During most of the Phanerozoic eon, which began about a half-billion years ago, there were few glacial intervals until the late Pliocene 2.75 million years ago. Beginning at that time, the Earth’s climate entered a period of instability with the onset of cyclical ice ages. At first these had a 41,000 year cycle, and about 1 million years ago the period lengthened to 100,000 years, which has continued to the present. Over this period of instability the climate has been extraordinarily sensitive to small forcings, whether due to Milankovitch cycles, solar variations, aerosols, or albedo variations driven by cosmic rays. The current interglacial has lasted for some ten thousand years, about the duration of past interglacials, and serious policy considerations arise as it nears its likely end. It is extremely unlikely that the current rise in carbon dioxide concentration–some 30% since 1750, and projected further increase over the next few decades–will significantly postpone the next glaciation.
A shorter version appeared in Energy &Environment VOLUME 22 No. 8 2011 (PDF)
A related Op-Ed: THE COMING OF A NEW ICE AGE (PDF)