Diesel's promise as a future weapon against global warming is being undermined by revelations that suggest the government underestimated the toll today's diesels are exacting on air quality and human health. The findings include a discovery, confirmed by federal regulators, that as many as 1 million of the nation's heavy diesel trucks are emitting vastly higher levels of air pollution than previously believed, a consequence that officials say, of a technology "fix" that allegedly allows engine manufacturers to get around federal pollution limits.