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The United States cut energy-related carbon dioxide pollution by 3.8 percent last year, the second-largest drop since 1990, according to the Department of Energy’s Energy Information Administration (EIA).
The only recent year with a larger drop was in 2009, which reflected, in part, the impact of the financial crisis on industrial output and lower overall energy use.
But this year's decline comes amid rising GDP, suggesting that environmental and cultural trends rather than economic factors alone could be behind the cut in emissions.
The EIA said U.S. cars and factories spewed 5.83 billion tons of carbon dioxide into the atmosphere in 2012, down from 6.06 billion in 2011, representing the lowest level for U.S. emissions since 1994.
Energy Department economist Perry Lindstrom said the reduction in carbon pollution was due to warm winter weather, more efficient cars because of new mileage requirements and an continuing shift from coal to natural gas to produce electricity.
"The emissions decline was the largest in a year with positive growth in per capita output and the only year to show a decline where per capita output increased 2 percent or more," the EIA said.
Energy consumption fell 2.4 percent from 2011 to 2012, while GDP rose 2.8 percent.
The coal shift was a big factor, as was a sluggish economic recovery, said Jay Apt, director of the Carnegie Mellon Electricity Industry Center. He said that in 1994 coal provided 52 percent of U.S. power and that now it is down to 37 percent. Burning coal produces far more carbon dioxide than burning natural gas.
Some earlier cuts in carbon pollution were mostly due to economic factors, like the 7.1 percent drop in 2009, Lindstrom said.
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, U.S. carbon pollution per GDP unit dropped by a record 6.5 percent, he said.
That shows the decrease was clearly not due to a recession, Lindstrom said.
In 2012 the United States 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."
Overall, the world is heading in the opposite direction. In 2011 the world's carbon dioxide emissions jumped 3 percent because of a large increase by China, the No. 1 carbon-polluting country. The U.S. is No. 2 in carbon emissions.
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