A Melting Antarctica Could Push Sea Levels to Rise More than 3 Feet by 2100

Sea-level rise is one of the scariest effects of climate change. Over coming decades, scientists say it will put millions of people at risk in the continental United States, not to mention other parts of the world, including large parts of Asia and many Pacific islands.

What's even scarier is that it could happen a lot faster than we predicted.

In a new study in Nature, published Wednesday, two US climate scientists say that a melting Antarctica could contribute more than 1 meter of sea-level rise by 2100 (that's over 3 feet), and more than 15 meters by 2500, if we keep pumping out greenhouse gases at current rates. In the worst-case scenario, their model doubles the best estimates of sea-level rise from the UN's Intergovernmental Panel on Climate Change, predicted over the next century.

For coastal cities like New York and Miami, "one to two meters would be very, very serious," said co-author David Pollard of the Pennsylvania State University, adding that these rising sea levels will contribute to storm flooding and Hurricane Sandy-like storms. Over a few hundred years, a 10-meter sea level rise "would be mindbogglingly catastrophic," he continued, and would make large swaths of our planet uninhabitable.

Floating ice shelves around Antarctica are a hugely important defence against sea-level rise, because they "buttress the ice sitting on land in the interior," Pollard told me. We know that warmer ocean water is eating away at the undersides of these ice shelves, and current models of future melt take that into account. But, according to Pollard, these models still aren't able to simulate enough melting to explain sea level rise during the Pliocene, the period some 3 million years ago when levels rose up to 10 or 20 meters.

So Pollard and DeConto added two other mechanisms to their model, which could become more important as the planet gets warmer. Summer temperatures around many of Antarctica's ice shelves already hover near 0°C, the new study says; with just a little more warming, there will be a lot more surface melt and rainfall on Antarctica. Pooling water at the surface of the continent's ice shelves will drain down into cracks and crevasses and "force them open" until the ice shelf disintegrates, said Pollard.

Once these ice shelves break away, the second mechanism comes into effect—vast, exposed walls of ice that crumble under the pressure of their own massive weight, collapsing into the sea. "If the floating ice is removed, you're left with a vertical cliff where the ice meets the ocean," he said. As it falls apart, the cliff will erode backwards "several kilometers per year."

"Things will happen quickly after that."

We've seen this process happening already at certain places in Greenland, he said.

The Arctic and Antarctica are very different—one's surrounded by water, the other by land, and sea ice behaves differently in both places—but both are being impacted in a major way by climate change. On Tuesday, the National Snow and Ice Data Center said that Arctic sea ice reached its annual maximum on March 24. And according to satellite imagery, this year's maximum set another new record low, beating out last year's.

Between December and February, air temperatures over the Arctic Ocean were 2-to-6°C above average, according to the NSIDC, based in Boulder. "I've never seen such a warm, crazy winter in the Arctic," said director Mark Serreze. "The heat was relentless."

As for Antarctica, scientists already know that meltwater and collapsing ice cliffs can contribute to ice retreat. But they aren't major contributors there right now, according to Pollard. The updated model, which fits the picture of what happened during the Pliocene, was driven forward into the future, as the scientists plugged in various scenarios for how much our climate will change.

They did come up with some good news. If we manage to limit the global rise below 2°C, which is what climate scientists are pushing for, "our model predicts these mechanisms won't be triggered," Pollard said, at least not in such an extreme fashion.

In the face of a worst-case scenario that would leave millions of people displaced, Pollard won't say whether he's a pessimist. "I don't have any special knowledge about social impacts. I just concentrate on the physics," he said. "We're indicating some very scary uncertainties, and what should be looked at in the future."

Whether the world can even stay below that 2°C threshold, though, isn't a sure thing. The new study closes off by saying that the warming ocean means that any lost ice won't be recovered—not for thousands of years.