President Barack Obama signed an executive order on Monday requiring all arms of the federal government to reduce greenhouse gas output in an effort to curb climate change.

Each federal agency will have to set its own targets for reducing carbon emissions from its buildings, fleets and workers' commutes, and has 90 days to tell the White House how it plans to measure and limit emissions by 2020. Targets for employees' commutes and travel are due in June 2010.

"As the largest consumer of energy in the US 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 government mandate comes as the Obama administration begins to demand greenhouse gas reductions from car manufacturers and large industrial facilities. The White House is anxious to show some progress on emissions before more than 180 nations meet in Copenhagen in December to hammer out a new international treaty on global warming.

The president wants Congress to pass a bill setting mandatory limits, but this is unlikely to occur before the Copenhagen negotiations begin. The bill would require refineries, factories and power plants to reduce greenhouse gases by 20% by 2020, and roughly 80% by the middle of the century.

The latest order also compels agencies to curb petroleum use, conserve water and curtail waste, extending and expanding on an executive order issued by the former president, George Bush, in January 2007, which became law earlier this year.

Bush's order, unlike Obama's, did not require agencies to set emissions targets. Nevertheless, it was unclear how challenging the targets would be, or how much of a dent the proposed reductions would make in total US emissions.