Deciding how to evaluate a cap-and-trade programme raises some thorny questions, says David Goldston.

Despite huge obstacles, political forces in Washington may finally get greenhouse-gas legislation moving.

Last month, The Washington Post reported that President George W. Bush had personally intervened to weaken new regulations to control smog just as they were about to be announced by the Environmental Protection Agency (EPA). In response, advocates of tighter standards predictably charged that the president had overturned a scientific judgement. Carol Browner, who headed the EPA under President Bill Clinton, put the matter starkly, telling the Post that the Clean Air Act creates "a moral and ethical commitment that we're going to let the science tell us what to do'.