Melting of Ice Pole

It’s not exactly news that Greenland and Antarctica are shedding ice at record rates.

But in 2016, an eyebrow-raising idea ricocheted through the scientific community: It was possible, the authors said, that a warmer planet could push the towering ice cliffs at the fringes of the Antarctic ice sheet to essentially self-destruct, collapsing like a set of dominoes.

What was extra shocking was just how fast the ice could retreat under this runaway scenario, leading to about three feet of sea level rise fed from Antarctica alone by 2100—much faster than previous estimates, which generally proposed increases of only a few centimeters by the end of the century.

But two new pieces of research, published Wednesday in Nature, suggest a more measured retreat is likely in the coming decades. Both studies revise the estimates of just how much sea levels will rise by 2100 downward, suggesting that Antarctica could contribute somewhere between about three to 16 inches to the world’s oceans under the “worst case” scenarios.

Adding that to the other components that make up sea level rise—how the ocean expands as it warms (which will likely add about 10 inches), the melt from mountain glaciers (about six inches), and changes to the amount of water stored in lakes and rivers on land (one and a half inches), and the total is still a daunting number somewhere between just under two- to over three- foot range.

That is in no way a get-out-of-jail-free card, say the authors of both studies. It’s still an enormous amount of extra water that could slosh up onto coasts, enough to debilitate cities from Boston to Shanghai. But the most drastic impacts of sea-level rise, they say, are likely to kick in only after the turn of the century, giving communities around the world more time to adapt.

What’s more, changes to the ice sheets in both Greenland and Antarctica could also trigger planet-wide shifts in temperature, ocean circulation, and many other parts of the climate system, says says Nick Golledge, a climate scientist at the Antarctic Research Center of the University of Victoria, Wellington, and the lead author of one of the studies.

“The sea-level estimates maybe aren’t as bad as we thought, but the climate predictions are worse,” says Golledge.

What happens in the Antarctic…

In a separate analysis, the team led by Golledge found that their ice sheet model could match the modern and Last Interglacial records well—also without MICI. Warm water soaking the base of the ice sheets, they found, was enough to force key parts of the ice sheet to melt away.

They used that model to predict how the Greenland and Antarctic ice sheets will speed up their melting in the coming decades. If the world continues to burn greenhouse gases unabated, following the worst-case scenario, the authors predict that the two ice sheets will add about 10 inches to the world’s oceans by 2100.

That number is similar to what the IPCC projected for the “worst case scenario” in their last comprehensive report in 2013, predicting about nine inches of sea level rise from Greenland and Antarctica. It is smaller than the number predicted by the 2016 study, which said that Antarctica alone might feed more than three feet of sea level rise into the oceans by 2100.

The sea level rise estimates may be lower, but the overall picture of how melting ice sheets will affect climate is grim.

Golledge and his colleagues also attached their ice sheet model to a global climate model, in order to see how the impacts of ice melting at the poles would influence climate and oceans in farflung parts of the world (in the past, ice sheet models have traditionally been run separately, primarily because computers haven’t been powerful enough to link them together).

Changes in the ice sheets, they found, could influence global climate profoundly—slowing down major ocean circulation pathways, skewing air temperatures around the world, and somewhat surprisingly, making climate more variable from year to year.

“What happens in the Antarctic does not stay in the Antarctic, and that’s what they show very clearly,” says Pattyn.

The impacts are already leaking out of the poles. “We’re living in a time when, even in the last few years, we have seen extreme weather events become even more and more common,” says Golledge. “Dealing with steady warming is easier, in many ways. But if things are just unpredictable and extremely variable from year to year—well, that’s a much harder problem for society to solve.”