Faster ice sheet melting could bring more coastal flooding sooner

If you've ever built a sandcastle on the beach, you've seen how sea water in the sand can quickly undermine the castle. A new study by the British Antarctic Survey concludes warmer seawater may work in a similar way on the undersides of ground-based ice sheets, melting them faster than previously thought.

That means computer models used to predict ice-sheet melt activity in the Antarctic may underestimate how much the long reach of warming water under the ice contributes to melting, concludes the study published Tuesday in the journal Nature Geoscience.

Faster ice sheet melting could bring greater flooding sooner than expected to coastal communities along the U.S. East Coast, where they're already seeing more high tide flood days along the shore and coastal rivers.

The study is at least the second in five weeks to report warmer ocean water may be helping to melt ice in glaciers and ice sheets faster than previously modeled. Scientists are working to improve these crucial models that are being used to help plan for sea level rise.

Relatively warmer ocean water can intrude long distances past the boundary known as the "grounding zone," where ground-based ice meets the sea and floating ice shelves, seeping between the land underneath and the ice sheet, the new study reports. And that could have "dramatic consequences" in contributing to rising sea levels.

“We have identified the possibility of a new tipping-point in Antarctic ice sheet melting,” said lead author Alex Bradley, an ice dynamics researcher at the survey.  “This means our projections of sea level rise might be significant underestimates.”

“Ice sheets are very sensitive to melting in their grounding zone," Bradley said. "We find that grounding zone melting displays a ‘tipping point-like’ behaviour, where a very small change in ocean temperature can cause a very big increase in grounding zone melting, which would lead to a very big change in flow of the ice above it."

The study follows an unrelated study published in May that found "vigorous melting" at Antarctica's Thwaites Glacier, commonly referred to as the "Doomsday Glacier." That study, published in the Proceedings of the National Academy of Sciences, reported visible evidence that warm seawater is pumping underneath the glacier. 

The land-based ice sheets in Antarctica and Greenland gradually slide toward the ocean, forming a boundary at the edge of the sea where melting can occur. Scientists report melting along these zones is a major factor in rising sea levels around the globe.

Water intruding under an ice sheet opens new cavities and those cavities allow more water, which in turn melts even larger sections of ice, the British Antarctic Survey concluded. Small increases in water temperature can speed up that process, but the computer models used by the Intergovernmental Panel on Climate Change and others don't account for that, the authors found.

“This is missing physics, which isn’t in our ice sheet models. They don’t have the ability to simulate melting beneath grounded ice, which we think is happening," Bradley said. "We’re working on putting that into our models now."

The lead author of the previous study, published in May, Eric Rignot, a glaciologist at the University of California, Irvine, told USA TODAY there's much more seawater flowing into the glacier than previously thought and it makes the glacier "more sensitive to ocean warming, and more likely to fall apart as the ocean gets warmer."

On Tuesday, Rignot said the survey's research provides "additional incentives to study this part of the glacier system in more detail," including the importance of tides, which make the problem more significant.

"These and other studies pointing at a greater sensitivity of the glacier to warm water means that sea level rise this coming century will be much larger than anticipated, and possibly up to twice larger," Rignot said.

Contributing: Doyle Rice, USA TODAY