The theory that the changes of climate through geologic time (the paleoclimates) have been due to changes of land and sea distribution and orography combined with fluctuations of solar radiation of the order of 10%–20% on either side of the mean. Topographic changes may account for the occurrence of ice ages as a whole, but not for the more rapid alternations of glacial and interglacial periods. A combination of the topographic with the solar fluctuations seems to offer a nearly complete explanation of climatic changes, but it is not yet clear which plays the main part.