Computer study shows that changes in Earth's orbit affect monsoons

Those vital monsoon rains, on which the economies of large parts of Asia and Africa depend, may in turn be linked to the details of Earth's orbit. Climatologists have suspected that slight changes in that orbit can make the difference between more abundant rains and the somewhat drier conditions known today. Now, in what he calls a climatic ''sensitivity experiment,'' meteorologist John E. Kutzbach has made a computer study which tends to confirm these suspicions.

At the University of Wisconsin's Center for Climatic Research, which he heads , Kutzbach has programmed a mathematical model of climatic conditions some 9,000 years ago. At that time, the perihelion of Earth's orbit - the point closest to the sun - was reached on July 30 rather than Jan. 3. Earth's axis was tilted 24. 23 degrees with respect to its orbital plane instead of 23.45 degrees. And the eccentricity of the orbit - a measure of how much it departs from a circle - was 0.0193 rather than 0.0167.

In other words, Earth came closest to the sun in summer rather than winter. The sun swung farther north and south of the equator with the seasons. And a more ellipitical orbit carried our planet farther from and closer to the sun at the extreme orbital positions than happens today.

Running his simulation program on the computer at the National Center for Atmospheric Research at Boulder, Colo., Kutzbach found that these minor orbital changes made a major difference in monsoon weather. An average 6 percent greater solar energy income from June to August (with a compensating deficit in winter) was reflected in stronger land-sea temperature contrasts. Kutzbach explains that this ''generates a vast continent-scale sea breeze, with air moving from the 'cool' ocean to the 'warm' land. This stronger monsoonal circulation brings more moisture from the oceans to the land masses and results in heavier monsoon rains.''

In a report of his experiment in Science, Kutzbach notes that his results agree with geological and oceanographic evidence of greater rainfall and substantially larger lakes in Africa, the Middle East, and other monsoonal areas. This period of enlarged lakes ran from 10,000 to 5,000 years ago. Likewise, the increased summer heating peaked around 10,000 to 9,000 years ago and had returned close to its present level by 5,000 years ago.

Kutzbach considers his model preliminary. It simulates ice sheets poorly and neglects feedbacks from land surface conditions and carbon dioxide in the atmosphere. He urges followup studies with more sophisticated models.

Nevertheless, his study does add to understanding of how Earth's orbit affects climate. Thus it contributes to the building of computer models that can begin to predict future climate changes.