Summary of Key Points from Climate Change Indicators in the United States, 2014 Page: 1 of 2
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EPA's Climate Change Indicators in the United States, 2014, presents compelling evidence that the impacts of climate change are already occur-
ring 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
*5 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
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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United States. Environmental Protection Agency. Summary of Key Points from Climate Change Indicators in the United States, 2014, text, May 2014; Washington, D. C.. (digital.library.unt.edu/ark:/67531/metadc949195/m1/1/: accessed April 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.