Scientists watch anxiously for giant “doomsday” glacier in Antarctica

Cracks and fractures are raising fears that a rift could cause the world’s ocean level to rise by half a meter – or more.

Twenty years ago, a patch of ice believed to weigh nearly 500 billion tons abruptly broke away from the Antarctic continent and crashed into thousands of icebergs in the Weddell Sea.

It was known that the 1,255-square-mile (3,250-square-kilometer) Larsen B ice shelf was melting rapidly, but no one could have guessed that it would take just one month for the 200-meter-long thickness to completely collapse.

Glaciologists were shocked not only by the speed but also by the extent of the destruction. “It’s staggering. It just fell apart. It collapsed like a wall and disintegrated like hundreds of thousands of bricks,” one of them said.

Ice scientists gathered in New Orleans this week warned that something even more alarming is brewing in the West Antarctic Ice Sheet, a huge basin of ice on the Antarctic Peninsula. Years of research by teams of British and American scientists have shown that large cracks and fractures have opened both on top and underneath the Thwaites Glacier, one of the largest in the world, and there are fears that parts of it too could split and collapse, perhaps within five years or less.

Compared to Thwaites, the Larsen B Glacier looks like an icicle. It is about 100 times larger, the size of Great Britain, and contains enough water to raise the world’s ocean level by more than half a meter. It accounts for about 4 percent of the annual rise in global sea level, and has been called the most important glacier in the world, even a “doomsday” glacier. Satellite studies show that it is melting much faster than it did in the 1990s.

Thwaites is a concern, but there are many other large glaciers in Antarctica that are also retreating, thinning and melting as the Southern Ocean warms. Many of them are held back because Thwaites acts like a traffic jam, blocking them from reaching the sea. Scientists believe that if the Thwaites collapse, other ice will accelerate, causing the entire ice sheet to collapse and a catastrophic rise in sea level of several meters.

Whether and how quickly they might collapse is one of the most important questions of our time. Sea levels are rising rapidly: the annual rate of rise more than doubled from 1.4 mm to 3.6 mm between 2006 and 2015 and is accelerating. A few millimeters a year is not much, but the loss of even a small part of Thwaites Island will not only help accelerate this process, it will likely increase the strength of storm surges.

If all the glaciers of West Antarctica ever collapse, there is not a single coastal city in the world that will not eventually be inundated with ruinous losses to life and the economy.

Glaciologists used to agree that it would take centuries of global warming before Thwaites-sized glaciers would collapse and collapse, but so rapid and unexpected was the loss of sea ice on the opposite side of the Earth in the Arctic, and so sudden was the loss of Larsen B, that it is now thought possible that it could happen quickly in Antarctica as well.

The loss of ice in the Arctic has almost no effect on sea level because it is mostly formed at sea. Antarctic ice, however, is mostly on land, so any melting raises sea levels.

The tipping point for the Larsen B Ice Shelf came suddenly. How Thwaites and other glaciers will respond to global warming is not yet known, but these big global physical processes are well underway.

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