World Climate, Past and Future: A Desktop View

Reid Bryson likes to keep things simple.

He doesn't use elaborate programs that take many hours of expensive supercomputer time to model climate. He runs his simulations on a desktop machine in a matter of seconds in his office at the University of Wisconsin's Madison campus. His computer runs cost virtually nothing.

Dr. Bryson's approach reflects a fundamental difference in how climate is viewed.

Most climate models work by applying basic physical laws to small parcels of air at thousands of points throughout the atmosphere. Scientists calculate how each parcel will move and change during a short period of time. Then, starting with that new state of the atmosphere, they run the calculation again. This is done millions of times to construct how weather and climate develop over time on both a global and regional scale.

Bryson calls this method of building up climate from a multitude of small pieces the ``microphysical'' approach. For his own purposes, however, he prefers to start with the big picture, using what he calls the ``macrophysical'' approach. He starts with the major influences that drive the climate system as a whole.

These include changes in solar heating as Earth's orbit changes, the cooling effects of volcanic dust, and changes in the planet's ice volume. He then can simulate such major weather-determining factors as the position of jet-stream winds or of the subtropical high-pressure areas - including the famous ``Bermuda High.'' He also models the seasonal monsoons of West Africa and India.

These large-scale features enable Bryson to estimate climatic conditions for specific regions over thousands of years in the past with a resolution of as little as 200 years. The chart shown at right is his estimate of the peak flow of the Nile River at Aswan based on simulating precipitation at the headwaters of the Blue and White Niles.

To check out such an estimate, Bryson looks to the archaeological record of human occupation on the theory that, where there was little water, there were few people. As the chart shows, Egypt's ``dark ages'' coincided with times of relative drought.

Bryson is currently making such studies at a number of places around the world. He calls his work ``a first guess'' at how to study climate in this way rather than being a ``better'' approach than his colleagues' elaborate computer simulations. ``The ideal climate model has not yet been devised,'' he says.