Uncertainty over sea level predictions

Concerns about the rate at which the polar ice caps are melting have prompted a debate about future sea level rise. Some scientists think levels could be several metres higher by the end of the century, but others are more circumspect.

The past six months have brought a string of warnings about future sea level rise caused by climate change. Some researchers are now predicting increases over the next century of considerably more than the 18-59 centimetres put forward by the Intergovernmental Panel on Climate Change (ENDS Report 385, p 7 ). Others are more cautious, pointing to a series of natural limits they believe will moderate the rate of ice melt.

In its fourth assessment report, published in February, the IPCC suggests sea levels will rise by 18-38cm by 2100 even if greenhouse gas emissions fall relatively rapidly. If emissions continue to rise the figure will be 26-59cm. The report takes into account increases in ice-sheet melt between 1993 and 2003, but does not include the “full effects of changes in ice sheet flow, because a basis in published literature is lacking”.

Tim Lenton, a climatologist from the University of East Anglia, believes the ice sheet covering Greenland could melt much faster. He has been investigating “tipping elements” in the climate system – non-linear changes like the collapse of the Indian monsoon system or stalling of the Gulf Stream – and warns the ice sheet could collapse if temperatures rise another 1°C above current levels and be gone within 300-1,000 years.

Given the 0.6°C of unavoidable warming the earth faces as a result of past greenhouse gas emissions and the volume of water locked up in the Greenland ice sheet – enough to cause 7m of sea level rise – Professor Lenton’s worst-case scenario looks bleak.

James Hansen, who has led NASA’s Goddard Institute for Space Studies since the early 1980s, also thinks the IPCC’s figures are an underestimate. In a paper published in May,1 he claims the panel “assumes an inertia for ice sheets that, we argue, is incompatible with paleoclimate data and inconsistent with observations of current ice sheet behaviour.”

Temperatures 3°C higher than those of today – a level achievable by 2100 if emissions continue to grow – have in the past been accompanied by seas 25m above the present level. The ice sheets are already showing non-linear patterns of slippage and lurching and the temperature increase created by a business-as-usual emissions scenario “provides ample energy to melt ice corresponding to several metres of sea level per century,” the paper says.

Two other studies have brought more modest estimates – but ones that still exceed the IPCC’s predictions. In January, Stefan Rahmstorf of the Potsdam Institute in Germany extrapolated current rates of increase to give an estimate for 2100 of 0.5-1.4m (ENDS Report 386, p 28  ). And in a more recent study based on a closer analysis of the melting process, Mark Meier and colleagues from the University of Colorado at Boulder called for more attention to be paid to mountain glaciers and smaller ice caps – particularly those adjoining the sea.2

These currently account for about 60% of the additional water entering oceans and will continue to dominate over the next 100 years, Professor Meier said. Measurements suggest they are switching to a new dynamic mode with meltwater lubrication at their base resulting in a thinner, faster-moving ice mass from which more iceberg calving takes place.

If current melt rates continue through the next century, such systems will generate 10-25cm of sea level rise on their own. Adding meltwater from the Greenland and West Antarctic ice sheet would result in 56cm. This figure does not include seawater expansion in a warming world – responsible for around half of current annual sea level rise so the overall figure will be higher.

The Colorado estimate is much more realistic than the 2-5m rise put forward by some scientists, said Tad Pfeffer, a colleague of Professor Meier.

“These high numbers come from analogies with conditions in the past that don’t correspond well with conditions today,” said Professor Pfeffer, who has been working out the rate of ice melt required to deliver these figures. “In order for Greenland to produce such changes today, the outlet glaciers which drain ice from Greenland would have to move at rates that far exceed anything that’s ever been observed, and would require some exotic physics for which there is no evidence.”

The situation is different in the Antarctic, where “a very large fraction” of the ice sheet can flow directly into the ocean. “But it’s not clear that Antarctica will produce significant sea level rise in the next century because it’s too cold there, and will be for the foreseeable future,” he said.

Met Office ice specialist Jeff Ridley also dismisses the more extreme predictions. “The science just isn’t there yet to justify these concerns,” he said. “It could happen but I really don’t think Greenland will catastrophically collapse. Any collapse of Antarctica would take centuries rather than decades.”

The extrapolation of current melt rates is difficult to justify as melting is likely to slow once the glaciers retreat above sea level, he said. Big cracks were also recorded in Greenland’s ice sheets in the 1930s and have not yet been conclusively linked to global warming, he added.

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