Current Warming Pace Risks Irreversible Antarctic Ice Loss and Sea Level Rise, Study Warns

Robert DeConto, a polar climate scientist at the University of Massachusetts, puts the timing of the problem at about 2060. That is when, in his new Nature paper, Antarctica's ice loss takes an abrupt jump if the world keeps warming at the rate set by current government policies. The study warns the result would be "rapid and unstoppable" sea level rise, with coastal cities in greater danger.

The trigger is a global temperature rise of 3C (5.4F) above preindustrial levels. Many researchers consider that feasible by 2100 under today's policies. If it happens, the paper finds, Antarctic ice loss would be irreversible on multi-century timescales, and the continent alone would help raise the oceans by 17cm to 21cm (6.69in to 8.27in) by the end of the century.

The alternative is far smaller. If countries meet their Paris agreement commitments, Antarctica would add 6cm to 11cm by 2100, on a par with the current rate of ice loss. Under Paris, governments have pledged to hold warming below 2C above preindustrial times, while pursuing efforts to limit it to 1.5C.

DeConto, the lead author, explained why the loss cannot be undone. Once enough heat has gone into the climate system, he said, the ice shelves will be lost, and once that is set in motion it cannot be reversed. The oceans would have to cool before the ice sheet could heal, which would take a very long time. "On a societal timescale it would essentially be a permanent change," he said. "It's really the next few decades that will determine the sea level rise from Antarctica."

The stakes are large. If all of Antarctica's ice melted, global sea levels would rise by 57 metres, enough to submerge the world's coasts, though that is not expected on any foreseeable timescale. Even a small increase in loss would be felt, and scientists are especially concerned about the western part of the ice sheet. There they are studying the decline of the Thwaites glacier, known as the doomsday glacier. It is about the size of Britain and 1km deep, and in a worst case its loss would raise sea levels by 65cm.

The ice is being worn down by a warming atmosphere and by heating oceans. Warmer seawater enters crevasses and erodes the "pinning points" that hold enormous bodies of ice to the submerged bedrock. A rapid acceleration could set off a cascade, with huge amounts of ice and water flowing unimpeded into the Southern Ocean.

The effects would play out over centuries. "In the century after 2100 it's potentially catastrophic," DeConto said. With no emissions reductions at all, he said, Antarctica alone could produce 5 metres of sea level rise by 2200, at which point you would have to remap the world from space. The paper relies on a model that combines temperature rise and ice loss with the dynamic processes in Antarctica, which make precise prediction difficult.

A second Nature paper, published the same Wednesday by scientists at King's College London, finds sea level would rise by 0.5cm every year by 2100 if global temperature reaches 3C. DeConto called it an impressive piece of work but noted it differs from his, which includes the compounding effects of ice shelf loss. "Neither of these papers are the last word, this is ongoing work," he said. The real question, he added, is whether the world will face sea level rise at a manageable or unmanageable rate.

Andrea Dutton, a sea level expert at the University of Wisconsin–Madison who was not involved, said the paper addresses an important and pressing question about what the Paris targets mean for sea level. A major acceleration in Antarctic melting, she said, would bring coastal retreat and migration on a scale never before witnessed. "We will not be able to just adapt because it is impossible to just engineer our way out of this," she said, calling the finding a reminder of the urgency of deep and sustained emissions cuts.