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Computers: window onto past climate

For climatologists studying ice ages, a computer is like a window in time. The view may be clouded by inadequacies of climatic theory and distorted by simplifications in representing those theories in a computer. Yet it provides insights into what may have happened many thousands, or even millions, of years ago.

One of the more ambitious of these numerical experiments is being run at the Pacific Institute of Geography of the Academy of Sciences, at Vladivostok in the USSR. Already it has shown the crucial role that continental ice sheets may play. The great mass of the ice, along with the massive heat storage capacity of the ocean, sets up climatic modes that oscillate between ice ages and warm interglacial periods, such as the present.

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V. Ya. sergin, chief of the Laboratory for Mathematical Modeling of the Climate at the institute, points out in the journal Science that the studies make no assumption as to the cause of the cooling that sets up ice sheets in the first place. This means that the laboratory's mathematical model of the glacier-ocean-atmosphere (GOA) global system should be valid whatever the ultimate cause of climatic cooling may be.

For much of its history, Earth has been relatively warm. The last such warm epoch, beginning 180 million years ago, featured tropical and subtropical conditions in middle latitudes and temperate weather toward the poles. Then about 30 million years ago a cooling set in that led to the present glacial-interglacial regime.

The GOA model indicates how to translate such a general cooling into a glacial climate. As Earth's temperature drops over tens of million of years, a point is reached a few million years ago at which continental ice sheets appear and oscillations between ice ages and interglacials set in.

Sergin explains that this shows the importance of what you could call thermal "inertia" in the climatic system. As long as there is only one element with massive heat capacity and slow response time -- namely the ocean -- climate is stable. As soon as a second comparable element -- ice sheets -- appears, the climate begins to oscillate.

A number of such computer studies are under way in various countries, each helping to elucidate different aspects of climatic change. For example, a recent study by Tzvi Gal-Chen at the US National Center for Atmospheric Research indicates that a shift from forests to tundra at higher latitudes may be a key influence in initiating a specific ice age. It suggests that Earth may be moving into such an icea age in another thousand years.

But the entire planet is unlikely to be ice-covered. Sergin says, according to his studies, that complete glaciation of Earth is impossible. And Gal-Chen points out that man-made environmental change and pollution may overshadow natural forces and become the biggest climatic concern of all.Scientists have only begun to figure out how to take account of that in their computer studies.