Meanwhile, the global surface temperature increased 0.6° Celsius (1.8° Fahrenheit) in the last century. Oceans have become warmer, too, expanding while storing heat. This has caused sea levels to rise 4 to 8 inches (10 to 20 centimeters) in the last hundred years.
While most scientists agree that higher greenhouse gas concentrations, particularly carbon dioxide, are causing global warming, a few scientists argue that atmospheric carbon dioxide levels may not be the only culprit, because they have remained relatively steady for the past 30 years.
At the present temperatures, about half of the snow that falls on Greenland melts and runs off as water. The rest of it stays and is discharged in the form of icebergs.
An increase of 3° Celsius (5.4° Fahrenheit) would change that equation, producing an increase in melting that will outweigh the increase in snowfall, according to Gregory.
"The warmer it gets, the more melting there is," he said. "You would also expect more precipitation, but most studies suggest that the increase in melting would be bigger. Beyond that threshold [of a 3° Celsius temperature increase], the ice sheet will likely not be viable and would just get smaller and smaller."
The study considered the climate sensitivity of a range of climate models and a range of carbon dioxide scenarios, from 450 parts per million, the lowest level considered by the United Nation's Intergovernmental Panel on Climate Change, to a thousand parts per million, or four times the pre-industrial carbon dioxide concentration.
"They demonstrate that a warming of about 3° Celsius for ongoing melting of the Greenland ice sheet is exceeded by 2100 for the majority of cases considered," said John Church, a climate scientist at the Australian government CSIRO Marine Research center in Hobart who was not involved in the study.
In the most extreme scenario, using a carbon dioxide level of 1,000 ppm, the study predicts temperatures to rise by 8° Celsius (18° Fahrenheit) by the year 2050. This, in turn, would raise sea levels by 7 meters (23 feet) in a thousand years.
"This is a high concentration, but it is within the range of scenarios that people have considered," Gregory said. "It's not a completely outrageous number."
If rising temperatures were to cause melting of Greenland's ice sheet, the process would be gradual. There's no evidence that the ice sheet would catastrophically disintegrate. Even the worst scenario is unlikely to alter the world map.
"If you were to raise the sea levels by seven meters [23 feet] and look at a map of the world, you probably wouldn't think it was startlingly [different]," Gregory said. "But of course many of the places where a lot of people live are close to sea level."
Many cities and communities along the U.S. Eastern Seaboard are at least partly below this level.
"Sea level rise has the potential to affect millions of people living in low-lying coastal regions, particularly the inhabitants of megacities developing on coasts around the world and those living on deltas of major rivers and small island nations," Church said.
Gregory and other scientists warn that even if the composition of the atmosphere could be reversed to pre-industrial conditions, the sea level rise could be irreversible. Once it's gone, the Greenland ice sheet is unlikely to be reestablished.
"Sea level represents one of the longer time scale responses found in the climate system," said Ronald J. Stouffer at the U.S. National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey.
"It may not be possible to stop or reverse this process once it has been underway for a period of time, even though the climate returns to a relatively cool state," Stouffer said.
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