Field Notes From a Catastrophe: Man, Nature, and Climate Change Read Online Free PDF Page B

centers in Scandinavia, Siberia, and the highlands near Hudson Bay, spreading gradually across what is now Europe and Canada. By the time the sheets had reached their maximum southern extent, most of New England and New York and a good part of the upper Midwest were buried under ice nearly a mile thick. The ice sheets were so heavy that they depressed the crust of the earth, pushing it down into the mantle. (In some places, the process of recovery, called isostatic rebound, is still going on.) As the ice retreated, at the start of the current interglacial—the Holocene—it deposited, among other landmarks, the terminal moraine known as Long Island.
    It is now known, or at least almost universally accepted, that glacial cycles are initiated by slight, periodic variations in the earth’s orbit. These orbital variations, which are caused by, among other things, the gravitational pull of the other planets, alter the distribution of sunlight at different latitudes during different seasons and occur according to a complex cycle that takes a hundred thousand years to complete. Orbital variations in themselves, however, aren’t sufficient to produce the sort of massive ice sheet that picked up the Madison Boulder.
    The crushing size of that ice sheet, the Laurentide, which stretched over some five million square miles, was the result of feedbacks, more or less analogous to those now being studied in the Arctic, only operating in reverse. As the ice spread, albedo increased, leading to less heat absorption and the growth of yet more ice. At the same time, for reasons that are not entirely understood, as the ice sheets advanced, CO 2 levels declined: during each of the most recent glaciations, carbon dioxide levels dropped almost precisely in sync with falling temperatures. During each warm period, when the ice retreated, CO 2 levels rose again. Researchers who have studied this history have concluded that fully half the temperature difference between cold periods and warm ones can be attributed to changes in the concentrations of greenhouse gases.
    While I was at CRREL, Perovich took me to meet a colleague of his named John Weatherly. Posted on Weatherly’s office door was a bumper sticker designed to be pasted—illicitly—on SUVs. It said, i’m changing the climate! ask me how! Weatherly is a climate modeler, and for the past several years, he and Perovich have been working to translate the data gathered on the Des Groseilliers expedition into computer algorithms to be used in climate forecasting. Weatherly told me that some climate models—worldwide, there are about fifteen major ones in operation—predict that the perennial sea-ice cover in the Arctic will disappear entirely by the year 2080. At that point, although there would continue to be seasonal ice that forms in winter, in summer the Arctic Ocean would be completely ice-free. “That’s not in our lifetime,” he observed. “But it is in the lifetime of our kids.”
    Later, back in his office, Perovich and I talked about the long-term prospects for the Arctic. Perovich noted that the earth’s climate system is so vast that it is not easily altered. “On the one hand, you think, It’s the earth’s climate system; it’s big, it’s robust. And, indeed, it has to be somewhat robust or else it would be changing all the time.” On the other hand, the climate record shows that it would be a mistake to assume that change, when it comes, will come gradually. Perovich offered a comparison that he had heard from a glaciologist friend. The friend likened the climate system to a rowboat: “You can tip and then you’ll just go back. You can tip it and just go back. And then you tip it and you get to the other stable state, which is upside down.”
    Perovich said that he also liked a regional analogy. “The way I’ve been thinking about it, riding my bike around here, is, You ride by all these pastures and they’ve got these big granite boulders in the middle of