The U.S. Environmental Protection Agency (EPA) said greenhouse gas (GHG) emissions decreased 1.6 percent in 2011 from the previous year, according to the EPA's 18th Annual U.S. Greenhouse Gas Inventory.
GHG emissions in 2011 were also 6.9 percent below 2005 levels. Total emissions of the six main GHGs — carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride — were equivalent to 6,702 million metric tons of carbon dioxide.
The downward trends can be attributed to reducing emissions in the electricity generation industry, improvements in fuel efficiency and year-to-year changes in the weather.
The "Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2011" tracks annual GHG emissions at the national level and presents historical emissions from 1990 to 2011. The inventory also calculates carbon dioxide emissions that are removed from the atmosphere by forests, vegetation, soils and other natural processes called carbon sinks.