July 21, 2024

A newly identified tipping point for the loss of ice sheets Antarctica and elsewhere may mean future sea level rise is significantly higher than current projections.

A new study has investigated how warm seawater intrudes between coastal ice caps and the land on which they rest. The warm water melts cavities in the ice, allowing more water to flow in, further expanding the cavities in a feedback loop. This water then lubricates the collapse of sea ice, which pushes up sea levels.

The researchers used computer models to show that a “very small increase” in the temperature of the intruding water can lead to a “very large increase” in the loss of ice—that is, tipping point behavior.

It is unknown how close the tipping point is, or if it has even been crossed. But the researchers said it could be caused by temperature increases of just tenths of a degree, and most likely by the increases expected in the coming decades.

Sea level rise is the biggest long-term impact of the climate crisis and will redraw the world map in the coming centuries. It has the potential to submerge many major cities, from New York to Shanghai, and affect billions of people.

The study addresses a key question about why current models underestimate the sea level seen in earlier periods between ice ages. Scientists think that some ice sheet melting processes should not yet be included in the models.

“[Seawater intrusion] could basically be the missing piece,” said Dr Alexander Bradley of the British Antarctic Survey, who led the research. “We don’t really have many other good ideas. And there’s a lot of evidence that when you do include that, the amount of sea level rise that the models predict could be much, much higher.”

Previous research showed that seawater intrusion can double the rate of ice loss from some Antarctic ice shelves. There is also real evidence that seawater intrusion is causing melting today, including satellite data showing drops in the height of ice sheets near land zones.

“With every tenth of a degree of ocean warming, we’re getting closer and closer to passing this tipping point, and every tenth of a degree is tied to the amount of climate change that’s happening,” Bradley said. “So we need very dramatic action to limit the amount of warming that is occurring and prevent this tipping point from being passed.”

The most important action is to reduce the burning of fossil fuels to net zero by 2050.

Bradley said, “Now we want to sit down [seawater intrusion] in ice sheet models and see if that twofold sea level rise plays out when you analyze the whole of Antarctica.”

Scientists have warned in 2022 that the climate crisis will bring the world to the brink of several “disastrous” tipping pointsincluding the collapse of Greenland’s ice sheet and the collapse of a key current in the North Atlantic, disrupting rain that billions of people depend on for food.

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Research in 2023 found that accelerated ice melting in west Antarctica was inevitable for the rest of the century, no matter how much carbon emissions are reduced, with “terrible” implications for sea levels.

The new research, published in the journal Nature Geoscience, found that some Antarctic ice sheets were more vulnerable to seawater intrusion than others. The Pine Island Glacier, currently Antarctica’s largest contributor to sea level rise, is particularly vulnerable, as the base of the glacier slopes inland, meaning that gravity helps seawater penetrate. The large Larsen ice sheet is also at risk.

The so-called “Doomsday” glacier, Thwaites, was found to be among the least vulnerable to seawater intrusion. This is because the ice is already flowing so fast into the sea that any cavities in the ice that are melted by seawater intrusion are quickly filled with new ice.

Dr Tiago Segabinazzi Dotto, from the UK’s National Oceanography Centre, welcomed the new analysis of the sea ice feedback loop under ice sheets.

“The researchers’ simplified model is useful for showing this feedback, but a more realistic model is highly needed to evaluate both positive and negative feedback,” he said. “An improvement in observations at the ground zone is also essential to better understand the key processes associated with the instability of ice shelves.”

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