May 15, 2009
Study Halves Prediction of Rising Seas
By ANDREW C. REVKINA new analysis halves longstanding projections of how much sea levels could rise if Antarctica’s massive western ice sheets fully disintegrated as a result of global warming.
The flow of ice into the sea would probably raise sea levels about 10 feet rather than 20 feet, according to the analysis, published in the May 15 issue of the journal Science.
The scientists also predicted that seas would rise unevenly, with an additional 1.5-foot increase in levels along the east and west coasts of North America and the east coast of southern Africa. That is because the shift in a huge mass of water away from the South Pole would subtly change the shape and rotation of the Earth, the authors said.
Several Antarctic specialists familiar with the new study had mixed reactions to the projections.
But they and the study’s lead author, Jonathan L. Bamber of the British Glaciology Center, agreed that the odds of a disruptive rise in seas from warming over the next century or so remain serious enough to warrant the world’s attention.
They also uniformly called for renewed investment in ice-probing satellites 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, would 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 underlying bedrock, both direct and by satellite.
It did not assess the pace or 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, at NASA’s Jet Propulsion Laboratory, described the new paper as “good solid science,” but added that the sea-level estimates cannot be verified without renewed investment in satellite missions and other initiatives that are 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 longtime specialist in polar ice at NASA’s Goddard Space Flight Center, said the study only provided 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 thattime.
“Even in Bamber’s world,” he said, referring to the study’s author, “there is more than enough ice to cause serious harm to the world’s coastlines.”
