October 12, 1999

Global Warming Not to Blame for Melting of Huge Ice Sheet


WASHINGTON -- The huge West Antarctic Ice Sheet may be headed for a complete meltdown in a process that a new study indicates was started thousands of years ago, and not as a result of global warming or other changes brought on by humans.

The study, however, did not address the possibility that actions of people -- like the burning of coal, oil and natural gas -- could help accelerate the process.

As scientists have been increasingly able to document melting and the discovery of icebergs breaking off from Antarctica in recent years, concerns have risen that a change in climate could be damaging the Antarctic ice sheet.

But the future of the West Antarctic Ice Sheet "may have been predetermined when the grounding line retreat was triggered in early Holocene time," which was about 10,000 years ago, according to a team of scientists that was led by Dr. Howard Conway of the University of Washington in Seattle. They reported their findings on Friday in the journal Science.

The grounding line is the boundary between floating ice and ice thick enough to reach the sea floor, and the scientists found that line has receded about 800 miles since the last ice age, withdrawing at an average of about 400 feet per year for the last 7,600 years.

Referring to the melting, Dr. Conway said, "It seems like the rate that been going on since the early Holocene is similar to the rate right now." Dr. Conway said.

"Collapse appears to be part of an ongoing natural cycle, probably caused by rising sea level initiated by the melting of the Northern Hemisphere ice sheets at the end of the last ice age." Dr. Conway added in a telephone interview.

Continued shrinking of the ice sheet, perhaps even complete disintegration, "could well be inevitable," the report concluded.

According to estimates, the ice sheet's complete melting could raise the global sea level by 15 to 20 feet, swamping low-lying coastal communities worldwide.

But, at the current rate of melting, that would take about 7,000 years, the researchers estimated.

Dr. Conway said the melting annually contributed about one twenty-fifth of an inch to sea-level rise.

Although the study indicates that global warming is not the cause of the melting, changes in climate remain a problem, Dr. Conway said.

"Global warming could well speed the process," Dr. Conway said.

"Our study," he added, "doesn't address that problem."

Some environmentalists have grown concerned that industrial chemicals added to the atmosphere are trapping heat like a greenhouse, causing the temperature of the earth to rise.

But there is disagreement about just what is causing any rise in temperature and how great a hazard is being posed.

Dr. Conway's report is one of three in this issue of Science, which focuses on the Antarctic ice sheet.

In the another, scientists studying satellite-based measurements found a complex system of tributaries feeding major rivers of ice on the West Antarctic Ice Sheet.

The web of tributaries forms a transition zone between the sluggish inland ice and the swiftly moving ice streams closer to the margins.

Still other researchers, using the ages of volcanic debris that erupted onto the West Antarctic Ice Sheet, reconstructed the past elevation of the ice sheet as it began to melt just after the end of the last ice age. They concluded that the sheet was not the source of a huge flow of meltwater into the oceans 10,000 years ago.

West Antarctica, a section of the continent south of the tip of South America, is covered by an ice sheet that extends over about 360,000 square miles -- about the combined areas of Texas and Colorado.

Dr. Conway's team calculated the movement of the grounding line using evidence gathered from raised beaches and radar imaging of subsurface ice structures.

The timing of start of the melting was determined by carbon-14 dating of samples that were found on raised beaches.

Copyright 1999 The New York Times Company