Sea level rise not as high as expected

Tad Pfeffer and colleagues calculated that existing models of sea-level rise would require extremely fast delivery of ice sitting on land to the sea, something that is not plausible according to our current understanding of glacier dynamics. However they stress that the result is not cause to be complacent – a rise of 0.8 m would still be catastrophic, especially for the millions of people living near coastal areas around the world.

"We have estimated limits on sea-level rise during the next century by considering simple constraints on glacier and ice sheet motion," Pfeffer told environmentalresearchweb. "Our work suggests that 0.8 m of sea level rise is plausible, 2 m is only possible under extreme conditions and more than 2 m is unlikely."

Moving ice
Pffefer's team obtained the results by calculating the mass of water that glaciers and ice sheets required to produce rates of sea level rise ranging from 2 to 5 m in the next century. Next, the researchers subtracted the ice that would melt in this time and then calculated how fast the remaining ice would need to move to achieve these amounts of sea-level rise by 2100. They did this by considering the cross-sectional "gates" of the glaciers, which drain ice from land to sea. "Ice must drain through these gates, and we calculated how fast the ice would have to move through them," explained Pfeffer.

In a second analysis, the team calculated the sea-level rise that would occur as a result of losses from glaciers and ice sheets using "reasonable" assumptions of ice-flow speed. A range of projections from 0.8 to 2 m were obtained, taking the thermal expansion of ice into account.

Although the world's other glaciers and ice caps are more important when considering future sea-level rise, Pfeffer and colleagues focused their analysis on Greenland because researchers have already hypothesized sea-level contributions greater than 2 m in the next century from this land mass. "We wanted to test this hypothesis and the kind of data we needed to answer our questions are available for Greenland, but not for most of Antarctica or the remaining glaciers and ice caps," he said.

The researchers concluded that for Greenland to deliver 2 m of sea level to the ocean by 2100, all of the ocean-terminating outlet glaciers there would have to move at rates that are around 70 times faster than today, or about three times faster than an outlet glacier has ever been observed to flow. What’s more, this fast motion would have to start immediately and continue unabated until the end of the century. "While we cannot claim that this is a physical impossibility, we believe it is extremely unlikely – and not a good starting point for making predictions," said Pfeffer.

However the result is not an excuse to continue polluting and burning fossil fuels. "A rise of 0.8 m is still a very serious business indeed – especially for the millions of people living within a metre of the sea," he added. "There has been a growing tendency to think that if we do not see a Hollywood-style cataclysm in sea-level rise, we do not have a problem – that's not true. We are not trying to sell a movie script here, but trying to get the answer right!"

Policy makers need realistic and accurate projections of future sea level rise so they can direct resources towards solution and mitigation, he continued, and not spend billion of dollars solving the wrong problems.

The work was published in Science.