Global warming may be causing ocean levels to rise twice as fast as previously expected, warns a climate physicist in Germany. Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research has used sea-level and temperature data spanning the twentieth century to create a very simple model for predicting ocean levels. Then, by using temperatures from future global-warming scenarios published by the Intergovernmental Panel on Climate Change (IPCC), he calculated that the sea level in 2100 will be 0.5-1.4 m higher than it was in 1990 (Science Express DOI: 10.1126/science.1135456).
Rising seas could be the most devastating consequence of manmade global warming and the IPCC currently predicts that levels will rise 21-70 cm by 2100. While this is bad news for more than 50% of the world’s population who live in low-lying coastal regions, a rise of 1.4 m would be catastrophic and submerge many of the world’s largest cities.
The oceans have risen about 20 cm in the past century commensurate with a global warming of about 0.8 degrees. This is expected to continue as rising temperatures reduce the size of the Antarctic and Greenland ice sheets and cause the thermal expansion of the oceans. While scientists have tried to model how sea levels will respond to rising global temperatures, their understanding of the complex processes involved remains limited. Indeed, the state-of-the-art models used to produce the IPCC predictions appear to underestimate the effect of rising temperatures on sea level.
Instead of worrying about the physical processes behind sea-level rise, Rahmstorf looked for a purely mathematical relationship between sea levels and global temperatures in data covering the period 1880-2001. What he found is surprisingly simple – the rate at which the sea-level rose in given year was proportional to the difference between that year’s average temperature and a pre-industrial global average temperature. For example, if a year was 0.25 degrees warmer than average, the sea level would rise about 0.8 mm that year – and if it was 0.5 degrees warmer a 1.6 mm rise would occur.
Rahmstorf told Physics Web that rather than yielding deep insights into the physical processes of sea-level rise, his work provides a simple linear approximation to a very complicated phenomenon. While he believes that the relationship should hold until 2100, he identified several factors that could change this century. Mountain glaciers could all but disappear before 2100, ending their current contribution to sea level rise. And perhaps most importantly, the lubricating effects of increasing quantities of melt water under continental ice sheets could cause the ice to flow more rapidly towards the sea, releasing more water in a shorter time period.
