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The climate, it is achanging.

Ever since scientists realized that the earth's history is punctuated by fairly regular episodes of glaciation, they have tried to untangle the multitude of forces that could trigger climate change. One idea, called the Milankovitch theory, pins the responsibility on periodic changes in the earth's orbit, which alter the amount of solar radiation received by the planet; variations in the eccentricity, tilt and precession of the earth are thought to drive climate cycles at intervals of roughly 100,000, 1,000 and 23,000 years. The Milankovitch theory has had a controversial past, but within the last decade it has gained wide acceptance. Now, a number of papers further assure the theory a place in the club of forces that govern the earth's climate.

In the September GEOPHYSICAL RESEARCH LETTERS, Atsuyuki Yamamoto at the Osaka Electro-Communication University in Osaka, Japan, and colleagues discovered that the size of mineral grains that had been deposited over the last 260,000 years in Lake Biwa, one of the oldest lakes in the world, rose and fell at the same frequencies as Milankovitch cycles. According to the researchers, the grain size reflects erosion rates, which in turn depend on climate changes such as rainfall, temperature and wind; high precipitation rates, for example, increase erosion and the amount of coarse grains carried to the lake.

In another paper in the Sept. 12 NATURE, John Kutzbach of the University of Wisconsin in Madison and F. Alayne Street-PErrott of Oxford University in England conclude that Milankovitch cycles have been largely responsible for changes in the levels of tropical lakes in the Northern Hemisphere. The researchers used a computer model of the atmosphere to simulate how variations in the earth's tilt changed January and July monsoon circulation and rainfall pattersn over the last 18,000 years. In general, their model agreed with long-term changes observed in the geologic record; shorter fluctuations in lake levels lasting 1,000 years or so still must be explained. Complementing this work, Edward Pokras and Alan Mix of Lamont-Doherty Geological Observatory in Palisades, N.Y., recently found Milankovitch rhythms in the abundance of land-based dust in deep-sea cores taken off the coast of Africa; peaks in such dust correspond to times of arid land conditions.
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Title Annotation:Malankovitch theory puts responsibility on periodic changes in earth's orbit
Publication:Science News
Date:Oct 19, 1985
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