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The Vanishing Ice Sheets

Page 4 of 6

IV. Ocean

You don't have to spend much time in the company of ice scientists before you notice a marked generational divide. Those older than 50 got into the field when you needed to be a mountaineer to conduct meaningful research — to travel to the globe's end, to stick a pole in the ground and to suffer through brutal weather while the data accumulated. But the most profound insights, over the past decade, have come from satellite data. "Remote sensing technology has become so powerful that it allows us to observe the ice sheets in ways that would be impossible to replicate in the field," Eric Rignot, senior research scientist at the Jet Propulsion Laboratory, says with a hint of triumph. The younger ice scientists seem less like explorers and more like mathematicians. The new aim is to build a computer model that more perfectly mimics the Earth; if you take 10 graduate students in glaciology, each of them will be eager to go into the field, but only two or three will have the mathematical brain to synthesize the data. The older ice scientists always thought about ice — where it forms, how it moves, its fundamental properties and underlying mechanics. The younger ones are trained to think in terms of climate. And if you are thinking about the climate, you consider everything.

David Holland, director of the Center for Atmosphere Ocean Science at New York University, falls firmly on the modern side of this divide. Holland started out as an academic by building mathematical models of the movements of oceans, but slowly, over time, he found himself drawn to the rhythms of fieldwork — the adventure and the engineering challenge. Holland, who has a clipped Canadian accent, is ironic and contrarian. He grew up in Newfoundland, raw country strafed by the storms of the Labrador Sea; there are holes along the cragged coast where the same molecules of water have lain for hundreds of years, too dense and salty to get out. It is a place that impresses upon you the power of the ocean to shape the land and the society built there.

In 2006, Holland got a call from Bob Thomas, who ran NASA's polar science division. Something was bothering Thomas about the Jakobshavn Glacier in Greenland. Since scientists had documented the glacier's speedup, the assumption had been that its cause was the warming climate, which had melted pools of water on top of the ice sheet. That meltwater, the theory went, had drained through to the bottom of the ice and lifted the glacier off its rocky bed, sending it rushing to the sea, as slick and purposeful as a python.

But Thomas had spent nearly a decade studying Jakobshavn, had noticed something else. The glacier hadn't just sped up. The edge of it that lay in the water, the floating ice tongue, had thinned dramatically. At the time, thinning of a few feet a year was considered remarkable. Jakobshavn was thinning by more than 250 feet a year. The meltwater alone couldn't account for that much thinning. Something else must be helping to melt the glacier. What separated the ice tongue from the rest of the glacier, Thomas observed, was that it lay in the ocean. What if the key change hadn't happened on top of the ice sheet but beneath it? What if the larger problem wasn't warm air attacking the ice sheet from above but the ocean swallowing it from below?

NASA's satellites can't penetrate salty water, so Thomas couldn't see what was happening underneath the glacier. Was there a way, he asked Holland, to get into the polar fjord off Disko Bay where Jakobshavn's thinning tongue was bathing, to measure the water there and to see if something had changed?

Holland and Thomas talked through the problem. Even in summer, the fjord is too clogged with icebergs for a ship to get in. Thomas suggested giving instruments to the natives who went out on the ice by dog sled, digging holes to fish for halibut.

Holland had a better idea. In Ilulissat, a nearby town, he rented a helicopter and had the pilot fly over the fjord, dropping down low to clear a hole of ice with wind generated by the whirring rotors. Then, as the helicopter hovered 500 feet above the water, Holland leaned out the side and released a small metal probe, the size of a can of Coke. Sometimes he missed the hole, and the probe stuck in the surface ice, its small parachute flopping in the wind. But when he managed to drop the probe into the water, it left an FM radio transmitter on the surface before sinking to the bottom of the fjord, sending back temperature, salinity and depth readings as it went. Holland got the readings on his laptop instantly. In most places in Greenland, he knew, the water was about 34.7 degrees. But everywhere he looked in the fjord, it was 37.9 degrees. "For a glacier," Holland says, "that's absolutely intolerable."

The unexpected thing about the oceans is that their movements are as regular and fixed as subway lines. The physics of the atmosphere conspire to sort water into giant bands called currents — each hundreds of feet deep and thousands of miles long — which share the same temperature and salinity. Like subway lines, ocean currents may pass over or under one another, but the water inside one seldom mixes with another. When a buoy in Greenland detects that the water passing by is slightly saltier and slightly warmer than it has been for decades, it doesn't just mean that some water has sloshed around in the bay. It means that something more fundamental has changed: An entire subway line has moved. If Holland was right — if the ocean was responsible for melting Jakobshavn — then the threat extended far beyond Disko Bay. Warm air alone would never melt Antarctica. But if warmer water could find its way to Greenland and destroy the ice shelves, it could do the same in Antarctica, the world's great lockbox of ice.

When he returned from Greenland at the end of the summer, Holland and some colleagues built a computer model to try to predict how much ice the warmer water from Disko Bay might melt. In each experiment, the model produced melting rates of more than 250 feet a year — the same amount of thinning that Thomas had observed by satellite. "Now we knew that it was the oceans that were driving the ice," Holland says. "And the question became, what is driving the oceans?"

That winter, by e-mail and phone, Holland and a few other scientists tried to find all the data they could on the waters around Disko Bay. When, precisely, had it gotten warmer? They had little luck. Then a Danish oceanographer named Mads Ribergaard mentioned another source of data. For two decades, as fishermen trawled for cod and shrimp along the bottom of the continental shelf in Western Greenland, as much as 2,000 feet below the surface, they had attached small sensors to their nets that measured temperature and salinity, and then returned the sensors to the Greenland Institute of Natural Resources, which was using the data to build a record. When Ribergaard and Holland assembled the data, they noticed a single, stunning change. During the early years of the program, the temperatures at the mouth of Disko Bay were steady, at about 34.7 degrees. Then, in 1997, the temperature jumped, to 37.9 degrees, and stayed there. The next summer, the speedup at Jakobshavn had begun. "To see a graph like that is very rare in ocean science," Holland says.

Holland looked more closely at the data set. He could see in the records that this pulse of warm water had crept north during the summer of 1996. This was, he knew, a branch of the Gulf Stream called the Irminger Current — very heavy, very warm water that usually cycled back into the North Atlantic far south of Disko Bay. But in 1997 something had changed; instead of turning back, this pulse of warm water had crept along the Greenland Shelf, farther and farther north. In other places in the fisheries data, you could see oblique references to this pulse: One species of cod, which favored warmer waters, began appearing in unprecedented numbers up the coast, and another species, which prefers the cold, was retreating. Something had changed the Irminger Current.

There is an international set of weather data that has been building for 50 years, composed of wind patterns collected by ships crisscrossing the sea and weather balloons launched at airports. Scientists have subjected this data to a rigorous analysis, plugging it into massive computers to build a model of the Earth's wind field over time. It is a clean model, beautiful in its simplicity, the best that climatologists can construct. Among many other features, it provides a record of the North Atlantic Oscillation, a mysterious element of the climate that governs the power of the winds that blow across the North Atlantic, from west to east: For 10 years or so, those winds will be strong, and then in the course of a month, they will inexplicably shift to weak and may stay that way for another decade.

It took Holland only a few minutes of paging through the records to discover what he was looking for. In December 1995, the oscillation changed, and the winds suddenly shifted from strong to weak. By the next summer, the Irminger Current had crawled so far north that it was just outside Disko Bay. The summer after that, the ice at Jakobshavn was racing for the sea. "It's all right there," Holland says. "That's how it works. The atmosphere controls the ocean. The ocean controls the ice. You could see it right in front of you."

On a recent afternoon, Holland sat in his office at NYU, overlooking Washington Square Park. He had just come back from a month in Antarctica, where he had gone hoping to install a weather station and some GPS devices on Pine Island Glacier, one of the continent's largest rivers of ice, already moving rapidly through its basin, already thinning at its edges. It had been a frustrating trip. Antarctica is a complicated logistical operation, run by the National Science Foundation and the U.S. military, and day after day Holland had sat at an airfield, waiting for a flight to the glacier's edge. One day the planes couldn't fly because of the storms. The next, a fuel pump was broken, and they had to wait for new parts. Then the pilots had to take a rest day. After a month of waiting, Holland wound up spending only four hours on Pine Island Glacier.

The experience made him sensitive to the limits of polar exploration. As he sees it, the oceans themselves are resistant to clear descriptions of cause and effect, and some of the most essential questions remain shut inside black boxes: How fast does the wind blow in the seas that surround Antarctica? How will ocean currents respond to the changing climate? We don't know, because the effort to figure it out has been spotty; too many of the critical spots in Antarctica haven't even been mapped. If we took the problem seriously, Holland thinks, then science wouldn't be delayed a year because a plane in Antarctica had a broken fuel pump, the scientists stranded in a base camp, anxiously watching the winds.

"Let me show you this," Holland says. He has Google Earth up on his computer screen, and he rotates the satellite photos so we are looking at a tiny outcropping on the coast of Antarctica. "This is called Sulzberger Ice Shelf, after the publisher of The New York Times," he says. "We know there's warm water here, warm enough to kill an ice shelf." He traces his fingernail across the screen, to the right. "Thirty miles away is the Ross Ice Shelf — the largest in the world," he says. If it were to flow into the ocean, the Ross would release enough ice to alter the shape of the world's map.

There are two alternatives at the Sulzberger shelf, Holland explains. "Either the warm water stays where it is," he says, "or the warm water moves. You could say, 'The warm water's been there a long time, and it hasn't come in yet, so it's unlikely.' On the other hand, we are changing the ocean's circulation in ways that we don't understand and whose consequences we aren't prepared for." The unique feature of Antarctica, he points out, is that much of the ice lies on bedrock that has always been beneath sea level. "It's a question," he says, "of whether the ocean wants its territory back."

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