The Energy Information Administration, the statistical arm of the Energy Department, said energy-related CO2 emissions declined 3.8 percent in 2012.
"The 2012 [economic] downturn means that emissions are at their lowest level since 1994 and over 12 percent below the recent 2007 peak," it said in a report published Monday. "After 1990, only the recession year of 2009 saw a larger percentage emissions decrease than 2012."
The EIA said half of the overall decline in energy demand came from the residential sector. The agency attributed the decline to a "very warm" first quarter of 2012.
The National Oceanic and Atmospheric Administration said March 2012 was the warmest on record for the month. The national average temperature of 51.1 degrees Fahrenheit was 8.5 degrees warmer than the 20th century average.
The EIA said energy-related CO2 emissions have declined in five of the last seven years in the United States.