Key Issues > Understanding Climate Change
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Understanding Climate Change

Congressional and public interest in climate change has grown over the past 20 years. Over this period, several issues have emerged, particularly in the areas of emissions data and science policy.

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Certain gases in Earth’s atmosphere—carbon dioxide (CO2) and methane—are known as greenhouse gases because they trap energy from the sun that would otherwise escape Earth’s atmosphere. In the atmosphere, greenhouse gases absorb and reemit radiation causing a greenhouse effect, which, in turn, causes a warming of Earth’s atmosphere. Table 1 shows the current and projected impacts of climate change in the United States.

GAO-12-116, Table 1

Various human and natural activities emit greenhouse gases, with the production and burning of fossil fuels for energy contributing around two-thirds of man-made global emissions in 2005. CO2 is the key greenhouse gas affected by human activity, accounting for about three-quarters of global emissions in 2005. Other greenhouse gases include––in order of their prevalence by volume––methane (CH4), nitrous oxide (N2O), and ozone (O3). The interactive feature below 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

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 below shows current estimates of the equilibrium transfer of energy.

Figure 2: Global Average Energy Budget of Earth's Atmosphere


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