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