The Great Debate: How Fast Will Sea Levels Rise?

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October 23, 2007

The Great Debate: How Fast Will Sea Levels Rise?

Sealevels_3_5 "We need to go right back to the drawing board on what the ice sheets are about. Fifteen years ago, we thought ice sheets wouldn't respond quickly to global warming because the melting would happen at the surface. This was true, but what we didn't count on was fracturing. This permits water to get to the base of the ice, all the way through the ice sheet. We were really surprised to see this even where the ice core is well below freezing. The water allows glaciers to flow more rapidly, dumping the ice into the sea."

~ Ted Scambos, lead scientist at the National Snow and Ice Data Center at the University of Colorado, Boulder.

A team of scientists are now hard at work in a race against the clock at a remote outpost in northern Greenland. They are attempting to answer one of the most critical questions of global warming: how fast will sea levels rise? It’s a question the experts are eager to find answer for, as the rate at which some glaciers are melting away into the ocean has already doubled, far outpacing former estimates.

The two great polar landmasses, Greenland and Antarctica, together comprise nearly 99 percent of the planet's landlocked ice. The giant ice sheets contain enough frozen water to raise sea levels some 80 meters. But even if only 10 percent of this ice melted, it would flood the world's coasts at levels comparable to those seen in post-Katrina New Orleans. No one is predicting a catastrophe on that scale anytime this century, but scientists are concerned that melting might accelerate beyond all expectation as the planet warms. Especially worrisome is a scenario that glaciologists and climate scientists are still piecing together: rather than slowly but steadily melting, the ice sheets could rapidly break apart into massive chunks.

Recent observations show that some of the major glaciers on Greenland and the West Antarctic Ice Sheet are sliding increasingly seaward. But the processes involved are not well understood, and therefore have not been incorporated into the computer models currently relied on to predict how much sea levels could rise.

Unfortunately, humankind’s current knowledge of the Earth’s geology is not broad enough to adequately gauge the risk of having ice sheets break apart, nor how fast such a breakup would raise sea levels. Much of the bedrock beneath Greenland and Antarctica is unmapped. No one knows how much liquid water lies under the ice. Even a small amount could dramatically speed the breakup of the ice sheets by making the surface below them slippery and less stable. Across both landmasses, scientists are striving to take more precise measurements. Some have installed GPS stations on the ice sheets and the bedrock surrounding the coasts to more accurately calculate loss of ice mass. Others are measuring snowfall and studying how snow becomes ice. Basically, they are hoping to develop a much more realistic estimate for just how much inland ice there is, and how fast it is slipping into the ocean.

Currently, the IPCC's sea-level estimates are based on dated math that use only a few well-understood processes. One is the expansion of seawater as it warms. Another is the melting of mountain glaciers in temperate zones--places like the Alps, Andes, and Himalayas. Third is the melting of the ice sheets' surfaces and the glaciers' seaward migration under the pull of gravity. But the problem is that some of these other processes may actually prove to be much more consequential.

Warmer and higher oceans undermine the glaciers that surround Greenland and Antarctica, pulling them away from their seabed anchor. With those bulwarks softened, inland glaciers slide much faster toward the sea. In a synergystic process, water gushes down through fractures and holes in the Greenland ice sheet, making it even easier for the glaciers to slide. As the glaciers reach lower, warmer elevations, the melting and sliding will only accelerate.

And that's what already appears to be happening. Recent observations have shown that the movement of major glaciers in both Greenland and the West Antarctic Ice Sheet is in fact speeding up.

"The current dynamical changes that we are seeing on the ice sheet are not captured in any climate model," says Prasad Gogineni, director of the Center for Remote Sensing of Ice Sheets at the University of Kansas. "That seems to indicate a huge uncertainty." Today's climate models, he says, simply can't be relied on to predict what will happen to the great ice sheets.

The IPCC acknowledges that there are many questions still waiting for answers that could greatly alter the equation, and that we need to keep and open mind to the possibilities revealed through future research. However, one of the IPCC’s central messages is that humans can divert some of the unpleasant predictions if we start making global changes early on.

IPCC Chairman, Dr R.K. Pachauri has stated, “Let me emphasize that adaptation alone will not do. We need to bring about mitigation actions to start in the short term even when benefits may arrive in a few decades. And there are huge co-benefits from mitigation action in terms of energy security, in terms of local environmental benefits. The cost of adaptation and impacts, I might mention, will keep going up as the global temperature goes up. 

But some worry that the IPCC predictions, dire as they are, may actually be presenting too rosy of a picture. Greenland's melting ice sheet is now contributing more than half a millimeter per year to sea-level rise, according to a study coauthored by Eric Rignot, a senior research scientist at NASA's Jet Propulsion Laboratory who has collaborated with Gogineni and others at the University of Kansas. The new data is noteworthy because it's more than twice the upper limit of Greenland's contribution as estimated by the IPCC in its report earlier this year. "The [existing models] don't assume any change in velocity in the glaciers, except on very long time scales," Rignot says. "What we are seeing today is that those glaciers do speed up in a significant fashion in response to climate warming."

The melting of the ice sheets is really just getting started, says Rignot. The resulting loss of ice will dwarf any increases in snowfall, Rignot says. Rignot disagrees with the conclusions drawn by the IPCC. He believes oceans will rise more than a full meter before the end of the century, nearly twice the upper bound of the IPCC's predictions. "We have to acknowledge that we don't have reliable ways to predict what ice sheets will do, but that they will certainly react much more strongly to climate warming in the future," he says. "There is no reason to alarm people that the end of the world is coming. But there is no reason to reassure them, either, that there is nothing to worry about with the ice sheets."

Meanwhile, the scientists are also seeking clues from the past. Next summer, the scientists will return to the Greenland base and start drilling out an ice core, boring about 2,500 meters to bedrock. They are particularly interested in one key geologic period, called the Eemian interglacial. During this stretch of time, from around 130,000 to 115,000 years ago, the planet warmed. Greenland hit temperatures 7 to 8 ºC warmer than today's, and sea levels surged at least three to five meters higher than they are now. If this happened today, much of south Florida, Bangladesh, and many other low-lying coastal areas and islands would be completely submerged. A sample of ice spanning the whole Eemian period would provide a wealth of information. Trapped air bubbles inside ice contain samples of the old atmosphere. The thickness of ice layers can reveal how much snow fell. And bits of dust and organic matter will allow for much more accurate dating. Armed with a wealth of new information, perhaps finally the riddle will have a chance at being solved.

Posted by Rebecca Sato. Photo Credit: Google Earth.

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links:

http://www.technologyreview.com/Energy/19504/?nlid=616
http://www.ipcc.ch/

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