Glaciers melt as land, seas rise
Global warming
Sea levels would rise by about 10 feet rather than the earlier projection of 20 feet, according to a recent study, writes Andrew C Revkin. In Alaska, as the glaciers melt, land rises, points out Cornelia Dean
A new analysis halves long-standing 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 recent 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. That is because the shift in a huge mass of ice away from the South Pole would subtly change the strength of gravity locally and the 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 Bristol Glaciology Center in England, agreed that the odds of a disruptive rise in seas over the next century or so from the build-up of greenhouse gases remained serious enough to warrant the world’s attention.
They also uniformly called for renewed investment in satellites measuring ice and field missions that could within a few years substantially clarify the risk.
How fast?
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 modelling 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 the 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.
Robert Bindschadler, a specialist in polar ice at NASA’s Goddard Space Flight Center, said the prime question that remains is what will happen in the next 100 years or so.
“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.”
Here, land rises as glaciers melt
Global warming conjures images of rising seas that threaten coastal areas. But at Juneau in Alaska, as almost nowhere else in the world, climate change is having the opposite effect: As the glaciers here melt, the land is rising, causing the sea to retreat. Morgan DeBoer, a property owner, opened a nine-hole golf course at the mouth of Glacier Bay in 1998, on land that was underwater when his family first settled here 50 years ago.
“The highest tides of the year would come into what is now my driving range area,” DeBoer said. Now, with the high tide line receding even farther, he is contemplating adding another nine holes. “It just keeps rising,” he said.
Relieved of billions of tons of glacial weight, the land has risen much as a cushion regains its shape after someone gets up from a couch. The land is ascending so fast that the rising seas, a ubiquitous byproduct of global warming, cannot keep pace. As a result, the relative sea level is falling, at a rate “among the highest ever recorded,” according to a 2007 report by a panel of experts convened by Bruce Botelho, Juneau’s mayor.
Greenland and a few other places have experienced similar effects from widespread glacial melting that began over 200 years ago, geologists say. The effects are more noticeable in and near Juneau, where glaciers are retreating 30 feet a year or more.
The region faces unusual environmental challenges. As the sea level falls relative to the land, water tables fall, too, and streams and wetlands dry out. Land is emerging from the water to replace the lost wetlands, shifting property boundaries and causing people to argue about who owns the acreage and how it should be used. And meltwater carries the sediment scoured long ago by the glaciers to the coast, where it clouds the water and silts up once-navigable channels.
The topographical changes have threatened crucial ecosystems and even locally vital species like salmon. Relative to the sea, land here has risen as much as 10 feet in little more than 200 years, according to the 2007 report. As global warming accelerates, the land will continue to rise, perhaps three more feet by 2100, scientists say.
New York Times News Service
