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The Vanishing Ice Sheets

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II. Glacier

Over the next century, strange as it is to contemplate, the Earth's surface will be forcibly reshaped by those parts of the planet that remain the most inaccessible and the least understood. The ice sheets of Antarctica and Greenland are so barren and unbroken that they seem more like geometric abstractions than continents. They impose on visitors a near-total sensory deprivation. Because there is virtually nothing living — no trees, no grass, no animals — there is nothing to smell. Even time is distended at the poles: Scientists are generally able to come only at the height of summer, when it is light for nearly 24 hours a day, and they find their workdays slipping later and later into the night. From the interior of an ice sheet, the arc of the horizon is so long and so constant that you stop fully registering the empty landscape, and you focus on the only things that change, which are the clouds. When one drifts past, you imagine it as a more permanent formation — a rock outcropping or a distant mountain. Three weeks or so on the ice sheet is as much of this isolation as most glaciologists can take, and so they race against that limit, science against time.

Ice is a curiously fragile substance; the tiniest shifts in its surroundings — the temperature and pressure of the air, the salinity of the frozen water — can trigger fundamental transformations. "Much of the ice in the world is quite close to a phase change," says Joel Harper, a professor of geosciences at the University of Montana. "It doesn't take much to move it from solid to liquid." At times, these changes can seem the product of the ice's interior will. When a glacier, moving downhill, encounters a small obstacle — a rock a few inches across — it simply melts, allowing it to pass over the stone, then refreezes on the downstream side. When it encounters a large obstacle — perhaps a boulder the size of a house — the ice deforms so that it can move around the rock like a syrupy liquid would. Years later, you can still see in the ice the marks of this change.

Changes like these are almost never witnessed by humans; on the rare occasions when they are observed, they become legends, told and retold. Glaciologists still talk about the moment in 1983 when scientists on top of Variegated Glacier, in Alaska, watched the ice beneath their feet dissolving into a web of small stream in the space of a few minutes. In 1995, Harper and his team drilled a bore hole into the ice in Alaska's Worthington Glacier. A few nights later, they awoke to a rumble as loud as a 747; an unseen lake had quietly drained, migrated and then exploded through the bore hole, sending a geyser hundreds of feet into the sky.

The ice sheets are such unique workshops that glaciologists must invent their own tools and experiments each time they arrive. You can measure the speed of glaciers by tossing dark rocks onto moving ice and tracking them with surveying equipment from a nearby rock outcropping. You probe the interior of glaciers by jerry-rigging a jet of hot water (a home heater, a pressure pump and a long flexible hose) that drills down into the ice sheet, melting a perfect vertical bore hole. You track the snow that has accumulated, from one year to the next, by using a coffee can, a GPS and a length of wire. But technical discoveries like these were lucky accidents, and they provided only partial glimpses along a glacier's edge. No one knew what the entire ice sheet was doing; its most essential changes were hidden beneath those vast blocks of ice, unseen.

That began to change in 1978, when scientists sent a satellite hurtling around the Earth to map the extent of ice in Antarctica and Greenland — what was frozen and what was open sea. NASA engineers, working from a half- decommissioned rocket-launching base on a barrier island in eastern Virginia, also rigged an old naval patrol plane with lasers and GPS, to record how high the ice was at certain points and how far it extended. By mapping the ice grid by grid, and tracking any changes over the years, they could begin to see, for the first time, the workings of the ice sheets.

As the contours of climate change have started to come into focus, glaciologists — a tiny band of scientists in a long-neglected field — have suddenly found themselves briefing Congress, consulting with the United Nations. Perplexed graduate students, stuck in the field in Greenland, were asked to educate visiting dignitaries. The dawning realities of global warming made it evident that one of the gravest threats facing the planet depended upon a field of science that most people had never even heard of. "How fast will the ice sheets lose their mass into the sea?" asks Peter Clark, a professor of geosciences at Oregon State University. "That's the million-dollar question."

Searching for answers, scientists soon focused their attention on the largest glaciers, whose leading edges are hundreds of feet thick and many miles wide, floating mostly submerged in the water. Some glaciologists were beginning to believe that these ice shelves act like corks in champagne bottles, keeping the gigantic rivers of ice behind them from flowing into the sea. If an ice shelf was somehow removed, they argued, the glacier behind it could slide out into the ocean, far more quickly and catastrophically than had been imagined. The data from the thinning glacier in Greenland was particularly alarming. "It looked like we might be loosening the cork," says Bob Thomas, who ran the polar science research program for NASA.

But most scientists disagreed with the cork theory. The prevailing model held that if the ice shelves were removed, the glaciers behind them would stay in place, kept there by the friction between the ice and the rocky trough in which it sat. The question was impossible to resolve in the abstract, however, and so for years it just hung there as a hypothetical, a suggestion at the edges of science, a conversation point when glaciologists were in their second week in the desolate, frozen field and were looking for things to talk about. What was missing was a test case — a place where the cork had been removed. Scientists doubted that nature would ever provide a conclusive demonstration. Then one day — in a dramatic display at the southernmost reaches of the planet — it did.

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