James Hansen, a respected global warming expert and director of NASA’s Goddard Institute for Space Studies, has spent nearly two decades urging countries to reduce their emissions of carbon dioxide (CO2) to help curb climate change, but now he’s started singing a different tune. In research to be published this week in the Proceedings of the National Academy of Sciences, Hansen and colleagues say the present emphasis on CO2, by far the most abundant greenhouse gas, may be misplaced, and a faster and less costly approach to battling climate change might be to cut other potent, heat-trapping emissions first — methane, chlorofluorocarbons, diesel exhaust, and coal soot. Some climatologists worry that Hansen’s study could be misused by global warming skeptics and opponents of the Kyoto climate change treaty. Hansen stresses that he still believes human-caused global warming is underway, and that CO2 emissions will still have to be cut over the coming century.