Newly identified tipping point for ice sheets could mean greater sea level rise

A satellite image of ice-pack breakup at the mouth of the Pine Island glacier in Antarctica. Photograph: Planet Observer/Universal Images Group/Getty Images

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

A new study has examined how warming seawater intrudes between coastal ice sheets and the ground they rest on. The warm water melts cavities in the ice, allowing more water to flow in, expanding the cavities further in a feedback loop. This water then lubricates the collapse of ice into the ocean, pushing up sea levels.

The researchers used computer models to show that a “very small increase” in the temperature of the intruding water could lead to a “very big increase” in the loss of ice – ie, tipping point behaviour.

It is unknown how close the tipping point is, or whether it has even been crossed already. But the researchers said it could be triggered by temperature rises of just tenths of a degree, and very likely by the rises expected in the coming decades.

Sea level rise is the greatest long-term impact of the climate crisis and is set to redraw the world map in coming centuries. It has the potential to put scores of major cities, from New York City to Shanghai, below sea level and to affect billions of people.

The study addresses a key question of why current models underestimate the sea level seen in earlier periods between ice ages. Scientists think some ice sheet melting processes must not yet 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 it, the amount of sea level rise the models predict could be much, much higher.”

Previous research has shown that seawater intrusion could double the rate of ice loss from some Antarctic ice shelves. There is also real-world evidence that seawater intrusion is causing melting today, including satellite data that shows drops in the height of ice sheets near grounding zones.

“With every tenth of a degree of ocean warming, we get closer and closer to passing this tipping point, and each tenth of a degree is linked to the amount of climate change that takes place,” Bradley said. “So we need very dramatic action to restrict the amount of warming that takes place and prevent this tipping point from being passed.”

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

Bradley said: “Now we want to put [seawater intrusion] into ice sheet models and see whether that two-times sea level rise plays out when you analyse the whole of Antarctica.”

Scientists warned in 2022 that the climate crisis had driven the world to the brink of multiple “disastrous” tipping points, including the collapse of Greenland’s ice cap and the collapse of a key current in the north Atlantic, disrupting rains upon which billions of people depend for food.

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 cut, with “dire” 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 especially vulnerable, as the base of the glacier slopes down inland, meaning gravity helps the seawater penetrate. The large Larsen ice sheet is similarly 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 flowing into the sea so fast already that any cavities in the ice melted by seawater intrusion are quickly filled with new ice.

Dr Tiago Segabinazzi Dotto, of the UK’s National Oceanography Centre, welcomed the new analysis of the ocean-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 feedbacks,” he said. “An enhancement of observations at the grounding zone is also essential to better understand the key processes associated with the instability of ice shelves.”