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Ice-Age Theory Gets Dusted

A study of interplanetary dust has dealt a setback to an upstart proposal about what drives the ice ages. Calculations reported in today's Science suggest that dust does fall to Earth in a 100,000-year cycle that matches the ice ages. But the tiny amount of dust hitting Earth varies by only 2- to 3-fold--hardly enough to trigger anything so dramatic as an ice age.

The advance and retreat of ice sheets every 100,000 years is commonly attributed to cyclic variations in the shape of Earth's orbit, which slowly redistribute the sun's heat across the globe. But physicist Richard Muller of the University of California, Berkeley, has argued that another cycle, the changing tilt of the orbital plane, could dip Earth in and out of an interplanetary dust cloud that could somehow alter climate (Science, 11 July 1997, p. 183).

Estimating how much dust may fall to Earth is not easy: You have to know the orbital behavior of individual dust specks. So Stephen Kortenkamp of the Carnegie Institution of Washington's Department of Terrestrial Magnetism and Stanley Dermott of the University of Florida, Gainesville, set out to calculate dust orbits by simulating how planetary gravity, sunlight, and the solar wind disperse dust as it is generated by collisions among asteroids. Then they simulated how Earth gravitationally sweeps up the dust as its orbit takes it through the cloud.

The researchers found that the dust cloud spreads too far above and below the plane of the solar system for the tilting of Earth's orbit by a few degrees--Muller's mechanism--to change the amount of dust that reaches Earth. Their calculations showed that the dust does vary in time with the ice ages, but it does so because of a different orbital cycle: regular changes in the orbit's shape. When the orbit is more round, Earth moves more slowly through the cloud and its gravity can more easily pull in dust particles. When the ice ages were at their zenith, so was the amount of atmospheric dust, according to the calculations. But the increase was small--too small to make much difference to climate, says atmospheric physicist Donald Hunten of the University of Arizona in Tucson.

Muller isn't ready to give up on his dust mechanism, saying that although Kortenkamp and Dermott's work "is a great start in doing these sorts of calculations, there's still a lot of room for uncertainty."