In 1981, a glaciologist at the University of Maine named Terry Hughes was examining the data that had emerged from the first, crude attempts to map Antarctica, and in particular its more vulnerable part. The West Antarctic Ice Sheet, these surveys found, contained three large drainage basins. Two of them ended in vast ice shelves, thick fists of ice as big as countries, which acted as corks, limiting the rate at which the interior of the continent flowed out into the oceans. But the ice shelf at the third portal, the Amundsen Sea Embayment, was very weak — not a broad fist of ice but a few skinny knuckles jutting out above the waterline. Hughes published a paper pointing this out, and he called the area the "weak underbelly" of West Antarctica. If climate change were ever to disintegrate Antarctica, he theorized, it would begin at the Amundsen Sea, by releasing the largest glacier that flowed into it, the Pine Island Glacier. This, he thought, this was the place.
For more than a decade, Pine Island has been accelerating, and it is now racing toward the sea: 2.6 miles a year, 38 feet a day, more than a foot an hour, 10 times the rate of the other large Antarctic glaciers. Because of these speeds, Pine Island is the first of the great Antarctic glaciers to begin disappearing. And so, slowly, glaciologists have begun to incline their attention here, to look, in Pine Island, for hints of how much of Antarctica might be at risk.
Pine Island Bay was named for an exploratory naval ship that was sent to Antarctica to help map the Amundsen Sea coast in 1947. The storms here are so regular and so violent that only one scientist has walked on the floating edge of the glacier: a NASA glaciologist named Bob Bindschadler, who touched down three Januaries ago on snow so tightly packed that the airplane's skis, as they carved a momentary runway, left almost no mark. ("As close to concrete as any snow I've ever stood on," Bindschadler recalls.) It was an impossibly still day, barely any wind at all. He spent 20 minutes on the glacier, then had to leave. The weather was too rough for another landing, and neither he nor anyone else has been back since. The basin that flows into the glacier is very deep and holds enough packed snow to raise global sea levels dramatically on its own, were the glacier to melt. Bindschadler thinks that much of Pine Island might "very well drain within our lifetime."
The accelerated melting in Antarctica has been discovered so recently, and its trajectory remains so hard to discern, that the estimates of sea-level rise still have a broad gap between the best- and worst-case scenarios. Some conservative predictions suggest that global seas will rise two feet by 2100. But if Pine Island Glacier drains completely, that alone will raise the seas another nine inches. Estimates that include other vulnerable glaciers in Antarctica put the total rise in sea levels at more than six feet. Pine Island is the pivot, the point at which the scenarios diverge into best and worst, and the future comes into clearer relief.
Every geographic section of ocean is composed of fat belts, different kinds of water layered neatly on top of one another, arranging themselves by gravity. In the Amundsen Sea, the shallowest water is very cold; some of it, which has just melted off the ice sheet, is nearly as fresh as stream water. But the deepest waters, more than 1,800 feet below the surface, are both saltier and warmer. Bindschadler says it is this water — at some places more than five degrees above the freezing point — that "is killing the ice sheet."
Some scientists believe that Pine Island Glacier has been thinning for 50 years; all that's known for sure is that it's been getting thinner for at least 15 years. "We knew the ice was thinning, and we knew the ocean water in front of it was warm," says Adrian Jenkins of the British Antarctic Survey. "But the ocean cavity beneath the ice was a black box, and to understand what is going to happen to the glacier, and what will happen to sea level, we needed to somehow see inside."
Few scientists have ever managed to get into Pine Island Bay; so much of the water remains frozen from one year to the next that it takes a warm and lucky summer to make the sea passable. But Jenkins got in two summers ago, on an American icebreaking vessel. He brought with him a torpedo-shaped remote-controlled submarine called the Autosub and, three miles from the edge of the glacier, lay it quietly into the cold sea. The sub dove and began to make its way toward the glacier, quickly losing contact with Jenkins and his team. Thirty hours later, they heard a series of beeps on their receiver — the Autosub had completed its circuit. Jenkins' team sent a signal directing it to resurface. A few tense minutes later, the sub breached the surface, like a tiny, sleek whale, and the crew brought it onboard.
When Jenkins downloaded the data from the seafloor, he discovered something startling. Scientists had thought that the ice on the underside of Pine Island Glacier was anchored to a ridge near the mouth of the bay. But the Autosub had made its way 30 miles inland, probing along the base of the glacier. The ice wasn't anchored to the ridge at all; the glacier had come unstuck, and was floating. That meant the warm water in the bay wasn't just lapping against the edge of the ice shelf but attacking the glacier's underbelly. What's more, Jenkins found, the water under the ice sheet was too warm to have been sitting there for years — it must be the result of warmer currents from the north being driven into the bay over and over again. "You couldn't just have had a one-time input of warm water onto the continental shelf, sometime long ago," Jenkins says. "We think this process is repeating itself regularly."
If you were to stand on a particular spot along the Antarctic coast for a day, or a week, you wouldn't always feel the wind blowing in any particular direction; the atmosphere is a chaotic system of storms, sudden and unpredictable, their dispersing energies sending air in every direction. But over time, it is possible to see a trend emerge, a subtle preference of the wind to move around the continent from east to west and to push the ocean currents in the same direction. As the winds go faster — energized by humans turning the dials, raising temperatures in the atmosphere and destroying the ozone layer around Antarctica — the ocean currents grow stronger and more turbulent and more likely to send fingers of warm water up over the sill of the continental shelf, grasping for and then gripping the ice.
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