Consider the case closed on global warming. The assessment released this month by the U.N.'s Intergovernmental Panel on Climate Change (IPCC) concluded for the first time that evidence of the earth's rising temperatures was "unequivocal" and that this warming was more than 90% likely to be the result of human activity. The report's authors--some 600 scientists from 40 countries--also noted that we are locked into more climate change. Even if all greenhouse-gas emissions miraculously ended today, the earth would continue to warm through the rest of the century because of the amount of carbon we have already added to the atmosphere. Now that the IPCC has fingered the culprits behind global warming, the question becomes how will world leaders respond. And the policy debates are shaping up to be even more contentious than the scientific ones. China, which could become the world's largest carbon emitter by 2010, reiterated on Feb. 6 that it wants First World polluters to take primary responsibility for cutting emissions--a stance that doesn't sit well with the U.S., which refuses to give large developing countries like China a pass. But as the IPCC report shows, the price of inaction will be enormous for all of us.
Depending on greenhouse-gas emissions, global temperatures will probably rise between about 2˚F and 12˚F by 2100. If carbon levels double from the preindustrial norm--which many experts deem very likely--the IPCC predicts a warming of about 5˚F as well as longer and more intense heat waves.
The report predicts sea levels will rise 7 in. to 23 in. Bad, but not too dire, right? But this assessment doesn't account for the possibility of accelerated glacial melting, which a recent Science study estimated could raise seas more than 4 ft. by the end of the century--enough to swamp low-lying coastal cities.
The IPCC said it is "more likely than not" that man-made factors are responsible for the increased intensity of tropical storms--a much stronger statement than scientists have made in the past. For instance, the World Meteorological Organization this fall said it couldn't link stronger storms to global warming.
Surprisingly, the report suggests that by reflecting solar energy, visible airborne particles like sulfates from coal-burning power plants could actually have a cooling effect. But while better filters are reducing pollution's protective haze, invisible--and harmful--carbon emissions are still on the rise.