CLIMATE CHANGE
INDICATORS
2016
IN THE UNITED STATES
United States
Environmental Protection
Agency
FOURTH EDITION
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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.
-------
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.
-------
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 >
-------
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
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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
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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
-------
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
-------
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
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-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
-------
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
-------
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
-------
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
-------
*
- "• 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
-------
"\
• 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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
0) C
u g, .5
jo g
(0 'IS
•s ^ -io
« *
10 >-
£ 2 -is
-20
S2
3 <5 -25
-30
1940
0)
M- 3 50
O (A
i- «
Q) Q)
A £ 25
•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
-------
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
-------
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
-------
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
-------
'
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
-------
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
-------
•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
-------
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.
-------
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.
-------
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.
\
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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.
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
-------
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
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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.
-------
•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-
house gas emissions and sinks: 1990-2014. EPA 430-R-16-002. www3.epa.
gov/climatechange/ghgemissions/usinventoryreport.html.
4. WRI (World Resources Institute). 2014. Climate Analysis Indicators Tool (CAIT)
2.0: WRI's climate data explorer. Accessed May 2014. http://cait.wri.org.
5. FAO (Food and Agriculture Organization). 2014. FAOSTAT: Emissions—land use.
Accessed May 2014. http://faostat3.fao.orq/faostat-qateway/qo/to/download/
G2/VE.
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.
Raynaud, J. Jouzel, H. Fischer, K. Kawamura, and T.F. Stacker. 2008. High-res-
olution carbon dioxide concentration record 650,000-800,000 years before
present. Nature 453:379-382. www.ncdc.noaa.gov/paleo/pubs/luethi2008/
Iuethi2008.html.
Law Dome, Antarctica, 75-year smoothed: approximately 1010CEto 1975 CE
Etheridge, D.M., LP. Steele, R.L. Langenfelds, R.J. Francey, J.-M. Barnola, and
V.I. Morgan. 1998. Historical C02 records from the Law Dome DE08, DE08-2,
and DSS ice cores. In: Trends: A compendium of data on global change. Oak
Ridge, TN: U.S. Department of Energy. Accessed September 14,2005. http://
cdiac.ornl.gov/trends/co2/lawdome.html.
Siple Station, Antarctica: approximately 1744 CE to 1953 CE
Neftel, A., H. Friedli, E. Moor, H. Lb'tscher, H. Oeschger, U. Siegenthaler, and
B. Stauffer. 1994. Historical carbon dioxide record from the Siple Station ice
core. In: Trends: A compendium of data on global change. Oak Ridge, TN: U.S.
Department of Energy. Accessed September 14,2005. http://cdiac.ornl.gov/
trends/co2/siple.html.
Mauna Loa, Hawaii: 1959 CE to 2015 CE
NOAA (National Oceanic and Atmospheric Administration). 2016. Annual mean
carbon dioxide concentrations for Mauna Loa, Hawaii. Accessed April 14,2016.
ftp://ftp.cmdl.noaa.qov/products/trends/co2/co2 annmean mlo.txt.
Barrow, Alaska: 1974 CE to 2014 CE
Cape Matatula, American Samoa: 1976 CE to 2014 CE
South Pole, Antarctica: 1976 CE to 2014 CE
NOAA (National Oceanic and Atmospheric Administration). 2016. Monthly mean
carbon dioxide concentrations for Barrow, Alaska; Cape Matatula, American
Samoa; and the South Pole. Accessed April 14,2016. ftp://ftp.cmdl.noaa.gov/
data/trace gases/co2/in-situ/surface.
Cape Grim, Australia: 1992 CE to 2006 CE
Shetland Islands, Scotland: 1993 CE to 2002 CE
Steele, L.P., P.B. Krummel, and R.L. Langenfelds. 2007. Atmospheric C02
concentrations (ppmv) derived from flask air samples collected at Cape Grim,
Australia, and Shetland Islands, Scotland. Commonwealth Scientific and
Industrial Research Organisation. Accessed January 20,2009. http://cdiac.esd.
ornl.gov/ftp/trends/co2/csiro.
Lampedusa Island, Italy: 1993 CE to 2000 CE
Chamard, P., L. Ciattaglia, A. di Sarra, and F. Monteleone. 2001. Atmospheric
carbon dioxide record from flask measurements at Lampedusa Island.
In: Trends: A compendium of data on global change. Oak Ridge, TN: U.S.
Department of Energy. Accessed September 14,2005. 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
1. MOM (National Oceanic and Atmospheric Administration). 2016. National Centers
for Environmental Information. Accessed February 2016. www.ncei.noaa.gov.
2. NOAA (National Oceanic and Atmospheric Administration). 2016. National Centers
for Environmental Information. Accessed February 2016. www.ncei.noaa.gov.
3. Melillo, J.M.J.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.
4. National Research Council. 2011. Climate stabilization targets: Emissions,
concentrations, and impacts over decades to millennia. Washington, DC: National
Academies Press.
5. NOAA (National Oceanic and Atmospheric Administration). 2015. U.S. Climate
Extremes Index. Accessed December 2015. www.ncdc.noaa.gov/extremes/cei.
6. Sarofim, M.C., S. Saha, M.D. Hawkins, D.M. Mills, J. Hess, R. Horton, P. Kinney,
J. Schwartz, and A. St. Juliana. 2016. Chapter 2: Temperature-related death and
illness. The impacts of climate change on human health in the United States: A
scientific assessment. U.S. Global Change Research Program. https://health2016.
globalchange.gov.
7. NOAA (National Oceanic and Atmospheric Administration). 2016. U.S. Climate
Extremes Index. Accessed May 2016. www.ncdc.noaa.gov/extremes/cei.
8. Blunden, J., and D.S.Arndt(eds.). 2016. State of the climate in 2015. B.Am.
Meteorol.Soc.97(8):S1-S275.
9. NOAA (National Oceanic and Atmospheric Administration). 2016. National Centers
for Environmental Information. Accessed February 2016. www.ncei.noaa.gov.
10. Melillo, J.M.J.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.
11. NOAA (National Oceanic and Atmospheric Administration). 2016. U.S. Climate
Extremes Index. Accessed January 2016. www.ncdc.noaa.gov/extremes/cei.
12. Bell, J.E., S.C. Herring, L. Jantarasami, C. Adrianopoli, K. Benedict, K. Conlon, V.
Escobar, J. Hess, J. Luvall, C.P. Garcia-Pando, D. Quattrochi, J. Runkle, and C.J.
Schreck, III. 2016. Chapter 4: Impacts of extreme events on human health. The
impacts of climate change on human health in the United States: A scientific
assessment. U.S. Global Change Research Program. https://health2016.global-
13. Melillo, J.M.J.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.
14. Melillo, J.M.J.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.
15. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013:
The physical science basis. Working Group I contribution to the IPCC Fifth Assess-
ment Report. Cambridge, United Kingdom: Cambridge University Press, www.
Ipcc.ch/reporl/ar5/wa1.
16. Emanuel, K.A. 2016 update to data originally published in: Emanuel, K.A. 2007.
Environmental factors affecting tropical cyclone power dissipation. J. Climate
20(22):5497-5509.
17. IPCC (Intergovernmental Panel on Climate Change). 2012. Managing the risks of
extreme events and disasters to advance climate change adaptation. Cambridge,
United Kingdom: Cambridge University Press. http://ipcc-wg2.gov/SREX.
18. Mallakpour, I., G.Villarini. 2015. The changing nature of flooding across the
central United States. Nature Climate Change 5:250-254.
19. Slater, L., and G.Villarini. 2016 update and expansion to data originally published
in: Mallakpour, I., G.Villarini. 2015. The changing nature of flooding across the
central United States. Nature Climate Change 5:250-254.
20. Slater, L,and G.Villarini. 2016 update and expansion to data originally published
in: Mallakpour, I., G.Villarini. 2015. The changing nature of flooding across the
central United States. Nature Climate Change 5:250-254.
21. Mallakpour, I., G.Villarini. 2015. The changing nature of flooding across the
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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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