Earlier this week, the earth’s northern ice cap also showed the impacts of the climate crisis. Arctic sea ice reached its annual minimum, at the second lowest extent seen in four decades. On 15 September, the ice was measured at 3.74m sq km, which marked only the second time that the extent has fallen below 4m sq km in the current record, according to the US National Snow and Ice Data Center.
Scientists said the melting ice was a stark sign of how humans were changing the planet. Twila Moon, a research scientist at the University of Colorado at Boulder, said: “It’s devastating to see yet another Arctic summer end with so little sea ice. Not only is there a very small area of sea ice, but it is also younger and more vulnerable overall. The Arctic is a changed place. All hope rests on humans to act on climate and slow this alarming pace of ice loss.”
While the Antarctic ice sheet will take centuries to melt in response to temperature rises, the new Nature paper showed how difficult it would be to reverse.
Antarctica’s vast ice cap, which covers about as much of the earth as North America and is close to three miles (5km) thick, holds more than half of the earth’s fresh water. Some of it is floating sea ice, which does not cause sea level rises in the way of ice melting from land, and is subject to melting from above and below because of the warming sea.
The researchers examined how ice over land in the region can be expected to melt, and found a strong “hysteresis” effect, which makes it harder for ice to re-form than to melt. When the ice melts, its surface sinks lower down and sits in warmer air, so it requires lower temperatures for the ice to reform than it did to keep the existing ice stable.
If temperatures rose by 4C above pre-industrial levels, which some predictions say is possible if the world fails to reduce greenhouse gas emissions quickly, then the sea level rise would be 6.5 metres from the Antarctic alone, not counting the contribution from Greenland and other glaciers. That would be enough eventually to inundate all of the world’s coastal cities and cause devastation on a global scale.