A major Antarctic glacier is at risk of disintegrating irreversibly if it passes a key tipping point, which could trigger the collapse of the entire West Antarctic ice sheet 鈥� and we can鈥檛 say when it might happen.
Pine Island glacier is one of聽two glaciers flowing into Pine聽Island Bay, part of the Amundsen Sea off West Antarctica. The other is Thwaites glacier. Both have retreated rapidly due to climate聽change, contributing to聽rising sea levels.
鈥淥f all glaciers around Antarctica, we believe Pine Island glacier has contributed most, so far,鈥� says Sebastian Rosier at Northumbria University in the UK. 鈥淧eople are very concerned.鈥�
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Worse, the two glaciers are the weak point of the West Antarctic ice sheet, which sits on聽bedrock below sea level. A dramatic glacier retreat could let water get under the ice and thus collapse the entire ice sheet, leading to more than 3聽metres of聽sea level rise, over centuries.
The main reason the Pine Island glacier is retreating is a current of warmer water that now periodically flows under its聽floating tip, melting it from below. Because the main part of the glacier sits on rocks that are below sea level, there is nothing to stop this warmer water getting ever further under the glacier.
Rosier and his colleagues simulated the glacier鈥檚 behaviour as the ocean water at聽its tip slowly warmed. They found that it passed through not one but three tipping points. The first two both led to聽rapid ice loss, even if the ocean was later cooled. The third caused the glacier to collapse entirely. This couldn鈥檛 be stopped by cooling the ocean (EarthArXiv, ).
鈥淭ipping point three is sort聽of聽game over,鈥� says Ted Scambos at the University of聽Colorado in Boulder.
In the model, the third tipping point occurred when the聽ocean water had warmed by 1.2掳C. A 2014 study found that .
However, Rosier says the 1.2掳C聽threshold is only 鈥渁 rough ballpark鈥�. He emphasises that the simulation deliberately warmed the ocean slowly, to tease out the tipping points.
鈥淚t鈥檚 highly likely that things might happen over a quicker period of time,鈥� he says. Rapid ocean warming could even trigger a cascade, in which the聽first tipping point caused enough melting to unleash the聽second, and then the third.
The real challenge is that the Amundsen Sea isn鈥檛 warming as if a thermostat were being turned up, says Scambos. Instead, warmer water that used to be kept out by currents and winds is entering in pulses, controlled by shifts in winds across the Pacific Ocean.
Could we see the tipping point coming? In the model, the聽researchers were able to spot聽warning signs: Pine Island glacier became slow to respond to perturbations in ocean temperature. But they needed 300 years of data for this to work. Detailed records of the glacier only go back to the 1970s, with a smattering of older ones. That is a problem, says Rosier.
It may be possible to find out how the glacier behaved earlier in its history by studying sediment cores from the ocean floor just off Antarctica, says Scambos. This could reveal when the ice was grinding over the seabed, and when the seabed was exposed to water.
