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Extreme heat: who's most at risk?
A person's chances of being harmed by extreme heat depend on a combination of exposure, sensitivity, and capacity to
adapt. Depending on these factors, certain populations face higher risks of heat-related illness and death. Increases in the
frequency and intensity of heat waves due to climate change will raise this risk. Specifically:
• The population of adults aged 65 and older, which is expected to continue to grow, has a higher-than-average risk of
heat-related illness and death clue to factors such as underlying health conditions.6 This group accounted for more
heat-related hospitalizations than any other age group in EPA's leat-Related Illnesses indicator.
• Children are at higher risk because their bodies are less able to adapt to heat than adults', and they must rely on others
to help keep them safe. They also tend to spend more time outdoors than adults.7
• People with certain chronic diseases, such as cardiovascular and respiratory illnesses, are at higher risk for heat-related
health impacts. Data also suggest a higher incidence of heat-related illnesses among non-Hispanic Black people.8
• People who lack access to cool spaces or air conditioning, either due to poor housing or from experiencing
homelessness, are at higher risk. The risk increases in neighborhoods where hot temperatures are magnified by the
heat island effect.
• Many workers face higher exposure to extreme heat because they work outdoors in industries like agriculture,
landscaping, construction, and delivery services or work in hot indoor environments that lack adequate air conditioning,
such as manufacturing plants, warehouses, and other facilities.9 For instance, OSHA has found that workers in
agriculture are 35 times more likely to die from heat exposure than the average American worker.10
Learn more about who's most at risk from extreme heat on EPA's website, Climate Change and Human Health: Who's Most
at Risk?, as well as in the "Human Health" chapter of the Fifth National Climate Assessment.
Climate Change Indicators in the United States
17
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Heat waves are more frequent, long-lasting, and intense
than they used to be.
What's happening
A period of several unusually hot days in a row is referred to
as an extreme heat event or a heat wave. Unusually hot days
and heat wave events are a natural part of day-to-day variation
in weather, but as the Earth's climate warms, heat waves are
expected to become more frequent and intense. High humidity
adds to this hazard. EPA's Heatwaves indicator uses more
than 60 years of detailed data on apparent temperatures
(actual temperature plus humidity, similar to the heat index) to
examine how heat wave frequency, duration, season length,
and intensity have changed. In this indicator, a heat wave is
defined as a period of two or more consecutive days when
the daily minimum apparent temperature in a particular city
exceeds the 85th percentile of historical July and August
temperatures (1981-2010) for that city.
• Heat waves are occurring more often than they used to
across 50 major cities measured, including Chicago, Dallas,
Miami, Boston, Phoenix, Memphis, and Seattle. Their
frequency has increased steadily, from an average of two
heat waves per year during the 1960s to six per year during
the 2010s and 2020s (Figure 9).
• The average heat wave season (the number of days
between the first heat wave of the year and the last) across
the 50 cities measured is about 46 days longer now than it
was in the 1960s (Figure 9).
• In recent years, the average heatwave in major U.S. urban
areas has been about four days long. This is about a day
longer than the average heat wave in the 1960s.
• Heat waves have become more intense over time.
Compared with the 1960s, the average heat wave in the
2010s and 2020s is hotter across the 50 cities measured.
Figure 9. Heat Waves in Major U.S. Cities, 1960s vs.
2020s
Heat waves in major U.S. cities
1960s vs. 2020s
2010s 2020s
1990s
2000s
Frequency
Average number
of heat waves per year
across major cities
Hi
in the 1960s
in the 2020s
2010s
1970s
Season
Average length
of heat wave season
across major cities
Data source: NOAA, 2024J
days longer now
compared to the 1960s
The timing of heat waves can matter. Extreme heat events that occur earlier in the spring or later in the fall can catch people off-
guard and increase exposure to the health risks associated with extreme heat. For instance, people who live in multi-family buildings
might not have control of when their air conditioning operates.
-------
The heat island effect
Cities have buildings, pavement, and other infrastructure that absorbs and re-emits the sun's heat more than natural
landscapes such as forests and water bodies. As a result, urban areas can become "islands" that are warmer than
surrounding areas and can hold that heat for longer into the night. This phenomenon is known as the heat island effect.
Heat islands heighten the risk of heat-related illness and death. They also increase energy demand for cooling, which can
increase greenhouse gas emissions and air pollution and can pose a financial burden for many people—particularly low- or
fixed-income households.
The heat island effect is heightened in neighborhoods that lack tree cover and green space. In many urban areas, this
contributes to disproportionate impacts on lower-income communities and people of color, populations who already
face other burdens and challenges. Studies have found that heat island effects are often more severe in neighborhoods
with lower median incomes and a higher proportion of people of color. Some of this can be traced back to the legacy of
"redlining" that began in the 1930s—a discriminatory practice in which the federal government labeled many non-white
neighborhoods as undesirable for real estate investment. Public and private lenders often withheld loans and other services
from people in those areas, depriving residents of opportunities to grow their wealth. Even though redlining was outlawed
in 1968 with the Fair Housing Act, it contributed to residential segregation patterns, homeownership disparities, and
deteriorating infrastructure that are still present today. Current neighborhoods that were once redlined continue to have
less tree cover, higher temperatures, and greater proportions of residents with lower income.12
Learn more about heat islands, and about how strategies such as investing in cooler infrastructure have been used to
mitigate the effects of heat islands, at EPA's Heat Islands website.
Climate Change Indicators in the United States
19
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Warmer temperatures increase the risk of heat-related
illnesses and deaths.
What's happening
Heat is the leading weather-related killer in the United States, even though most heat-related illnesses and deaths are preventable
through outreach and intervention. Extreme heat can exacerbate underlying health conditions including cardiovascular disease,
respiratory diseases like asthma, and diabetes. EPA's Heat-Related Deaths indicator tracks annual deaths classified as "heat-related"
by medical professionals in the 50 U.S. states and the District of Columbia.
• Between 1979 and 2022, the rate of deaths directly resulting from exposure to heat (the underlying cause of death) generally
hovered between 0.5 and 2 per million people; this number spiked in certain years (Figure 10).
• Overall, more than 14,000 Americans have died from heat-related causes since 1979, according to death certificates.
• It is difficult to determine if the United States has experienced a meaningful increase or decrease in deaths classified as
"heat-related" over time. While dramatic increases in heat-related deaths are closely associated with the occurrence of hot
temperatures and heat waves, deaths may not be reported as "heat-related" on death certificates and there is considerable
year-to-year variability in the data.
• While heat-related deaths are the more severe outcome, heat-related conditions such as heat exhaustion and heat stroke send
more Americans to the hospital every year. EPA's Heat-Related Illnesses indicator shows that from 2001 to 2010, annual heat-
related hospitalization rates ranged up to nearly four cases per 100,000 people in some states.
• EPA's Cold-Related Deaths indicator notes that rising winter temperatures are expected to reduce the number of cold-related
deaths, but the decrease is projected to be smaller than increases in heat-related deaths.13
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Outdoor workers, particularly those engaging in strenuous physical activity,
disproportionately face heat-related health threats.16 EPA's A Closer Look:
Heat-Related Workplace Deaths tracks the number of workers who have
died as a result of heat exposure on the job in the United States.
• From 1992 to 2022, a total of 986 workers across all industry sectors
in the United States died from exposure to heat. This represents an
average of 34 deaths per year (Figure 11).
• The construction sector accounted for about 34 percent of all
occupational heat-related deaths from 1992 to 2022. During this time
frame, 334 construction workers died due to heat exposure on the job.
• The total number of heat-related worker deaths per year has ranged
from 11 (reported in 1992) to 61 (2011). The number of heat-related deaths
in the construction sector peaked at 18 in 2010 (and 18 again in 2011).
70
All other industry sectors
60 Construction
All industry sectors
(construction merged into total)
¦c 50 ' I 1
1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 2020 2022
Year
Figure 11. Heat-Related Workplace Deaths in the United States, 1992-2022
This graph shows the number of reported heat-related workplace deaths in the United States each year. The blue portions
of the bars show deaths in the construction sector and the orange portions shovz deaths in all other sectors of the economy.
The dashed lines indicate when the U.S. Bureau of Labor Statistics (BVS) changed how it classifies "construction " (1996/1997
and 2002/2003) or changed its thresholds for reporting individual industry totals (2018/2019). Total construction sector deaths
in 2019 fell below the new threshold; for that year, BLS combined construction into a single "all-industry" total rather than
reporting a sector total. BLS has indicated that data collection for 2020 fell below reporting thresholds. Data source: BLS,
2024.17
Climate Change Indicators in the United States
21
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Taking action: adapting to extreme heat
With unusually hot summer temperatures becoming more common and heat waves
becoming more frequent and intense, the risk of heat-related deaths and illness is expected
to increase unless people take steps to adapt.
The best defense against extreme heat is to be prepared, and remember:
• Get ready. Take steps now to prepare your home, workplace, and community for future
heat events. Consider ways to avoid or postpone outdoor activities, especially during the hottest part of the day.
• Get set. Know the symptoms of heat-related illnesses and what to do in an emergency.
• Go. Check on those who may be most at risk during an extreme heat event, like children, older adults, people with
disabilities, homebound neighbors, outdoor workers, people with chronic medical conditions, and pregnant people.
Communities across the United States are taking steps to prepare for extreme heat and protect their residents. In
response to several severe heat waves in the early 1990s—one of which, in 1993, led to more than 100 deaths-
Philadelphia, Pennsylvania, formed a "Heat Task Force" that implemented several successful strategies that are still
protecting lives today. These strategies include:
• "Heatline" activation. A special helpline is available for residents to call for assistance during a heat emergency.
• Home visits by health department staff. Mobile heat health teams can be dispatched to help people in need.
• Halt to service shutoffs. Residential electric and water shutoffs are stopped during a heat health emergency.
• Cooling centers/senior refuge. Hours of operation at air-conditioned senior centers are extended to provide a
refuge for those otherwise lacking access to air conditioning.
For more information, see:
• www.heat.gov (offering resources from multiple federal agencies).
• EPA's Excessive Heat Events Guidebook.
• EPA and CDC's Climate Change and Extreme Heat: What You Can Do to Prepare.
• CDC's extreme heat resources.
• Philadelphia's hot weather preparedness website.
Climate Change Indicators in the United States
22
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Warmer temperatures increase the demand for cooling.
Why it matters
Temperature affects our health, comfort level, and demand for heating and air conditioning. Collectively, heating and cooling
the spaces in which we live accounts for about half of the energy that American households use every year. Heating and
cooling degree days 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
As climate change drives an increase in average temperatures, an increase in unusually hot days, and a decrease in unusually cold
days, 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. EPA's
Heating and Cooling Degree Days indicator measures the difference between outdoor temperatures and a temperature that people
generally find comfortable indoors. These differences in temperature suggest how much energy people might need to use to heat
and cool their homes and workplaces.
• Heating degree days have declined overall in the contiguous United States, particularly in recent years, as the climate has
warmed (Figure 12).
• Overall, cooling degree days have increased over the past 100 years. The increase is most noticeable over the past few decades
(Figure 12).
Figure 12: Heating and
Cooling Degree Days in
the Contiguous 48 States,
1895-2023
This graph shows the
average number of heating
and cooling degree days per
year across the contiguous
48 states. Data source:
NOAA, 2024,18
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1990
2010
2030
Year
Climate Change Indicators in the United States
23
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More cooling can require more electricity use.
Why it matters
As the need for air conditioning increases, people will likely have to spend more money on electricity for cooling. Increased
use of air conditioning could also lead to more greenhouse gas emissions and further climate change, as more electricity
must be generated to meet this increased demand. Hot days are also the most vulnerable days for the nation's power grid:
increased electricity demand for cooling on these days can increase the likelihood of brownouts or blackouts.
What's happening
As climate change contributes to an increase in average
temperatures and unusually hot days, Americans are expected
to use more energy—mostly electricity for air conditioning. EPA's
Residential Energy Use indicator examines trends related to home
air conditioning and heating by tracking the amount of electricity
used by U.S. homes in the summer and energy used in the winter.
• The amount of electricity used by the average American at
home during the summer has nearly doubled since 1973.
It appears to have leveled off somewhat in recent years,
concurrent with increases in energy efficiency (Figure 13).
• Year-to-year fluctuations in electricity use generally follow
changes in cooling degree days (see Heating and Cooling
Degree Days). Total cooling degree days have also increased
since 1973, though they have not increased as dramatically as
electricity use (Figure 13).
• The amount of natural gas used by the average American
at home during the winter has decreased since 1974. Total
heating degree days have also decreased since 1974, though
they have not decreased as dramatically as natural gas use.
Extreme heat and equity
Extreme heat can have a variety of effects on
people's physical and cognitive health. Impacts are
not felt equally and can have a greater effect on
populations who are most at risk. Extreme heat can
also have economic implications and be a financial
burden. Rising energy costs make it difficult for
people, especially low-income households, to afford
to cool their homes.
This energy burden can also affect children's
learning, which in turn means lost future income. A
recent report found that installing air conditioning
in schools generally costs less than this lost income,
but still only partially mitigates the effect on learning.
Rather, A/C in homes is also necessary to reduce
learning losses from extreme heat. Low-income
households and communities are more likely to have
lower rates of A/C usage and access, making them
more likely to experience impacts from extreme heat
disproportionately.
As temperatures continue to rise, Americans are expected to use
less fuel energy for heating their homes during the winter and more electricity for cooling buildings in the summer.19
Figure 13. Residential Summer Electricity Use
per Capita and Summer Cooling Degree Days
in the United States, 1973-2022
This graph shows the amount of electricity used
by the average American during the summer
months (June, July and August) of each year
from 1973 to 2022. The solid line shows average
summer electricity use per capita. It represents
all 50 states plus D.C. For reference, the dashed
line shows the average number of cooling
degree days for the same months across the
contiguous 48 states plus D.C. Data sources: EIA,
2022;20 BEA, 2022;21 NOAA, 2022.22
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Summer electricity Summer cooling degree
use per capita (50 states) days (contiguous 48 states)
1,200
1,050
900
750
600
450
300
150
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020 2025
Year
Climate Change Indicators in the United States
24
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Extreme
Rising global average temperature is associated with
widespread changes in weather patterns. Extreme
events such as heavy rainstorms, hurricanes, floods,
droughts, and wildfires have happened throughout
history, but human-induced climate change is
expected to make these events more frequent and/or
intense. While risks vary across the country, these
events are among the nation's costliest disasters,
sometimes causing great damage to ecosystems,
communities, and the economy.
Indicators featured in this chapter:
Heavy Precipitation, Drought, Tropical Cyclone Activity, Wildfires
25
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Heavy precipitation events are becoming more common.
Why it matters
Heavy precipitation events can cause crop damage, soil erosion, and flooding. An increase in flood risk due to heavy rains can
lead to injuries, drownings, respiratory health impacts from exposure to mold, and other flood-related health effects.1 Heavy
precipitation also can overwhelm drainage systems, and the runoff can wash pollutants from land into water bodies, which
can reduce water quality and affect the health of people and ecosystems.
What's happening
Climate change can affect the intensity and frequency of precipitation. Warmer oceans increase the amount of water that evaporates
into the air. When more moisture-laden air moves over land or converges into a storm system, it can produce more intense
precipitation—for example, heavier rain and snowstorms.2 As warmer temperatures cause more water to evaporate from the land
and oceans, changes in the size and frequency of heavy storms could in turn affect the size and frequency of river flooding (see EPA's
River Flooding indicator).
"Heavy precipitation" refers to storms that deliver much more rain or snow in a short period than a particular location is used
to. EPA's Heavy Precipitation indicator tracks the frequency of heavy precipitation events in the United States. The exact amount
that counts as "heavy" varies by location and season, depending on what is normal. An increase in heavy precipitation does not
necessarily mean the total amount of precipitation at a location has increased—just that more of it is falling in more concentrated
periods of time.
Climate change increases the odds of extreme weather
There have been changes in extreme weather events in the United States over the last several
decades, including more extreme heat, less extreme cold, more heavy precipitation, and an increased
potential for intense hurricanes. This rise in extreme weather events fits a pattern one can expect
with a warming planet, where average temperatures are increasing and the atmosphere holds more
moisture. Scientists project that climate change will make some of these extreme weather events
more likely to occur and/or more likely to be severe.
Establishing the most likely causes behind an extreme weather event can be challenging, since
these events are due to combinations of factors, some of which occur naturally.
Nevertheless, scientists have been able to draw a connection between some
types of extreme climate patterns—and even some individual events—and
climate change. The movement of heat in the atmosphere drives weather
patterns, so because added heat is a key feature of climate change,
weather patterns are expected to shift with this increase in energy. A
good way to think about this connection is to focus on whether an
extreme weather event was made more likely by climate change.
Climate change also increases the likelihood
of "whiplash," where conditions shift abruptly
between opposite extremes. For example, a shift
from intense drought to a deluge of rain can
heighten the risk of flooding and landslides if the
drought leaves the ground so hardened that it
can't absorb much of the rainwater. Landslides
can also occur if heavy rain follows a wildfire.
When extremely wet seasons that lead to plant
overgrowth are followed by drier periods, this
creates conditions conducive to wildfires.3
Climate Change Indicators in the United States
26
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See the Water Resources at Risk chapter for more information about
changing precipitation patterns, including information from EPA's U.S.
and Global Precipitation indicator.
• The prevalence of extreme single-day precipitation events
remained fairly steady between 1910 and the 1980s, but has risen
substantially since then (Figure 14).
• In recent years, a larger percentage of precipitation has come
in the form of intense single-day events. Looking at more than
100 years of data, nine of the top 10 years for extreme one-day
precipitation events have occurred since 1995 (Figure 14).
Year
Figure 14. Extreme One-Day Precipitation Events in the Contiguous 48 States, 1910-2023
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.
This graph shows the percentage of the land area of the contiguous 48 states where a much greater than normal portion of
total annual precipitation has come from extreme single-day precipitation events. The bars represent individual years, while the
orange line is a nine-year weighted average. Data source: NOAA, 2024,"
Climate Change Indicators in the United States
'
27
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Historically dry areas in the West are at increased risk of
drought.
Why it matters
Drought conditions can negatively affect agriculture, water supplies, energy production, human health, and many other
aspects of society. The impacts vary depending on the type, location, intensity, and duration of the drought. For example,
effects on agriculture can range from slowed plant growth to severe crop losses, threatening the livelihoods of farmers and
farm workers. Water supply impacts can range from lowered reservoir levels and dried-up streams to major water shortages
that affect access to drinking water. Lower streamflow and groundwater levels can also harm ecosystems more broadly, by
harming plants and animals and increasing the risk of wildfires.
What's happening
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.
As average temperatures have risen because of climate change, the rate of water evaporation from the Earth's surface (including
soil, lakes, and reservoirs) and transpiration (the process of losing water) from plants has risen as well. This has sped up the Earth's
water cycle. An increase in evapotranspiration (the transfer of water from land to the atmosphere) makes more water available in the
air for precipitation, but it also leaves less moisture in the soil.
• EPA's Drought indicator shows that from 1900 to 2023, drought conditions have increased in southwestern states such
as California, Arizona, and New Mexico. Over the same period, the eastern United States—particularly the Midwest and
Northeast—has experienced generally wetter conditions. From 2000 through 2023, roughly 10 to 70 percent of the U.S. land area
experienced conditions that were at least abnormally dry at any given time (Figure 15).
• EPA's A Closer Look: Temperature and Drought in the Southwest has more information about drought in the U.S. Southwest.
For example, for extended periods from 2002 to 2005 and from 2012 to 2020, nearly the entire region experienced abnormal
drought conditions. The region's rapid population growth has added to the challenge of managing water.
Figure 15. Average Change in Drought (Five-Year
SPEI) in the Contiguous 48 States, 1900-2023
This map shows the total change in drought
conditions across the contiguous 48 states, based on
the long-term average rate of change in the five-year
Standardized Precipitation Evapotranspiration
Index (SPEI) from 1900 to 2023, SPEI measures the
combination of water supply (precipitation) and
atmospheric water demand (evapotranspiration,
which is based on temperature) to determine
whether a certain area is experiencing extreme
drought, extreme moisture, or conditions in between.
Data are displayed for small regions called climate
divisions. Blue areas represent increased moisture;
brown areas represent decreased moisture or drier
conditions. Data sources: Abatzoglou et a!„ 2017f
Western Regional Climate Center, 2024,6
More extreme drought More extreme moisture
Change in SPEI:
Climate Change Indicators in the United States
28
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Drought: elevated risks for Indigenous Peoples
Prolonged droughts pose a particular threat to the health and well-being of
Indigenous Peoples because of their economic and cultural connection to the land
and water, and because historical actions moved many Tribes to lands that were
already water-stressed.
Warming and drought can reduce water quality and availability, increasing the risk
of waterborne illnesses, and can threaten medicinal and culturally important plants
and animals. These climate threats intersect with other challenges, like a lack of
access to drinking water and wastewater infrastructure on some Tribal lands.
Tribes and Indigenous Peoples are leading action to prepare for and adapt to
these changes. For example, in 2018, the Navajo Nation released its first climate
adaptation plan, which outlines strategies to preserve and enhance natural
resources and provide a resilient future for the Navajo communities.7
For more information, see the "Tribes and Indigenous Peoples" chapter of the Fifth
National Climate Assessment and the Status of Tribes and Climate Change Report.
Tropical cyclones have become more intense in recent
years.
Why it matters
The effects of hurricanes and other tropical cyclones are numerous and well known. At sea, storms disrupt and endanger
shipping traffic. When cyclones make landfall, their intense rains and high winds can cause severe property and infrastructure
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, erosion, and destruction.
What's happening
Hurricanes, tropical storms, and other intense rotating storms
fall into a general category called cyclones. There are two main
types of cyclones: tropical and extratropical (those that form
outside tropical regions like Nor'easters). Tropical cyclones get
their energy from warm tropical oceans, They are most common
during the Atlantic hurricane season, which runs from June
through November. Climate change is expected to affect tropical
cyclones by increasing sea surface temperature, which is a key
factor that influences the formation and behavior of these storms.
Climate Change Indicators in the United States
29
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• EPA's Tropical Cyclone Activity indicator focuses on tropical cyclones in the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico.
It shows that while there is no clear trend for the number of tropical cyclones per year, tropical cyclones are becoming more
intense.
• The Power Dissipation Index is one method of measuring tropical cyclone frequency, strength, and duration using wind speed
measurements. It shows that cyclone intensity has risen noticeably over the past 20 years (Figure 16).
Figure 16. North Atlantic Tropical
Cyclone Activity According to
the Power Dissipation index,
1949-2022
This graph shows annual values of
the Power Dissipation Index, which
accounts for cyclone strength,
duration, and frequency. Sea surface
temperature trends from the tropical
Atlantic are provided for reference.
Note that sea surface temperature is
measured in different units, but the
values have been plotted alongside
the 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 (2018-2022) is plotted at
2020. Data source: Emanuel, 2023.8
Year
Wildfires are burning more land.
Why it matters
Wildfires can harm property, livelihoods, and public health. Wildfire-related threats are increasing, especially as more
people live in and near forests, grasslands, and other natural areas.9 According to the National Oceanic and Atmospheric
Administration, between 1980 and 2023 the United States had 22 wildfire events that individually caused more than $1 billion
in damage; 18 of those have occurred since 2000.10 Over the past few decades, the United States has routinely spent more
than $1 billion per year to fight wildfires, including $3.5 billion in 2022." These efforts have resulted in the deaths of hundreds
of firefighters.12 Even in communities far downwind, wildfire smoke has been directly linked to poor air quality that can lead to
significant health effects and costs to society including emergency department visits, hospital admissions, and deaths.13
What's happening
While wildfires occur naturally and play a long-term role in the health of ecosystems such as forests, shrubland, and grassland,
changing wildfire patterns threaten to upset the status quo. Many studies have found that climate change has already led to an
increase in wildfire season length, wildfire frequency, and burned area.14,15 This is a result of factors such as higher temperatures,
longer summer dry seasons and increased drought conditions, and warmer spring weather with less snowpack to supply water
throughout the summer.16
Climate Change Indicators in the United States
30
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Wildfires and climate-warming emissions
Beyond the human and societal impacts, wildfires also affect the Earth's climate. Forests in particular store large amounts
of carbon. When they burn, they immediately release carbon dioxide and other gases into the atmosphere, which in turn
contributes to climate change. After burning, forests also release carbon dioxide more gradually through decomposition.
Factors other than climate change also play an important role in wildfire frequency and intensity: land use, large-scale insect
infestation, fuel availability (including availability of invasive species such as highly flammable cheatgrass), and management
practices (including fire suppression). All these factors vary greatly by region and over time, as do precipitation, wind, temperature,
vegetation types, and landscape conditions.
• EPA's Wildfires indicator shows that the extent of area burned by wildfires in the United States has increased since the 1980s
(Figure 17), with the largest increases occurring in the West and Southwest.
• According to National Interagency Fire Center data, of the 10 years with the largest acreage burned, all have occurred since
2004, including peak years in 2015 and 2020 (Figure 17). This period coincides with many of the warmest years on record
nationwide (see the Heat or the Rise chapter). The largest increases have occurred during the spring and summer months.
Figure 17. Wildfire Extent in the
United States, 1983-2022 12
This graph shows annual wildfire- s 10
burned area (in millions of acres) b
from 1983 to 2022. The two lines £ 8
represent two different reporting =
systems, though the orange line is E 6
shown only for comparison in the
early part of the graph. (The U.S. £
Forest Service stopped collecting E
statistics in 1997 and is not planning $
to update them.) Data sources: < 2
NIFC, 2024;17 Short, 2015.18
Forest Service
National Interagency Fire Center
/
111
25
1980 1985 1990 1995 2000 2005 2010 2015 2020 20
Year
Climate Change Indicators in the United States
31
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Taking action: preparing for wildfire smoke
Wildfire smoke can harm people's health. Microscopic particles in the smoke can get into
people's eyes and respiratory systems, where they can cause problems such as burning
eyes, runny nose, and illnesses like bronchitis. Smoke can also aggravate chronic heart and
lung diseases—and is even linked to premature deaths in people with these conditions.
To help people know when they may be at risk from wildfire smoke, EPA and the U.S.
Forest Service have built on the popular AirNow program to create the Air Now Fire and
Smoke Map, which provides comprehensive data on where wildfire smoke is affecting air quality in real time. The map
leverages hourly air quality data provided by AirNow's state, local, and Tribal air agency partners; crowd-sourced data
from low-cost air sensors; and satellite data. It uses these data to show information about whether air quality is getting
better or worse, fire locations, smoke plumes, smoke forecast outlooks (where available), and steps people can take to
reduce their exposure to smoke.
AirNow is a partnership between EPA and state, local, and Tribal air agencies; the U.S. Forest Service; the National
Oceanic and Atmospheric Administration; the National Park Service; the National Aeronautics and Space Administration;
the Centers for Disease Control and Prevention; and the U.S. Department of State.
Visit AirNow's wildfires page to learn what to do before, during, and after a fire.
Climate Change Indicators in the United States
32
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Water
Resources at
Risk
Clean fresh water is essential to life on the Earth,
and climate change is affecting the planet's
water resources. As the climate warms, changing
precipitation patterns, drought, decreasing amounts
of snow, and earlier snowmelt all pose risks to water
supplies in the United States, affecting communities,
livelihoods, and ecosystems.
Indicators featured in this chapter:
U.S. and Global Precipitation, Snowpack, Streamflow,
A Closer Look: Temperature and Drought in the Southwest
33
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Precipitation is increasing in some parts of the country
and decreasing in others.
Why it matters
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 particular place.
Changes in precipitation can disrupt a wide range of natural processes, particularly if these changes occur more quickly than
plant and animal species can adapt.
What's happening
As the Earth becomes warmer (see the Heat on the Rise chapter), more evaporation
and transpiration (water loss from plants) occur. Increased evaporation adds
more moisture to the atmosphere and, in turn, generally increases precipitation.
Therefore, a warming climate is expected to increase precipitation in many areas.
Just as precipitation patterns vary across the world, so do the effects of climate
change on precipitation. By shifting the wind patterns and ocean currents that
drive the world's climate system, climate change will also cause some areas to
experience decreased precipitation. In addition, areas with increased precipitation
will not necessarily have more water available for people and ecosystems, because
higher temperatures also lead to more evaporation.
• EPA's U.S. and Global Precipitation indicator shows that total annual
precipitation has increased in the United States and worldwide (Figure 18). Since
1901, precipitation in the contiguous 48 states has increased at a rate of 0.18
inches per decade and global precipitation has increased at an average rate of
0.03 inches per decade.
Precipitation and
water quality
Precipitation patterns don't just affect
water quantity—they also affect water-
quality. For example, runoff from
heavy rain can wash pollutants from
land into water bodies, impairing
water quality.
EPA's Office of Water is taking actions
to address climate change impacts in
the water sector.
Figure 18. Precipitation
Worldwide, 1901-2023
This graph 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 showing change.
Choosing a different
baseline period would
not change the shape of
the data over time. Data
source: NOAA, 2024.1
V)
a»
-c
u
c
_>> 1
-------
• Because precipitation changes vary by region, some parts of the country have experienced greater increases in precipitation than
others. A few areas, such as the Southwest, have seen a decrease in precipitation (Figure 19).
Figure 19. Change in
Precipitation in the United
States, 1901-2023
This map shows the percent
change in total annual
precipitation in different parts
of the United States since
the early 20th century (since
1901 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, 2024.2
Percent change
in precipitation:
30
The effect of temperature on water quality
For example, increases in harmful algae growth already
threaten Lake Erie's ecosystems and drinking water
supplies. The lake provides drinking water to 12 million people in the United States and Canada, and it is especially at risk
from harmful algae because of nutrient pollution from urban, industrial, and agricultural runoff. As water temperatures rise,
this risk could increase.4 Toledo, Ohio, is one city that has experienced contamination in its water supply from these algal
blooms. The National Oceanic and Atmospheric Administration produces harmful algal bloom forecasts for Lake Erie for July
to October, when water temperatures are warmest.
EPA's Great Lakes Water Levels and Temperatures
indicator explores how climate change is affecting water
temperatures in the Great Lakes, which supply drinking
water to more than 30 million people.3 It shows that
average temperatures at the water's surface are rising
slightly in each of the Great Lakes. This rise is mostly
driven by warming during the spring and summer.
Rising temperatures can affect water quantity by
increasing drought risk and reducing resources like
snowpack. However, like precipitation, temperature also
affects the quality of water resources. Warmer water
promotes the growth of algae and bacteria, including
some that produce toxins. These toxins can cause
illnesses and contaminate drinking water supplies.
Climate Change Indicators in the United States
35
-------
Snowpack is decreasing.
Why it matters
Snowpack is the amount or thickness of snow that accumulates on the ground. Mountain snowpack plays a key role in the
water cycle in western North America, storing water in the winter when the snow falls and releasing it as runoff in spring and
summer when the snow melts. Millions of people in the West depend on the melting of mountain snowpack for hydropower,
irrigation, and drinking water. In most western river basins, snowpack stores more water than human-constructed reservoirs.
What's happening
As the climate warms, more precipitation is falling as rain rather than snow in most
areas—reducing the extent and depth of snowpack. With warmer winter and spring
weather, the seasonality of snowpack is also changing. Higher temperatures cause
snow to melt earlier, which in turn affects the timing and availability of water. In other
words, less snowpack and earlier melting mean less water is stored in snowpack that
will last long enough to feed streams and reservoirs in the summer.
• April 1 is a date that is often used to measure snowpack for spring and summer
water supply forecasting. EPA's Snowpack indicator shows that large and
consistent decreases in April snowpack have been observed throughout the
western United States. Decreases have been especially prominent in Washington,
Oregon, northern California, and the northern Rocky Mountains (Figure 20).
• Almost 80 percent of the sites this indicator covers have experienced a shift toward
earlier peak snowpack. This earlier trend is especially pronounced in southwestern
states like Colorado, New Mexico, and Utah. Across all stations, peak snowpack has shifted earlier by an average of nearly seven
days since 1982.
• From 1982 to 2023, the snowpack season became shorter at 80 percent of the sites where snowpack was measured. Across all
sites, the length of the snowpack season decreased by about 15 days, on average (Figure 21).
Climate Change Indicators in the United States
Figure 20. Trends in April
Snowpack in the Western
United States, 1955-2023
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:
USDA Natural Resources
Conservation Service, 2024.5
Percent change:
•
>80
•
60 to 80
•
40 to 60
•
20 to 40
•
Oto 20
•
0 to -20
•
-20 to -40
•
-40 to -60
•
-60 to -80
•
<-80
-------
Figure 21. Change in
Snowpack Season Length in
the Western United States,
1982-2023
This map shows trends in the
length of the snowpack season
across the western United
States, in days. Blue circles
show where the season has
grown longer; red circles show
where it has become shorter.
Data source: USDA Natural
Resources Conservation
Service, 2023.6
Snowpack season
length change (days):
£ >40
£ 30 to 40
• 20 to 30
• 10 to 20
• OtolO
• Oto -10
• -10 to-20
• -20 to -30
£ -30 to -40
<-40
Streamflow levels and timing are changing in response to
warming conditions.
Why it matters
Streamflow naturally varies over the course of a year. For example, rivers and streams in many parts of the country have their
highest flow when snow melts in the spring and their lowest flow in late summer. The amount of streamflow is important
because very high flows can cause erosion and damaging floods, while very low flows can diminish water quality, harm fish,
reduce the amount of water available for people to use, and disrupt economic activities like river shipping. The timing of the
high flows is important for water supplies because it affects the ability of reservoir managers to store water to meet needs
later in the year.
What's happening
Climate change can affect streamflow in several ways. Less spring snowpack
and earlier melting of snow can reduce high spring streamflows and shift them
earlier. This is especially an issue for western areas that rely on snowpack as a
water source. Elsewhere, more precipitation is expected to cause higher average
streamflow in some places, while heavier storms could lead to larger peak flows.
More frequent or severe droughts could reduce streamflow in certain areas.
• EPA's Streamflow 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. In parts of the country that have substantial snowmelt, winter-spring
runoff is generally happening at least five days earlier than in the mid-20th
century (Figure 22).
• The Pacific Northwest and the Northeast have experienced the largest changes
in runoff timing (Figure 22).
Climate Change Indicators in the United States
37
-------
Figure 22. Timing of Winter-Spring
Runoff in the United States,
1940-2022 / t...„ J ) T ^
• '
This map shows changes in the timing T , T T 0
of annual high winter-spring flow
carried by rivers and streams from ^ y
1940 to 2022, This analysis focuses on
parts of the country where streamflow
is strongly influenced by snow melt.
Trends are based on the winter-spring
center of volume, which is the date
when half of the total January 1-July
31 streamflow (in the West) or half of
the total January 1-May31 streamflow
(in the East) has passed by each
streamflow gauge, reflecting the ~ v • " * * A
timing Of spring snowmen Data More than 5 to 10 2 to 5 2 days 2 to 5 5 to 10 More than
source: USGS, 2023.
7
10 days days days earlier to days days 10 days
earlier earlier earlier 2 days later later later later
Taking action: creating resilient water utilities
in Hershey, Nebraska
Climate change has prompted the Village of Hershey, Nebraska, to examine the drought
resilience of its drinking water system, which serves about 660 community residents.
Hershey has already experienced challenges due to drought, with an extended dry period
from 2011 to 2021. The utility is concerned that, under more intense drought conditions, the
system could experience reduced water supply due to aquifer withdrawal restrictions, as
well as decreased water quality.
These concerns led Hershey officials to investigate strategies to increase the system's drought resilience using EPA's
Climate Ready Evaluation and Assessment Tool (CREAT). CREAT helps water sector utilities assess climate-related risks to
their assets and operations, allowing them to evaluate future climate change for their local area and evaluate the cost-
effectiveness of different adaptive measures under these future changes. Hershey based its CREAT results on climate
model projections that predict hotter and drier conditions, and the utility assessed financial impacts on its system under
this future climate scenario. The utility identified several adaptive measures that could reduce drought impacts, including
installing water use meters, building new groundwater wells, and developing a drought contingency plan, all of which
CREAT indicated could be cost-effective.
Hershey's story is one of many adaptation case studies available from EPA's Creating Resilient Water Utilities initiative.
For case studies focused on equitable water utility climate adaptations, see the initiative's new Building Equity and
Environmental Justice StoryMap, which includes a dashboard bringing together socioeconomic and climate data as well
as other resources to inform equitable adaptation.
Climate Change Indicators in the United States
38
-------
In the Southwest, rising temperatures and drought
threaten already scarce water supplies.
Why it matters
The impacts of drought on water supplies can range from lowered reservoir levels and dried-up streams to major water
shortages. In the Southwest, water is already scarce, so every drop is a precious resource. People in the Southwest are
particularly dependent on surface water supplies like Lake Mead, which are vulnerable to losing water from evaporation.
Thus, even a small increase in temperature (which drives evaporation) or a decrease in precipitation in this already arid region
can seriously threaten natural systems and society.
What's happening
Drought risk is increasing in the Southwest. EPA's A Closer Look: Temperature and Drought in the Southwest indicator focuses on
six states that are commonly thought of as "southwestern" and characterized at least in part by arid landscapes and scarce water
supplies: Arizona, California, Colorado, Nevada, New Mexico, and Utah.
• Every part of the Southwest experienced higher average temperatures between 2000 and 2020 than the long-term average
(1895-2020).
• Large portions of the Southwest have experienced drought conditions since weekly Drought Monitor records began in 2000. For
extended periods from 2002 to 2005 and from 2012 to 2020, nearly the entire region was abnormally dry or even drier
(Figure 23).
• While drought conditions in the Southwest have varied since 1895, the Southwest has seen some of the most persistent droughts
on record since 1990.
To learn more about climate change and drought in the United States, see the Extreme Events chapter.
The Southwestern landscape
The American Southwest might evoke images of a hot, dry landscape—a land of rock, canyons, and deserts baked by the
sun. indeed, much of this region has low annual rainfall and high temperatures that contribute to its characteristic desert
climate. Yet this landscape actually supports a vast array of plants and animals, along with millions of people who call the
Southwest home. All of these plants, animals, and people need water to survive.
Climate Change Indicators in the United States
39
-------
Year
Category
Description
Possible Impacts
DO
Abnormally dry
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.
D1
Moderate drought
Some damage to crops or pastures; streams, reservoirs, or wells low; some water
shortages developing or imminent; voluntary water use restrictions requested.
D2
Severe drought
Crop or pasture losses likely; water shortages common; water restrictions imposed.
D3
Extreme drought
Major crop/pasture losses; widespread water shortages or restrictions.
D4
Exceptional drought
Exceptional and widespread crop/pasture losses; shortages of water in reservoirs,
streams, and wells, creating water emergencies.
Figure 23. Southwestern U.S. Lands Under Drought Conditions, 2000-2020
This graph shows the percentage of land area in six southwestern states (Arizona, California, Colorado, Nevada, New
Mexico, and Utah) classified under drought conditions from 2000 through 2020, This figure uses the U.S. Drought Monitor
classification system, which is described in the table. Data source: National Drought Mitigation Center, 2021.8
Climate Change Indicators in the United States
40
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Changing
Seasons
Although the timing, duration, and intensity of the
seasons vary naturally from year to year, climate
change is driving longer-term changes in seasonality
and fundamentally altering the ways in which humans
and natural systems experience and interact with
seasonal events. These changes lead to wide-ranging
impacts such as warmer winters, lakes thawing earlier,
longer growing seasons, and worsening allergies for
people.
Indicators featured in this chapter:
Seasonal Temperature, Snowfall, Lake Ice, Leaf and Bloom Dates,
Community Connection: Cherry Blossom Bloom Dates in Washington, D.C.,
Length of Growing Season, Growing Degree Days, Ragweed Pollen Season
-------
Temperatures are increasing in all seasons, with winter
warming the most.
Why it matters
Warming temperatures have profound, wide-ranging effects throughout the year—from reducing critical water supplies to
disrupting seasonal patterns that animals and plants rely on to survive. Hotter summers can lead to higher energy costs and
risk of wildfires, while milder winters can reduce snowfall; allow ticks to spread, exposing more people to Lyme disease; and
threaten winter recreation and tourism. These changes will increasingly affect people and ecosystems as warming continues.
What's happening
As the Earth warms, average temperatures increase throughout the year, but the increases may be larger in certain seasons and
regions than in others.
The winter is particularly sensitive to rising temperatures, because overall, minimum temperatures have increased at a higher rate
than average maximum temperatures.1 Much of the observed warming in the United States has taken place when and where it is
usually the coldest—namely, during winter,2 at night, and in the northern parts of the country.
• EPA's Seasonal Temperature indicator shows that average winter temperatures across the contiguous 48 states have increased
by about 3°F since 1896. Spring temperatures have increased by about 2°F, while summer and fall temperatures have increased
by about 1.6°F (Figure 24).
• Temperature changes vary by state, with larger seasonal increases across the northern states and the Mountain West, and
smaller increases in the South and Southeast (Figure 25).
• All 48 states experienced winter warming from 1896 to 2021. Most states experienced warming in the spring, summer, and fall,
but a few states (for example, Alabama) had little to no overall change or cooled slightly during those months (Figure 25).
Figure 24. Temperature Change
by Season in the Contiguous 48
States, 1896-2023
This graph shows changes in
the average annual temperature
by season for the contiguous
48 states from 1896 to 2023, in
degrees Fahrenheit. Seasons are
defined as follows: December,
January, and February for
winter; March, April, and May for
spring; June, July, and August for
summer; and September, October,
and November for fall. Data
source: NOAA, 2024.3
3.5
~ 2.5
o
0» 2
c
10
-C
^ 1.5
re
+*
.0
0.5
Winter
Spring Summer
Season
Fall
Climate Change Indicators in the United States
42
-------
Winter
Spring
tft.V
Summer
Fall
Total temperature change (°F):
-5 -4-3-2-10 1 2 3
Gray interval: -0.1 to0.1°F
Figure 25. Change in Seasonal Temperatures by State, 1896-2023
These maps show the total change in the average seasonal temperature for each of the contiguous 48 states from 1896
to 2023, in degrees Fahrenheit, Total change in temperature was calculated from the long-term average rate of change.
Seasons are defined as follows: December, January, and February for winter; March, April, and May for spring; June, July,
and August for summer; and September, October, and November for fall. Data source: NOAA, 2024,4
Climate Change Indicators in the United States
43
-------
More winter precipitation is falling as rain instead of
snow.
Why it matters
The amount of snow that falls in an area directly influences both snow cover and snowpack (snow that accumulates on the
ground). Many people depend on spring snowmelt to provide their drinking water and support other uses such as irrigation,
especially in the western United States. (Visit the Water Resources at Risk chapter for details about water quantity and quality.)
Many communities also rely on snow for winter recreation activities, like skiing, that support their local economies. Some
plants and animals depend on snow and snowmelt for survival. Changes in the amount of snow versus rain could raise or
lower streamflows at times that affect fish migration and spawning.
What's happening
Warmer temperatures cause more water to evaporate 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 causes more of this precipitation to fall in the form
of rain instead of snow. Some places could instead see more snowfall if temperatures rise but still remain below the freezing point,
or if storm tracks change. Areas near large lakes might also experience more snowfall as lakes remain unfrozen for longer periods,
allowing more water to evaporate. In contrast, other areas might experience less snowfall as a result of wintertime droughts.
EPA's Snowfall indicator shows that total snowfall has decreased in many parts of the country since widespread observations
became available in 1930.
• Nearly 80 percent of the locations studied across the contiguous 48 states have experienced a decrease in the proportion of
precipitation falling as snow (Figure 26).
• Snowfall trends vary by region. The Pacific Northwest has seen a decline in both total snowfall and the proportion of precipitation
falling as snow. Parts of the Midwest have also experienced a decrease, particularly in terms of the snow-to-precipitation ratio.
A few regions have seen modest increases, including some areas near the Great Lakes that now receive more snow than in the
past (Figure 26).
Figure 26. Change in Snow-
to .
/v P 0
to-Precipitation Ratio in the
Contiguous 48 States, 1949-2020
This map shows the percentage
change in winter snow-to-
precipitation ratio from 1949 to
2020 at 177 weather stations in
.* ° I o—L oo o »-•
1 ° ° 9 <§> °
< °°° £ J'jl K'J
O® *
°?£°o*<§
°°V a
• 0
m° °o
the contiguous 48 states. This ratio
measures what percentage of total
winter precipitation falls in the form
of snow. A decrease (red circle)
indicates that more precipitation is
falling in the form of rain instead of
Percent change:
snow. Solid-color circles represent
<-40 -30 -20 -10 -2 -2 2 10
to-40 to-30 to-20 to-10 to 2 to 10 to 20
• •
20 30 > 40
to 30 to 40
stations where the trend was
statistically significant. Data source:
Lower percentage of snow Higher
percentage of snow
NOAA, 2021.5
Filled circles represent statistically significant trends.
Open circles represent trends that are not statistically significant.
Climate Change Indicators in the United States
44
-------
Lake ice is thawing earlier in the spring.
Why it matters
Changes in ice cover can affect the physical, chemical, and biological characteristics of a lake. For example, ice influences
heat and moisture transfers between a lake and the atmosphere. Reduced lake ice cover leads to increased evaporation and
lower water levels, as well as an increase in water temperature and sunlight penetration. These changes, in turn, can affect
plant and animal life cycles and the availability of suitable habitat. Additionally, ice cover affects the amount of heat that is
reflected from the Earth's surface. Exposed water will absorb and retain heat, making the Earth's surface warmer, whereas an
ice- and snow-covered lake will reflect more of the sun's energy and absorb less. Less lake ice also reduces opportunities for
winter recreation, such as ice fishing, curling, and hockey.
What's happening
The formation of ice cover on lakes in the winter and its disappearance the following spring depend on climate factors such as air
temperature, cloud cover, and wind. A warming climate causes lake ice to thaw and break up earlier. EPA's Lake Ice indicator shows
the change in ice thaw dates for selected lakes in the United States with the longest and most complete historical records.
• Thaw dates for most of these lakes show a trend toward earlier ice breakup in the spring. Spring ice thaw is happening up to 24
days earlier since 1905 (Figure 27).
• Nearly all of the lakes selected were found to be thawing earlier in the year (Figure 27).
IT
'W\
Detroit Lake
^-0 days
Lake Superior at Bayfield
¦24 days
Moosehead Lake
-5 days
Lake
-0
• Shell Lake
Osakis 1 _6d
days 1
V.. ( /
ha
Lake Mendota
^-7 days
Lake Monona
-8 days
Cobbosseecontee Lake Damariscotta Lake
-10days v"-10days
Mirror Lake I / \Jy*-
-6 days
*\ Sebago Lake
Lake George \ j\ -i6days
-7 days % I
Otsego Lake
^3 days
VI
Geneva Lake
-5 days
Change in ice thaw date:
% Earlier • No change
Figure 27. Change in Ice Thaw Dates for Selected U.S. Lakes, 1905-2019
This map 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 2019. Twelve of the lakes have red circles with negative numbers, which represent earlier thaw dates. Larger circles
indicate larger changes. Two of the lakes had no change in thaw dates. Data source: Various organizations.6
Climate Change Indicators in the United States
45
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Learn more about seasonality and climate change
For more information on shifting seasons, EPA's Seasonality and Climate Change:
A Review of Observed Evidence in the United States explores in greater detail the
science behind seasonal events and how climate change can influence seasonal
trends. Published in December 2021, the report uses EPA's climate change indicators
to summarize changes that have been observed in the United States and describes
the implications of these changes. The report focuses on the following topics:
• Seasonal changes in temperature and precipitation
• Effects of warmer, shorter winters
• Biological responses and phenology in a warming world
• Seasonality and extreme events
Spring events, such as leaf emergence and flower blooms,
are starting earlier in the year.
Why it matters
Earlier spring events can have a variety of impacts on ecosystems and human society. For example, an earlier spring might
lead to longer growing seasons, more abundant invasive species and pests, and earlier and longer allergy seasons. Unusually
warm weather in late winter can create a "false spring" that triggers the new growth of plants too early, leaving them
vulnerable to any subsequent frosts. Because different plant and animal species respond differently to changes in seasonality,
seasonal interactions among species can become un-synchronized. Earlier springs can therefore increase the risk of mismatch
in the life cycles of different plants and animals. For example, the emergence of pollinators may no longer match the timing of
plant flowering.
What's happening
The timing of natural events, such as flower blooms and animal migration, is influenced by a combination of environmental factors,
including temperature, light, rainfall, and humidity. A warming climate is causing spring events to arrive earlier. Two particularly
useful markers of the timing of spring events are the first leaf emergence and the first flower bloom of the season in lilacs and
honeysuckles, two plants that are widely distributed across the United States. The first leaf date in these plants relates to the timing
of events that occur in early spring, while the first bloom date is consistent with the timing of later spring events, such as the start of
growth in forest vegetation.7 EPA's Leaf and Bloom Dates indicator examines changes in the timing of the first leaf dates and the first
bloom dates in lilacs and honeysuckles.
• In general, leaf and bloom events in lilacs and honeysuckles in the United States are happening earlier throughout most of the
contiguous 48 states and Alaska, but later in the South and part of the Upper Midwest (Figure 28). This observation is consistent
with many of the regional differences in temperature change (see U.S. and Global Temperature in the Heat on the Rise chapter).
Climate Change Indicators in the United States
46
-------
Figure 28. Change in First Leaf
Date Between 1951-1960 and
2014-2023
This map shows modeled trends
in lilac and honeysuckle first leaf
dates at weather stations across the
contiguous 48 states and Alaska. This
map compares the average first leaf
date for two 10-year periods. Data
source: 2024 update by M. Schwartz
to data from Schwartz et a I., 2 013.8
Change in first leaf date:
• • . ° •
More than 4 to 1 to Within 1 to 4 to More than
8 days 8 days 4 days 1 day 4 days 8 days 8 days
-< No change ~-
Earlier
Later
The timing of peak cherry blossom bloom in Washington, D.C., is shifting earlier
Washington has enjoyed cherry blossoms each
year dating back to 1912, when Japan gave about
3,000 cherry trees to the United States as a gift of
friendship. These trees attract more than 1.5 million
visitors to the area every year during the National
Cherry Blossom Festival, which is planned to coincide
with the peak bloom of the cherry trees. However,
earlier bloom dates can result in the annual festival
being out of sync with the peak cherry blossom bloom
and thus could affect tourism and the local economy.
EPA's Community Connection: Cherry Blossom Bloom
Dates in Washington, D.C., tracks how the annual
peak bloom date of Washington cherry trees has
changed overtime.
• Based on the entire 104 years of data, the average peak bloom date for Washington's cherry blossoms is April 3
(Figure 29).
• Peak bloom date for the cherry trees is occurring earlier than it did in the past. Since 1921, peak bloom dates have
shifted earlier by about eight days. The peak bloom date has occurred before April 3 in 15 of the past 20 years
(Figure 29).
• While the length of the National Cherry Blossom Festival has expanded over time, the cherry trees have bloomed near
the beginning of the festival in recent years. During some years, like 2024, the festival missed early peak bloom dates
entirely.
Climate Change Indicators in the United States
47
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Figure 29. Peak Bloom Date
for Cherry Trees Around
Washington, D.C.'s Tidal Basin,
1921-2024
This graph shows the timing of
peak bloom each year for the
cherry trees around the Tidal
Basin in Washington, D.C. The
peak bloom date occurs when 70
percent of the blossoms are in full
bloom. The shaded band shows
the timing of the annual National
Cherry Blossom Festival. The
festival began in 1934 but was
not held during World War II. Data
source: National Cherry Blossom
Festival, 2024;9 National Park
Service, 20241°
O March 21
.q
re
9)
a
«•-
o
to
D
-------
The length of the growing season has increased in almost
every state.
Why it matters
Changes in the length of the growing season can have both positive and negative effects on the yield and prices of particular
crops. A longer growing season could allow farmers to diversify crops or have multiple harvests from the same plot. However,
longer growing seasons could also limit the types of crops grown, encourage invasive species or weed growth, and increase
demand for irrigation. A longer growing season could also disrupt the function and structure of a region's ecosystems and
could alter the range and types of animal species in the area.
What's happening
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 longer growing seasons than others. Growing
season length is limited by many different factors. Depending on the region and the climate, the growing season is influenced by
air temperatures, frost days, rainfall, or daylight hours. Warming in winter, spring, and fall can mean an increase in growing season
length. EPA's Length of Growing Season indicator measures the length of the growing season in the contiguous 48 states.
• Between 1895 and 2023, the length of the growing season has increased in almost every state. States in the West (for example,
California and Washington) have seen the most dramatic increase. In contrast, the growing season has become slightly shorter in
Georgia (Figure 30).
• The average length of the growing season in the contiguous 48 states has increased by more than two weeks since the
beginning of the 20th century. A particularly large and steady increase has occurred since the 1970s.
Climate Change Indicators in the United States
Change in length of growing season (days):
No data
Figure 30. Change in
Length of Growing
Season by State,
1895-2023
This map shows the total
change in length of the
growing season from 1895
to 2023 for each of the
contiguous 48 states. Data
source: Kunkel, 2024,11
-------
The number of growing degree days is increasing,
worsening allergy season.
Why it matters
Plants typically grow only when the temperature is high enough; this makes growing degree days a useful way to look at
temperature changes from the perspective of plant growth and development. Temperature changes affect plant growth,
agricultural production, and the spread and impact of plant diseases and pests. One notable effect of warming temperatures
is pollen seasons that begin earlier and last longer, which can trigger allergies and asthma in people. These health effects
can lead to economic costs from medical expenses and missed workdays. An estimated 15.5 percent of all Americans are
sensitive to ragweed pollen,12 making it one of the most common environmental allergens. It can cause hay fever and trigger
asthma attacks, especially in children and older adults.13 Non-Hispanic Black and Non-Hispanic American Indian/Alaska Native
populations are particularly sensitive to longer pollen seasons because they have the highest rates of asthma in the United
States, and urban Black and Hispanic children tend to experience more symptoms of allergic rhinitis (hay fever).14
What's happening
Like growing season length, growing degrees days are a widely used measure for tracking plant growth and development. EPA's
Growing Degree Days indicator shows changes in growing degree days in the in the contiguous 48 states.
• Between 1948 and 2023, the number of growing degree days increased at 221 of the 280 long-term stations measured (79
percent) across the contiguous 48 states. The average change across all stations represents an increase of about 10 percent
(Figure 31).
• The largest increases in growing degree days from 1948 to 2023 occurred in the West and the Northeast. Fifty stations, mostly in
the West, have experienced an increase of 20 percent or more (Figure 31).
I
•* 5 • •• »V.#
I • . • * . •.
»' * •***. * $ Jtm
% 9% •••° • •• o
_ • tf?* ¥**—1 &m • .
Figure 31. Change in Growing _
Degree Days in the Contiguous • •
48 States, 1948-2023
This map shows trends in the _
total number of growing degree A •
days per year at 280 weather
stations. Growing degree days
are defined as the difference in
average daily temperature and a # *w ? • S__Sm
baseline of 50°F. For example, a j / # # iff
day with an average temperature / % (\ \ _•
of 70°Frepresents 20 growing
degree days. The color and size
of the symbols represent percent
vm v •
\ ••• • •
change between 1948 and 2023, V \ f 0 #
based on the long-term average
rote of change. Data source. PGrccntchcincjG!
NOAA, 2024.15
• • o • #
<-20 -10 to-20 -1 to-10 -Itol 1 to 10 10 to 20 >20
Climate Change Indicators in the United States 50
-------
As the climate warms and growing degree days increase, many locations could experience longer and more intense allergy seasons.
Multiple regions in the U.S. have experienced longer and more intense pollen seasons.16
Ragweed plants mature in mid-summer and produce small flowers that generate pollen. Ragweed pollen season usually peaks in
late summer and early fall, but these plants often continue to produce pollen until the first frost. Warmer fall temperatures extend
the growing season for ragweed and prolong pollen production. Warmer temperatures and increased carbon dioxide concentrations
also lead ragweed and other plants to produce more pollen, and the pollen they produce is more allergenic. EPA's Ragweed Pollen
Season indicator shows changes in the length of the ragweed pollen season in 11 cities in the central United States and Canada
between 1995 and 2015.
• Since 1995, ragweed pollen season has grown longer at 10 of the 11 locations studied (Figure 32).
• The increase in ragweed season length generally becomes more pronounced from south to north (Figure 32). This trend is
consistent with many other observations showing that climate is changing more rapidly at higher latitudes.17
• These trends 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 1995.18
Figure 32. Change in Ragweed Pollen
Season, 1995-2015
This map shows how the length of ragweed
pollen season changed at 11 locations in the
central United States and Canada between
1995 and 2015, Red circles represent a longer
pollen season; the blue circle represents a
shorter season. Larger circles indicate larger
changes. Data source: Ziska et al,, 2016.19
+24 days
+25 days
+21 days
+18 days
+15 days
^^^+15 days
+15 days
+25 days
+11 days
+6 days
Change in length
of pollen season:
-1 day
Decrease
Increase
Climate Change Indicators in the United States
51
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Indicators featured in this chapter:
Ocean Heat, Sea Surface Temperature, Marine Species Distribution,
Marine Heatwaves, Ocean Acidity
Ocean
Impacts
The heat-trapping greenhouse gases that humans
have added to the atmosphere are making the Earth's
oceans warmer and more acidic. Changes in the
oceans affect the Earth's climate and weather patterns
and threaten marine ecosystems and biodiversity and
the people whose livelihoods depend on them.
-------
Earth's oceans are warming.
Why it matters
Oceans regulate the Earth's climate by absorbing heat and transferring it around the world. Ocean temperature affects
this function—particularly sea surface temperature—because heat from ocean surface waters provides energy for storms
and thereby influences weather patterns. As water warms, it also expands slightly, causing the sea level to rise (see the
Rising Seas chapter). Increased heat absorption also changes ocean currents, which influence climate patterns and sustain
ecosystems that depend on certain temperature ranges and movement of nutrients.
What's happening
When sunlight and energy trapped by greenhouse gases reach the Earth's surface, oceans absorb some of this energy and store
it as heat. Water is much better at storing heat than air, meaning the oceans can absorb larger amounts of heat energy with only a
slight increase in temperature. As greenhouse gases trap more energy, the oceans are absorbing more heat. This heat is initially
absorbed at the surface, but some of it eventually spreads to deeper waters. Ocean currents move this heat around the world,
influencing climate patterns.
• The total amount of heat stored by the oceans is called "ocean heat content," and measurements of water temperature reflect
the amount of heat in the water at a particular time and location.
• EPA's Ocean Heat indicator shows a long-term trend of oceans becoming warmer since 1955. Four different organizations
analyzed ocean heat data, and all reached the same conclusion (Figure 33).
Figure 33. Heat Content in the Top 700 Meters of the World's Oceans, 1955-2023
This figure shows changes in heat content of the top 700 meters of the world's oceans between 1955 and 2023. 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 organizations in four countries: the United States'
National Oceanic and Atmospheric Administration (NOAA), Australia's Commonwealth Scientific and Industrial Research
Organisation (CSIRO), China's Institute of Atmospheric Physics (IAP), and the Japan Meteorological Agency's Meteorological
Research Institute (MRI/JMA). For reference, an increase of 1 unit on this graph (1x 1022joules) is equal to approximately 17
times the total amount of energy used by all the people on the Earth in a year (based on a total global energy supply of 606
exajoules in the year 2019, which equates to 6.06 x 1020 joules).1 Data sources: CSIRO, 2024;2 IAP, 2024f MRI/JMA, 2024,4
NOAA, 2024.5
Climate Change Indicators in the United States
53
-------
As the oceans absorb more heat, sea surface temperature
is rising.
Why it matters
Higher sea surface temperatures have led to an increase in evaporation over the oceans. This extra evaporation and the
associated heat fuel the formation of heavy rain and snow. Warmer surface waters can also promote the growth of certain
bacteria that can contaminate seafood and cause people to get sick. This results in beach and fishery closures and affects
ocean ecosystems and the people and communities—including Tribes and Indigenous Peoples—who rely on them for jobs or
subsistence.6,7 Many coastal areas have a large share of people who depend on income from fishing, tourism, or both. This is
especially common in areas that have fewer economic alternatives, including rural counties and the United States' Caribbean
and Pacific island commonwealths and territories.8
What's happening
Sea surface temperature—the temperature of the water at the ocean surface—is an important physical attribute of oceans. The
surface temperature of oceans varies mainly with latitude, with the warmest waters generally near the equator and the coldest
waters in the Arctic and Antarctic regions. As the oceans absorb more heat, sea surface temperature increases, disrupting the ocean
circulation patterns that transport warm and cold water around the globe.
• EPA's Sea Surface Temperature indicator shows that sea surface temperature increased during the 20th century and continues to
rise. From 1901 through 2023, temperature rose at an average rate of 0.14°F per decade (Figure 34).
• Sea surface temperature has also been consistently higher during the past three decades than at any other time since reliable
observations began in 1880. The year 2023 was the warmest ever recorded (Figure 34).
Figure 34. Average Global Sea
Surface Temperature, 1880-2023
This graph shows how the average _ 1-5
surface temperature of the world's 1-0
oceans has changed since 1880. This
graph uses the 1971 to 2000 average as f °-5
a baseline for showing change. Choosing is
a different baseline period would not |
change the shape of the data over time. j -0.5
The shaded band shows the range of a.
uncertainty in the data, based on the ® "1,°
number of measurements collected and A 5
the precision of the methods used. Data
source: NOAA, 20249 -2.0
18
/
A
t
1971
-2000
avera<
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fm
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80 1900 1920 1940 1960 1980 2000 2020
Year
Climate Change Indicators in the United States
54
-------
How sea surface temperature affects marine ecosystems
Changes in sea surface temperature can alter marine ecosystems in several ways.
For example, variations in ocean temperature can affect what species of plants,
animals, and microbes are present in a location, change migration and breeding
patterns, threaten sensitive ocean life such as corals, and change the frequency
and intensity of harmful algal blooms.10 Over the long term, increases in sea
surface temperature could also disrupt the circulation patterns that bring nutrients
from the deep sea to surface waters. Changes in reef habitat and nutrient supply
could dramatically alter ocean ecosystems and lead to declines in fish populations,
which in turn could affect people who depend on fishing for food or livelihoods.11
Warmer oceans affect marine life.
Why it matters
The world's oceans support an abundance of biodiversity, and rising water temperatures can affect the environments where
fish, shellfish, and other marine species live. The movement of species based on their preferred temperature conditions can
affect commercial and recreational fisheries, altering where species are available, resulting in reduced catch, complicating
fisheries management, and affecting livelihoods in communities that depend on these industries. Climate-related fishery
losses have already resulted in billions of dollars of lost catch in recent years, directly harming jobs, livelihoods, and local
culture.12
What's happening
Certain fish species migrate in response to seasonal temperature changes, moving to deeper, cooler waters in the summer and
migrating back during the winter. As climate change causes the oceans to become warmer year-round, however, populations of
some species may adapt by shifting away from areas that have become too warm and toward areas that were previously cooler.
Along U.S. coasts, this means a shift northward or to deeper waters that may have a more suitable temperature. As smaller prey
species shift their geographic range, larger predator species may follow them.
• EPA's Marine Species Distribution indicator shows that the average center of biomass for 157 marine fish and invertebrate
species shifted northward by nearly 17 miles between 1989 and 2019. These species also moved an average of 0.6 feet deeper
(Figure 35).
• Several marine species have shifted northward since the 1970s or 1980s, including several economically important species. In
waters off the northeastern United States, American lobster, red hake, and black sea bass have moved northward by an average
of 145 miles (Figure 36). In waters off the southeastern United States, Atlantic croaker, banded drum, and smooth butterfly ray
have moved northward by an average of 169 miles (Figure 37). In the Bering Sea, walleye pollock, snow crab, and Pacific halibut
have generally shifted away from the coast since the early 1980s and moved northward by an average of 41 miles (Figure 38).
Climate Change Indicators in the United States 55
-------
Northeast Southeast
Eastern Bering Sea Multi-region average
Figure 35. Change irt Latitude and Depth of Marine Species, 1974-2022
These graphs show the annual change in latitude (movement in miles) and depth (feet) of 41 marine species along the
Northeast coast, 58 in the eastern Bering Sea, and 58 along the Southeast coast. The multi-region average consists of 157
unique species. Changes in the centers of biomass have been aggregated across all species and by region. For each region,
the change in latitude and change in depth are set at zero for a base year, 1989. Data source: NOAA, 2024P
1974
2022
American lobster
Red hake
Black sea bass
Year
1970 1985 2000 2015
300
200
100
0
-100
1970 1985 2000 2015
300
200
100
0
-100
1970 1985 2000 2015
North
i
South
In vi/aters off the northeastern
United States, several
economically important species
(American lobster, red hake, and
black sea bass) have moved
northward by an average of 145
miles since the early 1970s. Dots
are shaded from light to dark to
show change overtime. Data
source: NOAA, 2024,14
Figure 36. Average Locations
of Three Fish and Shellfish
Species in the Northeast,
1974-2022
Climate Change Indicators in the United States
56
-------
Smooth
butterfly ra
A
a-
!
1985 1995 2005 2015 2025
North
I
South
1989
Atlantic croaker
M* Banded drum
Smooth butterfly ray
2019
300
Banded drum
200
100
0
-100
1985 1995 2005 2015 2025
Year
Figure 37. Average Locations
of Three Fish Species in the
Southeast, 1989-2019
In vi/aters off the southeastern
United States, several species
(Atlantic croaker, banded drum,
and smooth butterfly ray) have
shifted northward by an average
of 169 miles since the late 1980s.
Dots are shaded from light to
dark to show change over time.
Data source: N0AA, 20241s
® 300
u Atlantic croaker
c ^
re .2 200
s r~ \
73 -=• 100 /
QJ "D /
ffS 0 7
£ o
2 £ -loo
¦5 1985 1995 2005 2015 2025
Walleye pollock
Pacific halibut
1985 1995 2005 2015 2025 1985 1995 2005 2015 2025 1985 1995 2005 2015 2025
North
i
South
Figure 38. Average Locations
of Three Fish and Shellfish
Species in the Bering Sea,
1985-2022
In the Bering Sea, walleye
pollock, snow crab, and Pacific
halibut have generally shifted
away from the coast since
1985. They have also moved
northward by an average of
41 miles. Dots are shaded from
light to dark to show change
over time. Data source:
N0AA, 2024.16
0)
u
£
o
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>
E
<
Year
Climate Change Indicators in the United States
57
-------
Marine heat waves are occurring more often, lasting
longer, and becoming more intense.
Why it matters
Unusually hot ocean waters pose a risk to many creatures, especially those that cannot move, such as corals. These
organisms cannot escape or tolerate waters that become too warm for several days or more. Marine heat waves are
associated with some of the largest disruptions to marine life in recent years. For example, persistent marine heat wave
conditions in the northern Pacific have fed blooms of harmful algae that have affected food chains. Algal blooms have led to
the deaths of sea lions by contaminating the fish and squid they eat Toxins from algae also accumulated in Dungeness crabs
and other types of shellfish, rendering them hazardous for human consumption and leading to fishery closures. In 2015-2016,
harmful-algal-bloom-related closures of the Dungeness fishery resulted in millions of dollars in lost revenue.17
What's happening
Along with the overall trend of warming oceans, short-term spikes in ocean temperatures, known as marine heat waves, are also
becoming more prevalent. A marine heat wave is defined as a period of at least five days in a row during which the sea surface
temperature is much warmer than usual over a given location. Just like heat waves based on air temperature, marine heat waves are
an extreme condition that can severely disrupt life, especially for creatures that cannot move out of harm's way.
• EPA's Marine Heatwaves indicator combines the duration, intensity, and frequency of marine heat waves into a single metric
called cumulative intensity, measured in degree-days. If a location experiences an increase in annual cumulative intensity over
time, that means marine heat waves are becoming either more common, longer, more intense (hotter), or some combination of
the three.
• Between 1982 and 2023, the annual cumulative intensity of marine heat waves has increased in most coastal U.S. waters, with
the largest changes in waters off the northeastern U.S. and Alaskan coasts (Figure 39).
Climate Change Indicators in the United States
Total change in annual cumulative intensity, in degree (°F) days:
—i 1 1 1
-100 0 100 200 300 400 500
Gray interval: -0.1 to 0.1 degree (°F) days
Figure 39. Change in Annual
Cumulative Intensity of Marine
Heat Waves in the United States,
1982-2023
This map shows the change in
annual cumulative intensity of
marine heat waves along U.S. coasts
from 1982 to 2023. Cumulative
intensity is measured in degree
days—marine heat wave intensity
multiplied by duration. Areas with
increases are shown in red, with
darker colors indicating greater
change. The map shows total
change, which is the annual rate of
change multiplied by the number
of years analyzed. The boundaries
include the area within the U.S.
exclusive economic zone. Data
source: NOAA, 2024.18
-------
Florida's marine heat wave of 2023: corals, fish, and human health at risk
The Florida marine heat wave of 2023 caused unprecedented
warm ocean temperatures in the region, which harmed
coral reefs and other marine life and posed risks to human
health and safety. According to the National Oceanic and
Atmospheric Administration, the sea surface temperatures
around Florida reached the highest levels on record for July
since satellites began collecting ocean data.19 For example,
Manatee Bay in southwest Florida reached over 101°F.20
The heat wave exposed corals and other marine organisms
to prolonged, intense thermal stress. The extreme heat
triggered widespread coral bleaching, which occurs when
corals expel their algal food source and turn white. Coral
bleaching occurred in several locations in Florida.21 Corai
damage and death threaten the many ecological benefits that
coral reefs provide, including support for biodiversity, fishing,
and tourism and recreation.
Higher levels of carbon dioxide are making the oceans
more acidic.
Why it matters
Rising levels of carbon dioxide dissolved in the water can harm ocean creatures. Carbon dioxide reacts with sea water to
produce carbonic acid, and the resulting increase in acidity changes the balance of minerals in the water. This makes it more
difficult for corals, some types of plankton, and other creatures to produce a mineral called calcium carbonate, which is the
main ingredient in their hard skeletons or shells. Thus, rising ocean acidity can make it more difficult for these animals to
thrive. This can lead to broader changes in the overall structure of ocean and coastal ecosystems and can ultimately affect fish
and shellfish populations and the people and communities, including Tribes and Indigenous Peoples, who depend on them for
jobs or subsistence.23
What's happening
Oceans play an important role in regulating the amount of carbon dioxide in the atmosphere. As concentrations of carbon dioxide in
the atmosphere increase, some of this additional carbon dioxide dissolves in the ocean. Because of the slow mixing time between
surface waters and deeper waters, it can take hundreds to thousands of years to establish this balance. From 1750 to 2019, oceans
absorbed about 25 percent of the carbon dioxide emitted into the atmosphere by human activities.24
• Although the oceans help to slow the pace of climate change by storing large amounts of carbon dioxide, increasing levels of
dissolved carbon dioxide are changing the chemistry of sea water and making it more acidic.
• EPA's Ocean Acidity indicator demonstrates that ocean carbon dioxide levels have risen in response to increased carbon dioxide
in the atmosphere, leading to an increase in ocean acidity (Figure 40).
IflU UMWI
This map of the ocean waters around Florida shows the
difference between sea surface temperature on July 13,
2023 (during Florida's marine heat wave) and the long-
term average. Positive values indicate the temperature is
warmer than average, and negative values indicate the
temperature is cooler than average.22
Climate Change Indicators in the United States
59
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Figure 40. Ocean Carbon Dioxide
Levels and Acidity, 1983-2022
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;
lower pH means more acidity). The
data come from two observation
stations in the North Atlantic Ocean
(Bermuda and Canary Islands), one
in the Caribbean Sea (Cariaco), and
one in the Pacific (Hawai'i), The
up-and-down pattern shows the
influence of seasonal variations.
Data sources: Bates, 2016;25
Gonzalez-Davila, 2012;26 University
of South Florida, 2021;27 University of
Hawai'i, 2023 28
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Taking action: building resilience in the face of
ocean acidification
fig
Alaska's marine environment plays a critical role in the state's economy and traditional way
of life. Many Native Alaskans depend on aquaculture for their livelihoods and for cultural
practices. Ocean acidification leads to lower saturation levels of aragonite—the form of
carbonate that shellfish need to build and maintain their shells. As the ocean has become
more acidic, shellfish hatcheries in the Pacific Northwest and Alaska have experienced
challenges maintaining the health of their shellfish. Some have nearly collapsed due to economic losses.
In 2012, the University of Alaska Fairbanks Ocean Acidification Research Center and NOAA's Pacific Marine Environmental
Laboratory partnered with the Alutiiq Pride Shellfish Hatchery in Seward, Alaska, to develop a rapid response program to
monitor ocean acidification impacts on shellfish. Alutiiq Pride uses the data gathered to determine when aragonite levels
are too low in their hatcheries so they can take mitigating steps to keep their shellfish healthy. One such strategy used by
Alutiiq Pride is to make the water less acidic by adding soda ash.
Learn more about how the Alutiiq Pride Shellfish Hatchery is collaborating with federal and academic partners to
measure the impacts of ocean acidification on shellfish and develop possible adaptation strategies.
Climate Change Indicators in the United States
61
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As the temperature of the Earth changes, so does
sea level throughout the world's oceans. Water from
melting ice sheets and glaciers on land ultimately
flows into the ocean. Also, as water in the ocean
warms, it expands slightly, increasing the volume of
water in the ocean. Both of these factors contribute
to sea level rise, which increases coastal flooding and
other coastal risks.
Indicators featured in this chapter:
Glaciers, Ice Sheets, Sea Level, Coastal Flooding
62
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Glaciers in the United States and around the world are
shrinking.
Why it matters
In many parts of the world, glaciers provide a reliable source of fresh water to support ecosystems and give people drinking
water. This water is especially important in times of extended drought and late in the summer when seasonal snowpack has
melted away. When glaciers lose ice, they ultimately add more water to the oceans, leading to a rise in sea level.
What's happening
A glacier is a large mass of snow and ice that has accumulated over many years and is present year-round. Glaciers tend to
be smaller and narrower than ice sheets. They can occur at any latitude if the temperature conditions are right, usually at high
elevations and in well-shaded mountain valleys. At higher elevations, a glacier accumulates snow, which eventually becomes
compressed into ice. At lower elevations, the glacier naturally loses mass as the ice melts and as chunks break off (which is called
calving and is common in glaciers that end in a lake or at the ocean). Glacier mass balance is the net gain or loss of snow and ice
over the course of the year. If glaciers lose more ice through melting and calving than they can accumulate through new snowfall,
they add more water to the oceans, increasing sea level.
The world's small glaciers hold less ice than the two giant ice sheets that cover Greenland and Antarctica, but they are melting more
quickly. Between 1971 and 2018, glaciers added more water overall to the oceans than the ice sheets.1
• EPA's Glaciers indicator shows that glaciers worldwide have been losing mass since at least the 1970s, which in turn has
contributed to observed changes in sea level (Figure 41).
• A longer measurement record from a smaller number of glaciers suggests that they have been shrinking since the 1950s.
• The rate at which glaciers are losing mass appears to have accelerated since the early 2000s (Figure 41).
Figure 41. Average Cumulative Mass
Balance of "Reference" Glaciers
Worldwide, 1956-2023
This figure shows the cumulative change in
mass balance of a set of "reference" glaciers
worldwide beginning in 1956. 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 1956. 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, 2024.2
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Climate Change Indicators in the United States
63
-------
In the United States, glaciers can be found in the Rocky Mountains, in the Sierra
Nevada, in the Cascades, and throughout Alaska. Four glaciers in the United
States have been studied extensively for many years and are thought to be
representative of other glaciers nearby: South Cascade Glacier in Washington
State, Lemon Creek Glacier in southeastern Alaska, Wolverine Glacier near
Alaska's southern coast, and Gulkana Glacier in Alaska's interior (Figure 42).
• These four U.S. reference 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 over time (Figure 43).
• Trends for the four U.S. reference glaciers are consistent with the retreat of
glaciers observed throughout the western United States, Alaska, and other
parts of the world.3,4
AK
° Gulkana Glacier
O
Wolverine Glacier
° Lemon Creek Glacier
South Cascade Glacier
WA
Figure 42. Four Reference Glaciers
Studied in the United States
Figure 43. Cumulative Mass
Balance of Four U.S. Glaciers,
1952-2023
This graph shows the cumulative
mass balance of four U.S. reference
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 source: USGS, 2024.5
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1960 1970 1980 1990 2000 2010 2020 2030
Climate Change Indicators in the United States
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Greenland and Antarctica's ice sheets are also shrinking.
Why it matters
The vast ice sheets of Greenland and Antarctica are losing ice more quickly than new snowfall can replenish it. This overall
loss of ice adds fresh water to the ocean, increasing sea level and possibly changing ocean circulation that is driven by
differences in temperature and salinity. Because ice reflects more sunlight than bare ground due to its light color, the melting
of the ice sheets also causes the Earth's surface to absorb more energy from the sun and become warmer.
What's happening
Ice sheets are large bodies of ice on land that cover hundreds of thousands of square miles on Greenland and Antarctica. Greenland
and Antarctica's ice sheets together hold nearly 70 percent of the world's fresh water. When an ice sheet is in equilibrium, new snow
accumulation is balanced by melting at the surface, runoff, and calving (ice breaking off to form icebergs) into the ocean.
However, a warmer climate is affecting ice sheets in several ways. Warmer air is causing the ice sheets to melt more quickly and flow
more rapidly into the sea. Warmer ocean waters are melting the edge and underside of the ice sheets and accelerating the process
of calving. EPA's Ice Sheets indicator shows that Greenland and Antarctica have both lost ice overall since 1992, with Greenland
losing an average of about 175 billion metric tons of ice per year and Antarctica losing more than 90 billion metric tons of ice per
year (Figure 44).
• The total amount of ice lost by Greenland and Antarctica from 1992 to 2020 was enough to raise sea level worldwide by an
average of roughly three-quarters of an inch or more.6 By comparison, when factoring all contributions to sea level rise, global
average sea level increased by about 3 inches overall during this period.
• Although ice sheets naturally fluctuate with seasonal variations in temperature, precipitation, and other factors, the overa„
shrinking of the ice sheets far exceeds seasonal and year-to-year variations.
• Observations of ice sheets losing mass are consistent with trends in small glaciers.
Figure 44. Cumulative Mass Balance of
Greenland and Antarctica, 1992-2023
This graph shows the cumulative change in mass
in the ice sheets of Greenland and Antarctica
since 1992, The dark "combined" lines are
based on more than 20 different studies that
have been combined for each region. Shading
shows the uncertainty estimates from the various
data sets that feed into the combined average.
The two "NASA JPL" lines have been added to
show results from one commonly cited analysis,
including seasonal variations. All estimates
are centered at zero in 2002 to provide a
consistent point of reference. Thus, a downward
slope indicates a net loss of ice and snow. For
reference, 1,000 billion metric tons is equal to
about 260 cubic miles of ice—enough to raise
sea level by about 3 millimeters.7 Data sources:
Otosaka et ai, 2023f NASA, 2024.9
c
ru
-C
u
CD
>
.5
3
E
3
U
-4,000 — Antarctica (combined data)
Antarctica (NASA JPL for reference)
-5,000 —Greenland (combined data)
Greenland (NASA JPL for reference)
-6,000
1992 1996 2000 2004 2008 2012 2016 2020 2024
Year
Climate Change Indicators in the United States
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Measuring the changing mass of glaciers and ice sheets
EPA's Glaciers indicator arid Ice Sheets indicator draw from decades of data collection. Scientists use a variety of methods
to measure how glaciers and ice sheets have changed over time.
Field measurements. Scientists visit glaciers to collect measurements of snow depth and snow density on the glacier
surface. Comparing measurements from the same locations over time allows scientists to see changes in snow and ice
accumulation and loss. Scientists report these changes in meters of water equivalent. Because snow and ice can vary in
density, converting to the equivalent amount of liquid water provides a more consistent way to track and compare results.
Satellites. Unlike small glaciers, the vast ice sheets of Greenland and Antarctica are so large and remote that scientists
cannot visit them regularly and collect reliable field measurements. Satellites offer the most effective way to measure
changes in these ice sheets.
To get a more complete picture of how ice sheet mass balance has changed, scientists have combined many different
estimates. These estimates are based on three main measurement techniques:
• Measuring the height of the ice sheet surface relative to sea level. This technique is called altimetry, and it typically uses
radar or laser instruments mounted on aircraft or satellites. Increasing or decreasing ice surface elevation can show
areas of the ice sheet that are gaining or losing mass, respectively.
• Measuring changes in the weight of the ice on Greenland and Antarctica. Changes in the density of the Earth's crust and
the amount of ice or water overlying it will cause slight variations in the Earth's gravitational field. This technique uses
a pair of satellites, known as the GRACE Mission, which are orbiting next to each other and continually measuring the
distance between them. As the satellites pass over regions with different gravitational pulls, the distance between the
two satellites slightly changes. As ice sheets melt over time, the gravitational pull decreases, allowing a calculation of
the mass lost from ice sheets due to climate warming. Learn more about how the GRACE Mission measures changes in
the Earth's gravity.
• Using the "input-output" method, which combines various sources of information on ice accumulation (for example,
models of surface mass balance) and discharge into the ocean based on radar, satellite images, and other data.
Climate Change Indicators in the United States
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Sea level is rising around the world.
Why it matters
Changing sea levels can affect people and ecosystems in coastal areas. As sea level rises, coastal flooding happens more
often (even with high tide on a sunny day) and storm surge becomes higher and more damaging. Low-lying wetlands and
dry land can turn into open water, putting coastal ecosystems and the people living in coastal areas at risk. Rising sea levels
erode shorelines and increase the flow of salt water into estuaries and nearby groundwater aquifers, threatening some
aquatic plants and animals and contaminating freshwater supplies.
What's happening
Changes in sea level are a global phenomenon. As the temperature of the Earth changes, so does sea level. Temperature and sea
level are linked for two main reasons:
1. Changes in the volume of water and ice on land (namely glaciers and ice sheets) can increase or decrease the volume of water in
the ocean.
2. As water warms, it expands slightly—an effect that is cumulative over the entire depth of the oceans. (See the Ocean Impacts
chapter to learn more about warming oceans.)
Sea level changes that affect coastal systems involve more than just expanding oceans
Sea level can be affected by more than just melting glaciers and ice sheets and warming waters. Land can also rise and
fall relative to the oceans through geologic processes. Land can rise through processes such as sediment accumulation
(the process that built the Mississippi River delta) and geological uplift (for example, as glaciers melt, the land below can
rebound upward as it is no longer weighed down by heavy ice). In other areas, land can sink because of erosion, sediment
compaction, natural subsidence, groundwater withdrawal, or engineering projects that prevent rivers from naturally
depositing sediments along their banks. Changes in ocean currents such as the Gulf Stream can also affect sea levels by
pushing more water against some coastlines and pulling it away from others.
Scientists account for these types of changes by measuring 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 whether nearby land is rising or
falling. EPA's Sea Level indicator describes how absolute sea level averaged over the entire Earth's surface and relative sea level
along the U.S. coastline have changed over time.
• After a period of about 2,000 years of little change (not shown in the figure below),10 global average sea level rose roughly 6
inches through the 20th century, and the rate of change has accelerated in recent years (Figure 45).
• While absolute sea level has increased steadily overall, particularly in recent decades, regional trends vary, and absolute sea
level has decreased in some places.11
• Relative sea level rose along much of the U.S. coastline between 1960 and 2023, particularly the Mid-Atlantic Coast and parts of
the Gulf Coast, where some stations recorded increases of more than 8 inches (Figure 46). Meanwhile, relative sea level fell at
some locations in Alaska and the Pacific Northwest. At those sites, even though absolute sea level has risen, land elevation has
risen more rapidly.
• Relative sea level has not risen uniformly because of regional and local changes in land movement and long-term changes in
coastal circulation patterns.
Climate Change Indicators in the United States
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Figure 45. Global Average Absolute Sea Level
Change, 1880-2023
This graph shows cumulative changes in sea level for
the world's oceans since 1880, based on a combination
of long-term tide gauge measurements and recent
satellite measurements. This figure shows average
absolute sea level change, which refers to the height
of the ocean surface, regardless of whether nearby
land is rising or falling. Satellite data are based solely
on measured sea level, while the long-term tide gauge
data include a small correction factor because the
oceans slowly change in size and shape over time, (On
average, the ocean floor has been gradually sinking
since the last Ice Age peak, 20,000 years ago.) The
shaded band shows the likely range of values, based
on the number of measurements collected and the
precision of the methods used. Data sources: CSIRO,
2017;12 NOAA, 2024.13
1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020 2030
Year
12
10 — Trend based on tide gauges
— Satellite measurements
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Hawaii and
Pacific Islands
Relative sea level change (inches):
1
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>12
9.01 to 12
6.01 to 9
3.01 to 6
0.01 to 3
-2.99 to 0
-5.99 to -3
-8.99 to -6
-11.99 to-9
<-12
Figure 46. Relative Sea Level Change Along U.S. Coasts, 1960-2023
This map shows cumulative changes in relative sea level from 1960 to 2023 at tide gauge stations along U.S. coasts. Relative
sea level reflects changes in sea level as well as land elevation. Data source: NOAA, 2024,14
Climate Change Indicators in the United States
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Climate ready estuaries
Coastal communities face many effects of climate change, including
sea level rise and coastal flooding. EPA's Climate Ready Estuaries
program works to support National Estuary Programs and coastal
communities in becoming "climate ready" by helping them assess their
vulnerability to climate change and plan ways to adapt. The program
provides technical guidance and assistance about climate change
adaptation.
For example, the Mobile Bay National Estuary Program in Alabama
is implementing strategies in the Three Mile Creek watershed to
address climate vulnerabilities in low-lying, historically underserved
communities where residents are predominantly people of color.
Strategies include improving stormwater management, streambank
restoration, and other measures to increase the communities'
resilience to flooding.
Flooding is becoming more frequent along the U.S.
coastline.
Why it matters
Coastal flooding can damage homes, businesses, and infrastructure that is not designed to withstand frequent inundation or
exposure to salt water. Repeated flooding can result in frequent road closures, affecting the ability of people to get to work
or school, obtain food and supplies, and receive medical care. Coastal flooding can also make storm drains ineffective and
cause drinking water and wastewater treatment systems to fail, putting people at risk of exposure to harmful chemicals and
pathogens.
What's happening
As sea level rises, one of the most noticeable consequences is an increase in coastal flooding. Flooding typically occurs during high
tides and storms that push water toward the shore. In recent years, however, coastal cities are increasingly flooding on days with
less extreme tides or little wind, even on sunny days. Over time, recurrent coastal flooding can lead to permanent inundation. Dry
land can turn into wetlands or open water, as described in EPA's feature on land loss along the Atlantic Coast. EPA's Coastal Flooding
indicator demonstrates how flooding along the U.S. coastline has changed over time.
• Flooding is becoming more frequent along much of the U.S. coastline. EPA's indicator looks at 43 locations with long-term
records, and most of them have experienced an increase in coastal flooding since the 1950s (Figure 47).
• At more than half of the sites, floods are now at least five times more common than they were in the 1950s (Figure 47).
• The average number of flood events per year has progressively accelerated across the decades since 1950. The rate of increase
of flood events per year is the largest at most locations in Hawai'i and along the East and Gulf Coasts (Figure 48).
• Flooding has increased less dramatically in places where relative sea level has not risen as quickly as it has elsewhere in the
United States. Five sites in Alaska and along the West Coast have experienced a decrease in coastal flooding (Figures 47 and 48),
coinciding with decreasing relative sea level as the land itself is rising.
Climate Change Indicators in the United States
69
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f
Neah Bay, WA
l Friday Harbor, WA
Seattle, WA
Astoria, OR
i Crescent City, CA
San Francisco, CA
. Port San Luis, CA
Los Angeles, CA
LaJolla.CA
San Diego, CA
Ketchikan, AK
Adak Island, AK
Sitka, AK
i
Kahului,^ Hil°'HI
r Harbor, ME
Portland, ME
Sandy
Philadelphia, PA
Washingto
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Baltimore, MD - » ^ _
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^ M Newport, Rl
Annapolis,
Wilmington, NC
Montauk, NY
Battery, NY
Atlantic City, NJ
Lewes, DE
Sewells Point, VA
Fort Pulaski, GA
Pensacola, FL ¦—«
J
I Charleston, SC
Fernandina Beach, FL
Mayport, FL
Cedar Key, FL
Galveston Pier 21, TX
Port Isabel, TX
St. Petersburg, FL
Average number of flood days per year:
3
is
Figure 47. Frequency of Flooding Along U.S. Coasts, 2014-2023 Versus 1950-1959
This map shows the average number of days per year in which coastal waters rose above a local threshold for flooding at 43
sites along U.S. coasts. Each small bar graph compares the first decade of widespread measurements (1950-1959, in orangej
with the most recent decade (2014-2023, in purple). Data source: NOAA, 2024.15
Climate Change Indicators in the United States
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m 42
Figure 48. Average Number of Coastal Flood Events per Year, 1950-2023
This graph shows the average number of days per year in which coastal waters rose above a local threshold for flooding at
43 sites along U.S. coasts. The data have been averaged over multi-year periods for comparison. Data source: NOAA, 2024,16
Taking action: oyster reefs as resilience tools
Restoration of oyster reefs is an effective nature-based approach to increasing shoreline
resilience. Oysters cluster on underwater structures, such as rocks, and fuse together as
they grow, forming reefs. As a protective barrier, oyster reefs guard coastal communities
by reducing wave energy that causes erosion. In contrast to structures like bulkheads and
seawalls, communities can create "living shorelines" using oyster reefs as natural barriers
that benefit the shoreline ecosystem while buffering waves.
U.S. military leaders have started implementing oyster reef restoration efforts to better protect coastal military assets.
Naval Weapons Station Earle is working to restore oyster reefs in New Jersey's Raritan Bay. The naval station is
repopulating oysters in the estuary through the creation of a 10-acre oyster reef, located under the station's pier, that
will protect the shoreline from erosion. Marine Corps Recruit Depot Parris Island in South Carolina is also stabilizing its
shorelines with oyster reefs. The construction of oyster reefs at Parris Island will reduce wave energy and erosion, in
addition to benefitting the water quality and other wildlife.
Climate Change Indicators in the United States
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Alaska s
Warming
Climate
The Arctic is warming more quickly than the rest of
the world, as is Alaska—the northernmost U.S. state.
Alaska is also uniquely vulnerable to climate change
due to its frozen features. Alaska is the only state with
widespread permafrost (underlying 80 per cent of
its land) and significant sea ice extent, which are an
integral part of life. Entire ecosystems, communities,
and Indigenous ways of life could vanish as these
frozen features shrink or disappear.
Indicators featured in this chapter:
U.S. and Global Temperature, Arctic Sea Ice, A Closer Look:
The Black Guillemots of Cooper Island, Permafrost, Community Connection:
Ice Breakup in Three Alaskan Rivers, Leaf and Bloom Dates
-
-------
Alaska is warming more quickly than the rest of the
country.
Why it matters
The temperature in particular places can have wide-ranging effects on human life and ecosystems (see the Heat on the
Rise chapter to learn more). Temperature patterns affect what types of plants and animal species can survive in particular
locations. In Alaska, where snow and ice are key features of the environment, warming can have a large impact on
ecosystems, infrastructure, and the people who depend on the landscape for sustenance and cultural survival—particularly
Alaska Native communities.
What's happening
Average temperatures at the Earth's surface are increasing and are expected to continue rising. Some parts of the country—including
Alaska—are warming more rapidly than others.
• EPA's U.S. and Global Temperature indicator shows that Alaska has warmed more quickly than any other state over the past
century. Regionally, Alaska's North Slope has warmed at the fastest rate of all (Figure 49).
• Alaska has also warmed much more quickly than the global average.
Rate of temperature change
(°F per century):
4.5
4
-0 Gray interval:
-0.1 to 0.1 °F
Figure 49. Rate of Temperature Change in Alaska, 1925-2023
This map shows how annual average air temperatures have changed in Alaska since 1925, The data are shown for climate
divisions, as defined by the National Oceanic and Atmospheric Administration, Data source: NOAA, 2024J
Climate Change Indicators in the United States
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As temperatures rise, Arctic sea ice is decreasing.
Why it matters
Alaska's northern coast lies along the Arctic Ocean, where sea ice is an integral part of life. Arctic mammals such as polar
bears and walruses rely on sea ice for hunting, breeding, and migrating. These animals face the threat of declining birth rates
and restricted access to food sources because of reduced sea ice coverage and thickness. These changes matter to people,
too. Impacts on Arctic wildlife, as well as the loss of ice itself, are already restricting the traditional subsistence hunting
lifestyles of Indigenous Peoples such as the Yup'ik, Inupiat, and Inuit.
What's happening
During the dark winter months, sea ice, which is sea water that freezes from the ocean surface down to several feet below, covers
almost the entire Arctic Ocean. In summer, some of this ice melts because of warmer temperatures and long hours of sunlight. Sea
ice typically reaches its minimum thickness and extent in mid-September, when the area covered by ice is roughly half as large as
it is at the end of winter. The ice then begins to expand again during fall and winter when temperatures fall and sunlight is limited.
However, warmer air and water temperatures are reducing the amount of sea ice present.
Because sea ice is light-colored, it reflects more sunlight (solar energy) back to space than liquid water, thereby playing an important
role in maintaining the Earth's energy balance and helping to keep polar regions cool. As the amount of sea ice in the Arctic region
decreases, the cooling effect is reduced. This can magnify the original warming due to more absorption of solar energy by the
oceans, leading to even more loss of sea ice.
• EPA's Arctic Sea Ice indicator shows a long-term decrease in the extent of sea ice during all months over the past several
decades. The largest year-to-year decreases have occurred in the summer and fall months.2,3
• The September 2023 sea ice extent was the fifth smallest on record. It was about 789,000 square miles less than the historical
1981-2010 average for that month (Figures 50 and 51)—a difference almost three times the size of Texas. September 2012 had
the lowest sea ice extent ever recorded. For March, the lowest sea ice extent on record was in 2017 (Figure 51).
• Evidence suggests that fewer patches of older, thicker sea ice are surviving through one or more melt seasons and persisting
through multiple years. A growing percentage of Arctic sea ice is only one or two years old. Young, multi-year ice implies that the
ice cover is thinning, which makes it more vulnerable to further melting.
• The length of the melting season for Arctic sea ice has grown by 37 days since 1979. On average, Arctic sea ice now starts melting
seven days earlier and starts refreezing 30 days later than it has historically.
Figure 50. Dwindling Arctic Sea Ice, September 1979 vs. September 2023
Source: NASA, 2023.4
Climate Change Indicators in the United States
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Year
Figure 51. March and September Monthly Average Arctic Sea Ice Extent, 1979-2023
This graph shows Arctic sea ice extent for the months of September and March of each year from 1979 through 2023,
September and March are when the minimum and maximum extents typically occur each year. Data source: NSIDC, 2023.5
Climate Change Indicators in the United States
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Shrinking sea ice threatens Arctic species like the black guillemot
Decreasing sea ice has major impacts on Arctic species and
ecosystems. One example of this is a bird called the black
guillemot. A colony of black guillemots have made Cooper
Island near Utqiagvik (formerly Barrow), Alaska, home for at
least 50 years. The area's unique landscape and sea ice are
integral components of this bird's habitat. The black guillemot
spends the winter on Arctic sea ice, and it breeds on land near
the edge of the ice in summer.6 One of the main food sources
for the birds' chicks is the Arctic cod, which thrives in cold,
ice-covered waters.
Warming temperatures and reduced sea ice have contributed
to population declines of these birds and the Arctic cod that
serves as their preferred food source. EPA's A Closer Look;
The Black Guillemots of Cooper Island explores changes in the
population of black guillemot.
• While the black guillemot population on Cooper island
reached a peak of more than 200 pairs in the late 1980s,
the number has decreased by more than 80 percent since
then. In 2023, scientists observed only 24 breeding pairs
(Figure 52).
• The decline over the last three decades has coincided
with reduced breeding success,7 earlier egg laying,8 and a
decrease in the presence of sea ice in the region.9'10
Arctic Ocean
Russia
Chukchi Sea
... .° Beaufort Sea
Utqiagvik
Alaska
Bering Sea
Yukon
The black guillemot is just one species, but its story illustrates
a broader trend. Shrinking sea ice has wider implications for
animals in the Arctic that depend on marine prey, such as polar
bears, and for people who depend on Arctic animals for food and other resources.
Cooper Island lies off Alaska's northern coast.
Figure 52. Cooper Island Black
Guillemot Breeding Pairs, 1972-2023
This figure shows the number of breeding
pairs in the black guillemot colony that
inhabits Cooper Island off the north
coast of Alaska, measured at the peak of
breeding season (blue line). The dashed
orange line indicates the number of
installed nest site structures available to
black guillemots on Cooper Island each
year. Data source: Divoky 2024
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1980
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Climate Change Indicators in the United States
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Rising temperatures are causing permafrost to thaw.
Why it matters
In addition to snow and ice, frozen ground—called permafrost—is a crucial feature of the Alaskan landscape. A thawing
permafrost layer can lead to severe impacts on people and the environment. For instance, as permafrost thaws, it can turn
into a muddy slurry that cannot support the weight of the soil and vegetation above it. That instability also risks damaging the
infrastructure on top of it, such as roads, buildings, and pipes. Infrastructure damage and erosion, due in part to permafrost
thaw, has already caused some communities in western and southern Alaska to have to relocate. Additionally, organic matter
(like the remains of plants) currently frozen in the permafrost will start to decompose when the ground thaws, resulting in the
emission of methane and carbon dioxide into the atmosphere. These emissions further contribute to climate change.
What's happening
Permafrost is defined as ground that remains at or below the freezing point of 32°F for two or more years. It often contains ice.
Permafrost usually lies below an "active layer" of ground that freezes and thaws every year with seasonal changes in temperature.
In Alaska, about 80 percent of the ground has permafrost underneath it. A warming climate has brought higher temperatures to
Alaska and other areas with permafrost, causing some of the permafrost to thaw.
• EPA's Permafrost indicator shows that between 1978 and 2022, permafrost temperatures increased at 14 out of 15 locations
measured across Alaska (Figure 53).
• In general, permafrost has warmed more quickly in northern Alaska than interior Alaska. This is consistent with changes in air
temperatures, which have increased more quickly in northern Alaska than in other parts of the state (Figure 53),12
Figure 53. Change in
Utqiagvik (Barrow) West Dock Qaaktugvik (Kaktovik)
Permafrost Temperatures in
Alaska, 1978-2022
Deadhorse-W FrJjnB|uffs
Northern Alaska Happy Valley
This map shows the change
/f,fer/or - - - fl^GSIbfaith Lake
asKa T'eedrinjik (Chandalar Shelf)
in permafrost temperature
Coldfoot •
per decade for 15 long-term
Old Man£
borehole sites in Alaska. Larger
• Livengood
circles correspond to greater
Troth Yeddha' (College Peat) •
9 Birch Lake
rates of warming per decade.
•
Deenaalee (Healy)
The blue circle represents
one site with a cooling trend.
•
C'uul C'ena' (Gulkana)
Borehole measurement depths
range from 49 to 85 feet. Data
source: University of Alaska
Fairbanks, 2023P
Change in temperature per decade (°F):
• • #
-0.3 to Oto 0.3 to 0.6 to 0.9 to 1.2 to
0 0.3 0.6 0.9 1.2 1.5
Climate Change Indicators in the United States
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Thawing permafrost forces a move to more solid ground
The community of Niugtaq, located in western Alaska, faces big threats
from thawing permafrost and erosion. Because of the risks these changes
pose to structures and critical infrastructure, the community is being
relocated to a new site—a large but necessary undertaking.
As a result of the degraded permafrost, buildings are sinking into the
tundra, and the drinking water system no longer works. The community
also faces severe storm surge from the Ninglick River during times of high
water, leading to loss of land along the river bank. As the climate warms,
these threats to the community's health and safety have grown.
Niugtaq's residents have been working with the state of Alaska, federal agencies, and private funders since the 1990s to
move to safer ground in Mertarvik, 9 miles across the river. Such a move required extensive site assessments and planning.
Creating a new community also takes significant funding for infrastructure, buildings and housing, power, transportation,
and more. Some Niugtaq residents have moved into new homes, but many steps remain to relocate the entire community.14
River ice is breaking up earlier in Alaska.
Why it matters
River ice breakup is an important time of transition for communities in Alaska that rely on relatively remote and free-flowing
wild rivers for transportation, subsistence hunting and fishing, and other needs. Early thawing can lead to severe ice
movement, jamming, damage to infrastructure, and destructive floods.15
What's happening
Regions in the far north, including Alaska, are warming more quickly than
other parts of the world, and this pattern is expected to continue.16'17 The
Tanana, Yukon, and Kuskokwim rivers iri Alaska provide a particularly
noteworthy record of northern climate.
Some towns have annual competitions to guess when ice breakup will
occur. Since 1917, the Nenana Ice Classic competition on the Tanana River
in central Alaska has paid several million dollars in winnings to the people
who come closest to guessing the exact date and time when the river ice
will break up. Similar traditions exist on the Kuskokwim River in Bethel,
where breakup dates have been recorded since 1924, and in Dawson City
on the Yukon River, just across the border in Canada, where breakup dates
have been recorded since 1896. The data collected by these communities
highlight how river ice breakup dates have changed over time.
• EPA's Community Connection: Ice Breakup in Three Alaskan Rivers
shows that all three of these rivers demonstrate long-term trends toward earlier ice breakup in the spring. Looking at the average
change over all years of data, the ice breakup dates for all three rivers have shifted earlier by eight to nine days (Figure 54).
• At all three locations, the earliest breakup dates ever recorded have occurred within the past four years (Figure 54).
Climate Change Indicators in the United States
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Tanana River
Yukon River
Kuskokwim River
June 4
1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005 2015 2025
Year
Figure 54. Ice Breakup Dates for Three Alaskan Rivers, 1896-2022
This graph shows the date each year when ice breaks up at three locations: the town ofNenana on the Tanana River, Dawson
City on the Yukon River, and Bethel on the Kuskokwim River, The annual data for each river have been plotted using a nine-
year moving average. Data sources: Nenana Ice Classic, 2022f Yukon River Breakup, 2022;'" Kuskokwim Ice Classic, 2022;20
National Weather Service, 2022.21
April 5
April 20
May 20
Dawson City,
Yukon
Spring events are arriving earlier in Alaska.
Why it matters
The earlier arrival of spring can have a variety of impacts on ecosystems and human society. For example, as the Changing
Seasons chapter explains, an earlier spring might lead to longer growing seasons, more abundant invasive species and pests,
and earlier and longer allergy seasons. Changes in timing of warmup, and the way different animals and plants respond
to them, can also cause important events in animals' life cycles to become out of sync with the food sources they depend
on. This timing is especially critical in Alaska and for Alaska Native communities, where many economically and culturally
significant ecological events are precisely matched to the state's dramatic temperature differences and short growing
seasons. These events include salmon runs, caribou migration and calving, and the arrival of hundreds of species of birds that
breed in Alaska every year.
What's happening
As Alaska's physical environment warms earlier each spring, that change can cause certain biological events to happen earlier
too. Plant growth, flower blooms, animal migration, and animal breeding activities are all examples of seasonal events that can be
influenced by changes in climate. Different plant and animal species respond to different environmental cues, such as temperature,
light, rainfall, and humidity.
An earlier section of this report introduced the Leaf and Bloom Dates indicator, which tracks the first leaf dates and the first bloom
dates of lilacs and honeysuckles. These flowering plants grow throughout most of the United States, and local observers across
the country have carefully recorded their "first leaf" and "first bloom" dates for many decades—making lilac and honeysuckle
leaf and bloom dates a useful general indicator of the timing of spring events. While there are fewer direct historical observations
Climate Change Indicators in the United States
79
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from Alaska, and these plants only grow in certain parts of the state, the data that are available have helped scientists understand
how lilac and honeysuckle leaf and bloom dates typically correspond to local air temperatures. This allows scientists to use local
temperature data to "fill the gap" where needed.
The results in EPA's Leaf arid Bloom Dates indicator show that spring events in Alaskan plants are shifting earlier, especially over the
last few decades (Figure 55). This shift has been much more dramatic in Alaska than in other parts of the country.
Figure 55. First Leaf and Bloom Dates in
Alaska, 1950-2023
This figure shows modeled trends in lilac
and honeysuckle first leaf dates and
first bloom dates across Alaska, 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: 2024 update by M. Schwartz to
data from Schwartz et al„ 2013.22
Leaf date (smoothed)
Leaf date (annual)
Bloom date (smoothed)
Bloom date (annual)
1981-2010
average
-20
1950 1960 1970
1980 1990 2000 2010 2020
Year
Later
t
I
Earlier
Taking action: engaging communities on
climate change in the Arctic
Arctic communities have been affected greatly by climate change. The Local Environmental
Observer (LEO) Network provides a way for the Tribal health system and local observers to
document and share knowledge about environmental conditions and climate change in the
region.
Launched at the Center for Climate and Health at the Alaska Native Tribal Health Consortium in 2012, the LEO Network
helps engage the local community and technical experts on climate and environmental events. Using the LEO Network,
local observers and experts can collect and share observations, as well as connect with other community members
and experts from many different organizations. By combining traditional knowledge, conventional science, and modern
technology, LEO Network members help raise awareness about environmental conditions that affect Arctic communities.
Climate Change Indicators in the United States
SO
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I
Conclusion
The indicators in this report present compelling evidence that the composition of the atmosphere and many fundamental aspects of
our climate in the United States are changing. These changes include rising air and water temperatures, more heavy precipitation,
more frequent heat waves, and more intense Atlantic hurricanes. Many observed climate changes are linked to rising levels of
greenhouse gases in our atmosphere, caused by human activities.
Climate change is affecting the environment in ways that have significant impacts on the health and well-being of people and
ecosystems. For example, as temperatures increase, the frequency of extreme heat days and heat waves also increases, which puts
people at greater risk for heat-related illnesses and deaths. Less snowpack and increased glacier melt affect water resources for
both ecosystems and human use. Changes in the timing and character of seasons affect the number of days suitable for growing
crops and increase pollen that triggers seasonal allergies. These changes will not be experienced equally, as some communities
have faced and will continue to face disproportionate impacts of climate change due to existing vulnerabilities, including
socioeconomic disparities, historical patterns of inequity, and systemic environmental injustices.
The indicators highlighted in this report do not represent all possible measures of the causes and effects of climate change. The
report does not present all of the climate change indicators found in the scientific assessment literature, not does it present all of
the indicators EPA itself maintains. The full suite of EPAs climate change indicators is available on EPA's Climate Change Indicators in
the United States website. The collection covers a broad range of topics, including greenhouse gases, weather and climate, oceans,
snow and ice, health and society, and ecosystems.
Looking ahead, EPA will continue to work in partnership with other agencies, organizations, and individuals to maintain and improve
the climate change indicators resource. As new indicator data become available, EPA plans to continue to update the indicators
online and explore additional indicators that can more comprehensively document climate change and its effects, including its
disproportionate impacts. Identifying and analyzing indicators improves our understanding of climate change, validates projections
of future change, and, importantly, assists EPA in its efforts to slow climate change and adapt to its impacts.
Climate Change Indicators in the United States 81
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To learn more about climate change and what EPA is doing to protect human health and the environment, please visit EPA's Climate
Change website. On this site you can explore resources about climate change science, impacts, and actions.
Climate Science and Greenhouse Gases
• View the latest information about EPAs climate change indicators and download figures as well as accompanying technical
documentation.
• Learn more about greenhouse gases and the science of climate change.
• Read about sources of greenhouse gas emissions and learn about the Inventory of U.S. Greenhouse Gas Emissions and Sinks.
• Search EPAs database of frequently asked questions about climate change.
• Find climate change resources for educators and students.
Climate Impacts
• Discover the varied impacts of climate change on society and ecosystems.
• Explore the many ways that climate change threatens human health.
• Learn about climate equity, environmental justice, and the disproportionate impacts of climate change.
Actions to Address Climate Change
• Learn about EPAs climate change regulatory actions and initiatives as well as its partnership programs.
• Read about EPAs climate adaptation efforts.
• Explore what you can do to help reduce greenhouse gas emissions.
Other Resources
For more information, explore these websites with authoritative scientific information about climate change:
• The Intergovernmental Panel on Climate Change (IPCC) is the international authority on climate change science. The IPCC
website summarizes the current state of scientific knowledge about climate change.
• The U.S. Global Change Research Program (USGCRP) 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.
• NOAA's Climate.gov site provides scientific data and information about climate science, adaptation, and mitigation.
Climate Change Indicators in the United States 82
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Introduction
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Mauna Loa, Hawai'i: 1959 CE to 2023 CE
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Barrow, Alaska: 1974 CE to 2023 CE
Cape Matatula, American Samoa: 1976 CE to 2023 CE
South Pole, Antarctica: 1976 CE to 2023 CE
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Cape Grim, Australia: 1977 CE to 2023 CE
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Shetland Islands, Scotland: 1993 CE to 2002 CE
Steele, L. P., Krummel, P. B„ & Langenfelds, R. L. (2007).
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Lampedusa Island, Italy: 1993 CE to 2000 CE
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Water Resources at Risk
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Changing Seasons
1. Marvel, K„ Su, W„ Delgado, R„ Aarons, S„ Chatterjee, A., Garcia,
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Climate Change Indicators in the United States
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• Cover artwork by Jill Pelto: "Landscape of Change" and "Replanting Resilience."
• p. 1: Artwork by Jill Pelto: "Replanting Resilience."
• p. 2; Artwork for "Ocean Impacts" chapter by Jill Pelto: "Gulf of Maine Temperature Variability."
• p. 2: Artwork for "Rising Seas" chapter by Jill Pelto: "Climate Change Data."
• p. 8: Illustration from EPA's "Climate Change Science" site (https://www.epa.gov/climatechanqe-science).
• p. 31: Forest fire photo by Mike Lewelling, National Park Service.
• p. 32: Screenshot from the AirNow Fire and Smoke Map (https://fire.airnow.gov).
• p. 35: Lake photo by Zachary Haslick, Aerial Associates Photography, Inc.
• p. 52: Artwork by Jill Pelto: "Gulf of Maine Temperature Variability."
• p. 59: Image from NOAAs Coral Reef Watch (https://coralreefwatch.noaa.gov).
• p. 62: Artwork by Jill Pelto: "Climate Change Data."
• p. 66: GRACE satellite rendering by NASA Jet Propulsion Laboratory.
• p. 71: Oyster-planting photo by Patrick Bloodgood, U.S. Army.
• p. 75: Satellite images of Arctic sea ice from NASA's Scientific Visualization Studio (https://svs.gsfc.nasa.gov).
• p. 78: House construction photo from the Office of Senator Lisa Murkowski.
Climate Change Indicators in the United States
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I' United States
. Environmental Protection
-!-~I m m Agency
EPA430-R-24-003
July 2024
www.epa.gov/climate-indicators
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