Climate is a complex, non-linear system with many variables that are not well understood. Interactions and feedbacks affect many aspects of climate, which scientists attempt to model mathematically.

Take clouds. These fluffy balls of condensation can be thick or thin, broad or narrow, towering thunderheads or wispy trails. They also are among the most complex and difficult problems facing climate modelers. Everyone in the field knows clouds impact climate and weather, but no one is certain how.

Clouds both warm and cool the planet, for instance. When heat reaches the Earth's surface, clouds tend to act like a blanket, reflecting the heat back to the surface as it tries to escape into space. Water and ice droplets, of which clouds are made, are potent greenhouse components.

On the other hand, clouds help to prevent the sun's heat from reaching Earth's surface in the first place. So they wield a cooling effect. Which effect is dominant? No one can say for sure.

There also are feedbacks. Will a changing climate result in thicker or thinner cloud cover? What impact will that have on the changing climate in return - intensifying or mitigating? Once again, no one can say for sure, and models are very poor at dealing with these issues.

Clouds are very intangible things, John Mitchell, chief scientist at the meteorological office of the United Kingdom in Exeter, told United Press International. It is hard to say even where they begin or end. Clouds are the result of all the other processes, and although these processes show up in clouds, it affects all the other processes.

In a climate model, scientist attempt to use the known physical processes and interactions of the atmosphere to create a virtual climate, which - if the model works - mimics the dynamics of the real thing.

Scientists usually test their models by running them backward - seeing if they will recreate the known climate of the past before they are used in a predictive way.

The National Center for Atmospheric Research in Boulder recently released the latest version of its Community Climate System Model - CCSM3. Its predecessors have been among several mathematical models operated by supercomputers to study climate. The newest version exemplifies the promise and problems that face people who try to predict Earth's future.

The many criticisms leveled at computer climate models notwithstanding, it is important to remember the science still is in its infancy. Even a couple of decades ago, modeling even the simplest climate would have been impossible.

CCSMs 1 and 2 used data from thousands of points in the atmosphere, oceans, sea ice and land surface, applying the information to mathematical formulas that calculate the known physical processes and interactions of the atmosphere.

CCSM3 requires about 3 trillion calculations to simulate a single day's worth of global climate, and can run about four years of climate in a day. Modeling, say, a thousand years of climate history can take quite a while, even though it is running on the world's fourth largest and fastest computer system.

This model makes substantial improvements in simulating atmospheric, oceanic and terrestrial processes, said William Collins, the NCAR scientist who oversaw the CCSM3's development. It has done remarkably well in reproducing the climate of the last century, and we're now ready to begin using it to study the climate of the next century.

Looking at a global climate map produced by CCSM3, some of the correspondences are remarkable. One of the model's main triumphs is its complex calculations have predicted a fairly close correlation of the extent of Arctic sea ice - a major terrestrial climate feature but one with which other models have had trouble.

It also shows a nice band of heat along the equator in the tropics, right where it should be - although according to CCSM3 the band is forked in the tropical Pacific, while the actual data show a single band.

CCSM3 produces clouds, too. Alas, the clouds are too thick or too thin or not in the right places, said Philip Merilees, an NCAR scientist who presented a paper on the model at the recent CLIVAR 2004 conference on climate change in Baltimore.

Because of its inconsistencies, there is reason for caution about CCSM3's disturbing news about the future. The model estimates global average temperature will warm by 4.7 degrees Fahrenheit (2.6 degrees Celsius) in response to a doubling of atmospheric carbon dioxide. This is considerably more than CCSM2 had estimated.

Computer power remains a limitation of climate models. An unsatisfactory alternative is to increase the resolution of the model by collecting more data at points closer together. The problem is more data requires either more computing power or more time.

Certain properties of the climate might be predictable (but the) challenge is to derive that predictability, said Jagadish Shukla, director of the Center for Ocean-Land-Atmosphere studies at George Mason University in Fairfax, Va.

Estimates of climate predictability are still model dependent, he added.

A further consideration is no one really cares about global climate. What people want to know is what will happen in their area - the drought in the American Southwest, the heat waves in Europe, or the coastal flooding in Bangladesh.

Models have to do things right locally, said Seigfried Schubert, a meteorologist at NASA Goddard Space Flight Center in Greenbelt, Md. How the large-scale models interact with local phenomenon is a challenge for models.

A more clinical description of the difficulties appear in Climate Models: A Primer, a recent report by William O'Keefe and Jeff Kueter of the George C. Marshall Institute.

Climate models are a mathematical representation of the physical and chemical processes occurring the climate system, they wrote. Because our understanding of these processes is incomplete, current climate models do no accurately represent the climate system.

The models are useful to some extent, O'Keefe and Kueter wrote.

Based on our understanding of the climate of the last century, it is reasonable to assume that climate may be predictable for a few decades into the future, but unpredictable centuries in the future.

In other words, though the long-term climate horizon may remain clouded, there is hope this blossoming science has begun to give a glimpse of the near future.

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