It’s obvious that a warming climate will mean less glacial ice and higher sea level, but putting a precise number on these things is another matter. The landscapes concealed beneath the great ice sheets of Greenland and Antarctica are complex in hugely important ways. The interplay of ocean currents, which deliver warmer water to eat away at the underside of floating ice shelves, also varies regionally and even locally. And the ice itself is a dynamic thing, flowing in response to changes at the coastal edges.

So while we use measurements in the present and records from the past to forecast the future, we're stuck with scientific uncertainty, which means we need the language of risk analysis to discuss things sensibly. What is the possible range of sea level rise? And what are the probabilities for different parts of this range?

While some work in this regard has already been done, we're continually sharpening those assessments. As data and knowledge accumulate, our models of the ice sheets become more reliable guides to the future.

Last year, researchers published a study that added two key physical processes to an ice sheet model. First, they added the ability of meltwater to fill up crevasses in the ice until the pressure at the bottom of the crevasse is so great the ice breaks apart. Second, the new model accounted for the fact that the cliff of ice at the front of a glacier can’t hold itself up if it grows beyond a certain height. Together, these processes greatly accelerated the rate at which floating ice shelves could disappear and destabilize the rest of the glacier.

The researchers used the model to simulate a time during the Pliocene period when global sea level was as much as 20 meters higher than it is today. Other models were unable to get ocean levels that high without cranking up the heat to temperatures far above the warmth of the Pliocene climate. But with these added processes, the researchers got 17 meters (rather than several) of sea level rise from the melting of Antarctic ice.

Now, Penn State’s David Pollard and University of Massachusetts Amherst researcher Rob DeConto have applied an updated version of that model to the all-important question of Antarctica’s future. They simulated Antarctica’s ice out to the year 2500 for three of the common greenhouse gas emissions scenarios: the business-as-usual growth scenario that produces roughly 4 degrees Celsius of global warming this century, an extreme mitigation scenario that keeps warming well below 2 degrees Celsius, and an intermediate scenario.

In that lowest emissions scenario, Antarctica looks pretty much the same at the end of this century. But in the high emissions scenario, things get, well, interesting. Ice loss kicks into high gear around the middle of this century, resulting in almost 80 centimeters of global sea level rise by 2100. Remember, that's for Antarctica alone. For reference, the conservative estimates for total sea level rise from all sources in the latest IPCC report called for 50 to 100 centimeters by 2100, with Antarctica contributing very little of that.

Moving beyond this century, the melt rate continues to accelerate, with Antarctica’s contribution to sea level rise growing to more than 4 centimeters per year. That's a full meter of extra ocean every 25 years. The vulnerable West Antarctic Ice Sheet, much of which lies in bedrock basins that dip below sea level, falls apart over a couple of centuries. And even with atmospheric CO 2 stabilizing around the year 2300, Antarctica raises sea level by over 12 meters before the year 2500.

To put useful bounds on the uncertainty in these numbers, the researchers play with the mechanics of the new processes in the model. After simulating sea level rise during the Pliocene as well as the last interglacial period about 120,000 years ago as a test, they systematically twiddled knobs to produce a pile of different possible simulations. Since our estimates of sea level from those time periods also span a range, they identified model configurations that hit the low and high end of those sea level ranges. Then they re-ran the simulations of the future with each version of the model.

For the high emissions scenario, the range of sea level rise from Antarctic melt in 2100 spans about 60 to 115 centimeters. Taking the intermediate emissions scenario, where global warming stays below 3 degrees Celsius this century, Antarctica adds about 25 to 50 centimeters by 2100. The West Antarctic Ice Sheet disintegrates in that scenario, as well—it just takes about twice as long to do it (this scenario ends with about 5 meters of sea level rise from Antarctica by 2500). And remember, that’s separate from contributions from melting ice from Greenland and mountain glaciers, as well as the expansion of seawater as the oceans warm.

While there is plenty of science left to be done here, this does mark a significant jump. And that allows us to put a finer point on the risks of sea level rise as we drive global temperatures upward. The consequences are significant, and our actions today commit us to consequences that last for generations.

Nature, 2016. DOI: 10.1038/nature17145 (About DOIs).