Climatologists regularly issue confident warnings about impending atmospheric disasters. The secret of their wizardry: sophisticated computer models, which are no more than mathematical representations of the world's climate and the conditions that scientists think may contribute to a specific phenomenon like, say, ozone depletion. Unfortunately, when all the variables are fed into the computer, the predictions can fail miserably to match reality.
Take the Antarctic ozone hole, for example. Before it was discovered, climate modelers trying to simulate ozone loss in the atmosphere had not yet factored in the presence of ice clouds in the Antarctic stratosphere. Thus their models failed to predict the existence of the ozone hole. After the hole was finally stumbled upon two years ago, Susan Solomon, a chemist at the National Oceanic and Atmospheric Administration in Boulder, and Rolando Garcia, of the National Center for Atmospheric Research, plugged more numbers into NCAR's computer model to account for the Antarctic ice clouds. Bang! The hole appeared.
Does that mean, as one critic put it, that models projecting climatic change are "just the opinion of their authors about how the world works"? Not necessarily. That the model eventually proved accurate, if only in hindsight, was a tribute to the powers of computer climate models -- and a demonstration of their shortcomings. The models attempt to reduce the earth's climate to a set of grids and numbers, then manipulate the numbers based on the physical laws of motion and thermodynamics. The sheer number of calculations involved is mind-boggling. A three-dimensional model, for example, requires more than 500 billion computations to simulate the world's climate over one year.
Not surprisingly, the earliest models in the 1960s were hopelessly simplistic. The earth's surface was often reduced to one continent with one ocean, fixed cloud cover and no seasons. But as computing power grew, so did the complexity of climate modeling. Continents were added. So were mountain ranges, deeper oceans and surface reflectivity.
Even so, climate modelers admit, building a completely realistic mock earth is an impossibly tall order. "You divide the world into a bunch of little boxes," explains Michael MacCracken, an atmospheric scientist at Lawrence Livermore National Laboratory. The size of the geographic box -- the degree of detail called for -- limits the model. Smaller grids dramatically increase the number-crunching power required. "The state of the art would be to get down to small areas so we can say what's going to happen in Omaha," says Livermore's Stanley Grotch. "The models just aren't that good yet."