GLACIAL CYCLES AND ASTRONOMICAL FORCING

Narrow spectral features in ocean sediment records offer strong eviden ce that the cycles of glaciation were driven by astronomical forces. T wo million years ago, the cycles match the 41,000-year period of Earth 's obliquity. This supports the Croll/Milankovitch theory, which attri butes the cycles to variations in insolation. But for the past million years, the spectrum is dominated by a single 100,000-year feature and is a poor match to the predictions of insolation models. The spectrum can be accounted for by a theory that derives the cycles of glaciatio n from variations in the inclination of Earth's orbital plane.