‘Greenhouse’ Threat to Sea Level Scaled Back

Global warming probably will trigger a sea level rise of no more than a foot by the middle of the next century, only a third as great as some previous scientific projections, a team of experts concluded at the end of a special symposium here Thursday.

While any increase in sea level could pose problems for coastal areas, the degree of the threat has been scaled back dramatically, the scientists said. Many projections had predicted an increase of at least three feet over the next 50 years or so.

“The (expected) rise in sea level is much less than we had thought a few years ago,” Mark Meier, director of the University of Colorado’s Institute of Arctic and Alpine Research, said during the fall meeting of the American Geophysical Union.

In a related development, scientists for the National Oceanic and Atmospheric Administration reported in an article in Nature on Thursday that the greenhouse effect will warm the planet unevenly, with temperatures rising more quickly in the Northern Hemisphere. The new climate model showed there will be almost no surface warming in the sub-polar southern ocean, indicating that the Antarctic ice sheet is unlikely to collapse and cause disastrous global flooding.

The National Oceanic and Atmospheric Administration group attributed the uneven distribution of the greenhouse effect in part to the Southern Hemisphere’s larger ocean mass--which will absorb more heat--and to the circulation patterns of the ocean.

The National Oceanic and Atmospheric Administration researchers estimated a rise in sea level of about two feet over the next century, which is consistent with the estimates at Thursday’s symposium.

Jeff Dozier of UC Santa Barbara, who is chairman of the National Academy of Science’s committee on glaciology, organized the symposium because of a growing conviction among the experts that the role Antarctica will play in any global warming scenario had been greatly understated.

The scientists who participated in the San Francisco symposium based their projections of sea level rise on the belief that much of what other areas of the world add to the seas, Antarctica will take away.

According to their scenario, ocean water will evaporate into the warmer atmosphere, lowering sea levels, and then be dumped on Antarctica as snow. And even with rising global temperatures, Antarctica will remain too cold for the snow to melt.

So even if global temperatures rise by several degrees over the next few decades, causing glaciers and snow around the world to melt and flow into the ocean, Antarctica would just get bigger, the scientists said.

A century or so down the road, if global warming were to continue unchecked, the role of Antarctica could change. But the scientists emphasized that they now believe there is considerably less to worry about in terms of sea level rise over the next few decades than they had thought just a few years ago.

Global warming is believed to be caused by the emission of carbon dioxide and other gases from automobiles and industry. These gases accumulate in the atmosphere and act like a greenhouse, trapping heat from the sun.

Dozier said he will seek to have a special panel of the National Academy of Sciences officially endorse Thursday’s conclusions.

“What people overlook is that glaciers can grow in a warmer climate because they will get more snow,” said Jay Zwally, a physicist with the Goddard Space Flight Center who has studied Greenland extensively.

“A warmer atmosphere can transmit more moisture,” he said. Much of that moisture will be carried to high latitudes where it will fall as snow on both the north and south polar regions.

In the early years, both poles will reduce the effect on the seas of global warming, the scientists said.

Because of this, the great glaciers of Greenland will actually thicken during the earliest stages of global warming, said Mike Kuhn, a meteorologist at the University of Innsbruck.

There could come a time in the decades ahead when the temperature at the high northern latitudes would become warm enough to cause much of that snow to melt, and Greenland would begin adding water to the seas.

But that would not happen at Antarctica because of its massive expanse of ice and the uneven warming patterns.

“At Antarctica, the situation is different,” said Charles Bentley, a glaciologist with the University of Wisconsin and an expert on the giant, frozen continent.

Even with global warming, temperatures at Antarctica would remain so cold that the new snowfall would not melt, Bentley said.

“And when the air gets warmer, it will be able to carry a great deal more moisture, so snowfall will increase at Antarctica,” he said. The moisture for that snow will come from the ocean, thus causing sea levels to fall, offsetting snowmelt elsewhere in the world, he added.

Rafe Pomerance, senior associate of the Washington-based World Resources Institute, an environmental think tank, said the symposium’s report was in keeping with 1983 estimates by U.S. Environmental Protection Agency scientists that the seas would rise anywhere from slightly more than eight inches up to four feet by the middle of the next century, assuming the Antarctica ice sheet does not melt.

Even at the lower end, the rise will still cause “significant shoreline retreat,” Pomerance said.

“And I assume that if you don’t stop global warming, the sea level rise keeps growing,” he said. “Sea level estimates have been varying for some time, but I don’t think anybody is saying that the greenhouse effect is not a problem. It will affect precipitation, agriculture, floods, hurricanes--the entire climate system.”

Dye reported from San Francisco and Dolan from Los Angeles