Greenhouse gases occur naturally, though human activities like burning fossil fuels also emit the gases into the atmosphere. The substances trap heat to the earth's surface, making the world warmer, according to scientists.
The leading source of 2011 greenhouse gas emissions in the U.S. was electricity production, including burning natural gas, coal and oil, which made up a third of the overall emissions. Electricity emissions have increased about 18 percent since 1990.
Other major emitters include fuel burned for transportation, responsible for 28 percent of the nation's emissions, and industrial emissions, which account for 20 percent of the country's overall output.
Total emissions of the six main greenhouse gases add up to 6,702 million metric tons of carbon dioxide.
The EPA's report on greenhouse gas emissions was delivered to the United Nations as part of a framework on climate change ratified by the United States in 1992.