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]."
Free Email News Updates
Best Online Newsletter, 2006 Codie Awards
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES