Ice Sheet Melting for Centuries, Study Says

The massive West Antarctic ice sheet may be headed for a complete meltdown in a process that a new study indicates was triggered thousands of years ago, not as a result of global warming.

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 human-induced climate change 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,” about 10,000 years ago, a team of scientists led by Howard Conway of the University of Washington reports in today’s edition of 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.

“It seems like the rate [of melting] that has been going since the early Holocene is similar to the rate right now,” Conway said in an interview. “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.”

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

The ice sheet’s disappearance is of concern because of estimates that its complete melting could raise the global sea level by 15 to 20 feet, swamping low-lying coastal communities around the world.

At the current rate of melting, that will take about 7,000 years, the researchers estimate. Conway said the melting annually contributes about 1 millimeter--nearly one-twenty-fifth of an inch--to sea-level rise.

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

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

Although the study indicates global warming is not causing the melting, climate change remains a problem, Conway said, adding: “Global warming could well speed the process. Our study doesn’t address that problem.”

Environmentalists have grown concerned that industrial chemicals added to the atmosphere are trapping heat like a greenhouse, causing the Earth’s temperature to increase. There is disagreement, however, about the process and how great a hazard it may pose.

Conway’s report is one of three in this issue of Science focusing on the Antarctic ice sheet. In the others:

* Scientists studying satellite-based measurements found a complex system of tributaries feeding major rivers of ice on the West Antarctic Ice Sheet. This web of tributaries forms a transition zone between the sluggish inland ice and the swiftly moving ice streams closer to the margins.

* Using the ages of volcanic debris that erupted onto the ice sheet, researchers reconstructed the past elevation of the West Antarctic Ice Sheet as it began to melt when the last ice age ended. They concluded the sheet was not the source of a massive flow of meltwater into the oceans 10,000 years ago.