Elizabeth Kolbert, The sixth extintion

For the last forty million years ago or so, the earth has been in a general cooling phase. It’s not entirely clear why this is so, but one theory has it that the uplift of the Himalayas exposed vast expanses of rock to chemical weathering, and this in turn led to a drawdown of carbon dioxide from the atmosphere. At the start of this long cooling phase, in the late Eocene, the world was so warm there was almost no ice on the planet. By around thirty-five million years ago, global temperatures had declined enough that glaciers began to form on Antartica. By three million years ago, temperatures had dropped to the point that the Arctic, too, froze over, and a permanent ice cap formed. Then, about two and a half million years ago, at the start of the Pleistocene epoch, the world entered a period of recurring glaciations. Huge ice sheets advanced across the Northern Hemisphere, only yo melt away again some hundred thousand years later.

Even after the idea of ice ages was generally accepted -it was first proposed in the eighteen-thirties by Louis Agassiz, a protégé of Cuvier- (…)

It would take another three-quarters of a century for the question to be resolved. It is now generally believed that ice aged are initiated by small changes in the earth’s orbit, caused by, among other things, the gravitational tug of Jupiter and Saturn. These changes alter the distribution of sunlight across different latitudes at different times of year. When the amount of light hitting the far northern latitudes in summer approaches a minimum, snow begins to build up there. This initiates a feedback cycle that causes atmospheric carbon dioxide levels to drop. Temperatures fall, which leads more ice to build up, and so on. After a while, the orbital cycle enters a new phase, and the feedback loop begins to run in reverse. The ice starts to melt, global CO2 levels rise, and the ice melts back farther.

During the Pleistocene, this freeze-thaw pattern was repeated some twenty times, with world-altering effects. So great was the amount of water tied up in ice during each glacial episode that sea levels dropped by some three hundred feet, and the sheer weight of the sheets was enough to depress the crust of the earth, pushing it down into the mantle. (In places like northern Britain and Sweden, the process of rebound from the last glaciation is still going on.)

How did the plants and animals of the Pleistocene cope with these temperature swings? According to Darwin, they did so by moving. In On the Origin of Species, he describes vast, continental-scale migrations. 

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Darwin’s account has since been confirmed by all sorts of physical traces. researchers studying ancient beetle casings, for example, have found that during the ice ages, even tiny insects migrated thousands of miles to track the climate. (To name just one of these, Tachinus caelatus is a small, dullish brown beetle that today lives in the mountains west of Ulan Bator, in Mongolia. During the last glacial period, it was common in England.)

In its magnitude, the temperature change projected for the

coming century is roughly the same as the temperature swings of the ice ages. (If curren emissions trends continue, the Andes are expected to warm by as much as nine degrees.) But if the magnitude of the change is similar, the rate is not, and, once again, rate is key. Warming today is taking place at least ten times faster than it did at the end of the last glaciation, and at the end of all those glaciations that preceded it. To keep up, organisms will have to migrate, or otherwise adapt, at least ten times more quickly In Silman’s plots, only the most fleet-footed (or rooted) trees, like the hyperactive genus Schefflera, are keeping pace with rising temperatures. How many species overall will be capable of moving fast enough remains an open question, though, as Silman pointed out to me, in the coming decades we are probably going to learn the answer, whether we want to or not.