IT’S melting from above and below and crumbling at the edges. Antarctica is in trouble. Its frozen edges, or ice shelves, are disappearing into the ocean faster than we thought. Some have thinned by up to 18 per cent in the past two decades, and the process is accelerating. The most rapidly melting ones are likely to be gone within a century.
As this happens, the ice sheets sitting over Antarctica’s land – which hold the equivalent of 60 metres of sea level – will speed up their descent into the ocean, causing it to rise globally.
These new findings are the culmination of a torrent of studies over the past few years that have boosted our understanding of what is happening to Antarctica. And it’s rarely been good news.
There is still a lot of uncertainty over how ice sheets will respond to climate change, which confounds our ability to predict sea level rises, says Fernando Paolo from the University of California, San Diego. That realisation has led to an explosion of research on the topic, he adds.
As for the ice shelves, their melting does not directly raise sea levels because they are already floating on the ocean. But they act like the crust at the edges of a lemon tart, preventing the filling from seeping outwards. In the case of Antarctica, the filling corresponds to the ice sheets, which will flow into the ocean and cause sea level rise once the ice shelves are gone.
Already, the loss of ice shelves in West Antarctica has led to a 59 per cent increase in ice discharge from some regions.
So far, the rate at which the ice shelves are melting has been a matter of debate, one confused by a plethora of satellite observations that cover short periods of time.
Paolo and colleagues have developed a model that combines data from three satellite missions, so that they could look at all the ice shelves over a period of 18 years. This allowed them to produce the first continuous plot of how the ice volume of the whole system was evolving (Science, doi.org/278).
“We show not only that the total ice shelf volume is decreasing, but we see an acceleration in the last decade,” says Paolo.
For the first nine years of the study, the average thinning of the ice shelves across Antarctica was negligible. Consistent losses in vulnerable West Antarctica were mostly offset by gains in East Antarctica, which Paolo says might be a result of increased snowfall there – itself something that is forecast to happen as the globe warms.
But the second half of the study, covering 2003 to 2012, was completely different. Ice shelf loss off West Antarctica increased by 70 per cent over the previous nine years, and even East Antarctica showed a small loss, too. On average, some 310 cubic kilometres of ice was lost from the ice shelves each year during this time, and the process sped up as it went along.
“The fact that the rate of their thinning is increasing is alarming,” says Ben Galton-Fenzi from the Australian government’s Antarctic Division in Kingston.
The accelerating ice shelf loss in West Antarctica is mostly down to warmer water flowing underneath, says Paolo. That is happening because the sea surrounding Antarctica is warming, and the winds that push it against the ice are getting stronger (see diagram).FIG-mg30153701.jpg
Changes in East Antarctica are harder to understand because we have less data for that region, says Paolo. More snowfall in the first nine years of the study might have masked any thinning, he says, but the factors that are melting West Antarctica could be starting to play a role in East Antarctica too.
Some of the disappearing ice shelves are especially important for holding back the ice sheets. In particular, ice shelves in the Amundsen Sea, which regulate a lot of West Antarctica’s ice, lost 8 per cent of their volume on average over the 18 years of the study.
Instead of taking hundreds of years to respond to warming, ice shelves will respond in a matter of years, according to Paolo. “We are seeing large changes over the course of decades,” he says. “At current rates of change, some of these ice shelves could disappear within 100 years.”
We are seeing large changes over the course of decades. Some of these ice shelves could disappear
East versus west
Some parts of Antarctica are melting much faster than others, as revealed by Fernando Paolo from the University of California, San Diego, and his colleagues, who looked at ice loss between 1994 and 2012 (see main story). Some ice shelves collapsed during this time, and now others are in danger of melting fast. West Antarctica has fared worse so far, but Paolo’s study hints that the East may start catching up.
Ben Horton from Rutgers University in New Jersey says that as West Antarctica loses mass, sea levels will rise unevenly, with North Atlantic coastlines bearing the brunt of it. This means that coastal cities like New York will be worst affected.
West antarctica crumbles
This part of Antarctica has been warming quickly. If the ice held here melted completely, it would raise sea level by more than 3 metres. Some studies suggest that the complete collapse of glaciers within the next 200 to 1000 years is now inevitable. The Antarctic peninsula, which juts towards South America, could be most affected: it has seen some of the fastest warming in the world, of about 2.5 °C over the past 50 years.
The Larsen collapses
Once there were three connected Larsen ice shelves on the Antarctic peninsula. Larsen A collapsed in 1995, Larson B in 2002 and Larsen C is at risk. Paolo’s study shows that it is thinning and could separate from the continent within 100 years, if the current rate of loss persists.
The southernmost collapse
In 2009, the Wilkins ice shelf all but completely collapsed. It does not hold in any ice sheet, so the event will not contribute to sea level rise, but the collapse was significant as it was the closest yet to the South Pole.
Crosson and Venable ice shelves
Each of these has lost some 18 per cent of its thickness over the 18 years of the study, and could disappear within the next 100 years.
Ice shelves in the Bellingshausen Sea have thinned at more than 7 metres per decade.
Ice shelves in the Amundsen Sea have been thinning at almost 20 metres per decade. This, together with melting in the Bellingshausen Sea, accounts for 85 per cent of West Antarctica’s ice loss over the study period. The Amundsen shelves matter as they are slowing the flow of some major glaciers.
Getz ice shelf
Part of the Amundsea Sea, Getz has thinned by a whopping 66.5 metres per decade, losing some 6 per cent of its volume over the 18 years of the study. It alone accounts for 30 per cent of the ice volume lost from West Antarctica.
East antarctica clings on
East Antarctica, facing the Indian Ocean, has experienced some of the slowest warming on Earth. That’s fortunate, because it makes up two-thirds of Antarctica and holds most of the continent’s ice. But not everything is hunky-dory.
Mixed news from Totten
Totten glacier drains ice from a huge part of East Antarctica, dumping about 70 billion tonnes of ice into the ocean each year, which is roughly on par with the amount of snowfall, keeping things in balance. The ice locked up in this glacier alone is equivalent to about 3.5 metres’ worth of sea level. Earlier studies had suggested that the adjoining ice shelves could be thinning (Nature, doi.org/27v) although the Paolo study did not find any thinning in the 18 years to 2012. But this month it emerged that a tunnel under the ice shelf may be letting warm water reach the glacier (Nature Geoscience, doi.org/27w). That could be increasing the flow of the glacier and hence sea level rise.
This article appeared in print under the headline “Antarctic ice is fading ever faster”
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