Study Halves Prediction of Rising Seas

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They also uniformly called for renewed investment in satellites measuring ice and field missions that could within a few years substantially clarify the risk.

There is strong consensus that warming waters around Antarctica, and Greenland in the Arctic, will result in centuries of rising seas. But glaciologists and oceanographers still say uncertainty prevails on the vital question of how fast coasts will retreat in a warming world in the next century or two.

The new study combined computer modeling with measurements of the ice and the underlying bedrock, both direct and by satellite.

It did not assess the pace or the likelihood of a rise in seas. The goal was to examine as precisely as possible how much ice could flow into the sea if warming seawater penetrated between the West Antarctic ice sheet and the bedrock beneath.

For decades West Antarctic ice has been identified as particularly vulnerable to melting because, although piled more than one mile above sea level in many places, it also rests on bedrock a half mile to a mile beneath sea level in others. That topography means that warm water could progressively melt spots where ice is stuck to the rock, allowing it to flow more freely.

Erik I. Ivins, a scientist at NASA’s Jet Propulsion Laboratory, described the new paper as “good solid science,” but added that the sea-level estimates could not be verified without renewed investment in satellite missions and other initiatives that were currently lagging.

A particularly valuable satellite program called Grace, which measures subtle variations in gravity related to the mass of ice and rock, “has perhaps a couple of years remaining before its orbit deteriorates,” Dr. Ivins said. “The sad truth is that we in NASA are watching our Earth-observing systems fall by the wayside as they age — without the sufficient resources to see them adequately replaced.”

Robert Bindschadler, a specialist in polar ice at NASA’s Goddard Space Flight Center, said the study provided only a low estimate of Antarctica’s possible long-term contribution to rising seas because it did not deal with other mechanisms that could add water to the ocean.

The prime question, he said, remains what will happen in the next 100 years or so, and other recent work implies that a lot of ice can be shed within that time.

“Even in Bamber’s world,” he said, referring to the study’s lead author, “there is more than enough ice to cause serious harm to the world’s coastlines.”