Research shows that a warmer earth may not melt polar ice and cause sea levels to rise, as has been feared so far.
THERE is widespread belief that global warming will melt the polar ice caps and submerge large areas of inhabited lands. But a review of the latest scientific research published in New Scientist (Vol 135 No 1833) points to a totally opposite possibility. Says Garry Davidson of the geology department at the University of Tasmania in Australia, "Overall, the evidence seems conclusive that past ice sheets grew when the average temperature was higher. So the future of the ice at least looks less bleak than some early estimates of the impact of global warming have suggested."
Researchers have been trying to look back into the past to understand what may happen in the future. During the past 1.6 million years, the world switched from a glacial period to a milder interglacial period about 9 times largely because of the changes in the earth's orbit and tilts in its axis.
David Sudgen at the University of Edinburgh and George Denton and David Marchant at the University of Maine, studied the East Antarctic ice sheet, a volume of ice 4 kilometres thick. It is the behaviour of ice sheets such as these which will determine future sea levels. If all the ice in these sheets melted, ocean levels would rise by 50-60 metres.
The team visited the McMurdo area of the trans-Antarctic mountains, where dry ground sticks out though the ice. The perimeter of the ice has retreated and expanded at various times, showing how the ice sheets behaved during warmer and cooler periods. According to Sudgen, the ice sheet expanded during warm periods.
Nick Hulton of Edinburgh University, who has studied the Greenland ice sheet using a computer model, claims that the ice sheet will shrink at the edges but grow in the centre, as a result of increased snowfall. The net effect on the sea level will be negligible.
Says Davidson: "The most likely explanation is that mild global warming brings a net increase in the amount of snow at the poles rather than a net melting. In a warmer world, more water evaporates from the oceans, to be transported to the poles to become snow." The extra melting during the hotter summer is obviously not enough to override the effect of extra snow due to air circulation, says Davidson.
Interesting evidence has also come from scientists drilling for sediments in the Antarctic waters. Studies conducted by Eugene Domack of Hamilton College in New York, Timothy Jull of the University of Arizona and Seizo Nakao of the Geological survey of Japan showed that the types of sediment found near ice-covered continents change with climate. When the ocean is free of ice, the sediments have large quantities of microscopic algae called diatoms -- as is the case with sediments of the last 4,000 years. But from 4,000 to 7,000 years ago, when there was an ice shelf over the ocean, the sediments largely consisted of silty sands and gravels. Before about 7,500 years ago, the algae were again present in large quantities. This evidence surprised the researchers because it showed the Antarctic ice sheets were expanding when the world was, on an average, 20C warmer than today.
The modern world is, however, giving out confusing signals. During the past century, the earth has warmed by 0.60C. and indeed, snow lines in regions such as Arctic Canada, Baltin Island and Alaska are spreading out and moving to lower altitudes. The Greenland ice sheet has been thickening and some coastal and interior sites in Antarctica have accumulated ice over the past 80 years. But glaciers in mountains have been receding and some ice shelves in the Antarctic peninsula are disintegrating.
Davidson points out that detailed information to predict the effects of global warming is still lacking. It is also a matter of conjecture whether the past can predict the future.
Temperature change because of carbon dioxide increases in the atmosphere could be far more rapid than the changes in temperature that took place in the past because of changes in the earth's orbit and axis. Moreover, temperatures may well increase higher than ever in the past and this could tip the balance of polar ice towards melting, leading to an inexorable rise in sea levels, warns Davidson.
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