The New York Times, by Andrew C. Revkin, March 7, 2003
New evidence from a rapidly warming part of Antarctica suggests that ice can flow into the sea much more readily than had been predicted, perhaps leading to an accelerated rise in sea levels from global warming.
Many polar and ice experts said the new study, to be published in the journal Science, suggested that seas might rise as much as several yards over the next several centuries. They called that prospect a slow-motion disaster, the cost of which -- in lost shorelines, salt in water supplies, and damaged ecosystems -- would be borne by many future generations.
The new analysis focuses on the recent breakup of one of the floating ice shelves fringing the 1,000-mile Antarctic Peninsula after decades of warming temperatures there. The loss of the coastal shelves caused a "drastic" speedup of the seaward flow of inland glaciers, the researchers say.
The peninsula, which stretches north toward South America, has warmed an average of 4.5 degrees over the last 60 years, so much so that ponds of melted water now form in the southern summer atop the flat ice shelves.
The warming there has not been linked definitely to a global warming trend that scientists attribute in part to emissions of carbon dioxide and other heat-trapping gases from human activities. Indeed, temperatures in most other parts of Antarctica have remained stable or cooled, and other ice shelves and glaciers show no signs of deterioration.
But if the warming spreads to more of Antarctica, the freeing of other glaciers could greatly accelerate the amount of ice flowing into the ocean and contributing to rising sea levels around the world.
In the new study, two Argentine researchers report on aerial surveys they conducted in 2001 and 2002, which found that the collapse of the sprawling Larsen A ice shelf in 1995 led to a sudden surge in the seaward flow of five of the six glaciers as if a doorstop had been removed or a dam breached.
Geological evidence shows no signs of similar ice breakups along the peninsula in many thousands of years, the researchers and other experts said.
Indeed, the recent disintegration of ice shelves along both coasts of the peninsula with another one, Larsen C, poised to go next has come after thousands of years of stability, said Pedro Skvarca, an author of the study and the director of glaciology at the Antarctic Institute of Argentina.
"We are witnessing a very significant warning sign of climate warming," he said.
Other experts said the long-term risk of rising sea levels was still unclear. But they added that the new study underscored the importance of clarifying the relationship between Antarctica's many fringing ice shelves and the glaciers behind them.
The glaciers act like frozen rivers, transporting compacted snow from the interior to the ocean. The balance between added snow and departing ice can raise or lower the global sea level by several yards.
For 30 years scientists have been debating whether the glaciers are held back by the fringing ice, which resembles the tattered brim of a sombrero, or by friction with the land.
Now it is clear that at least some ice shelves act as a brake, said the study's authors and other glacier and polar experts who have seen the work.
The new study is the latest of several analyses in recent years that illustrate how a slow warming can lead to abrupt changes in the planet's frozen zones.
The other author was Hernán De Angelis, a geologist at the Argentine institute. Both researchers contributed last year to a study that provided the first indications of the accelerating slide of these glaciers, but this analysis provides new detail, they said, including images of terraces of "stranded ice" stuck 60 to 100 feet up on slopes that showed the glaciers' elevation just a few years ago.
The sliding could be abetted not only by the loss of the ice-shelf blockade, they said, but also by another unpredicted result of warming noted by other scientists in Antarctica and in Greenland: the rapid percolation of water from summertime ponds high on the ice sheets down through cracks to the base. There the water acts as a lubricant, facilitating the slide of glacial ice over the earth below.
For the moment, the breakdown of ice shelves has been restricted to the Antarctic Peninsula.
Still, the continent has many other systems of ice shelves and glaciers that appear to behave similarly to the one that was the focus of the new study. The resemblance gives the findings great significance, said Dr. Theodore A. Scambos, a glacier expert at the National Snow and Ice Data Center, a joint operation of the Commerce Department and the University of Colorado.
The probability and timing of such an outcome remain unknown, but the new work has shed some light on the question, and it has increased some scientists' level of urgency.
More evidence could come in the next year or so, if the next ice shelf in line down the peninsula's coast, Larsen C, falls apart, experts said.