Sea level could rise from 0.75 to 1.9 meters this century

The authors, Martin Vermeer of Helsinki University of Technology in Finland and Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany, based their analysis on measurements of sea level and temperature taken over the past 130 years. In those data they identified a strong link between the rate of sea level rise and global temperature.

"Since 1990 sea level has been rising at 3.4 millimetres per year, twice as fast as on average over the 20th Century," says Stefan Rahmstorf. Even if that rate just remained steady, this would already lead to 34 centimetres rise in the 21st century. "But the data show us clearly: the warmer it gets, the faster sea level rises. If we want to prevent a galloping sea level rise, we should stop global warming as soon as possible," adds Rahmstorf.

The link between the rate of sea level rise and global temperature was originally proposed by Rahmstorf in an article in the journal Science in 2007. The new study refines this idea. It adds a second term to the equation in order to capture the short-term response of sea level, leading to greater physical realism as well as a much greater precision. Vermeer and Rahmstorf also added the latest data sets, including satellite measurements up to 2008 and a correction for water storage in man-made reservoirs, which overall lowers global sea level by 3 centimetres.

Their results show that even for a relatively low greenhouse gas emissions scenario with just 2 degrees Celsius warming over the 21st century, sea level is likely to rise by more than one meter. Their highest scenario, with over 4 degrees Celsius warming over the 21st century, would lead to over 1.4 meters of sea level rise by 2100. When the full set of emissions scenarios and estimated uncertainties are considered, waters may rise by anything between 75 centimetres and 1.9 metres by the year 2100 -- consistent with another recent estimate of an upper limit of 2 metres, based on consideration of ice sheet dynamics.

"More noteworthy even than the very high figures for sea level rise is the almost clockwork precision by which, on climatic time scales, temperature drives sea level rise," says Martin Vermeer. The results of the study also demonstrate the quality of the existing sea level and temperature time series used, "painstakingly constructed from measurements at stations around the globe for well over a century," Vermeer notes.

The projected rise is about three times as much as estimated in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change in 2007, which did not fully include the effects of ice loss from Greenland and Antarctica. To avoid such a large sea level rise, which would be an existential threat to many large coastal cities as well as a number of small island nations, drastic and rapid cuts in greenhouse gas emissions will be required.

The study finds that delays in emissions reductions will come at a high cost, since early emissions cuts are much more effective in limiting sea level rise than later cuts. The emissions reductions needed to keep sea level rise below 1 meter will likely be considerably more ambitious than those needed to limit global warming to 2 degrees Celsius, a policy goal now supported by many nations.