Life In The Greenhouse

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Then they loaded those estimates into the even larger, more powerful computer programs that attempt to model the planet's climate. Because no one climate model is considered definitive, they used seven different versions, which yielded 235 independent predictions of global temperature increase. That's where the range of 2.5[degrees]F to 10.4[degrees]F (1.4[degrees]C to 5.8[degrees]C) comes from.

The computer models were criticized in the past largely because the climate is so complex that the limited hardware and software of even a half-decade ago couldn't do an adequate simulation. Today's climate models, however, are able to take into account the heat-trapping effects not just of CO2 but also of other greenhouse gases, including methane. They can also factor in natural variations in the sun's energy and the effect of substances like dust from volcanic eruptions and particulate matter spewed from smokestacks.

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 shield, temperatures should go up even faster.

The models still aren't perfect. One major flaw, agree critics and champions alike, is that they don't adequately account for clouds. In a warmer world, more water will evaporate from the oceans and presumably form more clouds. If they are billowy cumulus clouds, they will tend to shade the planet and slow down warming; if they are high, feathery cirrus clouds, they will trap even more heat.

Research by M.I.T. atmospheric scientist Richard Lindzen suggests that warming will tend to 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.

It won't take the greatest extremes of warming to make life uncomfortable for large numbers of people. Even slightly higher temperatures in regions 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.