Post by Bozur on Mar 20, 2009 23:51:20 GMT -5
Scientists: Ice cores show Antarctica is melting fast
chicagotribune.com — A 40,000-year cycle is out of whack--it should be cooling, but Antarctica is 4.5 degrees warmer than a century ago, and melting. The last time it melted this fast, ice cores show atmospheric CO2 was at 400 parts per million. Everything melted. We're now at 386 parts per million, and rising 1 ppm every year. More… (Environment)
-------
Great polar melt-off feared
Global warming, human activity speed Antarctic thaw, experts say
By William Mullen | Tribune reporter
March 19, 2009
For the last 5 million years, the frozen polar ends of the Earth have melted on a regular basis, raising sea levels dramatically to heights that, if achieved today, would inundate most of the world's major cities and coastal areas where billions of people live.
Scientists studying those polar freeze/thaw cycles reported in two papers in Thursday's edition of the research journal Nature that it appears Earth is headed toward another thaw—and this time, it's being hurried along by carbon dioxide pollution in the atmosphere.
The research dealt specifically with the ice sheets that sit atop West Antarctica, which contain enough water that world sea levels would rise 16 feet if it all melted. Such a thaw would take a thousand years at least, a long time in human terms but a blink in geological time.
The new papers both drew on core samples extracted from the Antarctic Ocean floor in 2006 as part of the ANDRILL project, one of the largest scientific undertakings ever for the continent. The project involved 53 scientists and was co-directed by Northern Illinois University geologist Ross Powell.
By examining millions of years' worth of sediments, researchers found that the ice in West Antarctica collapsed and melted about every 40,000 years during the Pliocene epoch 3 to 5 million years ago—a time when there were warm spells "similar to those projected to occur over the next century," Powell said.
When the polar ice began melting on a massive scale, atmospheric carbon dioxide levels were up to around 400 parts per million, Powell said. "We are now at 386 parts per million and rising," he said, and it grows by one part per million every year.
The concern, he said, is that the current rise in carbon dioxide levels—driven by human activity over the last 200 years, mostly the burning of fossil fuels—is causing unprecedented global warming and putting West Antarctica on the fast track to melting.
The Earth's average annual temperature has risen 1.4 degrees Fahrenheit in the last 100 years, but over Antarctica, which holds 70 percent of the world's fresh water as ice, it has risen 4.5 degrees.
"Even if it might take a thousand years or more for the West Antarctic Ice Sheet to disappear, the melting before then could be significant enough that humans should really be taking note ... as we worry about our future generations," Powell said.
Natural polar freeze/thaw cycles occur because of a periodic shift in the tilt of the Earth's axis, known as the Milankovitch Cycle.
"The tilting changes the amount of radiation absorbed into each hemisphere of the Earth, depending on which hemisphere is tilted closest to the sun," said Powell. With the change comes a gradual buildup of atmospheric carbon dioxide that ANDRILL records show eventually caused drastic loss of ice in West Antarctica.
But now, human activity appears to be having its own effect on the world's climate by driving temperatures higher than they otherwise would be, said Northern Illinois University geologist Reed Scherer, also on the ANDRILL research team. Some climatologists believe global temperatures should even be slightly cooling at this time.
"If something is an external cycle, it should be predictable," Scherer said. "But it is much more complicated than that, and we seem to be throwing the pattern off balance now."
Also in Nature was a report from David Pollard of Pennsylvania State University and Robert DeConto of the University of Massachusetts, who used ANDRILL data to simulate Antarctic ice sheet variations over the past 5 million years.
The two climate modelers found that the ice sheet atop West Antarctica could move between full, intermediate and collapsed states over only a few thousand years.
Today, even a partial melt-off raising sea levels by 4 feet would put at risk an estimated half a billion people who live along shorelines.
DeConto said warming ocean temperatures play a key role in how fast polar ice melts, both the ice sheets and the floating ice shelves to which they are attached. The shelves extend for miles into the ocean around Antarctica.
"The next big step," said DeConto, "is to determine what is happening to the ocean temperatures under the ice shelves and around the ice sheet. We really need that information."
www.chicagotribune.com/news/local/chi-antarctica-climate-change-19mar19,0,7871508.story