Climate is the product of so many variables—rising and falling carbon dioxide levels, the shifts of continents, solar activity, the stately wobbles of the Milankovitch cycles—that it is as difficult to comprehend the events of the past as it is to predict those of the future. Much is simply beyond us. Take Antarctica. For at least twenty million years after it settled over the South Pole Antarctica remained covered in plants and free of ice. That simply shouldn't have been possible.

No less intriguing are the known ranges of some late dinosaurs. The British geologist Stephen Drury notes that forests within 10 degrees latitude of the North Pole were home to great beasts, including Tyrannosaurus rex. "That is bizarre," he writes, "for such a high latitude is continually dark for three months of the year." Moreover, there is now evidence that these high latitudes suffered severe winters. Oxygen isotope studies suggest that the climate around Fairbanks, Alaska, was about the same in the late Cretaceous period as it is now. So what was Tyrannosaurus doing there? Either it migrated seasonally over enormous distances or it spent much of the year in snowdrifts in the dark. In Australia—which at that time was more polar in its orientation—a retreat to warmer climes wasn't possible. How dinosaurs managed to survive in such conditions can only be guessed.

One thought to bear in mind is that if the ice sheets did start to form again for whatever reason, there is a lot more water for them to draw on this time. The Great Lakes, Hudson Bay, the countless lakes of Canada—these weren't there to fuel the last ice age. They were created by it.