News•November 28, 2012

Sea Level Rise Accelerating Faster than Initial Projections

Sea level is rising as the planet warms up, but how much it will rise, and how fast is still something climate scientists are working out. And according to study released late Tuesday in Environmental Research Letters the ocean is already rising faster than the most recent authoritative report from the U.N. Intergovernmental Panel on Climate Change (IPCC) was projecting as recently as 2007.

“Results show that global temperature continues to increase in very good agreement with the best estimates of the IPCC,” the authors of the new study write. “The rate of sea level rise of the past decades, on the other hand, is greater than projected by the IPCC models. This suggests that IPCC sea level projections for the future may also be biased low.”

Storm surge on a Louisiana highway shows the affects of rising sea levels.
Credit: NOAA.

The IPCC issues comprehensive reports every five to seven years, with the next one due out in 2013-2014. The reports summarize the state of scientific knowledge on climate change, and are used as the underpinning of interational climate talks aimed at reducing greenhouse gas emissions. The latest round of these negotiations is currently underway in Doha, Qatar.

The IPCC’s projections showed a worst-case scenario of just less than 2 feet of sea level rise by 2100. “Lots of people felt that the IPCC was too conservative,” said co-author Grant Foster, of the consulting firm Tempo Analytics, in Garland, Maine. “The IPCC explicitly stated in the 2007 report that its models excluded 'future rapid dynamical changes in ice flow.' ” In other words, it didn’t allow for the fact that ice flowing into the sea from Greenland and Antarctica might speed up as the planet warms.

That doesn’t entirely account for the lower prediction, according to the study's lead author, Sefan Rahmstorf, of the Potsdam (Germany) Institute for Climate Impact Research. “We also have five more years of data,” Rahmstorf said. And much of that data is from satellites, which are more comprehensive than the tide gauges used in the pre-satellite era.

Still, many experts, including Rahmstorf, were convinced several years ago that the increase in sea level by 2100 should be more than 3 feet (assuming, that is, that no serious measures are taken to reduce heat-trapping greenhouse gas emissions). Even the 1-foot rise in the New York area from 1900 to the present was enough to boost the destructive power of Hurricane Sandy’s storm surge; 3 feet of extra sea level could prove truly catastrophic. Their calculations were based, not on additional sea level data, but on more sophisticated analyses of the relation between sea level and temperature, based in part on evidence from ancient climate conditions.

The newly published figures don’t change that 3-foot projection for 2100: they look instead at the measured rate of sea level rise from 1993-2011, pegging it at about 3.2 millimeters per year. That’s 60 percent faster than the 2 millimeters per year the IPCC’s computer models suggested should have happened over that same period — a clear indication that those models weren’t especially accurate.

Even at 3.2 millimeters per year, sea level would only go up by a foot by 2100; the extra 2 feet almost everyone expects will come from an accelerating sea level rise caused by ever increasing temperatures.

But 3 feet of sea level rise by 2100 is a projection; it isn’t necessarily destiny. “A lot depends on what emissions path we follow,” Foster said. “If we get our acts together, it doesn’t have to go that high.”