Alarm as ice loss from Antarctica triples in the past five years | Tech News
The forecasts they are a-changin’. In 2007, the official view was that there would be no net ice loss from Antarctica over the next century. By 2012, it was clear that Antarctic was already losing ice. And now ice loss has tripled, according to the most comprehensive study to date.
“There has been a sharp increase, with almost half the loss coming in the last five years alone,” says Andrew Shepherd at the University of Leeds, UK, one of the 84 researchers involved in the study. “The outlook for the future is looking different to what it was.”
Antarctica’s contribution to rising sea level is still small: just 7.6 millimetres between 1992 and 2017. What’s alarming is that the rate of ice loss is increasing.
Up until 2012, Antarctic ice loss was contributing just 0.2 mm per year to sea level, and did not appear to be increasing. “We could not detect any acceleration,” says Shepherd.
But since 2012 the rate has tripled to 0.6 mm per year. “I was completely surprised,” says team member Pippa Whitehouse of Durham University, UK. “The threefold increase was out of the range we were expecting.”
How bad is bad?
Ice loss is now tracking close to the worst-case Antarctic scenario set out in the 2013 report by the Intergovernmental Panel on Climate Change, in which the Antarctic alone contributes 150 mm of sea level rise by 2100.
And that worst case is no longer the worst case. There have been a series of alarming findings since that report came out.
In particular, a 2016 computer modelling study concluded that Antarctica alone could lose enough ice by 2100 to raise sea level by 2 metres. This means overall sea level could rise by more than 3m by 2100. It will keep rising long after that, perhaps by 20m or more.
While many glaciologists are sceptical about the 2016 study, nobody knows for sure how fast the seas will rise. The problem we have is that the computer models are essentially the only way we have to forecast how much ice Antarctica will lose. And the only way to check those models are in the right ballpark is to compare their predictions to what is actually happening.
“The models must be calibrated and tested,” says team member Michiel van den Broeke of Utrecht University in the Netherlands. This requires an accurate, long-term picture of what is happening. That is the main point of the latest work, which draws on all previous studies.
It is based on surveys by satellites that measure changes in the height of the ice sheet and changes in gravity as ice mass is lost. But using this data to calculate ice loss is far from simple. For instance, changes in snowfall can affect the apparent elevation of ice sheets, while the rebounding of land as ice is lost affects the gravity signal.
The team conclude that Antarctica lost 3 trillion tonnes of ice between 1992 and 2017 as the planet warmed. Almost all the ice loss was from the West Antarctic, where the ice sheet rests on the seafloor rather than on land and is thus exposed to warming waters.
“Warmer water is eating into the ice sheet,” says Whitehouse. “Warming oceans are really the major problem.”
Sea level has risen around 0.2m in the past century. Most of this is due to ice loss from mountain glaciers and Greenland, and from the expansion of the warming oceans.
Faster and faster?
The trillion-dollar question is whether the increase in Antarctic ice loss is a temporary blip, or if it will continue to increase in a non-linear fashion: rapidly rising to several millimetres per year, then to several centimetres per year and so on.
Climate scientist James Hansen of Columbia University, New York, described the danger in a 2007 article in New Scientist:
“The primary issue is whether global warming will reach a level such that ice sheets begin to disintegrate in a rapid, non-linear fashion on West Antarctica, Greenland or both. Once well under way, such a collapse might be impossible to stop, because there are multiple positive feedbacks. In that event, a sea level rise of several metres at least would be expected.”
Journal reference: Nature, DOI: 10.1038/s41586-018-0179-y
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