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Antarctic input to rising oceans ‘overestimated’

14 May 2009
Not as bad we thought?

Sea–level rises resulting from the collapse of the unstable western Antarctic ice sheet (WAIS) under climate change could be half the generally accepted value, according to researchers from the UK and the Netherlands.

Jonathan Bamber of the University of Bristol, together with colleagues from the Delft Institute of Earth Observation and Space Systems and the Delft University of Technology, has used updated survey data and theory to calculate that the maximum contribution to sea levels would be about 3 m, and not 5 to 6 m as had been assumed. However, the calculations suggest that any sea–level rise would be 25% worse along the Pacific and Atlantic seaboards of the US.

“If [this research] is correct, it constrains the potential impacts of sea–level rise over the coming centuries,” says Robert Nicholls at the Tyndall Centre for Climate Change Research at the University of Southampton. “As the WAIS has the potential to collapse relatively quickly this has important implications for worst case sea–level scenarios.”

Runaway effect

Sea-level rise is one of the more dangerous symptoms of climate change. Scientists estimate that a rise of just 1.5 m would be enough to displace 17 million people in Bangladesh alone. The rise would be caused partly by the expansion of water as it warms, but mostly it would result from the melting of glaciers, including ice sheets.

Three places are host to major ice sheets: Greenland, east Antarctica and west Antarctica. Of these only the WAIS is inherently unstable, which means that a little warming could potentially detach vast regions of the land ice and let it collapse into the ocean — a runaway effect that would take place over hundreds, not thousands, of years.

The 5–6 m estimate for a contribution to sea levels due to such a collapse was made over 30 years ago, and Bamber says it has “fallen into the vocabulary” of climate–change scientists ever since. Now, with recent observations that provide more accurate tomographical data of the bed and surface of the ice sheet, Bamber’s group has been able to revise the volume of ice that could fall into the oceans. The researchers assumed the most unstable regions — and therefore the regions prone to collapse — would be those grounded below sea level where the bedrock slopes downwards inland (see figure).

Bamber’s group calculated a collapse of the ice sheet would give a maximum sea-level rise of 3.3 m, and claim the previous study “overestimated.” However, they calculated that the change in the Earth’s gravity field due to the lost ice over the Antarctic means that oceans would build up greatest at a latitude of 40°, particularly along the western Atlantic and eastern Pacific. This peak would be 25% greater than the mean, and would exist regardless of the total sea–level rise.

‘Good news is offset’

Richard Alley, a geoscientist at Penn State University who specializes in ice sheet stability, described the study as “very nice work,” although he thinks it will not change researchers’ understanding of ice–sheet systems or potential sea-level rises “hugely.”

“In the absence of a suite of well-validated ice–flow models to assess the future path of ice–sheet loss in response to emissions scenarios, Bamber et al have revisited the data on the susceptible versus ‘safe’ parts of the ice sheet relative to the marine instability,”

“By doing the calculation very well, they find a reduced potential contribution from the [the WAIS],” Alley told “Although, this ‘good news’ is partially offset by an improved calculation showing that most of the coastal people of the world live in places that will see somewhat larger sea-level rise than the global average,” he warned.

Alley also points out that Bamber’s group has not provided any estimate of how rapidly sea levels could rise or how much sea levels could rise in total, although Bamber told that they are working on that next.

Richard Tol, an economist who looks at the impacts of climate change, thinks the study is “very important” because it suggests sea–level rise might be small enough to be contained by dikes, which take governments longer to install depending on how high they are required. In addition, he says it “breaks the trend of ever more gloomier scenarios of climate change and its impacts.”

This research was published in Science

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