ACCORDING to the astronomical or Milankovitch theory, the glacial fluctuations of the Pleistocene are the climatic response to secular variations in the obliquity, eccentricity, and longitude of perihelion of the Earth's orbit. Variations in these orbital parameters during the past several hundred thousand years can be computed with great precision1, and the resulting changes in incident radiation at the top of the atmosphere are easily obtained. If one can predict the climatic response to perturbations in incident radiation, one can, therefore, test the Milankovitch theory by comparing the predicted climatic changes with those inferred from the geological record. No other theory of the ice ages admits such a straightforward check on its validity. We have made a preliminary attempt at verifying the Milankovitch theory using a zonally symmetric energy-balance climate model forced with seasonally varying insolation and obtain generally favourable results.
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