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Like any other area of science, the case for human-induced global warming has uncertainties--and like many pro-business lobbyists, President Bush has proclaimed those uncertainties a reason to study the problem further rather than act. But while the evidence is circumstantial, it is powerful, thanks to the IPCC's painstaking research. The U.N.-sponsored group was organized in the late 1980s. Its mission: to sift through climate-related studies from a dozen different fields and integrate them into a coherent picture. "It isn't just the work of a few green people," says Sir John Houghton...
Measuring the warming that has already taken place is relatively simple; the trick is unraveling the causes and projecting what will happen over the next century. To do that, IPCC scientists fed a wide range of scenarios involving varying estimates of population and economic growth, changes in technology and other factors into computers. That process gave them about 35 estimates, ranging from 6 billion to 35 billion tons, of how much excess carbon dioxide will enter the atmosphere...
That is one reason the latest IPCC predictions for temperature increase are higher than they were five years ago. Back in the mid-1990s, climate models didn't include the effects of the El Chichon and Mount Pinatubo volcanic eruptions, which threw enough dust into the air to block out some sunlight and slow down the rate of warming. That effect has dissipated, and the heating should start to accelerate. Moreover, the IPCC noted, many countries have begun to reduce their emissions of sulfur dioxide in order to fight acid rain. But sulfur dioxide particles, too, reflect sunlight; without this...
...make cirrus clouds go away. Another critic, John Christy of the University of Alabama in Huntsville, says that while the models reproduce the current climate in a general way, they fail to get right the amount of warming at different levels in the atmosphere. Neither Lindzen nor Christy (both IPCC authors) doubts, however, that humans are influencing the climate. But they question how much--and how high temperatures will go. Both scientists are distressed that only the most extreme scenarios, based on huge population growth and the maximum use of dirty fuels like coal, have made headlines...
...that are already drought- or flood-prone would exacerbate those conditions. In temperate zones, warmth and increased CO2 would make some crops flourish--at first. But beyond 3[degrees] of warming, says Bill Easterling, a professor of geography and agronomy at Penn State and a lead author of the IPCC report, "there would be a dramatic turning point. U.S. crop yields would start to decline rapidly." In the tropics, where crops are already at the limit of their temperature range, the decrease would start right away...