Reuters News Service,Oct. 16, 2004
WASHINGTON - Some of Antarctica's glaciers are melting faster than snow can replace them, enough to raise sea levels measurably, scientists reported.
Measurements of glaciers flowing into the Amundsen Sea, on the Pacific Ocean side of Antarctica, show they are melting much faster than in recent years and could break up.
And they contain more ice than was previously estimated, meaning they could raise the sea level by more than predicted, the international team of researchers wrote in the journal Science.
"The ... Amundsen Sea glaciers contain enough ice to raise sea level by 1.3 meters (4 feet)," the researchers wrote in their report.
"Our measurements show them collectively to be 60 percent out of balance, sufficient to raise sea level by 0.24 mm (nearly 0.01 inch) per year."
And as the surrounding ice shelves melt, this process will speed up, the researchers said.
"The ice shelves act like a cork and slow down the flow of the glacier," said Bob Thomas of the NASA Goddard Space Flight Center's Wallops Flight Facility in Virginia.
Theirs is the second recent report to warn of rapidly melting glaciers in Antarctica.
A team at NASA and the University of Colorado reported that the 2002 breakup of the Larsen B ice shelf on the other side of the continent had accelerated the breakup of glaciers into the Weddell Sea.
Many teams of researchers are keeping a close eye on parts of Antarctica that are steadily melting.
Large ice shelves in the Antarctic Peninsula disintegrated in 1995 and 2002 as a result of climate warming. But these floating ice shelves did not affect sea level as they melted.
Glaciers, however, are another story. They rest on land, and when they slide off into the water they instantly affect sea level.
"The rates of glacier change remain relatively small at present," said Eric Rignot of NASA's Jet Propulsion Laboratory in Pasadena, Calif., who worked on the Amundsen study.
"But the potential exists for these glaciers to increase global sea level by more than 1 meter (3 feet). The time scale over which this will take place depends on how much faster the glaciers can flow, which we do not know at present."
The measurements also show the glaciers are thicker than once believed. This means more melting and more rapid melting, Thomas said.
"Our measurements show an increase in glacier thinning rates that affects not only the mouth of the glacier, but also 60 miles to 190 miles inland," Thomas said in a statement.
The researchers from NASA, the Centro de Estudios Cientificos in Chile, the University of Kansas and Ohio State University wrote their estimates based on satellite data and measurements from a Chilean P-3 aircraft equipped with NASA sensors.
Experts say that overall sea levels around the world are going up by about 1.8 mm or 0.07 inch a year. About half of this comes from melting ice in glaciers.
The melting into the Amundsen Sea is now more than the previous amount from all of Antarctica and more than the estimated contribution from Greenland, the researchers said.
Science, Vol 305, Issue 5692, 1897 , 24 September 2004
Richard A. Kerr
As the global climate warms up, glaciologists' big worry is polar ice, especially the ice sheet of West Antarctica, the muscular arm that juts from the huge mound of ice in East Antarctica. They aren't concerned about warmer air per se; even the thinner West Antarctic Ice Sheet (WAIS) would hold out against its effects for millennia. But researchers have long wondered whether warming could somehow get at the WAIS indirectly, destabilize it, and send its ice into the sea to melt, raising sea level up to a disastrous 5 meters in a few centuries. With the publication online of the latest survey of glaciers flowing into West Antarctica's Amundsen Sea, most glaciologists now allow that there probably is a way for warming to accelerate the movement of at least some of the WAIS ice toward the sea.
Glaciologist Robert Thomas of NASA contractor EG&G at the Wallops Island facility in Virginia and colleagues confirm that the half-dozen glaciers flowing into the Amundsen Sea have been getting thinner and thinner the past 15 years, and that one of them--the Pine Island Glacier--has been flowing faster and faster for more than 100 kilometers inland. "It's not necessarily a sign of [WAIS] collapse," says glaciologist Richard Alley of Pennsylvania State University, University Park, "but it could lead to a collapse."
However, no one can say whether the recent glacial acceleration will continue, whether it could reach more distant ice if it does continue, or whether other, more voluminous parts of the WAIS could suffer similar effects. "We're not running for the hills," says Alley, but "this is the wake-up call for the scientific community to get serious about it all."
Since the start of the 1990s, glaciologists have been closely monitoring the flow of ice from the Pine Island Glacier and nearby glaciers using motion-sensing radar, ice- penetrating radar, and laser and radar altimeters mounted on satellites and aircraft. By the end of the decade, the ice in at least some glacial channels nearing the sea seemed to be thinning and accelerating.
To learn more, Thomas and his colleagues, in cooperation with Centro de Estudios Científicos in Valdivia, Chile, rode an instrument-laden Chilean Navy P-3 aircraft 2700 kilometers to the remote Amundsen Sea coast. The onboard ice-penetrating radar found that the ice is far thicker than thought, on average 400 meters deeper than previously estimated near the coast. Combined with satellite radar velocity estimates from the late 1990s, those greater thicknesses implied that the glaciers are hauling away about 253 cubic kilometers of ice per year. That's about 90 cubic
kilometers more than accumulates each year from snowfall.
By analyzing recent satellite radar data, Thomas and colleagues confirm that ice withdrawals have been accelerating, at least through the Pine Island Glacier, the largest of the group. They calculate that it sped up by 3.5% between April 2001 and early 2003, making for a 25% increase since the mid-1970s. And the drawdown is not limited to areas near the coast. The P-3 data show a thinning, presumably induced by the faster flow, that extends along the main trunk of the Pine Island Glacier and averages about 1.2 meters per year between 100 and 300 kilometers inland.
These latest results from West Antarctica confirm an unsettling view of glacier behavior. For 30 years, glaciologists have debated whether one part of a glacier can "feel" what's happening in a distant part of the same glacier. At the coastal end of the Pine Island Glacier, for example, warmer water seems to be melting the underside of the glacier's floating ice shelf (Science, 24 July 1998, pp. 499 and 549), pushing landward the point at which the advancing glacier floats off the sea floor.
If an ice shelf pinned against an embayment's shore and floor helps slow a glacier's flow--as was hypothesized in the 1970s--and if changes at the coast could make themselves felt far up the glacier, then the Pine Island Glacier's so-called grounding line retreat would accelerate glacier flow well upstream. The researchers think that's what they're seeing. "I'm convinced the glacier feels what is happening a long way away," says Thomas. Similar accelerations struck after two other floating ice tongues recently broke up in West Antarctica and Greenland (Science, 30 August 2002, p. 1494).
"It's a very impressive piece of work," says Alley. "Too many different lines of evidence are agreeing now" for them to be wrong about the thinning or the speedup of the past 10 to 15 years. "Ice shelves may well play a role in the dynamics of glaciers," agrees geoscientist Michael Oppenheimer of Princeton University in New Jersey. But the next problem is that "we don't know why things are melting away at Pine Island Glacier." Oceanographers can't say whether the ocean warming that seems responsible is part of a cycle that will reverse itself or a long-term trend driven by greenhouse warming. And they can't say whether the WAIS's two largest ice shelves--the Texas-size Ronne and Ross ice shelves--could be melted as well.
Even if glaciologists knew what the ocean was going to do, their models for predicting glacier behavior are still so rudimentary that they can't say whether more distant, slower moving ice feeding the main ice streams will respond too. So plenty of uncertainties remain, notes Oppenheimer, but he adds, "I'm starting to get worried."
Science, Vol 306, Issue 5695, 403 , 15 October 2004
The global rise in sea levels that accompanied the melting of glaciers may owe more to the Antarctic Peninsula than had been thought, according to scientists at Rice University in Houston, Texas. New data show that the peninsula's ice sheet extended about 220 kilometers beyond its current shoreline--more than twice as far out as earlier estimates--during the last Ice Age before it began retreating about 16,000 years ago.
Marine geologist David Heroy determined the past extent of the ice sheet from the presence of glacial till-- accumulations of silt and rock deposited by glaciers--in sediment samples and by seismic data from the sea floor. Using a technique called multibeam swath bathymetry, scientists could discern the telltale grooves and humps made by glaciers scouring the seabed. They also found ridges up to 70 kilometers long left by ice streams draining the ice sheet's interior.
It's "hugely important" to get good estimates of how far ice sheets extended "to figure out how much seawater was tied up in Antarctica," says glaciologist David Vaughan of the British Antarctic Survey in Cambridge, U.K. Most estimates are "very shaky," he adds. The new data, which Heroy presented at a workshop at the Scott Polar Research Institute in Cambridge last month, indicate that the North American ice sheets were not the only big players in determining global sea level during the last Ice Age.