The Big Chill: Does Dust Drive Earth's Ice Ages?

Article excerpt

Earlier this year, as millions of people gazed upward to watch Comet Hale-Bopp sail through the heavens, some observers may have caught specks of space dirt in their eyes. More than 40,000 tons of extremely fine extraterrestrial dust rains down on the planet annually, gathering imperceptibly on windowsills, furniture, poodles, people, and every other available object.

In the long run, these interplanetary motes may have profound consequences for Earth and its inhabitants. Two scientists propose, in a radical new theory, that dust from space caused the last 10 ice ages, which have gripped the planet like recurrent cases of the flu over the last million years.

"I suspect that to really understand the climate in general, we're going to have to take into account the presence of this extraterrestrial dust, which right now is being ignored," says Richard A. Muller, a physicist at the University of California, Berkeley who collaborated with Gordon J. MacDonald of the International Institute for Applied Systems Analysis in Laxenburg, Austria.

In proposing their hypothesis, Muller and MacDonald have, in effect, picked a fight with the scientific establishment. The theory challenges the accepted explanation of the ice ages, first proposed by Scotsman James Croll in the late 1800s and then expanded by Serbian mathematician Milutin Milankovitch early in this century. The so-called Milankovitch theory holds that periodic flutters in Earth's orbit drive the ice ages by altering the strength of sunlight hitting the north.

"We have shown there is a very serious problem with Milankovitch. I think it's the most serious challenge that has ever been mounted. It's not easy for them to handle this," says Muller of those who hold to the old model.

Muller's strong rhetoric and his unorthodox views have won few converts among climate scientists who specialize in the ice ages. At the same time, opponents arguing from the standard Milankovitch theory cannot easily dismiss his points. This leaves a major gap in knowledge about both how the climate worked in the past and how it may behave in the future.

The new theory focuses on a climatic conundrum called the 100,000-year problem: Why have the last 10 ice ages struck every 100 millennia or so? Muller and MacDonald claim that the standard theory fails to account for this behavior, whereas extraterrestrial dust does. They present their argument in the July 11 Science.

To understand the disagreement, it's important to go back to 1914, when strife in the Balkans dragged Europe into what would become World War I. Milankovitch, who was held as a prisoner of war by the Austro-Hungarian army that year, escaped from the surrounding chaos by immersing himself in the predictable and stately wiggles of Earth's orbit. He isolated two orbital effects that he thought had a major influence on climate.

The first was the tilt of Earth's rotation axis. Currently 23.5 [degrees], the axis bobs toward the vertical and then dips toward the horizontal every 41,000 years. During times of the greatest dip, the amount of sunlight falling on the North and South Poles increases during summer and decreases during winter, causing the seasons to become more extreme.

The second factor, orbital precession, describes the time of year in which Earth comes closest to the sun. Every 23,000 years, the planet's orbit carries it nearest the sun during the Northern Hemisphere's summer solstice, thereby strengthening the sunlight bathing the Arctic.

Milankovitch reasoned that the intensity of summer sunlight hitting the far north is critical to the creation of an ice age. When Earth's orbital cycles conspire to weaken summer sunlight near the Arctic Circle, snow can accumulate from one winter to the next. As the ice sheet spreads, its surface reflects sunlight back into space and cools the climate even further.

In 1976, a trio of oceanographers discovered support for Milankovitch's hypothesis in cores of mud pulled from the seafloor. …