THE United States cut its energy-related carbon dioxide pollution by 3.8 per cent last year, the second biggest drop since 1990, the Department of Energy says.

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 tonnes 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 on MOnday that carbon pollution reduction was 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 that in 1994 coal provided 52 per cent of the US power and now it was down to 37 per cent. 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 per cent drop in 2009, Lindstrom said.

But this drop happened while the US economy was growing 2.8 per cent, as reflected by the gross domestic product, and its energy use was dropping by more than 2 per cent.

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 per cent, Lindstrom said.

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

"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."

The world is heading in the opposite direction. In 2011, the world carbon dioxide emissions jumped 3 per cent, because of a large increase by China, the number one carbon polluting country. The US is number two in carbon emissions.