Antarctic Ice May Melt, But Not For Millenia

Antarctic Ice May Melt, But Not For Millennia

by Richard Harris

JPL/NASA
Scientists are concerned that the melting of the West Antarctic ice sheet could contribute to sea levels rising by 15 or 20 feet, but it's likely to take at least 1,000 years for that to happen. Ben Holt, Sr.

“The West Antarctic ice sheet is, in some ways, the planet's Achilles' heel.”

The PBS series NOVA has teamed up with National Geographic on a project called "Extreme Ice" that follows adventure photographer James Balog and a team of scientists through the world's icy regions in the largest-ever photographic study of the cryosphere.
March 18, 2009In The Arctic, A Time-Lapse View Of Climate Change

“We certainly don't need a collapse of the ice sheet to cause major problems with sea level rise.”Stefan Rahmstorf, professor of physics of the oceans, Potsdam University

Up In Greenland...
Scientists have been studying how melting ice in Greenland is affecting the flow of ice sheets over the bedrock. Runoff meltwater from the surface makes its way underneath the ice, lubricating the ice sheet and making it flow faster to the coast where it ultimately contributes to rising sea levels. Below, photographs from Richard Harris' recent reporting trip to Greenland.

March 19, 2009 · A huge chunk of Antarctic ice can't withstand nonstop global warming, according to a new study published in the latest Nature magazine. And if it melts, the ice will raise the global sea level by 15 or 20 feet — or more.

The only good news here is the catastrophe isn't likely to unfold quickly.

The ice in question is called the West Antarctic ice sheet. In some ways, it's the planet's Achilles' heel. It holds a vast amount of water, locked up as ice, and it's sitting below sea level, so it's inherently unstable.

Research On The West Antarctic Ice Sheet

David Pollard at Penn State University says there has been intense research recently to figure out how the ice sheet has behaved over the past 5 million years.

"Before there was only a vague idea of how the West Antarctic ice sheet grew and decayed over those time scales," he says.

Now, a scientific drilling project has brought back sediment samples taken from underneath the ice sheet, allowing scientists to study the mud layers, like so many tree rings, to show what ice there has done over history.

"It's really exciting," Pollard says. "They've shown it really has collapsed and re-grown, multiple times."

Pollard and a colleague have taken that detailed information and asked what it portends for the future of the West Antarctic ice sheet.

"The main reason that it collapsed in the past is the ocean has gotten warmer around the periphery of Antarctica, increasing the rate of melting of these floating ice shelves which fringe West Antarctica," Pollard says.

These floating ice shelves act like buttresses to keep the much larger ice sheet pinned back. And whenever the shelves melt away, the ice behind them flows into the sea and sea levels rise.
Warming ocean water around Antarctica, by a maybe 2 to 5 degrees Celsius, could trigger that chain of events, Pollard says. That degree of ocean warming is not forecast for this century, but at the rate the planet is heating up, it seems inevitable at some point. But Pollard's study indicates that the West Antarctic ice sheet won't melt away too rapidly. He figures that will take at least 1,000 years, and more likely 2,000 to 3,000 years.

But instead of being reassured by this long time horizon, Pollard says, "I'd say I feel more nervous."

That's because there's now a clear history showing this massive ice sheet has melted before, under conditions that the Earth may soon experience. And while the full effect may not unfold for thousands of years, it would transform the planet into a place we would not recognize today.

Behavior Of Ice Still Unknown

Stefan Rahmstorf at Postsdam University in Germany says there are still so many unknowns about how Antarctic ice behaves that Pollard's study is surely not the final word on this subject. Nor does it have to be.

"We certainly don't need a collapse of the ice sheet to cause major problems with sea level rise," he says.

Even if Antarctica contributes little or no water to the oceans this century, Rahmstorf says, there's a growing consensus that seas are likely to rise by at least two or three feet — and quite possibly more — before the end of this century "unless of course we stop the global warming fairly soon."

Rahmstorf is not involved in the current Antarctica research, but he was at a scientific meeting last week in Copenhagen about rising seas and other aspects of global warming. Rahmstorf says Europe's global warming policy at the moment is built around a goal to limit global warming to about 4 degrees Fahrenheit.

"Just like we have this temperature limit, we should also have a sea level limit," he says.
He advocates setting that "sea level limit" at about three feet of sea level rise. Even if that can be accomplished, many vulnerable low-lying places on earth would be swamped, Rahmstorf acknowledges. But it's hard to imagine doing any better. And, as West Antarctica reminds us, we could easily do much, much worse.