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.

"We're seeing this all across the ice sheet," he said. "As the grounding line of the ice shelves moves back, the inland glaciers accelerate and raise global sea level."

Konrad and colleagues from University College London and the Helmholtz Centre for Polar and Marine Research in Germany measured how the grounding lines are shifting across 16,000 kilometers of coastline using data from the European Space Agency's CryoSat-2 satellites. Their animation above illustrates how the grounding line, tracked by satellite, is changing.

Receding at 5 Times Historical Average

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 Scenario

The 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 study showed 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.

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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 German climate 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.

Sea Level Rise and Other Feedback Effects

Communities around the world are already experiencing more coastal flooding caused by global warming, especially in big storms. Knowing how fast sea level will rise on a meaningful time scale is critical to planning and adaptation, and the new study helps fill gaps in areas where data from surface observations is sparse.

Columbia University researcher Pierre Dutrieux hopes to fill some of the gaps with data from recently deployed autonomous deep-sea drones that are spending nine months beneath West Antarctica's Dotson Ice Shelf to measure ocean flows and temperatures over a long period of time.

Current data shows that the ice shelves are responding to incursions of warmer water, but the record is short and the region has one of the most naturally variable climates on Earth.

Dutrieux said the new grounding line study is the first to bring a continent-wide view of grounding line retreat. "The findings support present understanding of the system, with big variations on a continental scale, but regional patterns of retreat that match increases in ocean temperatures," he said.

Along with rising sea level, there could be other climate feedback effects, Dutrieux said. Oceanographers have detected a trend of decreasing salinity in Antarctic waters fed by ice sheet melt. That affects the density of the deep, very cold waters that drive key ocean currents that affect climate at the surface.

Increasing freshwater at the edge of the ice sheet could also disrupt the timing of biological cycles, he said, starting with phytoplankton—the critical base of the Antarctic food web.