The oceans will rise, but when?

The original version to This story Featured in Quanta Magazine.

In May 2014, NASA announced in a press conference that part of the West Antarctic ice sheet appeared to have reached the point of irreversible retreat. Glaciers flowing toward the sea at the edges of the two-kilometre-thick ice sheet were losing ice faster than snowfall could replenish it, causing their edges to recede inward. However, the question is no longer if the West Antarctic Ice Sheet will disappear, but when. When those glaciers melt, sea levels will rise by more than a meter, submerging lands currently inhabited by 230 million people. This will be just the first act before the entire ice sheet collapses, which could raise sea levels by 5 meters and reshape the world’s coastlines.

At the time, scientists assumed that the loss of these glaciers would continue for centuries. But in 2016, a startling study appeared in nature He concluded that collapsing ice slopes could trigger a runaway retreat process, greatly accelerating the timeline. The Intergovernmental Panel on Climate Change has taken notice and created a new worst-case scenario: By 2100, melting water from Antarctica, Greenland, and mountain glaciers, combined with thermal expansion of seawater, could raise global sea levels by more than two metres. And that will be just the beginning. If greenhouse gas emissions continue unabated, sea levels will rise by a staggering 15 meters by 2300.

However, not all scientists are convinced by this wild scenario. Thus, tension has arisen over how long it will take until the huge glaciers of West Antarctica disappear. If their decline continues over centuries, humanity may have time to adapt. But if the process begins to destabilize rapidly in the coming decades through an unchecked and controversial process, the consequences may exceed our ability to respond. Scientists warn that major population centers — New York City, New Orleans, Miami and Houston — may not be ready.

“We certainly haven’t ruled it out,” said Karen Alley, a glaciologist at the University of Manitoba, whose research supports the possibility of this runaway process. He added, “But I am not ready to say that it will happen soon. I will also not say that it cannot happen.”

For thousands of years, humanity has thrived along the shore, unaware that we were living in a geological shell – an extraordinary wave of low-lying seas. The oceans will return, but when? What does science say about how the ice sheets are retreating, and therefore about the future of our ports, our homes, and the billions who live near the coast?

Based on the sea

In 1978, John Mercer, an eccentric glaciologist at Ohio State University who allegedly conducted bare field work, was among the first to predict that global warming threatened the West Antarctic ice sheet. He based his theory on the uniquely fraught relationship between the ice sheet and the sea.

Larger than the area of ​​Alaska and Texas combined, West Antarctica is divided from the eastern half of the continent by the Trans-Antarctic Mountains, whose peaks are buried in ice. In contrast to East Antarctica (and Greenland), where most of the ice lies on ground high above the water, the West Antarctica ice sheet rests in a bowl-shaped depression deep below sea level, with seawater flowing over its edges. This makes the West Antarctica ice sheet the most vulnerable to collapse.

An accumulating dome of ice, the ice sheet flows outward under its own weight through tentacle-like glaciers. But the glaciers don’t stop at the shore; Instead, huge floating sheets of ice hundreds of meters thick stretch over the sea. These “ice shelves” float like giant rafts, bound by drag forces and touching underwater ridges and ridges. They support glaciers against the relentless gravity of the sea.