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Understanding Climate Change

With congressional and public interest in climate change growing over the past 20 years, science policy and data on greenhouse gas emissions have emerged as particularly important issues to help inform climate policy.

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Certain gases, such as carbon dioxide, methane, and nitrous oxide, trap energy from the sun that would otherwise escape Earth's atmosphere. High concentrations of these gases create a greenhouse effect that raises average global temperatures. Global temperature increases may contribute to a gradual change in the balance of energy flowing into and away from Earth's surface. Earth's climate system is driven by energy from the sun and is maintained by complex interactions among the atmosphere, the oceans, and the reflectivity of Earth’s surface, among other factors. Earth's system maintains a constant average temperature only if the same amount of energy leaves the system as enters it. If more energy enters than leaves, the difference manifests as a temperature increase. The interactive feature (figure 1) shows current estimates of the equilibrium transfer of energy.

Global Average Energy Budget of the Earth's Atmosphere

Carbon dioxide is the key greenhouse gas affected by human activity, accounting for about 84% of U.S. emissions in 2011. The next interactive feature (figure 2) illustrates Earth’s carbon cycle, which regulates the flow of carbon between the atmosphere and land-based and oceanic sinks.


Depiction of the Global Carbon Cycle Changes Over Time

Since the 1700s, atmospheric carbon dioxide concentrations have risen from approximately 280 parts per million to approximately 400 parts per million (figure 3). According to the May 2014 National Climate Assessment, after decades of increases, carbon dioxideemissions from energy use (which account for 97% of total U.S. emissions) declined by about 9% from 2008 to 2012, largely due to a shift from coal to less carbon-intensive natural gas for electricity production. Based on observational trends and model simulations, climate changes since 1950 cannot be explained by natural factors or variability, and can only be explained by human factors.

Figure 3: Atmospheric Carbon Dioxide Concentration, 1700-Present    

Figure 3: Atmospheric Carbon Dioxide Concentration, 1700-Present

Note: Data from 1958 to present were measured at Mauna Loa, Hawaii. Data before 1958 were calculated by analyzing the carbon dioxide contained in ice cores.

These trends have led to measurable, widespread effects on the climate. Table 1 shows current and projected climate changes in the United States.

Table 1: Current and Projected Climate Changes in the United States


 Observed Climate Changes

 Projected Climate Changes


  • U.S. average annual temperature rose about 1.5 degrees Fahrenheit since recordkeeping began in 1895; more than 80 percent of this increase occurred since 1980. The most recent decade was the nation’s warmest on record.a
  • The frost-free season has lengthened since the 1980s, and rising temperatures have reduced ice volume on land, lakes, and sea. Minimum Arctic sea ice decreased by more than 40 percent since satellite records began in 1978.
  • U.S. temperatures are expected to continue to rise, with varying effects by region. In the next few decades, warming of 2 to 4 degrees Fahrenheit is expected for most of the nation.
  • The frost-free season is expected to increase by a month or more across most of the United States by the end of the century.
  • Antarctic sea ice is projected to continue declining in future decades.b,c


  • Annual precipitation across the United States has increased an average of 5 percent since 1900, but some areas have experienced decreases.
  • Heavy downpours increased in most regions of the United States, especially over the last three to five decades.
  • Projections of future precipitation indicate that northern areas will become wetter while southern areas, particularly the Southwest, become drier.
  • Further increases in the frequency and intensity of extreme precipitation events are projected for most of the nation.

Sea level rise and coastal erosion

  • Global sea levels rose about 8 inches since reliable recordkeeping began in 1880.
  • The current rate of global sea level rise is faster than at any time in the past 2,000 years.
  • Sea levels are projected to continue to rise, but the extent is not well understood.d
  • In the next several decades, sea level rise and land erosion could combine with storm surges and high tides to increase flooding in coastal regions.

Extreme weather events and storms

  • Certain types of extreme weather events—heat waves, floods, droughts—have become more frequent and intense in some regions.
  • In the eastern Pacific, the strongest hurricanes became more intense since the 1980s, while the total number of storms declined.
  • The intensity of the strongest hurricanes is projected to continue to grow as the oceans warm, causing wind, precipitation, and storm surges.
  • Other trends in severe storms, including the number of hurricanes and intensity and frequency of tornadoes, are uncertain and remain under study.

Sources: GAO analysis of USGCRP's 2009 and May 2014 National Climate Assessments and NRC's America's Climate Choices: Adapting to the Impacts of Climate Change, 2010.

aA report by the United Kingdom notes global mean surface temperatures rose rapidly from the 1970s, but have been relatively flat over the most recent 15 years to 2013. This has prompted speculation that human-induced global warming is no longer happening, or at least will be much smaller than predicted. Others maintain that this is a temporary pause in global temperatures and that they will again rise at rates seen previously. United Kingdom Met Office, Observing Changes in the Climate System: The recent pause in global warming (1): What do observations of the climate system tell us? (United Kingdom: July 2013).

bU.S. Climate Change Science Program (now known as USGCRP), Global Climate Change Impacts in the United States, Draft 2013 National Climate Assessment (Washington, D.C., 2013).

cLiu, J., Judith A. Curry, Accelerated Warming in the Southern Ocean and its Impacts on the Hydrological Cycle and Sea Ice. School of Earth and Atmospheric Sciences, Georgia Institute of Technology (Atlanta, GA: 2010).

dSea levels have been rising, and at an increasing rate, but understanding all of the dynamics involved is not sufficiently complete to allow for an accurate prediction of the likely total extent of sea level rise this century. For example, scientists have a well developed understanding of the contributions of thermal expansion of the oceans due to warming. However, other changes, such as ice sheet dynamics, are less well understood, and while this variable is expected to make a significant contribution to sea level rise, quantifying that contribution is difficult.

In addition, increasing carbon dioxide levels in the atmosphere and oceans are raising acid levels in the oceans. Ocean acidification could have a variety of potentially significant effects on marine species, ecosystems, and coastal communities. It could reduce the ability of some marine species, such as oysters, to form shells or it may alter their physiology or behavior. It could also alter marine ecosystems by disrupting predator and prey relationships and habitats, in turn disrupting the economy or culture of some communities by harming coastal fishing and tourism industries.

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Global Average Energy Budget of the Earth's Atmosphere
Depiction of the Global Carbon Cycle Changes Over Time
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