Antarctica's ice sheet is melting at a rapidly increasing rate, now pouring more than 200 billion tonnes of ice into the ocean annually and raising sea levels half a millimetre every year, a team of 80 scientists has reported.
The rate of melting has tripled in the past decade, the study concluded. If the acceleration continues, some of scientists' worst fears about rising oceans could be realised, leaving low-lying cities and communities with less time to prepare than they had hoped.
The result also reinforces that nations have a short window – perhaps no more than a decade – to cut greenhouse gas emissions if they hope to avert some of the worst consequences of climate change.
Antarctica, the planet's largest ice sheet, lost 219 billion tonnes of ice annually from 2012 to 2017 – approximately triple the 73 billion tonne melt rate of a decade ago, the scientists concluded. From 1992 to 1997, Antarctica lost 49 billion tonnes of ice annually.
The study is the product of a large group of Antarctic experts who collectively reviewed 24 recent measurements of Antarctic ice loss, reconciling their differences to produce the most definitive figures yet on changes in Antarctica.
Their results – known formally as the "Ice Sheet Mass Balance Inter-comparison Exercise" (IMBIE) – were published in the journal Nature on Wednesday.
"We took all the estimates across all the different techniques, and we got this consensus," said Isabella Velicogna, an Antarctic expert at the University of California, Irvine, and one of the many authors from institutions in 14 countries. The lead author was Andrew Shepherd of the University of Leeds in Britain.
"The detailed record shows an acceleration, starting around 2002," Beata Csatho, one of the study authors and a glaciologist at the State University of New York at Buffalo, said in an email.
Csatho noted that comparing the first and last five-year periods in the record reveals an even steeper acceleration.
"Actually, if you compare 1997-2002 to 2012-2017, the increase is even larger, a factor of more than 5!!"
For the total period from 1992 until the present, the ice sheet has lost nearly 3 trillion tonnes of ice, equating to just under 8 millimetres of sea level rise. Forty per cent of that loss has occurred in just the past five years, again underscoring the increase in losses recently.
Looking closer, the rapid, recent changes are almost entirely driven by the West Antarctic ice sheet, which scientists have long viewed as an Achilles' heel. It is known to be losing ice rapidly because it is being melted from below by warm ocean waters, a process that is rendering its largest glaciers unstable.
West Antarctica lost 159 billion tonnes of ice a year from 2012 to 2017, compared with just 65 billion tonnes from 2002 to 2007.
The growth is largely attributable to just two huge glaciers – Pine Island and Thwaites. The latter is increasingly being viewed as posing a potential planetary emergency, because of its enormous size and its role as a gateway that could allow the ocean to someday access the entirety of West Antarctica, turning the marine-based ice sheet into a new sea.
Pine Island is now losing about 45 billion tonnes per year, and Thwaites is losing 50 billion. Both numbers are higher than the annual losses for any other glacier anywhere in the world.
"The increasing mass loss that they're finding is really worrying, particularly looking at the West Antarctic, the area that's changing most rapidly and it's the area that we're most worried about, because it's below sea level," said Christine Dow, a glaciologist at the University of Waterloo in Canada, who was not involved in the research.
"If you start removing mass from there, you can have a very large scale evacuation of ice into the ocean and significant sea level rise," she said.
An additional increase in ice losses came from the smaller glaciers of the Antarctic Peninsula, which are also melting rapidly but contain less sea level rise potential.
Finally, the largest part of the continent, East Antarctica, has remained more stable and didn't contribute much ice to the ocean during the period of study, the assessment said. However, in the past five years, it too has begun to lose ice, perhaps as much as 28 billion tonnes a year, although the uncertainty surrounding this number remains high.
What's happening in East Antarctica is extremely important because it has by far the most ice to give, being capable of raising sea levels by well over 30 metres. A single East Antarctic glacier, Totten, has the potential to unleash as much total sea level rise as the entire West Antarctic ice sheet, or more.
"We cannot count on East Antarctica to be the quiet player, and we start to observe change there in some sectors that have potential and they're vulnerable," Velicogna said.
Scientists have previously raised fears about a scenario in which ice loss from Antarctica takes on a rate of explosive growth.
In a controversial 2016 study, former NASA scientist James Hansen and a team of colleagues (including Velicogna) found that the earth could see sea-level rise above one metre within 50 years if polar ice sheet loss doubles every 10 years. A tripling every decade, were it to continue, would reach that volume of sea level rise even sooner.
Granted, there's no proof the current rate of change in Antarctica will continue. Scientists can't see the future, but they do fear continuing and even worsening losses.
"I don't know if it's going to keep exactly tripling, but I think it has a lot of potential to keep significantly increasing," Velicogna said.
The changes will not be steady, in any case, said Knut Christianson, an Antarctic researcher at the University of Washington in Seattle, by email.
"We will not necessarily see solely rapid retreat," said Christianson, noting that as glaciers such as Pine Island retreat, they will sometimes encounter bumps that slow down their movement. So we should expect "periods of stability interspersed with rapid retreat", he said.
Under high greenhouse gas emissions, the worst-case projections of sea-level rise eventually reach more than a centimetre each year, said Rob DeConto, an Antarctic expert at the University of Massachusetts, Amherst, who was not involved in the current study.
We're nowhere near that point yet.
"We're still talking about roughly a half a millimetre per year," DeConto said. "That isn't going to sound horribly unmanageable. But remember, for the northern hemisphere, for North America, the fact that the location in West Antarctica is where the action is amplifies that rate of sea level rise by up to about an additional 25 per cent in a city like Boston or New York."
That's because, as Antarctica's mass shrinks, the ice sheet's gravitational pull on the ocean relaxes somewhat, and the seas travel back across the globe to pile up far away – with US coasts being one prime destination.
Whether the loss of Antarctic mass keeps worsening depends on choices made today, argues DeConto, who co-authored a separate paper in this week's Nature outlining two different visions for Antarctica's future in the year 2070.
Continuing high emissions could deliver massive sea level rise – but strong compliance with the Paris climate agreement, while unable to stop changes happening now, could help to control how much they worsen.
"The kinds of changes that we see today, if they were not to increase much more … then maybe we're talking about something that is manageable for coastal stakeholders," DeConto said.
Or alternatively, he continued, Antarctica could drive faster changes, ones that "begin to exceed what we're going to be able to cope with".
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