United States
Environmental Protection
Agency
Summary of Key Points from Climate Change Indicators in
the United States, 2014
EPA's Climate Change Indicators in the United States, 2014, presents compelling evidence that the impacts of climate change are already oc-
curring across the United States. Following is a summary of key points for 30 indicators that track signs of climate change.
U.S. Greenhouse Gas Emissions. In the United States, greenhouse
gas emissions caused by human activities increased by 5 percent
from 1990 to 2012. However, since 2005, total U.S. greenhouse gas
emissions have decreased by 10 percent. Carbon dioxide accounts for
most of the nation's emissions and most of the increase since 1990.
Electricity generation is the largest source of greenhouse gas emis-
sions in the United States, followed by transportation. Emissions per
person have decreased slightly in the last few years.
Global Greenhouse Gas Emissions. Worldwide, net emissions of
greenhouse gases from human activities increased by 35 percent from
1990 to 2010. Emissions of carbon dioxide, which account for about
three-fourths of total emissions, increased by 42 percent over this pe-
riod. As with the United States, the majority of the world's emissions
result from electricity generation, transportation, and other forms of
energy production and use.
U.S. and Global Temperature. Average temperatures have risen
across the contiguous 48 states since 1901, with an increased rate of
warming over the past 30 years. Seven of the top 10 warmest years
on record have occurred since 1998. Average global temperatures
show a similar trend, and the top 10 warmest years on record world-
wide have all occurred since 1998. Within the United States, tempera-
tures in parts of the North, the West, and Alaska have increased the
most.
High and Low Temperatures. Many extreme temperature condi-
tions are becoming more common. Since the 1970s, unusually hot
summer temperatures have become more common in the United
States, and heat waves have become more frequent—although the
most severe heat waves in U.S. history remain those that occurred
during the "Dust Bowl" in the 1930s. Record-setting daily high tem-
peratures have become more common than record lows. The decade
from 2000 to 2009 had twice as many record highs as record lows.
U.S. and Global Precipitation. Total annual precipitation has
increased in the United States and over land areas worldwide. Since
1901, precipitation has increased at an average rate of 0.5 percent
per decade in the contiguous 48 states and 0.2 percent per decade
over land areas worldwide. However, shifting weather patterns have
caused certain areas, such as Hawaii and parts of the Southwest, to
experience less precipitation than usual.
Ocean Heat. Three separate analyses show that the amount of
heat stored in the ocean has increased substantially since the 1950s.
Ocean heat content not only determines sea surface temperature, but
also affects sea level and currents.
Sea Surface Temperature. Ocean surface temperatures increased
around the world over the 20th century. Even with some year-to-year
variation, the overall increase is clear, and sea surface temperatures
have been higher during the past three decades than at any other
time since reliable observations began in the late 1800s.
Sea Level. When averaged over all the world's oceans, sea level has
increased at a rate of roughly six-tenths of an inch per decade since
Atmospheric Concentrations of Greenhouse Gases.
Concentrations of carbon dioxide and other greenhouse gases in
the atmosphere have increased since the beginning of the industrial
era. Almost all of this increase is attributable to human activities.
Historical measurements show that current levels of many greenhouse
gases are higher than any levels recorded for hundreds of thousands
of years, even after accounting for natural fluctuations.
Climate Forcing. Climate forcing refers to a change in the Earth's
energy balance, leading to either a warming or cooling effect. An
increase in the atmospheric concentrations of greenhouse gases
produces a positive climate forcing, or warming effect. From 1990 to
2013, the total warming effect from greenhouse gases added by hu-
mans to the Earth's atmosphere increased by 34 percent. The warming
effect associated with carbon dioxide alone increased by 27 percent.
Heavy Precipitation. In recent years, a higher percentage of pre-
cipitation in the United States has come in the form of intense single-
day events. Nationwide, nine of the top 10 years for extreme one-day
precipitation events have occurred since 1990. The occurrence of ab-
normally high annual precipitation totals (as defined by the National
Oceanic and Atmospheric Administration) has also increased.
Drought. Average drought conditions across the nation have varied
since records began in 1895. The 1930s and 1950s saw the most
widespread droughts, while the last 50 years have generally been
wetter than average. However, specific trends vary by region. A more
detailed index developed recently shows that between 2000 and
2013, roughly 20 to 70 percent of the United States experienced
drought at any given time, but this index has not been in use for long
enough to compare with historical drought patterns.
Tropical Cyclone Activity. Tropical storm activity in the Atlantic
Ocean, the Caribbean, and the Gulf of Mexico has increased dur-
ing the past 20 years. Increased storm intensity is closely related to
variations in sea surface temperature in the tropical Atlantic. However,
changes in observation methods over time make it difficult to know
for sure whether a long-term increase in storm activity has occurred.
Records collected since the late 1800s suggest that the actual number
of hurricanes per year has not increased.
1880. The rate of increase has accelerated in recent years to more
than an inch per decade. Changes in sea level relative to the land
vary by region. Along the U.S. coastline, sea level has risen the most
along the Mid-Atlantic coast and parts of the Gulf coast, where some
stations registered increases of more than 8 inches between 1960 and
2013. Sea level has decreased relative to the land in parts of Alaska
and the Northwest.
Ocean Acidity. The ocean has become more acidic over the past
few centuries because of increased levels of atmospheric carbon di-
oxide, which dissolves in the water. Higher acidity affects the balance
of minerals in the water, which can make it more difficult for certain
marine animals to build their skeletons and shells.
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Arctic Sea Ice. Part of the Arctic Ocean is covered by ice year-round.
The area covered by ice is typically smallest in September, after the
summer melting season. The minimum extent of Arctic sea ice has
decreased over time, and in September 2012 it was the smallest
on record. Arctic ice has also become thinner, which makes it more
vulnerable to additional melting.
Glaciers. Glaciers in the United States and around the world have
generally shrunk since the 1960s, and the rate at which glaciers are
melting has accelerated over the last decade. The loss of ice from
glaciers has contributed to the observed rise in sea level.
Lake Ice. Most lakes in the northern United States are freezing later
and thawing earlier compared with the 1800s and early 1900s. Freeze
dates have shifted later at a rate of roughly half a day to one day per
decade, while thaw dates for most of the lakes studied have shifted
earlier at a rate of half a day to two days per decade.
Snowfall. Total snowfall—the amount of snow that falls in a
particular location—has decreased in most parts of the country since
widespread records began in 1930. One reason for this decline is
that more than three-fourths of the locations studied have seen more
winter precipitation fall in the form of rain instead of snow.
Snow Cover. Snow cover refers to the area of land that is covered
by snow at any given time. Between 1972 and 2013, the average por-
tion of North America covered by snow decreased at a rate of about
3,500 square miles per year, based on weekly measurements taken
throughout the year. However, there has been much year-to-year vari-
ability.
Snowpack. The depth or thickness of snow on the ground (snow-
pack) in early spring decreased at about three-fourths of measure-
ment sites in the western United States between 1955 and 2013.
However, other locations saw an increase in spring snowpack.The
average change across all sites for this time period amounts to about
a 14 percent decline.
Heating and Cooling Degree Days. Heating and cooling degree
days measure the difference between outdoor temperatures and the
temperatures that people find comfortable indoors. As the U.S. climate
has warmed in recent years, heating degree days have decreased and
cooling degree days have increased overall, suggesting that Ameri-
cans need to use less energy for heating and more energy for air
conditioning. This pattern stands out the most in the North and West,
while much of the Southeast has experienced the opposite results.
Heat-Related Deaths. Over the past three decades, nearly 8,000
Americans were reported to have died as a direct result of heat-
related illnesses such as heat stroke. The annual death rate is higher
when accounting for other deaths in which heat was reported as a
contributing factor. Considerable year-to-year variability in the data
and certain limitations of this indicator make it difficult to determine
whether the United States has experienced long-term trends in the
number of deaths classified as "heat-related."
Lyme Disease. Lyme disease is a bacterial illness spread by ticks
that bite humans.Tick habitat and populations are influenced by
many factors, including climate. Nationwide, the rate of reported
cases of Lyme disease has approximately doubled since 1991. Lyme
disease is most common in the Northeast and the upper Midwest,
where some states now report 50 to 90 more cases of Lyme disease
per 100,000 people than they did in 1991.
Length of Growing Season. The average length of the grow-
ing season in the contiguous 48 states has increased by nearly two
weeks since the beginning of the 20th century. A particularly large
and steady increase has occurred over the last 30 years. The observed
changes reflect earlier spring warming as well as later arrival of fall
frosts. The length of the growing season has increased more rapidly in
the West than in the East.
Ragweed Pollen Season. Warmer temperatures and later fall
frosts allow ragweed plants to produce pollen later into the year,
potentially prolonging the allergy season for millions of people. The
length of ragweed pollen season has increased at 10 out of 11 loca-
tions studied in the central United States and Canada since 1995. The
change becomes more pronounced from south to north.
Wildfires. Since 1983, the United States has had an average of
72,000 recorded wildfires per year. Of the 10 years with the larg-
est acreage burned, nine have occurred since 2000, with many of
the largest increases occurring in western states. The proportion of
burned land suffering severe damage each year has ranged from 5 to
22 percent.
Streamflow. Changes in temperature, precipitation, snowpack, and
glaciers can affect the rate of streamflow and the timing of peak flow.
Over the last 73 years, minimum, maximum, and average flows have
changed in many parts of the country—some higher, some lower.
Nearly half of the rivers and streams measured show peak winter-
spring runoff happening at least five days earlier than it did in the
mid-20th century.
Great Lakes Water Levels and Temperatures. Water levels in
most of the Great Lakes have declined in the last few decades. Water
levels in lakes are influenced by water temperature, which affects
evaporation rates and ice formation. Since 1995, average surface wa-
ter temperatures have increased by a few degrees for Lakes Superior,
Michigan, Huron, and Ontario. Less of a temperature change has been
observed in Lake Erie.
Bird Wintering Ranges. Some birds shift their range or alter their
migration habits to adapt to changes in temperature or other environ-
mental conditions. Long-term studies have found that bird species in
North America have shifted their wintering grounds northward by an
average of more than 40 miles since 1966, with several species shift-
ing by hundreds of miles. On average, bird species have also moved
their wintering grounds farther from the coast, consistent with inland
winter temperatures becoming less severe.
Leaf and Bloom Dates. Leaf growth and flower blooms are ex-
amples of natural events whose timing can be influenced by climate
change. Observations of lilacs and honeysuckles in the contiguous
48 states suggest that first leaf dates and bloom dates show a great
deal of year-to-year variability. Leaf and bloom events are generally
happening earlier throughout the North and West but later in much of
the South.
For more information, see:
www.epa.gov/climatechange/indicators
EPA 430-F-14-016 • May 2014
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