For the first time, a “new and worrying” way that large ice sheets can melt has been characterised by scientists. Projections of sea level rise may thus be “significant underestimates”.

Relatively warm seawater can seep underneath Antarctic ice, accelerating the breaking away of chunks of ice into the ocean. That hastens melting.

Models used by the United Nation's Intergovernmental Panel on Climate Change (IPCC) do not take account of this phenomenon.

The research was by scientists at the British Antarctic Survey (BAS), and the findings are just published in the journal Nature Geoscience.

"We have identified the possibility of a new tipping-point in Antarctic ice sheet melting," says Alex Bradley, a researcher in ice dynamics at BAS and lead author of the paper. "This means our projections of sea level rise might be significant underestimates."

Beneath the ice sheet is the grounding zone, where the ice meets the ocean. The new study models how seawater can seep between the land and the ice sheet that rests on it, and how this affects the localised melting of the ice, lubricating the bed and influencing the speed at which it could slide towards the sea, says a press release.

The study looks at how this process accelerates with warming water

"Ice sheets are very sensitive to melting in their grounding zones,” Bradley says. “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,"

Ice sheets in Antarctica and Greenland seem to be shrinking faster than expected, and this could be why, he says. Including the results of the new work in such models could give more reliable estimates.

"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. We're working on putting that into our models now," he adds.