The Oceans Are Going to Rise—but When?

The original version of this story appeared in Quanta Magazine.

In May 2014, NASA announced at a press conference that a portion of the West Antarctic Ice Sheet appeared to have reached a point of irreversible retreat. Glaciers flowing toward the sea at the periphery of the 2-kilometer-thick sheet of ice were losing ice faster than snowfall could replenish them, causing their edges to recede inland. With that, the question was no longer whether the West Antarctic Ice Sheet would disappear, but when. When those glaciers go, sea levels will rise by more than a meter, inundating land currently inhabited by 230 million people. And that would be just the first act before the collapse of the entire ice sheet, which could raise seas 5 meters and redraw the world’s coastlines.

At the time, scientists assumed that the loss of those glaciers would unfold over centuries. But in 2016, a bombshell study in Nature concluded that crumbling ice cliffs could trigger a runaway process of retreat, dramatically hastening the timeline. The Intergovernmental Panel on Climate Change (IPCC) took notice, establishing a sobering new worst-case scenario: By 2100, meltwater from Antarctica, Greenland, and mountain glaciers combined with the thermal expansion of seawater could raise global sea levels by over 2 meters. And that would only be the beginning. If greenhouse gas emissions continue unabated, seas would rise a staggering 15 meters by 2300.

However, not all scientists are convinced by the runaway scenario. Thus, a tension has emerged over how long we have until West Antarctica’s huge glaciers vanish. If their retreat unfolds over centuries, humanity may have time to adapt. But if rapid destabilization begins in the coming decades through the controversial runaway process, the consequences could outpace our ability to respond. Scientists warn that major population centers—New York City, New Orleans, Miami and Houston—may not be ready.

“We’ve definitely not ruled this out,” said Karen Alley, a glaciologist at the University of Manitoba whose research supports the possibility of the runaway process. “But I’m not ready to say it’s going to happen soon. I’m also not going to say it can’t happen, either.”

For millennia, humanity has flourished along the shore, unaware that we were living in a geological fluke—an unusual spell of low seas. The oceans will return, but how soon? What does the science say about how ice sheets retreat, and therefore, about the future of our ports, our homes, and the billions who live near the coast?

Grounded by the Sea

In 1978, John Mercer, an eccentric glaciologist at Ohio State University who allegedly conducted fieldwork nude, was among the first to predict that global warming threatened the West Antarctic Ice Sheet. He based his theory on the ice sheet’s uniquely precarious relationship with the sea.

Bigger than Alaska and Texas combined, West Antarctica is split from the eastern half of the continent by the Transantarctic Mountains, whose peaks are buried to their chins in ice. Unlike in East Antarctica (and Greenland), where most ice rests on land high above the water, in West Antarctica the ice sheet has settled into a bowl-shaped depression deep below sea level, with seawater lapping at its edges. This makes West Antarctica’s ice sheet the most vulnerable to collapse.

A heaping dome of ice, the ice sheet flows outward under its own weight through tentacle-like glaciers. But the glaciers don’t stop at the shoreline; instead, colossal floating plates of ice hundreds of meters thick extend over the sea. These “ice shelves” float like giant rafts, tethered by drag forces and contact with underwater rises and ridges. They buttress the glaciers against an inexorable gravitational draw toward the sea.