WASHINGTON – President Barack Obama is putting the federal government on a greenhouse-gas diet.

In an executive order signed Monday, Obama directed all agencies to set the first-ever targets for reducing climate-altering pollution from government buildings, fleets and federal workers’ commutes.

The agencies will have 90 days to tell the White House how much they plan to measure and reduce greenhouse gases from buildings and vehicles by 2020. Targets for employees’ commutes and travel will be due June 2010.

“As the largest consumer of energy in the U.S. economy, the federal government can and should lead by example when it comes to creating innovative ways to reduce greenhouse gas emissions,” Obama said in a statement.

The order also compels agencies to curb petroleum use, conserve water and curtail waste – extending and expanding on an executive order issued by then-President George W. Bush in January 2007 that became law earlier this year.

Bush’s order, unlike Obama’s, did not require agencies to set emissions targets.

The government mandate comes as the Obama administration takes steps to require automakers and large industrial facilities to cut greenhouse gas emissions. It is also a way for the White House to show much-needed progress toward reducing greenhouse gases before more than 180 nations meet in Copenhagen, Denmark, in December to hammer out a new international treaty to slow global warming.

The president wants Congress to pass a bill setting mandatory limits, but its passage is unlikely in the Senate before negotiations begin. The Senate bill would require refineries, factories and power plants to reduce greenhouse gases by 20 percent by 2020 and roughly 80 percent by mid-century.

It was unclear how deep the targets would be for the federal government, or how much of a dent it would make in total U.S. greenhouse gas emissions.

The government operates nearly 500,000 buildings, more than 600,000 cars and purchases more than $500 billion per year in goods and services.

The White House said Monday that the actions taken would benefit the taxpayer by reducing energy costs.