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Antarctica, Warming, Looks Ever More Vulnerable |
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Publication Date: January 25, 2005 |
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Author: LARRY ROHTER |
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Source: The New York Times |
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Link: http://www.nytimes.com/2005/01/25/science/earth/25ice.html |
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Over the Abbott Ice Shelf Antarctica -- From an airplane at 500 feet, all that is visible here is a vast white emptiness. Ahead, a chalky plain stretches as far as the eye can see, the monotony broken only by a few gentle rises and the wrinkles created when new sheets of ice form. Under the surface of that ice, though, profound and potentially troubling changes are taking place, and at a quickened pace. With temperatures climbing in parts of Antarctica in recent years, melt water seems to be penetrating deeper and deeper into ice crevices, weakening immense and seemingly impregnable formations that have developed over thousands of years. As a result, huge glaciers in this and other remote areas of Antarctica are thinning and ice shelves the size of American states are either disintegrating or retreating - all possible indications of global warming. Scientists from the British Antarctic Survey reported in December that in some parts of the Antarctic Peninsula hundreds of miles from here, large growths of grass are appearing in places that until recently were hidden under a frozen cloak. "The evidence is piling up; everything fits," Dr. Robert Thomas, a glaciologist from NASA who is the lead author of a recent paper on accelerating sea-level rise, said as the Chilean Navy plane flew over the sea ice here on an unusually clear day late in November. "Around the Amundsen Sea, we have surveyed a half dozen glaciers. All are thinning, in some cases quite rapidly, and in each case, the ice shelf is also thinning." The relationship between glaciers (essentially frozen rivers) and ice shelves (thick plates of ice protruding from the land and floating on the ocean) is complicated and not fully understood. But scientists like to compare the spot where the "tongue" of a glacier flows to sea in the form of an ice shelf to a cork in a bottle. When the ice shelf breaks up, this can allow the inland ice to accelerate its march to the sea. "By themselves, the tongue of the glacier or the cork in the bottle do not represent that much," said Dr. Claudio Teitelboim, the director of the Center for Scientific Studies, a private Chilean institution that is the partner of the National Aeronautics and Space Administration in surveying the ice fields of Antarctica and Patagonia. "But once the cork is dislodged, the contents of the bottle flow out, and that can generate tremendous instability." Glaciologists also know that by itself, free-floating sea ice does not raise the level of the sea, just as an ice cube in a glass of water does not cause an overflow as it melts. But glaciers are different because they rest on land, and if that vast volume of ice slides into the sea at a high rate, this adds mass to the ocean, which in turn can raise the global sea level. Through their flights over this and other areas of Antarctica, NASA and the Chilean center hope to help glaciologists and other scientists interested in climate change understand what is taking place on the continent and why. To do that, they need to compile data not only on ice thicknesses but also the underlying geology of the region, information that is most easily obtained from the air. The flights are taking place aboard a Chilean Navy Orion P-3 plane that has been specially equipped with sophisticated instruments. The devices include a laser-imaging system that shoots 5,000 pulses of light per second at the ground to map the ice surface, as well as ice-penetrating radar to determine the depth of the ice sheets, a magnetometer and digital cameras. For most parts of Antarctica, reliable records go back less than 50 years, and data from satellites and overflights like the ones going on here have been collected over only the past decade or so. But that research, plus striking changes that are visible to the naked eye, all point toward the disturbance of climate patterns thought to have been in place for thousands of years. In 1995, for instance, the Larsen A ice shelf disintegrated, followed in 1998 by the collapse of the nearby Wilkins ice shelf. Over a 35-day period early in 2002, at the end of the Southern Hemisphere summer, the Larsen B ice shelf shattered, losing more than a quarter of its total mass and setting thousands of icebergs adrift in the Weddell Sea. "The response time scale of ice dynamics is a lot shorter than we used to think it was," said Dr. Robert Bindschadler, a NASA scientist who is director of the West Antarctic Ice Sheet Initiative. "We don't know what the exact cause is, but what we observe going on today is likely to be what is also happening tomorrow." Thus far, all of the ice shelves that have collapsed are located on the Antarctic peninsula. In reality a collection of islands, mountain ranges and glaciers, the peninsula juts northward toward Argentina and Chile and is "really getting hot, competing with the Yukon for the title of the fastest warming place on the globe," in the words of Dr. Eric Steig, a glaciologist who teaches at the University of Washington. According to a recent study published in Geophysical Research Letters, the discharge rate of three important glaciers still remaining on the peninsula accelerated eightfold just from 2000 to 2003. "Ice is thinning at the rate of tens of meters per year" on the peninsula, with glacier elevations in some places having dropped by as much as 124 feet in six months, the study found. But the narrow peninsula contains relatively little inland ice. Glaciologists are more concerned that they are now beginning to detect similar signs closer to the South Pole, on the main body of the continent, where ice shelves are much larger - and could contribute far more to sea level changes. Of particular interest is this remote and almost inaccessible region known as "the weak underbelly of West Antarctica," where some individual ice shelves are as large as Texas or Spain and much of the land on which they rest lies under sea level. "This is probably the most active part of Antarctica," said Dr. Eric Rignot, a glaciologist at the Jet Propulsion Laboratory in Pasadena, Calif., and the principal author of the Geophysical Research Letters paper. "Glaciers are changing rapidly and increasingly discharging into the ocean, which contributes to sea level rise in a more significant way than any other part of Antarctica." According to another paper, published in the journal Science in September, "the catchment regions of Amundsen Sea glaciers contain enough ice to raise sea level by 1.3 meters," or about four feet. While the current sea level rise attributable to glacier thinning here is a relatively modest 0.2 millimeters a year, or about 10 percent of the total global increase, the paper noted that near the coast the process had accelerated and might continue to do so. As a result, the most recent flights of NASA and the Chilean center have been directed over the Thurston Island and Pine Island zones of West Antarctica, near the point where the Bellinghausen and Amundsen Seas come together. The idea is to use the laser and radar readings being gathered to establish a base line for comparison with future measurements, to be taken every two years or so. "We're not sure yet how to connect what we see on the peninsula with what we observe going on further south, but both are very clearly dramatic and dynamic events," Dr. Bindschadler said. "On the peninsula, large amounts of melt water are directly connected to disintegration of the ice shelf, but the actual mechanism in West Antarctica, whether melt water, a slippery hill or a firmer bedrock, is not yet clear. Hence the need for more data." The information being gathered here coincides with the recent publication of a report on accelerating climate change in the Arctic, an area that has been far more scrutinized than Antarctica. That study, commissioned by the United States and seven other nations, found permafrost there to be thawing and glaciers and sea ice to be retreating markedly, raising new concerns about global warming and its impact. "The Arctic has lots of land at high latitudes, and the presence of land masses helps snow melt off more quickly," said Dr. Steig. "But there's not a lot of land to speak of in the high latitudes of the Southern Hemisphere," making the search for an explanation of what is going on here even more complicated. The hypotheses scientists offer for the causes of glacier and ice shelf thinning in Antarctica are varied. Rising air, land and ocean temperatures or some combination of them have all been cited. Some scientists have even proposed that a healing of the seasonal ozone hole over the South Pole and southernmost Chile, a phenomenon expected to take place in the next 50 years or so, could change the circulation of the atmosphere over the frozen continent in ways that could accelerate the thinning of Antarctic ice fields. But even without that prospect, the situation developing in Antarctica is already sobering, glaciologists agree. The data being collected here in West Antarctica and on the peninsula farther north make that obvious, they say, though the degree to which that should be cause for concern around the rest of the planet will become clear only with more research. "If Antarctica collapses, it will have a major effect on the whole globe," Dr. Rignot cautioned. He warned that "this is not for tomorrow, and Antarctica is such a big place that it's important to look at other areas" around the perimeter of the giant continent, but added, "Nature is playing a little experiment with us, showing us what could happen if the plug were to be removed." |
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