Surprise theory behind big Antarctic thaw

Amid increasing evidence of global warming, scientists have pointed to thawing polar icecaps in Antarctica and rising sea levels that threaten to flood low-lying islands the world over as side effects of the planet heating up.

But new evidence from Antarctica published today in the journal Science indicates the thawing of at least one major ice field is a natural event that has little to do with global warming.

Aside from challenging accepted theories behind global warming, these findings illustrate the complexities that go along with predicting how the earth will react to high levels of carbon-fuel consumption and deforestation, supposedly the two main causes of global warming.

At issue is the West Antarctic Ice Sheet. This frozen wasteland measures 360,000 square miles, an area larger than Texas and Colorado combined. The WAIS is one of the largest ice sheets on the planet, and for that reason, it could have an enormous effect on sea levels worldwide.

Unlike any other ice sheet, the WAIS is anchored to land below sea level - what's known as a marine ice sheet. Because of its ocean exposure and position on the sea floor, scientists say it is potentially unstable and could be more affected by changes in sea level or ocean temperature than other ice fields.

Sure enough, the WAIS is changing faster than any other ice sheet. It has shrunk at a rate of about 400 feet per year during the past 7,600 years, according to the scientists.

On the retreat

All told, the WAIS has receded about 800 miles since the height of the last ice age, 10,000 years ago. This hasty retreat has caught the eye of geologists, biologists, and other scientists due to the catastrophic implications.

"If the Western Antarctic ice sheets were to collapse, sea levels would rise" 16 to 20 feet, says Brenda Hall, a researcher at the University of Maine and co-author of the paper. According to the researchers' calculations, the Western Antarctic ice shield could completely collapse in the next 7,000 years.

But the thawing of the WAIS long predates the period when mankind began to change the chemistry of the atmosphere through fossil-fuel burning and deforestation. In fact, during most of the period of the ice sheet's thaw, global temperatures were relatively stable. Furthermore, the pace at which the WAIS is thawing has not appreciably sped up during the recent decades when global warming has really begun to show up. This implies that global warming, at least so far, has little to do with the shrinking of the WAIS.

"What we are suggesting is that the ice sheet may continue to retreat regardless of any global warming or future rise in sea level," says Dr. Hall.

Rather, scientists believe this phenomenon is linked to the thawing of ice sheets in the northern hemisphere that coincided with the end of the last ice age. The link is not well understood but could involve changes in ocean currents that occurred after the ice age ended, changing ocean temperatures.

Experts, however, are quick to point out that global warming could speed up the thawing of ice fields in the future.

"Certainly warming up the oceans or expansion of the ocean could help the ice sheet retreat more quickly," says Howard Conway, a University of Washington geophysicist and lead author of the Science article.

But Dr. Conway also admits that the thawing will probably continue regardless of whether mankind gets their global warming affairs in order. Alternatively, global warming could also make the ice fields grow.

"With continued warming there might be increased precipitation down there which would feed the ice sheet as well," says Conway, alluding to the scenarios that could all plausibly play out as a result of global warming.

Global warming's role

For their part, environmental groups believe that this research is a constructive contribution but not a turning point in the global-warming debate. Other recent findings have linked forest fires with droughts and jet-aircraft emissions with significant atmospheric changes that may augment global warning. And the volume of research that validates global warming grows almost weekly.

"Over the past century, average global temperatures have risen by 1 degree Fahrenheit. The 10 hottest years have all occurred since 1980. As a result, most scientists agree that's due in part to human pollution. Such a change is unlikely to have occurred through natural forces alone," says Jeff Fiedler of the Natural Resources Defense Council.

(c) Copyright 1999. The Christian Science Publishing Society