That is no mean feat because climate -- whether it is local, regional or global -- depends on a host of variables (defined) that do not lend themselves easily to computer simulation. To reasonably mimic climate, its components -- oceans, land, ice, atmosphere, biosphere -- must all be r educed to numbers that can be fed into a computer and processed in a logical way.

"Climate models should take all of those things into account," explains James Hurrell, a scientist who works with such models at the National Center for Atmospheric Research (NCAR) in Boulder, Colo. "But we're just getting to the stage where we can put all those things in."

Even so, the best the most comprehensive and sophisticated models can do is approximate the real world, says Hurrell. That's because it is extremely difficult -- some would say impossible -- to reduce nature to a number.

A cloudy crystal ball
Take clouds, for example. As one of the many variables that need to be plugged into a comprehensive, state-of-the-art model like the one used at NCAR, the role of clouds must be fully understood for the mod el to spit out an accurate snapshot of climate. But clouds, says Hurrell, and just how much solar radiation they soak up, is in dispute.

Recently, two groups of scientists, working independently and using the results of field studies of ocean and ground temperatures, arrived at the conclusion that clouds absorb a lot more of the energy from the sun than previously believed. If they are co rrect, then the people who build the general circulation models must alter them accordingly.

But not every scientist is convinced that those results are right, says Hurrell. "The results of the field experiments are new and still the subject of considerable scientific debate. There is no consensus on what the underlying physical mechanism is f or the extra absorption. Without a physical mechanism, it is impossible to model it in a physically meaningful way."

Indeed, the major weakness of the general circulation model is the coarseness with which it models the various components of climate. But there are ways to test the models, and refinements are made based on those tests and on the ever-increasing store o f climate knowledge.

One way to test the model, says Hurrell, is to ask it to model climate for a particular time, say 1990 to 1995, and then compare the model's output with what the climate was really like. Another way is to look into the more distant past and compare the results with historical evidence or other physical evidence of climate such as ice cores, paleontological evidence, or geochemical clues to past climate.

"From various historical records like geologic records and ice cores, we know what climate was like thousands and thousands of years go," says Hurrell. "You can build your climate model to simulate ancient climates. You can move things like continents ar ound, or change the amount of solar radiation. That's yet another way to test the veracity of the models."

Is it hot in here, or is it just me?
Clearly, the most controversial uses of the climate model are attempts to forecast climate. This is of special interest to many people because of the idea that humans, through the burning of fossil fuels and other activities, are altering the Earth's climate. Gases and other particles pumped into the atmosphere by humans, some scientists argue, are altering the climate system's heat storage through the so-called greenhouse effect. Indeed, things do seem to be warming up as the Earth's average surface temperature climbed to a record high in 1995, continuing a pattern of hotter mean temperatures for our planet.

Because of these concerns and patterns, many people would like to look ahead and project what climate might be like. The general circulation models are the tools to do that, but they are an imperfect crystal ball, Hurrell admits.

"We are getting much closer to an ability to (look ahead)," he says. "These models are really just tools, and if you interpret what they're telling you in an appropriate manner, they can give you some idea of what's in store. They are certainly helping us learn the physics of what might happen in the future."

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