One of the bigger risks we’re running with our planetary warming experiment is the melting of the vulnerable West Antarctic Ice Sheet, raising sea levels much more rapidly than we expect. It’s a scenario scientists assess both by studying present conditions and by examining the past.

One time period of particular interest is the previous interglacial—a warm intermission between ice ages—about 120,000 years ago. Sea level appears to have been 5 to 9 meters higher during this time when the configuration of Earth’s orbit made for a little more summer sunshine (and warmer temperatures) in the Northern Hemisphere.

The West Antarctic Ice Sheet is a candidate to have supplied about 3 meters of that sea level rise. Unfortunately, evidence of its history is hard to come by, as the regrowth of the ice sheet destroyed some of it and now conceals even more. Sediment cores show the ice sheet shrank drastically in the past, but it’s unclear when in the past.

A new study led by University of Washington researcher Eric Steig applies a suitably clever work-around to get at the ice sheet's history. It's based on the impact a collapse of the ice sheet (if it shrank down to a small remnant) would have had on local atmospheric circulation. Losing the ice sheet, after all, is like deflating an entire landscape. There are good reasons to expect that thickening the atmosphere by lowering the surface in that area would have consequences.

The researchers used four different types of climate models—some simpler, some more complex—to simulate the Antarctic atmosphere with and without the West Antarctic Ice Sheet. They hoped to find consistent patterns among the models, and they did. With the ice gone, winds brought more of the warmer air from over the Weddell Sea inland, with cold continental air heading the other direction, out over the Ross Sea. Some of the warmer air leaked across to East Antarctica, as well, even though no changes had been made to the ice sheet elevation there.

So we see a warmer West Antarctica on the Weddell Sea-side spilling into East Antarctica, and a cooler West Antarctica on the Ross Sea side. The researchers compared this pattern to what we see in ice core records from the last interglacial period. There are a number of such ice cores from the mighty East Antarctic Ice Sheet, but only one ice record (actually surface samples rather than a core) from the West Antarctic Ice Sheet contains ice that old. That ice happens to have come from the area adjacent to the Ross Sea.

Compared to the five East Antarctic cores, the record from near the Ross Sea shows significantly less warming during the last interglacial. That’s the same pattern you see in the model simulations if the West Antarctic Ice Sheet collapses, providing support for the idea that it really did collapse 120,000 years ago.

However, the researchers note this is a somewhat qualitative comparison. There are some potentially complicating factors they could not account for, and it’s unclear whether you need a full collapse of the ice sheet to explain the ice record measurements, or if a smaller amount of deflation might be enough.

But still, it’s another line of evidence to bring to bear on the question of what happened to the West Antarctic Ice Sheet back then, which can help us predict how it will respond to global warming. The models also show us how to further test the hypothesis, since they reveal where the most interesting ice core temperature records could be found, for example. Answers in science, whether from the past or the present, come from asking the right questions, and we could really use more answers about the vulnerability of the West Antarctic Ice Sheet.

Geophysical Research Letters, 2015. DOI: 10.1002/2015gl063861 (About DOIs).