Were the ice ages triggered by an influx of extraterrestrial dust or meteoroids hitting Earth's upper atmosphere? A controversial and still unproven new theory recently described by Muller and MacDonald  in Nature links the 100,000 year glacial cycle with changes in Earth's orbital inclination relative to the plane of the solar system. And, say the theory developers, the only logical mechanism they can find for the connection is increasing amounts of extraterrestrial material entering the atmosphere whenever Earth's orbit sweeps through the solar plane. Climate researchers are just beginning to test the model's predictions.
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