Believe it or not, this was an open question under the Bush Administration. In fact, during the Bush Administration, the EPA did nothing to regulate. It was so bad that several states (like Massachusetts) and environmental groups tried to sue the EPA to do its job. The EPA at the time argued that it did not have the authority under the Clean Air Act do regulate greenhouse gases.
The case went all the way up to the U.S. Supreme Court, which held in 2007 that the Clean Air Act did oversee greenhouse gases provided that the EPA concludes that greenhouse gases do endanger human health.
A new administration has taken the shackles off of the EPA, and today they are planning to announce that, in their view, greenhouse gases harm humans and therefore the EPA will start regulating them.
Speaking of the environment, all eyes are on Copenhagen this week. Kyoto didn't work out so well; the new agreement (one hopes) that will incolve China and other developing countries might be better. Krugman is optimistic; I am not.
UPDATE: Matt Yglesius writes that business groups are freaking out over the EPA's newfound commitment to control carbon emissions, adding that the EPA's power to seriously make effectatious changes in this area is overblown.