'Stable' Antarctic ice sheet may have started collapsing, scientists say
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Version 0 of 1. A vast slab of Antarctic ice that was previously stable may have started to collapse, according to new analysis of satellite data. Research published in the journal Science on Thursday found the Southern Antarctic Peninsula (SAP) ice sheet is losing ice into the ocean at a rate of 56 gigatons each year – about 8,500 times the mass of the Great Pyramid of Giza. This adds around 0.16mm per year to the global sea level. The sheet’s thickness has remained stable since satellite observations began in 1992. But Professor Jonathan Bamber of Bristol university, who co-authored the study, said that around 2009 it very suddenly began to thin by an average of 42cm each year. Some areas had fallen by up to 4m. “It hasn’t been going up, it hasn’t been going down – until 2009. Then it just seemed to pass some kind of critical threshold and went over a cliff and it’s been losing mass at a pretty much constant, rather large, rate,” said Bamber. Previous studies had found thinning was occurring on the SAP ice sheet but the cause has been a matter of conjecture. Ice sheets can thin for various reasons. Less snow can pile up on top of the sheet, the snow can become more compacted or the rate at which the ice is lost into the sea via glaciers can increase. The latter effect, to which the study attributes much of the observed thinning, can destabilise the sheet and cause it to collapse. Bamber said the findings were particularly concerning because the SAP ice sheet sits on land that slopes downward away from the sea. This allows currents of relatively warm water to push inland, undermining and further destabilising the sheet. “This part of Antarctica is believed to be inherently unstable. So what that means is you only need to give it a nudge and it will go into a different state. And it looks like its been nudged into a different state where it looks like it’s going to a considerable amount of mass for considerable amounts of time – years to decades,” he said. Bamber said further modelling of the sheet would be able to confirm whether or not the sheet had entered into a centuries-long irreversible collapse. The sheet contains enough water to raise the world’s oceans by 20cm – more than the rise caused by climate change over the whole 20th century. The SAP ice sheet is bordered by the ice sheets of West Antarctica and the Northern Antarctic Peninsula, both of which have begun catastrophic collapses that could raise sea levels by metres over the coming centuries. The mass currently being shed by the SAP ice sheet is more than half the amount being shed by the giant West Antarctic sheet. “If you add [the ice loss from Antarctic ice sheets] to what’s happening in Greenland you’re seeing a really worrying increase in mass loss from the ice sheets. It’s not plateauing, it doesn’t look like it’s stabilising. It just seems to be increasing,” said Bamber. But Andrew Shepherd, a professor of Earth observation at Leeds University, said the study had overestimated the effect of ice loss from below. He said a large reduction in snowfall and compacting of the snow could account for much of the thinning. “I think the new estimates of ice loss computed from them are far too high, because the glaciers in this sector just haven’t speeded up that much. It could be that a bigger chunk of the thinning is down to snowfall fluctuations than the authors have accounted for, and so I would be cautious about the new numbers until more information is to hand,” he said. “It’s very hard to see how we could have got the snowfall estimate wrong,” said Bamber. He said the study had used two sets of satellite data that measured different aspects of the ice – meaning the error would have to be replicated across both sets of results. “They are totally independent and they both give us the same result. That gave us a lot of confidence.” |