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Why, then, do scientists trust them? How do they assess their accuracy? "You compare them with reality," explains Princeton Climatologist Syukuro Manabe. "How well do they reproduce the movement of the jet stream, the geographical and seasonal distribution of rainfall and temperature? You can also reproduce climate changes from the past. Eighteen thousand years ago, there was a massive continental ice sheet. Given the conditions that we know existed, can we reproduce accurately the distribution of sea-surface temperatures then? The answer is, We can do this very well. It gives you some confidence." Large-scale phenomena can be modeled more easily than those affecting small areas. So when it comes to the global warming produced by the greenhouse effect, for example, the outlines are predictable but the specifics are not. Says Manabe: "All we can say is that maybe the mid-continental U.S. becomes dryer."
A major drawback of computer models is that the various data do not necessarily behave as a system. Coaxing ocean currents to interact with the atmosphere is no small matter. For starters, oceans heat and cool far more slowly than the atmosphere. "We've had a hard time coupling the two systems," admits Manabe. "Even though the atmospheric model and ocean model work individually, when you put them together, you get crazy things happening. It's taken us 20 years to get them together, and we're still struggling."
+ Offsetting the obvious weaknesses of climate models, says Warren Washington, who developed the model now used at NCAR, is one significant advantage. "They are experimental tools that allow us to test our hypotheses," he says. "We can ask such questions as 'What happens when a big volcano like El Chichon goes off?' and 'How much will the earth warm up by 2030 if we continue to dump CO2 into the atmosphere?' "
Models can also describe the effects of climatic phenomena that have never been seen. In 1983 a group of scientists that included Cornell's Carl Sagan calculated what would happen if the U.S. and the Soviet Union fought a nuclear war. Their conclusion: the dust and smoke from burning cities would blot out enough sunlight to plunge the land into a "nuclear winter" that would devastate crops and lead to widespread starvation.
The problem with their model was that it ignored such key factors as winds, oceans and seasons. When NCAR's Stephen Schneider and Starley Thompson ran the numbers through their agency's three-dimensional computer model, they found that the winter would be more like a "nuclear autumn." Schneider says the less dramatic conclusion does not change the fact that "nuclear autumn is not going to be a nice picnic out there on the rocks watching the leaves change color." Despite the limitations and omissions of climate models, he argues, scientists cannot afford to ignore their predictions. They are, he concedes, a "dirty crystal ball. The question is, How long do you wait to clean the glass before you act on what you see inside?"