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"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.
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While the more recent ice ages turned out to be more complicated than Milankovitch had envisioned, climate scientists credit his general theory with explaining the timing of the ice ages. "The Milankovitch orbital changes are the pacemaker determining when the large ice sheets grow and decay," says Imbrie, now at Brown University in Providence, R.l.

Muller and MacDonald, however thought they heard a different drummer setting the rhythm of the ice ages. When the two analyzed the pattern of glaciation for just the last million years, they uncovered something new. By reducing the ice-age variations to a series of frequencies, the scientists found that the dominant 100,000-year cycle was surprisingly exact. It formed a single sharp peak on their graph, with no hint of other frequencies playing a substantial role in the ice ages.

The standard model cannot explain this, they say, because orbital eccentricity varies in a complicated way, involving cycles with periods of 400,000 years and 125,000 years as well as the one close to 100,000. If eccentricity changes were driving the ice ages, all three separate beats should echo through the ice ages, but they do not.

While examining astronomical records, Muller happened on a different cycle that matched exclusively the 100,000-year pattern. The plane of Earth's orbit, he found, tilts gently with respect to the orbits of the other planets every 100,000 years. At times, Earth's orbit around the sun lines up with the plane of the solar system. At other times, Earth travels in a cockeyed orbit, its plane inclined 2.5 degrees.

Muller and MacDonald surmise that when Earth's orbit reaches a certain plane, the planet plows through an extrathick cloud of interplanetary dust. These small particles drift through the upper atmosphere and set in motion a series of climatic effects, perhaps blocking out sunlight, stimulating cloud growth, or weakening Earth's ozone layer. Such factors cool the planet enough to produce an ice age. Later, as the planet's orbit pulls out of the dust, the climate warms enough to fight back the ice.

Records of extraterrestrial dust offer some support for this hypothesis. Kenneth A. Farley of the California Institute of Technology in Pasadena has measured the variations in helium-3-a tracer for extraterrestrial dust-in seafloor sediments. His studies show that helium-3 values swing up and down over a 100,000-year-long cycle, as Muller and MacDonald predict.

What's more, the amount of helium-3 increased markedly between 2 million and 1 million years ago. This may explain why the 100,000-year climate cycle did not dominate before then, says Farley.

One interpretation of these findings is that large asteroids collided around a million years ago, creating a rich supply of dust that filled the inner solar system. Prior to that time, the ice ages were governed by the regular Milankovitch factors of orbital tilt and precession. When the dust appeared, it overwhelmed the other factors, making ice ages larger and longer.

The dust data do not totally support Muller and MacDonald's idea, however. "These variations in dust amount are quite small; at most, they go up and down by a factor of 3 to 5. I'm skeptical that such variations could change the climate. We're only looking at a little bit of extraterrestrial dust," says Farley.

Even more troubling is the question of how the dust alters climate. While Muller and MacDonald have general ideas, they lack a solid hypothesis to explain how adding dust to the upper atmosphere could plunge the planet into an ice age. "That is what we're most severely criticized for. We do a lot of hand waving at this point. There are many possibilities, but we don't have a good mechanism," says Muller.

When Muller and MacDonald came up with their dust theory in 1993, they had a hard time getting climate scientists to listen. Last year, Wallace S. Broecker of the LamontDoherty Earth Observatory in Palisades, N.Y., invited Muller to present his case to the top researchers interested in the ice ages. Broecker dubbed the meeting the "Mullerfest," and the discussions continued for weeks via Email.

Muller scored the most points at the meeting when he attacked a standard technique, called tuning, that oceanographers use for dating layers in sediment cores. The task of dating these strata is difficult because sediments may accumulate more quickly during some eras and more slowly in others. To tell the age of layers between known benchmarks, researchers often use the Milankovitch orbital cycles to tune the sediment record: They assume that ice volume should vary with the orbital cycles, then line up the wiggles in the sediment record with ups and downs in the astronomical record.

"This whole tuning procedure, which is used extensively, has elements of circular reasoning in it," says Muller. He argues that tuning can artificially make the sediment record support the Milankovitch theory.

Muller's criticisms hit home with many researchers. "He scared the hell out of them, and they deserved it," says Broecker.

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Most climate researchers do not think that extraterrestrial dust will provide the answer, but Imbrie, a longtime supporter of the Milankovitch hypothesis, says that Muller and MacDonald's concept may have some merit. It is possible that Earth's climate could respond to the unsteady orbital plane as well as to the kinds of astronomical twitches that preoccupied Milankovitch, he says. "These are not mutually exclusive ideas. Nature doesn't always follow a neat little paradigm."