Why should the earth's climate have oscillated for so long between very cold and reasonably mild, between long glacials and brief interglacials? Why should each change from one extreme to the other have been so sudden? Why, during each glacial interval, did less cold interstadials alternate with cold stadials, and why was the warm interglacial climate interrupted by little ice ages? How do we explain this flip-flop response of the climate system and what are the forces that make it behave so? We shall see in Chapter II that the Quaternary ice age came to us on soft feet, gently announcing itself some 30 million years ago, then advancing in distinct steps of which the last one, less than a million years ago, is the most striking feature of Figure 4.2.

The normal state of the world appears to be one of ice-free poles and a small equator-to-pole temperature gradient, but ours is not the only ice age the world has seen. The earliest major glacial deposits turn up between 3 and 2 by ago, but we know little about them and they may have been local. The ice ages of the late Precambrian and Paleozoic, however, are not in doubt; the wide distribution of their characteristic deposits permits no other conclusion. The late Precambrian ice age seems to have been one of the largest (Section 14.7), and descended to curiously low latitudes, while the Paleozoic ones left extensive marks on the continent of Gondwana around the South Pole of that time (Section 7.3).