Posted on 04/03/2002 9:11:05 AM PST by cogitator
Deciphering Contradictory Antarctic Climate Patterns
Antarctica is experiencing some of the fastest warming in the world. Antarctica is cooling.
Some of its glaciers are thinning. Some are thickening. Ice shelves are disappearing. More sea ice is forming.
Scientists have reported all this in recent months. It may all be true, even the contradictory parts.
"Confusing, isn't it?" asked Dr. Eric Rignot, a glacier expert at the NASA Jet Propulsion Laboratory in Pasadena, Calif.
Dr. Peter T. Doran, a professor of earth and environmental sciences at the University of Illinois at Chicago, agreed. "It's a mixed bag of signals."
The reason is that Antarctica is not a single, simple place. At 5.4 million square miles, it is one-third larger than the United States, and just as the Midwest may experience a heat wave while the Northeast is unusually cool, climate does not move in lock step across Antarctica. Those warning of dire consequences from global warming and those playing down the dangers of heat-trapping greenhouse gases can both find pieces of data to support their views.
"People forget that it's a continent," said Dr. David Vaughan, a glaciologist at the British Antarctic Survey. "We don't expect everything to be the same across Asia when climate changes. It's the same thing there."
Antarctica's role in climate and the oceans is largely a story of ice. Ninety percent of the world's ice lies either on the continent, in ice sheets that are on average 1.3 miles thick, or in sheets that have flowed offshore to form floating platforms of ice along the coast, hundreds to thousands of feet thick. The largest of these, the Ross Ice Shelf, covers 200,000 square miles, an area about the size of France.
The third component of Antarctic ice is a thin layer of frozen ocean, or sea ice, that grows and shrinks with the seasons. A few feet thick, sea ice covers one million square miles of ocean in summer and grows to six million square miles in winter, doubling the size of the continent.
News like the disintegration of an ice shelf the size of Rhode Island a month ago conjures a vision that a warming world will lead to doom by drowning — not from melting ice shelves, which like melting ice in a glass do not change water levels, but from melting ice sheets sending their fresh water flowing toward the sea. If all of Antarctica's ice sheets turned to water, the world's oceans would deepen by more than 200 feet.
That will not happen. Annual temperatures in the Antarctic interior average minus 70 degrees or colder. Even a 10-degree temperature rise — greater than climate models' worst-case predictions — would leave almost all of the ice frozen.
But ice sheets along the warmer coastal fringes of Antarctica are more vulnerable to melting. Even modest sea-level rises could increase flooding of coastal lands like Bangladesh, Florida and even Manhattan. Shifting ice could also divert the ocean currents that circle the continent, possibly disrupting the global flow of ocean water and altering the climate still further.
The changes in the Antarctic landscape do not have a single cause. Some are part of the natural cycles of the continent. Some are probably delayed effects of the end of the last ice age. Some may have been brought on by the warming trend of the last century.
On the spindly peninsula that stretches out toward South America, temperatures have risen rapidly, nearly 5 degrees Fahrenheit over the past 50 years, about 10 times as much as the average temperature rise worldwide.
The consequences have been quick and startling.
With summer temperatures now regularly rising above freezing, ice melts into puddles on the top of ice shelves along the peninsula. The water flows down into cracks in the ice, its weight forcing the cracks wider until large sections of the shelf shatter with surprising quickness.
"There's no doubt these ice shelves are disappearing because of this warming trend," said Dr. Rignot of NASA.
In 1995, researchers started noticing the disintegration of the Larsen Ice Shelf, on the northeastern side of the Antarctic Peninsula, when the northernmost section, known as Larsen A, shattered into shards. In 1998, the middle portion, Larsen B, started shrinking, losing 1,000 square miles over four years.
Then, in February, a 1,250-square-mile section — larger than Rhode Island — started splintering, and in just over a month, it was gone, sending billions of tons of ice floating into the ocean to melt. Scientists expect the remaining nub of Larsen B and C, the last section of the shelf, to fall apart in the coming years.
Although their destruction does not directly raise sea level, the shelves had acted like door jambs against inland ice sheets. The sheets may now flow more quickly to the sea; the new ice displaces water and raises sea levels, the way extra ice cubes raise the water level in a glass.
A melting ice shelf is not necessarily a sign of human-induced global warming. Ice shelves have grown and shrunk through the ages, mirroring the natural cooling and warming of the climate.
In a core of sediments taken from the sea floor that was once covered by the Larsen A Ice Shelf, researchers led by Dr. Eugene W. Domack, a professor of geology at Hamilton College in Clinton, N.Y., found the tiny fossils of marine algae. The finding indicates that this part of the ice shelf had been open water at least once before. The shelf probably melted about 6,000 years ago in a previous warm spell, Dr. Domack said, and remained open water until refreezing during the Little Ice Age about 700 years ago, then remained frozen until it fell apart in 1995.
Under the Larsen B, however, the researchers found no algae remains in the sediments, indicating that this shelf had remained intact since it formed during the last full-fledged ice age, more than 10,000 years ago, until its demise last month.
Dr. Theodore A. Scambos, a glaciologist at the National Snow and Ice Data Center at the University of Colorado, said the long life of Larsen B "makes you think there's something particularly unusual about this warming" — perhaps evidence that the warming has been brought on by artificial emissions of carbon dioxide and other greenhouse gases that trap heat in the atmosphere.
Almost at the same time of Larsen B's demise, further to the south, an iceberg the size of Delaware broke off the floating part of the Thwaites Glacier. A few months earlier, parts of the Ross Ice Shelf broke off. But these two events, scientists say, are not unusual. As glaciers flow into the water, the ice shelves grow until some point when a large iceberg breaks off, and then the process is repeated. The part of the Ross Ice Shelf where the iceberg broke away is now the same smaller size it was when the explorers Robert F. Scott and Ernest Shackleton observed it at the start of the 20th century.
The melting of the Larsen and other peninsula ice shelves will also not have worldwide repercussions. The amount of ice on the peninsula is relatively small, potentially contributing only a small rise in sea level.
The rest of Antarctica shows no signs of widespread warming. In an article in the journal Nature in January, Dr. Doran of the University of Illinois and his colleagues reported that temperatures in the McMurdo Dry Valleys, a rocky, ice-free area west of the Ross Ice Shelf, had cooled about 2 degrees Fahrenheit from 1979 to 1998. Extrapolating that data with other temperature measurements in other parts of Antarctica from the past 35 years, they concluded that Antarctica as a whole has cooled, too.
Scientists at NASA Goddard Space Flight Center have also reported that satellite measurements show that sea ice now covers about 2 percent more area around Antarctica than it did two decades ago, another suggestion of recent cooling.
Because climate naturally warms and cools on cycles lasting for decades, the observed Antarctic cooling does not disprove the idea of human-induced global warming. Scientists cannot say yet whether the cooling is a short-term blip masking a long-term warming trend or a long-term trend contrary to their predictions. "It may still warm," Dr. Doran said. "It's not really making up its mind yet."
In a comparison of 17 computer models of world climate, all predict global warming will kick in over Antarctica, and most indicate temperatures in the interior of the continent will rise faster than in the rest of the world, said Dr. Benjamin D. Santer, an atmospheric scientist at Lawrence Livermore National Laboratory. "There is a common warming signal," he said.
Because ice reflects most light and heat back into space, the exposed water will absorb more heat as sea ice melts, and further add to the warming.
Counterintuitively, global warming would actually lower sea levels at first. In warmer temperatures, evaporation of ocean water increases and more snow falls, more than offsetting the melting ice at the edges. But over the longer term — perhaps centuries, perhaps thousands of years — prolonged warmth in Antarctica would add to the ocean depths.
Of particular worry is West Antarctica, where most of the bedrock lies below sea level. That makes the ice sheet on top more vulnerable to warming waters lapping at its edges. The worry is not entirely theoretical. Scientists have found algae remains beneath the West Antarctic ice far inland from the present ocean, a sign that the ice sheet had entirely melted at some time in the last two million years. But the fossil evidence gives little hint how quickly the melt occurred or its cause.
Scientists do not have a good sense of the current trends, because until a few years ago, data came from only a few ground-based weather stations. With satellites, scientists now keep close watch of changes to Antarctica as they occur. "We can get images of what happened yesterday on the computer," said Dr. Rignot. "That changes everything for glaciologists."
In December, Dr. Rignot reported on satellite measurements indicating that the Pine Island and Thwaites glaciers in West Antarctica were thinning and speeding up, losing about 36 cubic miles in the last decade. A month later, other researchers, including Dr. Ian R. Joughin, an engineer at the Jet Propulsion Laboratory with an office near Dr. Rignot's, reported a change with opposite effect nearby. Two of the ice streams that flow in the Ross Ice Shelf have slowed, they said, and that area of Antarctica is gaining mass.
Neither the slowdown in the Ross ice streams nor the speed-up of the Pine Island and Thwaites glaciers results from climate changes, at least not recent ones. One Ross ice stream stopped flowing 150 years ago.
Even if the Antarctic warms in coming years, some glaciers will not be affected for a long time. Ice does not conduct heat well, and a rise of a few degrees in the air would take thousands of years to affect a glacier base a mile away, where it could lubricate the flow.
The thinning of the Pine Island and Thwaites glaciers "may be a direct impact of global warming that happened 20,000 years ago," said Dr. Robert Bindschadler, a glaciologist at Goddard. "It may be only now that Antarctica is getting around to its full-fledged response to that."
In Antarctica, climate change sometimes takes its time.
Possibly an early sign of El Nino: warmer coastal waters, more clouds/fog.
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