticks are active each year.21 Climate is not the only factor, however, that could influence the trans-
mission, distribution, and  incidence of  Lyme disease. Other factors include changes in the populations of host species such
as deer and white-footed mice, habitat changes, and the extent to which people take precautions to avoid getting infected.
   Reported Cases of Lyme Disease in the United States, 1991-2014
         12
          1990
                      1995
                                  2000
                                               2005
                                                           2010
                                                                        2015
                                        Year
  This figure shows the annual incidence of Lyme disease, which is calculated as the number
  of new cases per 100,000 people. The graph is based on cases that local and state health
  departments report to the Centers for Disease Control and Prevention's national disease
  tracking system. Data source: CDC, 201522

  Reported Lyme Disease Cases in 1996 and 2014
  These maps show the distribution of reported cases of Lyme disease in 1996 and 2014.
  Each dot represents an individual case placed according to the patient's county of resi-
  dence, which may be different than the county of exposure. The year 1996 was chosen as a
  reasonable starting point for comparison with recent years.  These maps focus on the parts
  of the United States where Lyme disease is most common. Data source: CDC, 20752J
ABOUT THE INDICATOR
•WHAT'S HAPPENING^
   • The incidence of Lyme
     disease in the United States
     has approximately dou-
     bled since 1991, from 3.74
     reported cases per 100,000
     people to 7.95 reported
     cases per 100,000 people
     in 2014.
   • Driven by multiple factors,
     the number and distribution
     of reported cases of Lyme
     disease have increased over
     time.
   HEALTH
   CONNECTION
   Outdoor workers,
   like farmers and
   landscapers, can be especially
   vulnerable to Lyme disease.
   They typically spend more
   time outside than other
   populations, which increases
   their exposure to ticks that
   may carry Lyme disease.24
This indicator examines the rate of confirmed Lyme disease cases nationwide over time. It is based on data collected by
state and local health departments, which track confirmed cases of Lyme disease that are diagnosed by health care provid-
ers. These agencies report total cases to the U.S. Centers for Disease Control and  Prevention, which compiled the national
statistics shown above. Nationwide reporting of Lyme disease began in 1991. The actual number of illnesses is likely greater
than what is reported to health officials and shown here, considering that Lyme disease can be difficult to diagnose.25 The
online version of this indicator also shows a map of changes in reported Lyme disease incidence by state.
                                                                                                                  65

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                         West Nile Virus
                       This indicator tracks the rate of reported West Nile virus disease cases across the United States.
            West Nile virus is the most common cause of mosquito-borne disease in the United States in most years. Infection with
            the virus can lead to symptoms such as headaches, body aches, joint pains, vomiting, diarrhea, and rash, as well as
            more severe damage to the central nervous system in some patients, causing encephalitis, meningitis, and occasionally
    death.26 Climate  change may raise the risk of human exposure to West Nile virus, which is transmitted between birds and mos-
    quitoes and causes human disease when infected mosquitoes bite people. Studies show that warmer temperatures associated
    with climate change can speed up mosquito development, biting rates, and  the incubation of the disease within a mosquito.27
    Mild winters and drought have also been associated with West Nile virus disease outbreaks.28'29 Climate change's effects on
    birds, the main hosts of the virus, may also contribute to changes in long-range virus movement, as the timing of migration and
    breeding patterns are driven by climate.
    ^WHAT'S HAPPENING-

      •  The incidence of West Nile
        virus neuroinvasive disease in
        the United States has varied
        widely from year to year. No
        obvious long-term trend can
        be detected yet through this
        limited data set.
      •  The years 2002, 2003, and
        2012 had the highest reported
        incidence rates, around one
        case per 100,000 people.
Reported Neuroinvasive Cases of West Nile Virus Disease in the United
States, 2002-2014
   1.2
                                                                                                    2014
                                                                                                             2016
                                       This figure shows the annual incidence of West Nile virus neuroinvasive disease, which is
                                       calculated as the number of new cases per 100,000 people. The graph is based on cases that
                                       local and state health departments report to the Centers for Disease Control and Prevention's
                                       national disease tracking system. Neuroinvasive cases, which occur for less than 7 percent
                                       of people infected with West Nile virus, are those that affect the brain or cause neurologic
                                       dysfunction. Data source: CDC, 2076JO
66

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  WHAT'S HAPPENING
  West Nile virus occurs
  throughout the contiguous
  48 states. Average annual
  incidence is highest in
  parts of the Southwest, the
  Mississippi Delta region, the
  Great Plains, and the  Rocky
  Mountain region.
Reported Neuroinvasive Cases of West Nile Virus Disease by State,
2002-2014
                                                 Average annual cases per 100,000 people:
                                               I
                       I
                                               0.5
                                                              1.5
                                                                             2.5
                                                                                         No cases
HEALTH CONNECTION
People who work or spend
large amounts of time out-
doors, like farmers, military
personnel, or homeless people,
can be especially vulnerable
to West Nile virus.52-55 A higher
risk of West Nile
virus infection is
also associated
with advanced age
and being male.54
                                This map shows the average annual incidence of West Nile virus neuroinvasive disease in
                                each state, which is calculated as the average number of new cases per 100,000 people per
                                year from 2002 to 2014. The map is based on cases that local and state health departments
                                report to the Centers for Disease Control and Prevention's national disease tracking system.
                                Neuroinvasive cases, which occur for less than 1 percent of people infected with West Nile
                                virus, are those that affect the brain or cause neurologic dysfunction.
                                Data source: CDC, 201&1
  ABOUT THE INDICATOR
  This indicator looks at the incidence of human cases of West Nile virus disease.
  It focuses on neuroinvasive cases, as the symptoms are noticeable and typically
  require medical care, which makes detection and reporting more consistent.
  West Nile became a nationally notifiable disease in 2002, which means health
  care providers are required to report confirmed cases to their local or state health
  departments. The U.S. Centers for Disease Control and Prevention compiles these
  reported data and calculates national and state-level totals and rates.
                                                                                                                   67

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                        Length of Growing  Season
                     This indicator measures the length of the growing season in the contiguous 48 states.
        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. Depending on the region and the climate, the growing season is influenced by air temperatures,
    frost days, rainfall, or daylight hours. Changes in the length of the growing season can have both positive and negative effects
    on the yield and prices of particular crops. Overall, warming is expected to have negative effects on yields of major crops, but
    crops in some individual locations may benefit.35 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.
    ^WHAT'S HAPPENING

      • The length of the growing
       season for crops has increased
       in almost every state. States in
       the Southwest (e.g., Arizona
       and California) have seen the
       most dramatic increase. In
       contrast, the growing season
       has actually become shorter in
       a few southeastern states.
Change in Length of Growing Season by State, 1895-2015
                                                    Change in length of growing season (days):
                                                 -20
                  -10
10
20
30
40
50
                                     This map shows the total change in length of the growing season from 1895 to 2015 for each
                                     of the contiguous 48 states. Data source: Kunkel, 201636
    ABOUT THE INDICATOR
    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. Temperature measurements come from weather
    stations in the National Oceanic and Atmospheric Administration's Cooperative Observer Program. Growing season length was
    averaged by state, while the timing of spring and fall frosts were averaged across the nation, then compared with long-term
    average numbers (1895-2015) to determine how each year differed from the long-term average. The online version of this indi-
    cator provides additional maps and graphs that track the length of the growing season nationwide and changes in the timing of
    the last spring frost and first fall frost by state.
68

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Timing of Last Spring Frost and First Fall Frost in the Contiguous 48
States, 1895-2015
                         
-------
                         Ragweed  Pollen  Season
                      This indicator depicts changes in the length of ragweed pollen season in the United States and Canada.
          One of the most common environmental allergens is ragweed, which can cause hay fever and trigger asthma attacks.
          Ragweed pollen season usually peaks in late summer and early fall, but these plants often continue to produce pollen
          until the first frost. Climate change can affect pollen allergies in several ways. Warmer spring temperatures cause some
    plants to start producing pollen earlier (see the Leaf and Bloom Dates indicator on p. 82), while warmer fall temperatures
    extend the growing season for other plants, such as ragweed (see the Length of Growing Season indicator on p. 68). Warmer
    temperatures and increased carbon dioxide concentrations also enable ragweed and other plants to produce more pollen.38 This
    means that many locations could experience longer allergy seasons and higher pollen counts as a result of climate change.
       WHAT'S HAPPENING

       Since 1995, ragweed pollen
       season has grown longer at 10
       of the 11 locations studied.
       The increase in ragweed
       season length generally
       becomes more pronounced
       from south to north. Ragweed
       season increased by 25 days in
       Winnipeg, Manitoba; 24 days
       in Saskatoon, Saskatchewan;
       21 days in Fargo, North Dakota;
       and 18 days in Minneapolis,
       Minnesota. This trend is
       consistent with many other
       observations showing that cli-
       mate is changing more rapidly
       at higher latitudes.39
       The trends shown 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 1995.40
Change in Ragweed Pollen Season, 1995-2015


               | +24 days

                           +25 days


                           j-21 days

                               +18 days
                              £ +15 days
                                  W A+15 days
                             +15 days
                            i
                               +25 days
          Change in length
          of pollen season:

            • Increase

            0 Decrease
                        +6 days{
                                +11 days
                         »-1 day
This figure shows how the length of ragweed pollen
season changed at 11 locations in the central United
States and Canada between 1995 and 2015. Red cir-
cles represent a longer pollen season; the blue circle
represents a shorter season. Larger circles indicate
larger changes. Data source: Ziska et a/., 2076*
    ABOUT THE INDICATOR
HEALTH CONNECTION
Longer pollen seasons
increase people's exposure to pol-
len and may lead to more asthma
episodes and other allergy-related
illnesses, especially for children.
Children are more sensitive than
adults to the effects of pollen and
other respiratory hazards because
of their level of physical activity
and body weight, and because
their lungs continue to develop
through adolescence.42
                           .-v.
    This indicator shows changes in the length of the ragweed pollen season in 11 cities that
    were selected as part of a study that looked at trends in pollen season at sites similar
    in elevation and across a range of latitudes from south to north. Air samples have been
    collected and examined at each location since at least the 1990s. Pollen spores are
    counted and identified using microscopes. Pollen count data have been compiled by the National Allergy Bureau in the United
    States and Aerobiology Research Laboratories in Canada. Trends were analyzed by a team of researchers that published a more
    detailed version of this analysis in 2011.43
70

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          Ecosystems
Ecosystems provide humans with food, clean water, and a variety of other
services that can be affected by climate change. This chapter looks at some
of the ways that climate change affects ecosystems, including changes
in wildfires, streams and lakes, bird migration patterns, fish and shellfish
populations, and plant growth.

WHY DOES IT MATTER?

Changes in the Earth's climate can affect ecosystems by altering the water
cycle, habitats, animal behavior—such as nesting and migration patterns—
and the timing of natural processes such as flower blooms. Changes
that disrupt the functioning of ecosystems may increase the risk of harm
or even extinction for some species. While wildfires occur naturally, for
example, more frequent and more intense fires can significantly disrupt
ecosystems, damage property, put people and communities at risk, and
create air pollution problems even far away from the source.

While plants and animals have adapted to environmental change for
millions of years, the climate changes being experienced  now could
require adaptation on larger and faster scales than current species
have successfully achieved in the past, potentially increasing  the risk of
extinction or severe disruption for many species.
                                                                          71

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                         Wildfires
                       This indicator tracks the extent of wildfires in the United States.
          Although wildfires occur naturally and play a long-term role in the health of forests, shrublands, and grassland, climate
          change threatens to increase the frequency, extent, and severity of fires through numerous factors, such as increased
          temperatures and drought (see the U.S. and Global Temperature and Drought indicators on pp. 18 and 28).
    Wildfires have the potential to harm property, livelihoods, and human health.  Beyond the human impact, wildfires also affect
    the Earth's climate.  Forests in particular store large amounts of carbon. When they burn, they release carbon dioxide into the
    atmosphere, which  in turn contributes to climate change.

    ABOUT THE INDICATOR
    The figures here show the total land area burned nationwide and by state. Data for the graph come from the National Inter-
    agency Fire Center,  which compiles reports from local, state, and federal agencies that are involved in fighting wildfires. The
    U.S. Forest Service tracked similar data using a different reporting system until 1997. Those data have been added to the graph
    for comparison.  Data for the map come from the Monitoring Trends in Burn Severity project, sponsored by the Wildland Fire
    Leadership Council. This project uses satellite images taken before and after wildfires to assess the severity of damage. Other
    parts of this indicator available online track the total  number of fires (frequency) and the degree of damage that fires cause to
    the landscape (severity).
      WHAT'S HAPPENING
        The extent of area burned by
        wildfires each year appears
        to have increased since the
        1980s. According to National
        Interagency Fire Center data,
        of the 10 years with the largest
        acreage burned, nine have
        occurred since 2000, including
        the  peak year in 2015. This
        period coincides with many of
        the  warmest years on record
        nationwide (see the U.S. and
        Global Temperature indicator
        on p. 18).
Wildfire Extent in the United States, 1983-2015

     12
 u
 9
     10
 &   6
 •c
 
-------
Average Annual Burned Acreage by State, 1984-2014
•WHAT'S HAPPENING>s

  •  Land area burned by
    wildfires varies by state.
    Fires burn more land in the
    western United States than in
    the East.
                        Change in annual burned acreage:
            0123456

             States colored light gray did not have any fires that were large enough
                            to be included in this analysis.

This map shows the average number of acres burned in each state per year as a pro-
portion of that state's total land area. Darker-shaded states have the largest proportion
of acreage burned. For reference, there are 640 acres in a square mile; therefore, an
average burned area of 6.4 acres per square mile would mean that fires burned 7 percent
of a state's total land area. A few states did not have any fires that were large enough to
be included in this analysis. Visit this indicator online at: www.epa.gov/climate-indicators
for an interactive version of this map. Data source: MTBS, 201&
 HEALTH CONNECTION
 Wildfires worsen air quality.
 Fine particles present in wildfire
 smoke can drift many miles away
 from the site of the fire. These
 air pollutants increase the risk of
 premature death as well as chronic
 and acute cardiovascular and
 respiratory health  problems.4

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                        Streamflow
                I
This indicator describes trends in the amount of water carried by streams across the United States, as well as the
timing of runoff associated with snowmelt.
        Streamflow is a measure of the rate at which water is carried by rivers and streams, and it represents a critical resource for
        people and the environment. Climate change can affect Streamflow in several ways. For example, changes in the amount
        of spring snowpack (see the Snowpack indicator on p. 52) and air temperatures that influence melting can alter the size
    and timing of high spring streamflows. Changes in precipitation and drought patterns could increase or reduce Streamflow in
    certain areas. 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.
       Seven-Day Low Streamflows in the United States, 1940-2014
            *»•
           o    o
            A*      T
                                            Morethan
                                           50% decrease
                                                                  20% to 50%
                                                                   decrease

                                                                      o
                                                                  20% decrease
                                                                 to 20% increase


                                                                  20% to 50%
                                                                   increase

                                                                     A
                                                                   Morethan
                                                                  50% increase
        This map shows percentage changes in the minimum annual Streamflow for rivers and streams
        across the country, based on the long-term rate of change from 1940 to 2014. Minimum Streamflow
        is based on the consecutive seven-day period with the lowest average flow during a given year. Data
        source: USGS, 2016s

        Three-Day High Streamflows in the United States, 1940-2014
                                  B^B^a
                                                         ,
                                                        *A
             T  °  A
                                                                   Morethan
                                                                  50% decrease
                                                                  20% to 50%
                                                                   decrease

                                                                      o
                                                                  20% decrease
                                                                 to 20% increase

                                                                      I
                                                                  20% to 50%
                                                                   increase

                                                                     A
                                                                   Morethan
                                                                  50% increase
•WHAT'S HAPPENING ^
    • During the past 75 years,
      seven-day low flows have
      generally increased in the
      Northeast and Midwest (in
      other words, on the days
      of lowest flows, streams in
      these areas are carrying
      more water than before).
      Low flows have generally
      decreased in parts of
      the Southeast and the
      Pacific Northwest (that
      is, streams are carrying
      less water than before).
      Overall, more sites have
      experienced increases
      than decreases.
    • Three-day high-flow
      trends vary from region
      to region across the
      country. For example,
      high flows have generally
      increased or changed
      little in the Northeast
      since 1940, whereas high
      flows have increased in
      some West Coast streams
      and decreased in others.
      Overall, more sites have
      experienced increases
      than decreases.
       This map shows percentage changes in the maximum annual Streamflow for rivers and streams
       across the country, based on the long-term rate of change from 1940 to 2014. Maximum Streamflow
       is based on the consecutive three-day period with the highest average flow during a given year.
       Data source: USGS, 20166
74

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    Annual Average Streamflow in the United States, 1940-2014
                                                                                •WHAT'S HAPPENING
                8
              o  o
       o    o
                               iB^l
o   

                        More than
                      50% decrease
                                                                20% to 50%
                                                                 decrease

                                                                    o
                                                               20% decrease
                                                              to 20% increase

                                                                    B
                                                                20% to 50%
                                                                 increase

                                                                   A
                                                                More than
                                                               50% increase
    This map shows percentage changes in the annual average streamflow for rivers and streams
    across the country, based on the long-term rate of change from 1940 to 2014. This map is
    based on daily streamflow measurements, averaged over the entire year.
    Data source: USGS, 20167


    Timing of Winter-Spring Runoff in the United States, 1940-2014

                                                                   T
                                                                More than
                                                               10 days earlier
                                                                   T
                                                                  StolO
                                                                days earlier
                                                                   V
                                                                  2to5
                                                                days earlier
                                                                   o
                                                               2 days earlier
                                                               to 2 days later

                                                                  2to5
                                                                days later
                                                                   A
                                                                  5 to 10
                                                                days later
                                                                   A
                                                                More than
                                                                10 days later

    This map shows changes in the timing of annual high winter-spring flow carried by rivers and
    streams from 1940 to 2014. This analysis focuses on parts of the country where streamflow is
    strongly influenced by snowmelt. Trends are based on the winter-spring center of volume, which
    is the date when half of the totalJanuary 1-July J7 streamflow (in the West) or half of the total
    January 1-May J7 streamflow (in the East) has passed by each streamflow gauge.
    Data source: USGS, 2016s
Annual average stream-
flow has increased
at many sites in the
Northeast and Midwest,
while other regions have
seen few substantial
changes. Overall, sites
show more increases
than decreases.
In parts of the coun-
try with substantial
snowmelt, winter-spring
runoff is happening
more than five days
earlier than in the mid-
20th century at most
gauges. The largest
changes occurred in the
Pacific Northwest and
Northeast.
ABOUT THE INDICATOR
This indicator is based on measurements taken by the U.S. Geological Survey using continuous monitoring devices called
stream gauges. The indicator focuses on sites where trends are not substantially influenced by dams, reservoir man-
agement, wastewater treatment facilities, or land-use change. The lowest flows each year are commonly calculated by
averaging the lowest seven consecutive days of streamflow, while the highest flows each year are commonly calculated by
averaging the highest three consecutive days of streamflow. Annual average streamflow is calculated by averaging daily
flows through the entire year. The fourth graph examines the timing of winter and spring runoff in areas where at least 30
percent of annual precipitation falls as snow. Scientists look at the total volume of water that passes by a gauge between
January 1 and July 31 for the western United States, and January 1 and  May 31 for the eastern United States, then determine
the date when exactly half of that water has gone by. This date is called the winter-spring center-of-volume date.
                                                                                                                     75

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                         Stream Temperature
                       This indicator shows changes in stream water temperature across the Chesapeake Bay region.
         Rising air temperatures (see the U.S. and Global Temperature indicator on p. 18), along with other factors such as land-
         use changes, can contribute to higher water temperatures in streams. This warming can affect water quality and aquatic
         life. Many plants, animals, and other organisms living in streams can flourish only in a specific range of water temperatures.
    Higher temperatures reduce levels of dissolved oxygen in the water, which can negatively affect the growth and productivity
    of aquatic life, and can accelerate natural chemical reactions and release excess nutrients into the water.9 A stream's water
    temperature can also influence the circulation or mixing patterns in the water it flows into, like bays and estuaries, potentially
    affecting nutrient levels and salinity. The Chesapeake Bay is the largest estuary in the United States, an important habitat for
    countless aquatic species, and a driver of the regional economy. Warmer stream water coming into the bay can stress plants and
    animals and worsen the effects of nutrient pollution that the bay is already facing.10
      -WHAT'S HAPPENING

      •  From I960 through
        2014, water temperature
        increased at 79 percent of
        the stream sites measured
        in the Chesapeake Bay
        region. More than half of
        these increases were sta-
        tistically significant. Only
        5 percent of stations had
        a significant temperature
        decrease over the same
        period.
      •  Since 1960, the Ches-
        apeake Bay region has
        experienced an overall
        increase in stream  water
        temperature. Temperature
        has risen by an average of
        1.2°F  across all sites and
        2.2°F at the sites where
        trends were statistically
        significant.
      •  Stream temperatures
        have  risen throughout the
        Chesapeake Bay region.
        The largest increases have
        occurred in the southern
        part of the region.
Changes in Stream Water Temperatures in the Chesapeake Bay Region,
1960-2014
                                  New York
                                        0
                                        o
            Ohio
jt  Pennsylvania

*•". *
       0%°
     Maryland °
   •   •*•
        • DC
                                             New Jersey
                                             ..Delaware

                                              •
            Chesapeake Bay Watershed
                      \
              West Virginia
   Virginia


    o


   North Carolina
Chesapeake
  Bay
                                                       Total change (°F):
                                                            -4 to -2
                                                            -2 to -1
                                                            -Itol
                                1to2
                    •
                    >4
                                                        Filled shapes represent
                                                      statistically significant trends.
                                                        Open shapes represent
                                                         trends that are not
                                                        statistically significant.
This map shows the change in water temperature at 129 stream gauges across the Chesapeake
Bay region from 1960 to 2014. Red circles show locations where temperatures have increased;
blue circles show locations where temperatures have decreased. Solid-color circles represent
sites where the change was statistically significant Data source: Jastram and Rice, 2015"
    ABOUT THE INDICATOR
    This indicator is based on an analysis developed by the U.S. Geological Survey. It uses water temperature data from a set of
    stream gauging stations in the Chesapeake Bay region. Field technicians visit each gauging station an average of eight times
    a year to measure various stream conditions, including water temperature. The data were analyzed in a way that accounts for
    variations in timing and makes it possible to compare average temperatures across many years. In addition to climate, changes
    to a stream's average water temperature can be influenced by other factors such as industrial discharges, changes to local hy-
    drology (such as construction and operation of dams and channels), and changes to land cover in the watershed (including the
    amount of shade that trees provide to the stream). Nonetheless, this study found that sites without many of these complicating
    factors warmed just as much as sites with more extensive human influence.12
76

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TRIBAL CONNECTION: TRENDS IN STREAM TEMPERATURE IN THE
SNAKE RIVER
Climate change has challenged and will continue to challenge some of the traditional ways of life that have sus-
tained indigenous peoples for thousands of years.  In the Pacific Northwest, warming river and stream tempera-
tures will threaten ecosystems and species, including salmon populations.13 Salmon play a particularly important
role in the diet, culture, religion, and economy of Native Americans in this region.14
Salmon are sensitive to water temperature at many stages of their lives. They spend much of their adult lives in
the ocean, then migrate inland to spawn. Salmon need cold water to migrate and for their young to hatch and
grow successfully. Warmer water can negatively affect fish, making it more difficult for them to swim upstream.
It can also make fish more susceptible to disease.15 River and stream temperatures in the Pacific  Northwest are
influenced by many factors, but are expected to rise as average air temperatures increase (see the U.S. and
Global Temperature indicator on p. IS).16'1718
The graph shows average August water temperatures at a site in the Snake River, in eastern Washington near
Nez Perce tribal lands. Several species of salmon use the Snake River to migrate and spawn. Between 1960 and
2015, water temperatures have increased by 1.4°F.
Average August Temperatures in the Snake River, 1960-2015
   78

   76
   64
   62
    1960
              1970
                       1980       1990
                                Year
                                          2000
                                                    2010
                                                             2020
This graph shows average August
water temperatures at a site along the
Snake River in eastern Washington.
Data source: USGS, 201&9
                                                                                                          77

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                        Great Lakes  Water  Levels
                      This indicator measures water levels in the Great Lakes.
        The Great Lakes, which are Lake Superior, Lake Michigan, Lake Huron, Lake Erie,
        and Lake Ontario, form the largest group of freshwater lakes on Earth. These lakes
        support a variety of ecosystems and play a vital role in the economy of the eight
    neighboring states and the Canadian province of Ontario, providing drinking water, ship-
    ping lanes, fisheries, recreational opportunities, and more. Water level (the height of the
    lake surface above sea level) is influenced by factors like precipitation, snowmelt runoff,
    drought, evaporation  rates, and people withdrawing water for multiple uses. Warmer
    water, reduced ice cover, and increased evaporation resulting from climate change could
    affect water levels, "lake-effect" precipitation, shipping, infrastructure, and ecosystems.
                                                                          • Direction of flow
                                                            Lake
                                                           Superior
                                                                  \
                                                              Lake
                                                             Michigan
                                                                       Lake
                                                                       Huron
                                                                                 Lake
                                                                                Ontario
                                                                        /
                                                                           /Lake
                                                                           Erie
    ABOUT THE INDICATOR
    This indicator analyzes water levels in the Great Lakes. Water levels are recorded by gauges along the shore of each lake, some
    of which have been operated since the 1800s. These data were provided by the National Oceanic and Atmospheric Administra-
    tion (NOAA) and the Canadian Hydrographic Service. Annual water level anomalies, or differences, in feet are compared with
    the average water levels in each lake from 1860 to 2015. Another component of this indicator available online tracks surface
    water temperatures in the Great Lakes  based on satellite imagery analyzed by NOAA's Great Lakes Environmental Research
    Laboratory.
       Water levels in the
       Great Lakes have
       fluctuated since
       1860. Over the
       last few decades,
       they appear to
       have declined for
       most of the Great
       Lakes. The most
       recent levels are all
       within the range of
       historical variation,
       however.
                                                                                    •WHAT'S HAPPENING-N
Water Levels of the Great Lakes, 1860-2015
                                                            Lake Erie
                             IB

                             o
                             c
                             IB

                            1
                             0)
                            I
    -4
    1860 1880
             1900  1920 1940 1960 1980 2000 2020 1860  1880 1900 1920 1940 1960 1980 2000 2020
                                                          Lake Ontario
Lakes Michigan and Huro
                                -4
                                I860 1880
                                         1900  1920 1940 1960  1980 2000 2020 1860  1880 1900 1920  1940 1960 1980 2000 2020

                                                                     Year
                            This figure displays how water levels in each of the Great Lakes have changed since 1860. For each year,
                            the shaded band shows the range of monthly average water levels, and the line in the middle shows the
                            annual average. The graph uses the 1981 to 2010 average as a baseline for depicting change. Choosing a
                            different baseline period would not change the shape of the data over time. Lakes Michigan and Huron are
                            shown together because they are connected at the same water level. Data source: NOAA, 201620
78

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                     Bird Wintering  Ranges
                   This indicator examines changes in the winter ranges of North American birds.
     Changes in climate can affect ecosystems by influencing animal behavior and ranges. Birds are a particularly good
     indicator of these changes because the timing of certain events in their life cycles—such as migration and reproduc-
     tion—is driven by cues from the environment. Changing conditions can influence the distribution of both migratory
and non-migratory birds as well as the timing of important life cycle events.21 If a change in behavior or range occurs across
many types of birds, it suggests that a common external factor, such as a change in the pattern of temperature or precipita-
tion, might be the cause. Birds are also a useful indicator because they are easy to identify and count, and thus people have
kept detailed records of bird distribution and abundance for more than a century.
   Change in Latitude of Bird Center of Abundance, 1966-2013
       80
   "v>
   *   70
   E
   ^   60
       50
o
c
•o
*
o
E

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                        Marine Species Distribution	
                      This indicator examines changes in the location offish, shellfish, and other marine species along U.S. coasts.
         Changes in water temperature can affect the environments where fish, shellfish, and other marine species live. As climate
         change causes the oceans to become warmer year-round (see the Ocean Heat and Sea Surface Temperature indicators
         on pp. 32 and 33), populations of some species may adapt by shifting toward cooler areas. Along U.S. coasts, this
    means a shift northward or to deeper waters. Marine species represent a particularly good indicator of warming oceans because
    they are sensitive to climate and have been studied and tracked for many years. Tracking the movement of multiple species is
    useful because if a change in behavior or distribution occurs across a large  range of species, it is likely the result of a systematic
    cause rather than a species-specific one.

    ABOUT THE INDICATOR
    This indicator tracks marine animal species in terms of their "center of biomass," which is a point on the map that represents the
    center of each  species' distribution by weight. If a fish population were to shift generally northward, the center of biomass would
    shift northward as well. Data for this indicator were collected by the National Oceanic and Atmospheric Administration's Na-
    tional Marine Fisheries Service and other agencies, which monitor marine species populations by conducting annual surveys at
    regular  intervals along the coast. By recording what they catch at each location, scientists can calculate each species' center of
    biomass in terms of latitude, longitude, and depth. This indicator focuses on two regions that have the longest, most consistent
    sampling: the Northeast and the eastern Bering Sea off the coast of Alaska. The species shown in the two maps were chosen
    because they represent a variety of habitats and species types (a mixture of fish and shellfish), they tend to be fairly abundant,
    and their population trends are not unduly impacted by overfishing.
    xWHAT'S HAPPENING
        The average center of bio-
        mass for 105 marine fish and
        invertebrate species shifted
        northward by about 10 miles
        between 1982 and 2015,  as
        shown in the chart. These
        species also moved an average
        of 20 feet deeper.
Change in Latitude and Depth of Marine Species, 1982-2015
                                           1980
                                                   1985
                                                          1990
                                                                 1995
                                                                         2000
                                                                                2005
                                                                                        2010
                                                                                               2015
                                        „  0
       This graph shows the annual
       change in latitude (movement in
       miles) and depth of 105 marine
       species along the Northeast
       coast and in the eastern Bering
       Sea. Changes in the centers of
       biomass have been aggregat-
       ed across all 105 species. Data
       source: NOAA and Rutgers
       University, 201624
                                                         2015
                                                                   North
                                                                                                         South
                                                                                                       2020
                                                                2020
80

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Average Location of Three Fish and Shellfish Species in the Northeast,
1968-2015
                                                              American lobster
                                                              Red hake
                                                              Black sea bass
                                                     1968
                                                           2015
                                                                           North
                                                                       •WHAT'S HAPPENING-^

                                                                           • In waters off the northeast-
                                                                             ern United States, several
                                                                             economically important
                                                                             species have shifted north-
                                                                             ward since the late 1960s,
                                                                             as shown in the map of the
                                                                             Northeast Atlantic coast.
                                                                             The three species shown
                                                                             (American lobster, red
                                                                             hake, and black sea bass)
                                                                             have moved northward by
                                                                             an average of 119 miles.
                                                                             This map shows the
                                                                             annual centers ofbiomass
                                                                             for three species in the
                                                                             northeastern United States
                                                                             from 1968 to 2015. Dots are
                                                                             shaded from light to dark
                                                                             to show change over time.
                                                                             Visit this indicator online at:
                                                                             www.epa. gov/climate-in-
                                                                             dicators for an interactive
                                                                             version of this map. Data
                                                                             source: NOAA and Rutgers
                                                                             University, 201625
                                                    -50
         1970   1985  2000   2015
                                1970  1985   2000

                                     Year
                                                2015   1970   1985  2000   2015 South
Average Location of Three Fish and Shellfish Species in the Bering Sea,
1982-2015
    ALASKA
              Alaska pollock
              Snow crab
              Pacific halibut
    1982   2015*
   •Alaska pollockdata only extend to 2014.
   .2  30
.2 £
•o —
& -o
§> 9
(Q JT
Sj £  -eo
1985  1995  2005  2015
                                        Snow crab
                                1985  1995  2005  2015
                                     Year
 60

 30

 0

-30

-60
                                                                 North

                                                                 I

                                                                 I
     Pacific halibut

1985  1995  2005  2015 South
                                                                             In the Bering Sea, Alaska
                                                                             pollock, snow crab, and
                                                                             Pacific halibut have gen-
                                                                             erally shifted away from
                                                                             the coast since the early
                                                                             1980s,  as shown in the map
                                                                             of the Bering Sea. These
                                                                             species have also  moved
                                                                             northward by an average
                                                                             of 14 miles.
                                 This map shows the annual
                                 centers ofbiomass for
                                 three species in the eastern
                                 Bering Sea from 1982 to
                                 2015. Dots are shaded
                                 from light to dark to show
                                 change over time. Visit this
                                 indicator online at: www.
                                 epa.gov/climate-indicators
                                 for an interactive version
                                 of this map. Data source:
                                 NOAA and Rutgers Univer-
                                 sity, 201626

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                         Leaf  and  Bloom  Dates
                       This indicator examines the timing of leaf growth and flower blooms for two widely distributed plants in the United
                       States.
        Phenology is the study of important seasonal events and their timing, such as flower blooms and animal migration. Pheno-
        logical events are influenced by a combination of environmental factors, including temperature, light, rainfall, and humidity.
        Because of their close connection with climate, the timing of phenological events can be used as an indicator of the
    sensitivity of ecological processes to climate change. Two particularly useful indicators are the first leaf dates and the first bloom
    dates of lilacs and honeysuckles in the spring. Scientists have high confidence that the earlier arrival of spring events is linked to
    recent warming trends in global climate.27
      •WHAT'S HAPPENING
        First leaf and bloom dates in
        lilacs and honeysuckles in the
        contiguous 48 states show
        a great deal of year-to-year
        variability, which makes it dif-
        ficult to determine whether a
        statistically meaningful change
        has taken place. Earlier dates
        appear more prevalent, howev-
        er, in the last few decades.
First Leaf and Bloom Dates in the Contiguous 48 States, 1900-2015
x
(Q
0)
01
a
o
o
00
CPi

E
o
**-
c
o
13
°>
                                                                      Later
                                                                                                             Earlier
                                                            1 Leaf date (smoothed)  •—• Bloom date (smoothed)
                                                          	Leaf date (annual)      — Bloom date (annual)
                                           -15
                                             1900  1910  1920  1930  1940  1950  1960 1970 1980  1990  2000  2010  2020

                                                                           Year

                                        This figure shows modeled trends in lilac and honeysuckle first leaf dates and first bloom
                                        dates across the contiguous 48 states, using the 1981 to 2010 average as a baseline. Positive
                                        values indicate that leaf growth and blooming began later in the year, and negative values
                                        indicate that leafing and blooming occurred earlier. The thicker lines were 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. Data source: Schwartz, 201628
    ABOUT THE INDICATOR
    This indicator shows trends in the timing of first leaf dates and first bloom dates in lilacs and honeysuckles across the contigu-
    ous 48 states. It is originally based on observations collected by the USA National Phenology Network, which collects ground
    observations from a network of federal agencies, field stations, educational institutions, and citizens who have been trained
    to log observations of leaf and bloom dates. Because many of the observation records in the United States are less than 40
    years long or contain gaps, computer models have been used to provide a more complete understanding of long-term trends
    nationwide. These models use temperature data from thousands of weather stations, and were developed and tested based on
    observed relationships between  leaf and bloom dates and daily temperatures. The online version of this indicator also presents
    maps that show how first leaf and bloom dates have changed across the country.
82

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 COMMUNITY CONNECTION: CHERRY BLOSSOM  BLOOM DATES IN

 WASHINGTON, D.C.
 In Washington, D.C., the arrival of spring brings a splash of color as the city's iconic cherry trees burst into bloom.
 The National Cherry Blossom Festival is planned to coincide with the peak bloom of the cherry trees and draws
 more than 1.5 million visitors to the area every year. The peak bloom date for the most common type of cherry tree
 around Washington's Tidal Basin—the Yoshino variety—has been carefully  estimated and recorded since 1921 by
 the National Park Service. The peak bloom date is defined as the day when 70 percent of the blossoms are in full
 bloom. Based on the entire 96 years of data, Washington's blossoms reach their peak on April 4 in an average year.
 The peak bloom date has shifted earlier by approximately five days since 1921. While the length of the National
 Cherry Blossom Festival has continued  to expand, the Yoshino cherry trees have bloomed near the beginning of
 the festival in recent years. During some years, the festival missed the peak bloom date entirely.


Peak Bloom Date for Cherry Trees Around Washington, D.C.'s Tidal Basin, 1921-2016

     March 1
                                              National Cherry Blossom Festival
    March 11
     April 30
          1920  1930  1940 1950  1960  1970 1980  1990  2000 2010  2020

                                      Year

This figure shows the peak bloom date each year for the main type of cherry tree around the Tidal Basin in Washington,
D.C. The peak bloom date occurs when 70 percent of the blossoms are in full bloom. The shaded band shows the timing of
the annual National Cherry Blossom Festival. The festival began in 1934 but was not held during World War II. Data sources:
National Cherry Blossom Festival, 2016;29 National Park Service, 2075JO
                                                                                                           83

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   Climate  cnange  Resource
   EPA's Climate Change website Cwww.epa.gov/climate-chanae) provides a good starting point for further exploration of
   this topic. At this site, you can:
      View the latest information about EPA's climate change indicators
      Cwww.epa.gov/climate-indicators) and download figures as well as
      accompanying technical documentation.
      Learn more about greenhouse gases and the science of climate
      change, discover the potential impacts of climate change on human
      health and ecosystems, read about how people can adapt to changes,
      and get up-to-date news.
      Read about greenhouse gas emissions, look through EPA's green-
      house gas inventories, and explore EPA's Greenhouse Gas Data
      Publication Tool.
      Learn about EPA's regulatory initiatives and partnership programs.
      Explore U.S. climate policy and climate economics.
      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.
      Learn how you, your family,  and your community can respond to and
      stay healthy in a changing climate.
      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 interna-
      tional authority on climate change science. The IPCC website Cwww.
      ipcc.ch/index.htm) summarizes the current state of scientific knowl-
      edge about climate change.
   •  The U.S. Global Change Research Program Cwww.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 and
      The Impacts of Climate Change on Human Health in the United States:
      A Scientific Assessment Chttps://health2016.globalchange.gov).
84

-------
   The National Academy of Sciences (http://nas-sites.org/
   americasdimatechoices) has developed many independent scientific
   reports on the causes of climate change, its impacts, and potential
   solutions. The National Academy's Koshland Science Museum
   (https://koshland-sdence-museum.org) provides an interactive online
   Earth Lab where people can learn more about these topics.
   The National Oceanic and Atmospheric Administration (NOAA) is
   charged with helping society understand, plan for, and respond to
   climate variability and change. Find out more about NOAA's climate
   indicators and other activities at: www.climate.gov.
   NOAA's National Centers for Environmental Information website
   (www.ncei.noaa.gov) 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 exten-
   sive information about the relationship between climate change and
   public health at: www.cdc.gov/climateandhealth/default.htm.
   The U.S. Geological Survey's Climate and Land Use Change website
   (www.usgs.gov/science/mission-areas/climate-and-land-use-change)
   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://earthobservatorv.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/crvosphere) provides more information about ice and
   snow and how they influence and are influenced by climate change.
   The Woods Hole Oceanographic Institution's website
   (www.whoi.edu/main/topic/climate-ocean) explains how climate change
   affects the oceans and how scientists measure these effects.
Climate change in
Plan ahead with a
For more indicators of environmental condition and human health, 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 exposure and health, and ecological systems.

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                                                                                                                       •ttltfA
                                                                                                                  .«"  /•'


                                                                                                  	,• •   "  . '.!
     INTRODUCTION
             Melillo, J.M..T.C. Richmond, and G.W.Yohe(eds.). 2014. Climate change impacts in the United States: The third National Climate Assessment. U.S. Global Change Research
             Program. http://nca2014.globalchange.gov.
             Crimmins, A., J.Balbus, J.L Gamble, C.B. Beard, J.E. Bell, D. Dodgen.R.J. Eisen, N. Fann.M.D. Hawkins, S.C. Herring, L. Jantarasami, D.M. Mills, S. Saha, M.C. Sarofim, J.
             Trtanj, and L. Ziska (eds). 2016. The impacts of climate change on human health in the United States: A scientific assessment. U.S. Global Change Research Program, https://
             health201 6.globalc hange.gov.
     UNDERSTANDING  GREENHOUSE  GASES
        1.    IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis. Working Group I contribution to the IPCC Fifth Assessment
             Report. Cambridge, United Kingdom: Cambridge University Press,  www.ipcc.ch/report/ar5/wg1.
     SUMMARY OF  KEY  POINTS
        1.    IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis. Working Group I contribution to the IPCC Fifth Assessment
             Report. Cambridge, United Kingdom: Cambridge University Press,  www.ipcc.ch/report/ar5/wg1.
     GREENHOUSE GASES
        1.    IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change
             2013: The physical science basis. Working Group I contribution to the IPCC Fifth
             Assessment Report. Cambridge, United Kingdom: Cambridge University Press.
             www.ipcc.ch/report/ar5/wq1.
        2.    U.S. EPA (U.S. Environmental Protection Agency). 2016. Inventory of U.S. green-
             house gas emissions and sinks: 1990-2014. EPA 430-R-16-002. www3.epa.
             gov/climatechange/ghgemissions/usinventoryreport.html.
        3.    U.S. EPA (U.S. Environmental Protection Agency). 2016. Inventory of U.S. green-
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        6.    EPICA Dome C and Vostok Station, Antarctica: approximately 796,562 BCE to
             1813 CE
             Liithi, D., M. Le Floch, B. BereiterJ. Blunier, J.-M. Barnola, U. Siegenthaler, D.
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             Law Dome, Antarctica, 75-year smoothed: approximately 1010CEto 1975 CE
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     data/trace  gases/co2/in-situ/surface.
     Cape Grim, Australia: 1992 CE to 2006 CE
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     Lampedusa Island, Italy: 1993 CE to 2000 CE
     Chamard, P., L. Ciattaglia, A. di Sarra, and F. Monteleone. 2001. Atmospheric
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     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.
7.    IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change
     2013: The physical science basis. Working Group I contribution to the IPCC Fifth
     Assessment Report. Cambridge, United Kingdom: Cambridge University Press.
     www.ipcc.ch/report/ar5/wg1.
8.    NOAA (National Oceanic and Atmospheric Administration). 2016. The NOAA
     Annual Greenhouse Gas Index. Accessed June 2016. www.esrl.noaa.gov/gmd/
     aggi.
86

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WEATHER AND  CLIMATE
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   4.    National Research Council. 2011. Climate stabilization targets: Emissions,
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      OCEANS
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             The physical science basis. Working Group I contribution to the IPCC Fifth Assess-
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             Ipcc.ch/report/ar5/wq1.
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88

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SNOW AND  ICE
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                                                                                                                                                                 89

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                                                                                                                                                                  91

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Photo Credits
  Beach house in Nags Head, North Carolina (front cover): James G. Titus, used by permission
  People observing nature at the shore (p. 4): U.S. Fish and Wildlife Service
  Ice forming on Lake Michigan (p. 7): M. McCormick, National Oceanic and Atmospheric Administration, Great Lakes
  Environmental Research Laboratory
  Flooding of the Wabash River (pp. 8 and 26): Ashley Brooks, National Weather Service
  Beach house (pp. 8 and 34): Martina McPherson, used by permission
  Coastal flooding (pp. 8 and 36): Chesapeake Bay Maritime Museum
  Wildfire (pp. 11 and 72): Howard McCullough, U.S. Fish and Wildlife Service
  House finch (pp. 11 and 79): U.S. Fish and Wildlife Service
  Black Forest Fire in Colorado (p. 11): National Wildfire Coordinating Group
  Flooding of the Red River in Minnesota (p. 26 and back cover): Dave Saville, Federal Emergency Management Agency
  Lake Mead (p. 30): Chris Lamie, used by permission
  Lake Oroville, California (p. 30): Kelly Grow, California Department of Water Resources
  Sun over Arctic ice (p. 40): Jeremy Potter, National Oceanic and Atmospheric Administration, Office of Ocean Exploration
  and Research
  Glacier research (p. 42): U.S. Geological Survey
  SNOTEL antenna in North Cascades National Park (p. 42): Melissa Webb, U.S. Department of Agriculture, Natural Resources
  Conservation Service
  Ducks on Lake Mendota (p. 46): Jeff Miller, University of Wisconsin-Madison.
  Forest fire (p. 73): Portland National Incident Management Organization, National  Park Service
  Salmon (p. 77): National Oceanic and Atmospheric Administration
  Fishing with a dip net (p. 77): Columbia River Inter-Tribal Fish Commission

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