If the earth's temperature heats up as mainstream scientists predict it will over the next century, the average level of the world's oceans could rise by about a foot and a half by 2100, and somewhat more after that, as glaciers partly melt and the heating makes sea water expand, inundating many low-lying coastal areas.
But it would be trivial compared with the melting or collapse of the huge West Antarctic ice sheet, the Godzilla of the game of rising seas. This would loose so much water into the oceans that they would rise by a truly catastrophic 16 to 20 feet. There, among other things, would go most of Florida and a sizable chunk of Manhattan.
Could that really happen?
Scientists have been unable to point with confidence to any instance in which it has happened in the past, in any sort of climate that compares with today's or with the climate that is expected in the decades and centuries just ahead.
In the past, natural forces have caused the earth's temperature to vary widely.
All the planet's permanent ice virtually disappeared during the super-high temperatures of the Cretaceous period, which ended some 65 million years ago. The extent to which it may have melted periodically since permanent ice became re-established as early as 15 million to 20 million years ago has been unclear.
Now a new study by investigators in Sweden and the United States has turned up what is described as evidence that the West Antarctic ice sheet, or part of it, did indeed collapse at least once during an ancient interglacial period, or interval of warmth between ice ages.
The world is in another such interglacial period now, and many scientists believe its warmth will be intensified by emissions of heat-trapping industrial waste gases like carbon dioxide.
The researchers at Uppsala University in Sweden and the California Institute of Technology, led by Dr. Reed P. Scherer, analyzed sediments recovered from the area beneath the West Antarctic ice and determined that they contained the remains of tiny marine algae called diatoms, along with a chemical tracer called beryllium-10.
Together, they report in the current issue of the journal Science, these clues "provide evidence that marine conditions prevailed deep within the West Antarctic interior" at some time within the last two million years. This, they wrote, suggests that the West Antarctic ice sheet had collapsed to some degree at least once within the same period.
The researchers nominate an unusually long interglacial interval about 400,000 years ago as the most likely time for the collapse of the West Antarctic ice sheet.
Scherer had made similar findings earlier on the basis of diatoms alone, but the findings were criticized on the ground that the diatoms could have got into the sediment samples by other means than melting of the ice.
That is true of the new study as well, said Dr. Lloyd Burckle, a micropaleontologist who works with diatoms at the Lamont-Doherty Earth Observatory at Columbia University.
But others said the new evidence, while not conclusive, was strong.
Collapse of the ice sheet is the best explanation for the placement of the diatoms and the beryllium-10, said Dr. Richard Alley, a paleoclimatologist at Pennsylvania State University.
Scherer's "original result was correct and he has strengthened it," Alley said.
The significance of the finding, he said, is that it "strengthens the possibility" that the West Antarctic ice sheet could collapse in the future; having done so once, it could do so again.
The reason the ice sheet is so vulnerable is that alone among the world's big ice sheets (including a much larger one in East Antarctica), it lies largely below sea level.
That means it is more open to melting as warming ocean waters lap at its margins; by comparison, warmer air melts the ice much more slowly. The ice sheet's margins consist of vast, floating ice shelves as big as Spain.
If these were to melt, they would not cause the sea level to rise, since they are already floating on the ocean.
But they are fed by large, moving rivers of ice that flow from the interior of the West Antarctic ice sheet, and scientists believe that the removal of the shelves could allow the rivers to flow faster, as if a cork were removed from a bottle.
This could deplete the interior reservoir of the West Antarctic ice sheet itself, which does not float but is instead grounded on an archipelago below sea level. If the flow of the moving ice streams did speed up, it might deplete the ice faster than freezing precipitation could replace it, leading to a net outflow of fresh water into the ocean.
How long this might take is a critical question.
The Scherer study does not shed any light on it.
"He can't demonstrate that it went in a hurry, just that it went," Alley said.
But few if any experts believe that the ice sheet is likely to collapse in anything less than a few centuries in response to global warming, whether natural or caused by human activity.
Scientists advising the United Nations predict an average surface warming of about 3.5 degrees Fahrenheit by the year 2100 if emissions of carbon dioxide and other heat-trapping gases are not reduced.
That amount of warming compares with a warming of 5 to 9 degrees since the depths of the last ice age.
With the predicted warming, which would continue beyond the 21st century by present calculations, it would probably take 500 to 700 years for the West Antarctic ice sheet to disintegrate fully, producing the rise in sea level of up to 20 feet, according to a review of research on the subject by Dr. Michael Oppenheimer, a senior scientist with the Environmental Defense Fund, published in the journal Nature in May. Alley said he considered this a reasonable assessment.
But the die could be cast much earlier, Oppenheimer calculated. The climate could warm enough by the end of the next century to make an eventual collapse of the ice sheet inevitable, he said. He called the latest Scherer study "critically important" because it suggests that the ice sheet disintegrated in global temperatures that could not have been much higher than current levels.
There are many uncertainties in all this, as Oppenheimer pointed out.
Not least, global warming is expected to result in more precipitation, as more water evaporates from the oceans. This is likely to build up the ice sheets more rapidly as precipitation falls and freezes. It is far from clear how the tug-of-war between increased buildup of ice on one hand and an accelerated flow of the ice streams on the other would turn out.
Two other new pieces of research, published in the current issue of Nature, suggest that the speed of the ice streams' flow would be dictated by the underlying geology of Antarctica. The ice streams flow along relatively narrow basins made slippery by a combination of fine sedimentary particles and melted ice water.
Several things can slow the flow, including rough protuberances sticking up through the slippery path and friction along the sides of the flow paths.
Moreover, said Alley, who is a co-author of one of the Nature papers, if the underlying topography contains many sedimentary basins along which more rapidly flowing ice can escape, the ice sheet would deplete faster.
But "if the ice streams can't get somewhere because the geology is wrong," he said, "that essentially buys us time because the ice can't change and move as rapidly." The primary authors of the Nature papers are Robin E. Bell of the Lamont-Doherty observatory and Sridhar Anandakrishnan of Pennsylvania State.
However fast the ice streams might flow if the floating shelves that cork it up were removed, Oppenheimer said, present knowledge suggests that the streams would probably flow not like water or wine out of a bottle but rather like stubborn ketchup.
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