Over the more than four-and-a-half billion years since the formation of the planet Earth, its climate has remained remarkably stable, and has apparently sustained life for about four billion of those years. Throughout that long period the oceans and the atmosphere have maintained an uneasy equilibrium; the sun has been a sufficiently steady source of heat so that the oceans have neither boiled their water away into space nor frozen down to the equator-fates that many other planets and satellites of the solar system have suffered.
Yet even in the recent past there have been dramatic shifts of climate. A mere 18,000 years ago most of Canada and northwestern Europe were covered by great ice sheets several kilometers thick in places (just as Greenland and the Antarctic are now). In fact, such ice ages and the kind of inter-glacial periods that we are in now have alternated approximately every 100,000 years for