Q: How long has Earth had polar ice caps? When was the last time Earth did not have polar ice caps? In the last 100 million years, has there been ice cap free periods, if so, for how long (percentage of 100 millon years)?
From Mr Robert Stewart (November 2009)
Reply by Dr Jan Zalasiewicz
The current Ice Age has been decidedly asymmetrical. At the beginning of the Oligocene Epoch, some 33 million years ago, the South Pole – Antarctica – went from being largely forested – a little like New Zealand, say, to being largely ice-bound in a mere few hundred thousand years. There is evidence for this in fossil shells of foraminifera, the chemistry of which indicates both global cooling (especially of deep ocean water, that refrigerated markedly) and the abstraction of water from ocean into polar ice. What caused this first large step into an Ice Age? Continental movements (to block equatorial warm currents and allow circum-polar cold currents) have been implicated, as have marked drops in atmospheric carbon dioxide levels. Evidence for the latter is seen in a sudden decrease in the corrosiveness (from dissolved carbon dioxide) of the world’s deep ocean waters towards calcium carbonate in sea floor oozes.
The Ice Age became truly bipolar around the (recently amended) Pliocene/Quaternary boundary, a little under three million years ago. The great Laurentide , Greenland and Scandinavian icecaps began to grow rapidly then, perhaps triggered by a ‘snowgun’ effect (where warm moisture-laden air blows across cold continental surfaces, triggering heavy snowfalls).
Prior to the Oligocene, and into the Mesozoic, the world had little or no polar ice (there is still debate as to the exact measure of ‘little or no’). Probably, there were small amounts of ice at least part of the time, for even in the late Cretaceous (generally regarded as a ‘greenhouse’ time) there were oscillations in sea level of a few tens of metres that seem best ascribed to the melting and re-forming of small polar icecaps.
The proportion of true ‘glacial’ time (even if mostly essentially unipolar) in the last 100 million years, however, may be taken as about one-third.