The agency won't rush in to control carbon emissions, but some wonder if Johnson is being methodical, or just stalling.

Last spring, the Supreme Court ruled that the EPA had responsibility to determine if emissions from cars and trucks were harmful to human health and the environment. But since then, very little has changed in the agency's approach to carbon emission control.

In a letter to House and Senate Environmental Committees, EPA Administrator Steven L. Johnson wrote, "Rather than rushing to judgment on a single issue, this approach allows us to examine all the potential effects of a decision with the benefit of the public insight."

The apparently slow pace has irked some on the Hill. "This is the latest quack from a lame-duck EPA intent on running out the clock… without doing a thing to combat global warming," said Rep. Edward Markey, D-Mass., and chairman of the House Select Committee on Energy Independence and Global Warming.

Johnson said he needed to gather as much information as possible before rushing into a decision, because in the event they find that the emissions cause harm, they would be obligated to regulate other sources of such gases as well. Such action would have to be coordinated with other agencies, and would impact many other industries, all of which must be allowed to comment.