Sea levels may rise three times more than first thought

Latest prediction on impact of climate change suggests that large tracts of Britain's coastline would end up under water

Science Editor,Steve Connor
Tuesday 08 December 2009 01:00 GMT

Sea levels may rise three times faster than the official predictions of the Intergovernmental Panel on Climate Change (IPCC) and the global average sea level may increase by as much as 1.9 metres (6ft 3in) by 2100, scientists said yesterday.

The new assessment comes just one week after another international scientific body concluded that the IPCC had been too conservative in estimating a maximum of 59 centimetres of sea level rise this century as a result of global warming.

Scientists believe earlier estimates failed to take into account gaps in the knowledge of how melting ice sheets will affect sea level, as well as technical errors in the calculations which have now been corrected, giving a much higher figure for estimated sea level rise than those published by the IPCC in its 2007 assessment.

A sea level rise of 1.9m would result in large tracts of eastern England being inundated with seawater, and would wipe out many low-lying island nations as well as making large parts of Bangladesh uninhabitable. It would also increase the chances of storm surges flooding major coastal cities, such as New York and London, even with the protection offered by the Thames Barrier.

The latest study, published in the Proceedings of the National Academy of Sciences, found that global average sea levels are likely to rise by between 75cm and 190cm by the end of the century, due to the thermal expansion of the warmer oceans and the melting of mountain glaciers and polar ice sheets.

Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany, and Martin Vermeer of Helsinki University of Technology in Finland, said sea levels are rising faster as a result of temperature increases, especially at the poles, which are warming at a faster rate than many other parts of the world.

"Since 1990, sea level has been rising at 3.4 millimetres per year, twice as fast as on average over the 20th century. Even if that rate just remained steady, this would already lead to 34 centimetres rise in the 21st century," Dr Rahmstorf said.

"But the data show us clearly – the warmer it gets, the faster the sea level rises. If we want to prevent a galloping sea level rise, we should stop global warming as soon as possible," he said.

Dr Rahmstorf published a study in 2007, which came too late for including in the IPCC's fourth assessment report, suggesting that global sea levels could rise by as much as 1.4m by 2100. However, he said that this earlier study was based on previous sea level rises that had failed to take into account the extra amount of sea level – about 3cm – that would have occurred had the freshwater held back by man-made reservoirs and dams been able to flow into the sea.

About two thirds of the additional 0.5m of maximum sea-level rise predicted in the latest study is due to this underestimate in previous calculations of past sea levels, Dr Rahmstorf said. The remaining third is due to refinements of the calculations producing more accurate estimates of sea level rise.

The 1.9m increase will occur if greenhouse gas emissions continue to increase on their present trajectory, which happens to be the worst-case scenario of the IPCC. Reducing emissions early this century will have a corresponding effect on reducing the maximum sea level rises, the scientists said.

"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," said Dr Vermeer.

Last week the Scientific Committee on Antarctic Research endorsed Dr Rahmstorf's 2007 assessment of future sea level rise, when it agreed that the IPCC will have to increase its future predictions. Dr Rahmstorf said that if the committee agreed with his earlier assessment, it is also likely to go along with his latest study, predicting a 1.9m rise.

One of the greatest difficulties in assessing future sea levels is the problem of gauging how fast it might take for polar ice sheets to disintegrate – whether they melt slowly, or slip quickly into the sea.

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