Annual Antarctic ice loss triples in just 5 years

The Thwaites Glacier in Antarctica is increasingly being viewed as posing a potential planetary emergency.
(Source: The Age)

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.

"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!!"

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.

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.

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