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.
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
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
Sea level rise and coastal erosion
Extreme weather events and storms
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.
GAO-16-122: Published: Oct 5, 2015. Publicly Released: Nov 3, 2015.
Federal agencies are enhancing understanding of climate-related risks to public health by (1) supporting and conducting research, (2) providing data and informational resources, and (3) communicating about risks. The Department of Health and Human Services’ (HHS) National Institutes of Health (NIH) supports a portfolio of research directly related to these risks. NIH reports awarding about $6 mi...
GAO-15-386T: Published: Feb 12, 2015. Publicly Released: Feb 12, 2015.
The National Oceanic and Atmospheric Administration's (NOAA) $11.3 billion Joint Polar Satellite System (JPSS) program has recently completed significant development activities and remains within its cost and schedule baselines; however, recent cost growth on key components is likely unsustainable, and schedule delays could increase the potential for a near-term satellite data gap. In addition, wh...
GAO-14-736: Published: Sep 12, 2014. Publicly Released: Oct 14, 2014.
Ocean acidification could have a variety of potentially significant effects on marine species, ecosystems, and coastal communities, according to six summary reports that GAO reviewed. The reports were developed by federal agencies and others and were based on extensive reviews of the scientific literature. The scientific understanding of these effects, however, is still developing, and uncertainty...
GAO-11-800: Published: Aug 31, 2011. Publicly Released: Sep 30, 2011.
The National Oceanic and Atmospheric Administration (NOAA) maintains a network of weather-monitoring stations known as the U.S. Historical Climatology Network (USHCN), which monitors the nation's climate and analyzes long-term surface temperature trends. Recent reports have shown that some stations in the USHCN are not sited in accordance with NOAA's standards, which state that temperature instrum...
GAO-10-818: Published: Jul 30, 2010. Publicly Released: Aug 5, 2010.
Nations that are Parties to the United Nations Framework Convention on Climate Change periodically submit inventories estimating their greenhouse gas emissions. The Convention Secretariat runs a review process to evaluate inventories from 41 "Annex I" nations, which are mostly economically developed nations. The 153 "non-Annex I" nations are generally less economically developed and have less stri...
GAO-18-206: Published: Nov 13, 2017. Publicly Released: Dec 13, 2017.
The expected impacts of weather effects associated with climate change pose operational and budgetary risks to overseas infrastructure according to the Department of Defense (DOD), but DOD does not consistently track the impacts' estimated costs. Operational risks (including interruptions to training, testing, and missions) and budgetary risks (including costs of repairing damages) are linked to t...
GAO-18-184T: Published: Nov 1, 2017. Publicly Released: Nov 1, 2017.
Federal agencies have established requirements to protect workers and the public against cancer and other harmful effects associated with low doses of radiation exposure. We testified that federal agencies have generally used the advice of scientific advisory bodies to develop and apply these requirements. Additionally, for fiscal years 2012 through 2016, the Department of Energy and other federa...
GAO-17-720: Published: Sep 28, 2017. Publicly Released: Oct 24, 2017.
Climate-related impacts, such as coastal property damage, have already cost the federal government billions of dollars, and these costs will likely rise in the future. We found that information on the economic effects of climate change is developing and imprecise, but it can convey insights into the nation's regions and sectors that could be most affected. As an initial step in establishing gover...
GAO-17-546: Published: Sep 26, 2017. Publicly Released: Oct 26, 2017.
Exposure to low doses of radiation may increase a person's risk of cancer. To help protect workers and the public against this risk, federal agencies set dose limits for power plants, issue guidance, and take other measures. The Department of Energy and other agencies have also invested millions to better understand the health effects of low-dose radiation, but uncertainty remains. Given the redu...
GAO-17-3: Published: Nov 30, 2016. Publicly Released: Jan 3, 2017.
The houses we live in, buildings we work in, and roads and bridges we use daily are supposed to be built to last—whatever the local forecast has in store. However, design standards and building codes generally use historical climate observations. Forward-looking climate information would help account for the changing frequency and intensity of extreme weather. Continuing to build with current...
GAO-16-834: Published: Sep 28, 2016. Publicly Released: Sep 28, 2016.
Climate assessments predict that floods and other extreme weather events will become more common and intense, putting coastal areas—which are home to more than half of the U.S. population—at risk. The wetlands, marshes, and mangroves that usually help protect coastal communities are also vulnerable. The National Oceanic and Atmospheric Administration provides funding, training, and other assi...
GAO-16-454: Published: May 12, 2016. Publicly Released: Jun 13, 2016.
Selected governments have approached enhancing resilience through climate change adaptation, and some have aligned adaptation with broader resilience efforts (see figure). All five selected governments have enacted laws and developed long-term plans as a part of their approaches to climate change adaptation. These plans established frameworks for addressing climate risks. For example, the European...
GAO-15-28: Published: Oct 29, 2014. Publicly Released: Nov 20, 2014.
Since GAO's 2007 report on flood and crop insurance, exposure growth in hazard-prone areas has increased losses, and climate change and related increases in extreme weather events may further increase such losses in coming decades. Scientific and industry studies GAO reviewed generally found that increasing growth and property values in hazard-prone areas have increased losses to date and that cli...
GAO-14-755: Published: Sep 16, 2014. Publicly Released: Oct 16, 2014.
The U.S. Department of Agriculture's (USDA) climate change priorities for agriculture include, among other things, providing better information to farmers on future climate conditions. These priorities generally align with national priorities set by the Administration, which include promoting actions that reduce greenhouse gas emissions, advancing climate science, developing tools for decision mak...
GAO-14-504T: Published: Jul 29, 2014. Publicly Released: Jul 29, 2014.
Climate change and related extreme weather impacts on infrastructure and federal lands increase fiscal exposures that the federal budget does not fully reflect. Investing in resilience—actions to reduce potential future losses rather than waiting for an event to occur and paying for recovery afterward—can reduce the potential impacts of climate-related events. Implementing resilience measures...