Stability Check on Antarctica Reveals High Risk for Long-Term Sea Level Rise

The warmer it gets, the faster Antarctica will lose ice, and at some point the losses will become irreversible. As a result, many of the world’s coastal cities and cultural heritage sites will disappear. That is what researchers from the Potsdam Institute for Climate Impact Research (PIK), Potsdam University and Columbia University’s Lamont-Doherty Earth Observatory say in a new study featured on the cover of the leading journal Nature,  in which they calculate how much warming the Antarctic Ice Sheet can survive.

In around a million hours of computation time, the unprecedentedly detailed simulations delineate where and at what warming levels the ice would become unstable and eventually melt and drain into the ocean. The researchers find a delicate concert of accelerating and moderating effects, but the main conclusion is that unmitigated climate change would have dire long-term consequences: If the global mean temperature level is sustained long enough at 4 degrees C above preindustrial levels, for instance, Antarctic melting alone could eventually raise global sea levels by more than six meters, or about 20 feet. More warming would bring even higher sea levels.

Antarctica holds more than half the earth’s fresh water, frozen in a vast ice sheet nearly 5 kilometers (3 miles) thick in many places. As the surrounding ocean water and atmosphere warm due to human greenhouse-gas emissions, the ice will almost certainly lose mass and eventually become unstable. The researchers calculate that at 2 degrees C of warming, melting and the accelerated ice flow into the ocean would eventually entail 2.5 meters (8 feet) of global sea level rise just from Antarctica alone. At 4 degrees, it would be 6.5 meters, and at 6 degrees almost 12 meters (almost 40 feet), if these temperature levels were sustained long enough.

Anders Levermann, a coauthor of the paper and researcher at PIK and Lamont-Doherty, said, “Antarctica is basically our ultimate heritage from an earlier time in earth’s history. It’s been around for roughly 34 million years. Our simulations show that once it’s melted, it does not regrow to its initial state even if temperatures eventually sank again. In other words, what we lose of Antarctica now is lost forever.”

The changes would be irreversible because of self-reinforcing mechanisms in the ice sheets’ behavior as conditions warm. In West Antarctica for instance, the current main driver of ice loss is warm ocean water penetrating under the floating ice shelves that fringe the continent. This leads to melting underneath the ice. This in turn could destabilize glaciers behind the shelves that are grounded on land, and cause them to slide into the ocean. The researchers say that once warming crossed the threshold of 6 degrees, effects from the ice surface would become more dominant. As gigantic mountains of ice slowly sink to lower elevations, where the air is warmer, this would lead to more melt at the ice surface—a phenomenon already observed in Greenland.

Ice loss and melting have already accelerated significantly over the last decades in Antarctica. Levermann said the paper is a reminder that international accords to limit warming to 2 below degrees are vital for the survival of the world’s coastlines as they now exist. “If we give up the Paris Agreement, we give up Hamburg, Tokyo and New York,” he said.

Adapted from a press release by the Potsdam Institute for Climate Impact Research.