The only recent year with a bigger percentage drop was in 2009, when America was in a large recession. American cars and factories spewed 5.83 billion tons of carbon dioxide in 2012, down from 6.06 billion in 2011. It is the lowest level for US emissions since 1994. Carbon dioxide is the chief man-made global warming gas.

Energy Department economist Perry Lindstrom said carbon pollution reduction is due to warm winter weather, more efficient cars because of new mileage requirements and an ongoing shift from coal-power to natural gas to produce electricity.

The coal shift is a big factor as is a sluggish economic recovery, said Jay Apt, director of the Carnegie Mellon Electricity Industry Centre. He said in 1994 coal provided 52% of the US power and now it is down to 37%.

Burning coal produces far more carbon dioxide than burning natural gas.

Some past cuts in carbon pollution were mostly due to economic factors, such as the 7.1% drop in 2009, Lindstrom said. But this drop happened while the U.S. Economy was growing 2.8%, as reflected by the gross domestic product, and its energy use was dropping by more than 2%.

Economists measure energy efficiency and how real reductions are in carbon pollution, by calculating carbon dioxide emissions per unit of GDP. And from 2011 to 2012, the United States carbon pollution per GDP dropped by a record 6.5%, Lindstrom said.

That shows this drop was clearly not due to a recession, Lindstrom said.

In 2012, America spewed more than 368,000 pounds of carbon dioxide per second.

"This latest drop in energy-related carbon emissions is reason for cautious optimism that we're already starting to move in the right direction," said Pennsylvania State University climate scientist Michael Mann. "But this alone will not lead us toward the dramatic carbon reductions necessary to avoid dangerous climate change.