WASHINGTON (Reuters) - The recession has slashed U.S. output of planet warming gases and puts the country on track to reach President Barack Obama’s short-term emissions goal, but cutting the pollution further will take more effort as the economy recovers.

A car drives past the Helanshan Wind Power Plant in Wuzhong, Ningxia province, September 23, 2009.REUTERS/Stringer

“Losing weight by starving is different than shedding pounds through exercise,” said Kevin Book, an analyst at ClearView Energy Partners, LLC.

He said as the economy recovers electricity demand should rise, pushing up emissions from that sector. That will require the world’s second largest emitter of greenhouse gases after China to move faster to low-carbon sources like renewable energy if Obama’s short-term goal is to be met, he said.

Obama is expected to pledge next week at a U.N. climate meeting in Copenhagen that the United States will cut output of gases blamed for warming the planet, including carbon dioxide, roughly 17 percent below 2005 levels by 2020.

On Tuesday the Energy Information Administration said in a monthly outlook that U.S. carbon dioxide output in 2009 will fall about 6.1 percent to 5.45 billion tonnes as the recession cuts demand for coal used to generate electricity.

That was about 8.9 percent below the 2005 level of 5.98 billion tonnes, putting the U.S. on track, at least for now, to reach Obama’s goal. Carbon dioxide emissions represent about 80 percent of overall U.S. greenhouse gas output.

A small increase in emissions could begin to show as early as next year, however. In 2010, the EIA said, U.S. carbon emissions are expected to rise 1.5 percent to 5.53 billion tonnes as the economy recovers.

Coal consumption by U.S. electricity generators fell nearly 12 percent for the first nine months of 2009, as the recession cut power demand. But emissions also were reduced by greater generation from lower-emission sources such as natural gas, hydropower and wind power, the EIA said.

Analysts said the United States need to speed up its moves toward a low-carbon economy.

“We have a drop in emissions as a result of the recession, but that does not mean that future cuts will come easily,” said Tim Cheung, an analyst at New Energy Finance in New York.

“Those will have to come from somewhere, such as the electricity or industrial sectors.”

U.S. emissions jumped steadily from 1990 to just before the recession. In May the EIA said U.S. energy-related carbon dioxide output had jumped nearly 16 percent from 1990 to 2008.