Anthropogenic US greenhouse gas emissions (GHG) amounted to a CO2-equivalent 6,702.3 million metric tons in 2011, down 1.6 percent from 2010 and 6.9 percent below 2005 levels. Longer term, US GHG emissions have increased at an annual average rate of 0.4 percent since 1990, according to the Environmental Protection Agency's (EPA) 18th annual US Greenhouse Gas Emissions and Sinks (Inventory) report, which was released April 15.
A decrease in the carbon intensity of fuels used in electricity generation due to increased use of natural gas as opposed to coal, a “significant increase in hydropower” generation, and “relatively mild winter conditions, especially in the South Atlantic Region of the US” were the main factors underlying the drop in national GHG emissions in 2011, according to the EPA's “The Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2011.”
Longer term trends from 1990 through 2011 were attributed to lower emissions from electricity generation, higher vehicle fuel efficiency and less in the way of miles traveled, and year-to-year changes in weather patterns.
Tracking trends in US GHG emissions
Anthropogenic, or human-caused, GHG emissions have been identified as the main drivers of climate change, the immediate economic, social and environmental costs and long-term threats of which have become increasingly apparent.
For 18 years, the US EPA has been tracking total emissions of the six main GHGs in its annual Greenhouse Gas Emissions and Sinks (Inventory) reports: carbon dioxide (CO2), methane, nitrous oxide, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride.
In 2002, the US government signed and Congress subsequently ratified the United Nations Framework Convention on Climate Change (UNFCCC), joining 195 other national governments in pledging to meet targets for reducing anthropogenic GHG emissions.
“Under this Administration, EPA has taken a number of common sense steps to help reduce GHG emissions. This includes increasing fuel efficiency for cars that will reduce America’s dependence on oil by an estimated 12 billion barrels by 2025, and increasing energy efficiency through the Energy Star program that saved Americans $24 billion in utility bills in 2012,” the EPA highlighted in a press release.
The primary agent of human-induced climate change, CO2 made up nearly 84 percent of anthropogenic GHG emissions in the US in 2011, fossil fuel combustion accounting for 5,277.2 (94 percent) of the total 5,612.9 million metric tons emitted in the US in 2011. That's up from 4,748.5 and 5,108.8, respectively, in 1990.
“As the largest source of U.S. greenhouse gas emissions, CO2 from fossil fuel combustion has accounted for approximately 78 percent of GWP-weighted emissions since 1990, and is approximately 79 percent of total GWP-weighted (Global Warming Potential) emissions in 2011,” EPA highlights in the report. “Emissions of CO2 from fossil fuel combustion increased at an average annual rate of 0.5 percent from 1990 to 2011.
“The fundamental factors influencing this trend include (1) a generally growing domestic economy over the last 22 years, and (2) an overall growth in emissions from electricity generation and transportation activities. Between 1990 and 2011, CO2 emissions from fossil fuel combustion increased from 4,748.5 Tg CO2 Eq. to 5,277.2 Tg CO2 Eq.—an 11.1 percent total increase over the twenty-two-year period. From 2010 to 2011, these emissions decreased by 130.9 Tg CO2 Eq. (2.4 percent).”
Image and graphics courtesy of EPA