A new analysis of satellite data has found “extreme” changes underway at eight of Antarctica’s major glaciers, as unusually warm ocean water slips in under their ice shelves.

The warmer water is eating away at the glaciers’ icy grasp on the seafloor. As a result, the grounding line—where the ice last touches bedrock—has been receding by as much as 600 feet per year, a new study shows. Behind the grounding line, the land-based ice then speeds up, increasing the rate of sea level rise.

The new continent-wide measurements of grounding lines suggests a widespread pattern of melting all around Antarctica, said University of Leeds climate researcher Hannes Konrad, lead author of the analysis published today in the scientific journal Nature Geoscience.

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Climate models suggest that the current rate of retreat could lead to “centennial-scale collapse of the inland catchment areas,” the study says. That suggests that huge areas of ice far from the ocean could collapse within 100 years, leading to unexpected pulses of sea level rise.

The land-based ice can also speed up in response to ice shelf thinning more than 500 miles away, according to a new study by British and Germanclimate scientists who showed that the effects of localized ice shelf thinning can reach across the entire shelf. Konrad said those findings help show where Antarctica is most vulnerable to future ocean warming, including the large Ross and Filchner-Ronne ice shelves.

Gravity pulls Antarctica’s three-mile thick ice sheet into the ocean along ocean-bottom canyons. The grounding line is where the ice begins to float, and the glaciers and ice sheets turn into ice shelves. When they loosen from the ocean floor, the flow of ice from land accelerates. Ice leaving firm ground and then melting in the oceans causes seas to rise around the world.

From 2010 to 2016, the scientists found that warm ocean water melted 564 square miles of ice from those crucial grounded-ice areas.

Using satellite measurements of ice sheet elevation combined with other factors, like the shape of the seafloor and the known buoyancy of ice, they calculated the grounding lines’ receding rate.

Eight of the frozen continent’s 65 major ice streams had retreated by more than 410 feet per year—five times the average rate of retreat since the end of the last ice age. The grounding line of some of the glaciers emptying into the Amundsen Sea had retreated by up to 600 feet per year.

Fears of Worst-Case Meltdown ScenarioThe grounding line retreat reinforces concerns about a worst-case Antarctic meltdown scenario, with global sea level rising 10 feet by 2100. Along with the melting from below caused by warm ocean water, a 2015 studyshowed how global warming is melting ice shelves from above by causing more surface melting. That lets water penetrate deep down into the ice sheets and shelves. When it refreezes, it fractures the ice sheet from within.

Some grounding line retreat is expected in the centuries following an ice age, the researchers wrote, but current levels are far outpacing normal melt rates. Typically, grounding lines should retreat about 82 feet (25 meters) a year, they said. However, some of the studied regions — particularly in western Antarctica — have been receding at up to 600 feet (180 meters) per year. In total, the researchers found that, between 2010 and 2016, warming ocean temperatures melted away about 565 square miles (1,463 square km) of underwater ice from Antarctica — roughly the area of the city of London, England.

The good news is, only about 2 percent of the entire Antarctic grounding line retreated at such high rates, and some parts of the continent aren’t seeing a retreat at all. The bad news is, these if these accelerated rates don’t slow down, they could lead to parts of Antarctica’s inland ice sheet totally collapsing into the ocean. According to a 2017 study, such a collapse would likely put the world on track for experiencing worst-case-scenario sea level rise of 10 feet (3 meters) by 2100.

Further study of Antarctica’s grounding lines is needed to understand why some regions of the continent are receding so drastically while others stand still. According to the researchers, the methods developed for their new study should make future observations of this invisible melting ice much easier.

The greatest declines were seen in west Antarctica. At eight of the ice sheet’s 65 biggest glaciers, the speed of retreat was more than five times the rate of deglaciation since the last ice age. Even in east Antarctica, where some scientists – and many climate deniers – had previously believed ice might be increasing based on surface area, glaciers were at best stable and at worst in retreat when underwater ice was taken into account.

“It should give people more cause for concern,” said Shepherd. “Now that we have mapped the whole edge of the ice sheet, it rules out any chance that parts of Antarctica are advancing. We see retreat in more places and stasis elsewhere. The net effect is that the ice sheet overall is retreating. People can’t say ‘you’ve left a stone unturned’. We’ve looked everywhere now.”

The results could prompt an upward revision of sea-level rise projections. 10 years ago, the main driver was Greenland. More recently, the Antarctic’s estimated contribution has been raised by the Intergovernmental Panel on Climate Change. But its forecasts were based on measurements from the two main west Antarctic glaciers – Thwaites and Pine Island – a sample that provides an overly narrow and conservative view of what is happening when compared with the new research.

The study’s lead author, Hannes Konrad, said there was now clear evidence that the underwater glacial retreat is happening across the ice sheet.

“This retreat has had a huge impact on inland glaciers,” he said, “because releasing them from the sea bed removes friction, causing them to speed up and contribute to global sea level rise.”

On the face of it, the results are pretty much as expected.

Of the 1,463km² of grounded ice that has been given up, most of it is in well documented areas of West Antarctica where warm ocean water is known to be infiltrating the undersides of glaciers to melt them.

Dr Konrad explained: “If you take 25m per year as a threshold, which is sort of the average since the end of the last ice age, and you say anything below this threshold is normal behaviour and anything above it is faster than normal – then in West Antarctica, almost 22% of grounding lines are retreating more rapidly than 25m/yr.

“That’s a statement we can only make now because we have this wider context.”

The new data-set confirms other observations that show the mighty Pine Island Glacier, one of the biggest and fast-flowing glaciers on Earth, and whose grounding line has been in retreat since the 1940s, appears now to have stabilised somewhat.

The line is currently going backwards by only 40m/yr compared with the roughly 1,000m/yr seen in previous studies. This could suggest that ocean melting at the PIG’s base is pausing.

Its next-door neighbour, Thwaites Glacier, on the other hand, is seeing an acceleration in the reversal of its grounding line – from 340m/yr to 420m/yr.

Thwaites is now the glacier of concern because of its potential large contribution to global sea-level rise. And the UK and American authorities will shortly announce a major joint campaign to go and study this ice stream in detail.

Elsewhere on the continent, 10% of marine-terminating glaciers around the Antarctic Peninsula are above the 25m/yr threshold; whereas in East Antarctic, only 3% are.

The significant stand-out in the East is Totten Glacier, whose grounding line is retreating at a rate of 154m/yr.

Overall, for the entire continent, 10.7% of the grounding line retreated faster than 25m/yr, while 1.9% advanced faster than the threshold.

One fascinating number to come out of the study is that grounding lines in general are seen to retreat 110m for every metre of thinning on the fastest flowing glaciers. This relationship will constrain computer models that try to simulate future change on the continent.