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Sea Level May Rise 40 Percent Higher Than Predicted, Study Says

John Roach
for National Geographic News
December 14, 2006
 
Global warming could push sea levels about 40 percent higher than
current models predict, according to a study that takes a new approach
to the calculation.

Most sea level models predict changes based on what we know about how ice sheets melt and warmer waters expand.

These models suggest that by 2100 sea level will be between 4 and 35 inches (9 and 88 centimeters) higher than it was in 1990.

But the physics of how ice sheets melt and how the oceans will expand in a warmer world is still poorly understood.

So Stefan Rahmstorf, an ocean physicist at Potsdam University in Germany, took a different approach: He used studied actual observations of changes in sea level collected in the 20th century to make predictions for the 21st century.

Current models don't jibe with actual sea level rise during recent decades, Rahmstorf says. So he crafted a formula based on a relationship between global temperature and sea level seen during the past hundred years.

"The more the temperature rises, the faster the sea level rises," he said.

In a paper published today in the online advance edition of the journal Science, Rahmstorf applied his formula to 21st-century warming scenarios from the Intergovernmental Panel on Climate Change (IPCC).

His results predict that by the end of the century sea level will rise between 20 and 55 inches (50 and 140 centimeters) above 1990 levels.

"We have much larger uncertainty than we previously thought about the sea level," Rahmstorf said.

Unforeseen Factors

Rahmstorf added that the actual range of uncertainty is probably larger than his calculations suggest.

The IPCC numbers are based on an older assumption that the ice sheets over Greenland and Antarctica will melt by a steady amount over time.

Recent research suggests, however, that ice sheets are melting faster (related news: "Global Warming Is Rapidly Raising Sea Levels, Studies Warn" [March 23, 2006]).

"If something dramatically new happens—something we haven't foreseen—then of course the whole approach [of using observations to make predictions] breaks down," Rhamstorf said.

"We may end up with more sea level rise."

Konrad Steffen is a professor of geography at the University of Colorado at Boulder who studies how melting ice sheets and glaciers contribute to sea level.

He said one wild card that could impact predictions is the so-called dynamic response of the ice sheets to warming.

In the last five to eight years, he noted, the speed at which Greenland's glaciers move toward the sea has sped up dramatically (explore Greenland's changing landscapes with a National Geographic Adventure magazine guide).

Scientists think that meltwater, which pools up on the ice, funnels down to the glacier bed. There, the water acts as a lubricant, allowing the ice to slip seaward more quickly.

The process may last five or ten years, or it may last decades, Steffen said.

"We have hypotheses on what is happening, but we can't model it for the future," he said. "That is where [Rahmstorf] is correct."

High Water Risk

Study author Rahmstorf notes in Science that a sea level rise of 39 inches (1 meter) is plausible if the 20th-century relationship between temperature and sea level holds true in the 21st century.

That much sea level rise would expose major coastal cities such as London and New York to greater storm surges, threatening life and property.

Reducing greenhouse gas emissions can impact such sea level rise, he noted.

"By implementing effective climate policy," he said, "we can stay below the lower end of my range [around 20 inches, or 50 centimeters]."

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