Global warming 'severe' risk to coastlines, researchers say

By Bryn Nelson
Newsday

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At that height, oceans likely would cover much of low-lying areas such as Florida, Bangladesh and the Netherlands.

A permanent loss of the ice cover on Greenland could be triggered by a rise in the island's average year-round temperature of 5.4 degrees Fahrenheit or more, the study suggests, an effect precipitated by increasing emissions of carbon dioxide and other greenhouse gases into the atmosphere.

"Even if atmospheric composition and the global climate were to return to pre-industrial conditions, the ice sheet might not be regenerated, which implies that the sea-level rise could be irreversible," the scientists write in today's issue of the journal Nature.

Climate-modeling studies traditionally have carried a bevy of uncertainties, but lead author Jonathan Gregory, a climate scientist at the Hadley Centre for Climate Prediction and Research in the United Kingdom, said the "real possibility" of the ice loss represents "a severe consequence, so that is something one should think carefully about."

Gregory said his study applies previously developed modeling applications to the specific question of what might happen to Greenland's ice sheet under an array of warming scenarios.

Scientists consider carbon dioxide the main greenhouse gas contributing to global warming. Pre-industrial levels of the gas have been estimated at 280 parts per million, with current levels at about 370 parts. Most climate studies have predicted gas levels beyond 450 parts by midcentury.

For the study, the researchers combined climate models based on atmospheric and ocean circulation patterns with a range of levels at which carbon-dioxide levels might stabilize over the next 350 years — from 450 parts per million to 1,000 parts per million.

In all but one of 35 resulting scenarios, Greenland's average annual temperature climbed by more than 4.9 degrees, compared with temperatures in 1990.

After this threshold, past studies indicate, snowmelt exceeds snowfall, initiating a contraction of the ice sheet. Beyond a 5.5-degree temperature rise, models have predicted shrinkage to the degree that only residual mountain glaciers would remain. The most extreme scenarios envision temperature gains of 14 degrees or more and a virtual meltdown in as little as 1,000 years.

The study contains several caveats. Experts say summer temperature increases — not average annual increases — are most relevant to ice melt. And no firm "point of no return" has been set for summer warming alone.
 
The researchers found that summer warming scenarios offer a somewhat rosier picture, with carbon-dioxide levels stabilizing at a level below the point of expected ice loss in 11 of 35 cases.

Michael Oppenheimer, a professor of geosciences and international affairs at Princeton University, said the findings are consistent with past conclusions. But Woods Hole Oceanographic Institution physical oceanographer Raymond Schmitt said the climate models used don't adequately account for ocean warming and heat capacity.

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