U.S. emissions of the main greenhouse gas rebounded nearly 4 percent last year as factories ran harder while the economy recovered and as consumers boosted air conditioning during the hot summer, the government said today.

U.S. carbon dioxide emissions from the burning of oil, coal, and natural gas, which accounts for about 80 percent of U.S. overall greenhouse gas output, rose 213 million tonnes, or 3.9 percent, last year, the Energy Information Administration said.

It was the first rise in the emissions blamed for global warming since the recession pushed them down in the previous two years and the largest increase since 1988.

"In 2010 manufacturing industries showed a strong recovery from the 2008-9 recession and energy-intensive manufacturing experienced high growth as well," which contributed to the rise, the EIA said in an annual report.

The emissions boost in 2010 "likely does not signal a new trend in emissions growth," as the total amount emitted was below levels hit in 2007 and 2005, the EIA said.

In fact, the United States has made some progress on reducing emissions as many utilities have been switching to burning natural gas, which releases less CO2 than coal.

In addition, U.S. CO2 emissions from vehicles could ease in coming years as the Obama administration has already passed efficiency standards and is considering making them tougher.

Still, last year's rise shows that U.S. emissions are now mostly dictated by changes in the economy. Without a comprehensive energy policy, President Barack Obama's goal of reducing greenhouse gas emissions about 17 percent by 2020 from the 2005 levels--which he made at an international conference on global warming--remains a challenge.

"The fact that emissions fell in 2008 and 2009 gave some people false comfort," said Michael Levi, an energy and environment expert at the Council on Foreign Relations.

"The reality remains that you're not going to hit the targets Obama has set out without concerted policy to achieve them."