The Totten and Moscow University glaciers in East Antarctica have lost billions of tons of ice since 2002. Between them they could raise sea levels by 5 meters.
6 August 2018
By Michael Marshall
Two huge glaciers in east Antarctica have been losing mass rapidly since 2002. The finding means that our forecasts for sea level rise this century will have to be revised upward, but it’s not clear by how much.
It is also further evidence that the ice in east Antarctica, which had long been thought to be stable even in the face of climate change, is in fact melting.
So far, most of the ice lost from Antarctica has come from west Antarctica – in particular the Antarctic Peninsula, which juts out to sea and is exposed to warm water. The Larsen C ice sheet, which infamously cracked in 2017, is in west Antarctica.
East Antarctica has been thought to be more stable because it is virtually cut off from the rest of the world by strong winds that spin around Antarctica, preventing heat from entering. So while Antarctica is warming, like every continent, it is not clear how much global warming is needed to melt east Antarctica.
The problem is that east Antarctica is so remote. Glaciologists have tried to estimate what is happening there but for decades the data has been all over the place. In 2015 one team even claimed that east Antarctica is gaining ice.
Adding to the confusion, the sea ice around Antarctica has grown in recent years – but the sea ice is minuscule compared to the ice sheet on the continent, and in the last two years it has shrunk.
Yara Mohajerani of the University of California, Irvine and his colleagues focused on two east Antarctic glaciers called Totten and Moscow University.
As recently as 2015, a major survey of Antarctica’s glaciers hedged its bets on whether Totten was shrinking. But since then evidence has accumulated. Calculations based on estimated snowfall and the glacier’s observed speed suggested it was losing mass.
A study published in November 2017 examined the Totten ice shelf – the region where the glacier meets the ocean and goes afloat – and found that it flowed into the sea faster between 2001 and 2006, when the water was a little warmer than usual. This suggested it was vulnerable. Then in March 2018 researchers announced that more of the glacier was floating than thought, meaning it was more exposed to warm currents.
“We used multiple lines of evidence to confirm significant mass loss from Totten and Moscow University glaciers,” says Mohajerani. In particular, they used data from the two GRACE satellites, which measure local changes in the strength of Earth’s gravity. Less ice in the glaciers means they have less mass and thus a weaker gravity field.
Between 2002 and 2016, the two glaciers lost an average of 18.5 gigatonnes of ice every year, the team estimates.
Rising seas
The two glaciers hold enough ice to raise sea levels 5 metres. Totten alone could cause a 3-metre rise.“It’s extremely unlikely that that amount of ice will emerge in this century,” says Sridhar Anandakrishnan at Pennsylvania State University. That’s because it takes time for warm seawater to eat its way back through the glacier.
But while we probably won’t get 5m by 2100, we could get a lot and we don’t know how much.
“Totten Glacier has a massive drainage sector [spanning] 537,900 square kilometres, most of which is grounded below sea level,” says Mohajerani. “Furthermore, this glacier is exposed to relatively warm salty ocean water, which may intensify the ice shelf melt. Therefore, this is a vulnerable area and as the glacier loses mass the basin may be increasingly vulnerable to ocean forcing.”
Anandakrishnan says it takes several hundred gigatonnes of ice to raise sea levels by 1 millimetre, and so far Totten is losing much less than that every year – putting it on course to raise sea level by a few extra centimetres by 2100. But it might accelerate, and we don’t know its top speed.
Warm water incoming
A study published in 2015 found an underwater channel that runs from under the Totten ice shelf out to the continental shelf, linking Totten to the wider ocean. That’s worrying. “The deeper the ocean floor is in front of these glaciers, the easier it is for warm water to come onto the continental shelf and make their way to the glacier,” says Anandakrishnan.
The other question is the shape of the bedrock on which Totten rests. It seems to slope downhill further inland, so warm water could flow under the glacier and melt it from below. But he cautions that the bedrock data, again, is scanty.
What is clear, says Anandakrishnan, is that glaciologists’ previous uncertainty about the state of east Antarctica has largely vanished. It is losing mass and has been for some years.
“I think the consensus was overly conservative and cautious,” he says. “Their measures were better than they were willing to admit to.” At this point, “the consensus is shifting quite strongly that East Antarctica is indeed losing mass, end of story,” he says.
Journal reference: Geophysical Research Letters, DOI: 10.1029/2018GL078173
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