This chapter compares the life that evolution produced during the past cold and warm climates of long duration. The dominant climatic state of the last several million years has been ice ages. James Lovelock has argued that these are a more favorable state for life on Earth than the present interglacials. This does not, however, sit well with various observations. During ice ages: (i) there was less land free of ice; (ii) much of the most productive parts of the ocean, the shelf seas, were turned into dry land by lower sea level; and (iii) the total mass of carbon locked up in vegetation and soils was only about half as much as today. Not only were ice ages unfortunate episodes for life on Earth, but also, conversely, the Earth during the Cretaceous was possibly even more congenial than it is today, although the evidence is not conclusive on this point.
