By Valerie Volcovici

WASHINGTON (Reuters) - The United States will not be able to meet its goal of slashing greenhouse gases 17 percent by 2020 from a 2005 baseline without taking additional steps to target emissions, a new report found.

The economic downturn and an increase in supplies of cheap natural gas, which has displaced coal in some power plants, have slashed carbon emissions but are only temporary trends, according to the World Resources Institute, a think tank that focuses on global environmental issues

WRI recently shared its findings with administration officials.

After President Barack Obama pledged in his inaugural address last month to respond to the "threat of climate change," analysts have said the Environmental Protection Agency and other federal agencies will be charged with the task of carrying out that goal, in the absence of Congressional action.

"The administration has multiple ways to move forward with smart policies to reduce U.S. emissions. The best opportunity is to enact new standards for existing power plants, which represent one-third of all U.S. emissions," said Nicholas Bianco, a senior associate at WRI and the report's lead author.

Power plant standards are one of four key measures the Obama administration can take to curb greenhouse gases without legislation, said the report, which was entitled "Can we get there from here?"

Other measures include reducing hydroflourocarbons (HFCs), used mainly in cooling and refrigeration systems; curbing methane leaks from natural gas production; and boosting energy efficiency.

Non-energy greenhouse gases, such as methane and HFCs, are projected to rise 18 percent above 2005 levels by 2020 and 36 percent above that level by 2035 if no policies are implemented, the WRI said.

CLIMATE PLEDGE

The United States pledged to reduce its greenhouse gas emissions 17 percent below a 2005 baseline by 2020 at U.N. climate negotiations in Copenhagen in 2009, at a time when the White House assumed Congress might pass climate change legislation.

Partisan fights in Congress killed a measure in 2010, forcing Obama's administration to tap its existing authorities to regulate the heat-trapping gases.

The EPA is expected to take its first steps to regulate greenhouse gas emissions from existing power plants later this year, after it finalizes emissions performance standards for the construction of new power plants this April.

The federal Clean Air Act does not specify how the EPA must regulate existing sources but says states need to determine the "best system of emission reduction."

"States can play a very important role in achieving reductions," Bianco said on Wednesday at an event to launch the report, adding that they will be in charge of implementing national standards set by the EPA.

NATURAL GAS BOOM

While the EPA is widely expected to regulate existing power plants, it is not yet clear if it plans to regulate methane as a greenhouse gas from natural gas systems - a significant unknown in light of the boom in the natural gas production called hydraulic fracturing, or fracking.

U.S. natural gas production has increased by more than 25 percent between 2005 and 2011 due to fracking, causing a significant release of methane gas.

"There is a great deal of uncertainty with regard to emissions for natural gas systems. This means that the absolute magnitude of abatement opportunities is uncertain," the report said.

The report did not assess the impact of a pending U.S. decision on whether to approve the Keystone XL pipeline, which would transport heavy crude oil from the carbon-intensive oil sands in western Canada to Texas.

(Reporting By Valerie Volcovici; Editing by Bob Burgdorfer)