NYT: The federal Clean Air Act, one of the most consequential pieces of environmental and health legislation in American history, celebrated its 40th birthday on Tuesday. The law, which has been attacked by business interests since its birth as overly costly and prescriptive, is under siege again as the Environmental Protection Agency begins to invoke the law to rein in the gases that contribute to global warming. Lisa P. Jackson, the E.P.A. administrator, delivered an impassioned defense …