Antarctica’s glaciers are melting from below, too

Antarctica isn’t what it used to be.

The ice that covers the continent and its surrounding ocean waters is always in flux with the seasons: Ice melts in the summer, and winter snowfall eventually gets packed down into new ice to replace what was lost. But in recent years, that snowfall hasn’t been able to make up the difference—in 2015, there was a deficit of 183 billion tons of snow, and scientists have predicted there will be even less snow each year following. Last year saw the calving of one of the largest icebergs ever—bigger than major cities, US states, or small countries depending on your comparison of choice. Both the snowfall deficit and the iceberg calving are related to warmer-than-usual ocean temperatures.

These are just the changes in ice visible at the surface level of the continent. Researchers have also now concluded that the ice attached to the seafloor surrounding the continent is receding fast—which could, in turn speed up the rate at which surface ice melts.

Antarctica is covered in glaciers, all of which flow like a slow (or not so slow) river out toward the sea, says Hannes Konrad, a geologist at the University of Leeds in England and lead author of the paper published (paywall) April 2 in Nature Geoscience. The glaciers’ movement is guided, in part, by what are called “grounding lines”—ice attached to the continental rock right at the sea floor, essentially forming rings of ice between the content and the surrounding Arctic ocean.

These ice lines are similar to an inchworm moving forward. When they extend, the rest of the glacier eventually catches up to it. The same effect works in reverse, too. “If you took a tiny bit of ice from the glaciers at this point, they move further inland. If you add a bit of ice, they move seaward,” Konrad says.

His team used satellite imaging to measure how the ice along the grounding lines had moved between 2010 and 2016. In total, their data showed, the grounding-line ice had shrunk 564 sq miles (1,435 sq km), which is roughly the size of London (or two New Yorks). The rates of ice lost varied across Antarctica: Some of the grounding lines on the eastern side of the continent appear to be stable, whereas some on the west coast have been retreating more than 600 ft (180 meters) per year.

Since icebergs are already floating in seawater, when they melt, they don’t contribute much to sea-level rise; they displace the same amount of water frozen or in liquid form. But grounded glacial ice, as it melts, will cause the sea level to rise.

Calculating out how far the grounding lines had retreated required records of where those lines were previously; those data were only available for a portion of the glaciers. So this particular study wasn’t able to say exactly how much retreating grounding lines are contributing to sea-level rise. That said, other oceanography experts say Konrad’s satellite-data measurement method seems to be valid. If applied to more glaciers over a longer period of time in the future, it could help make more definitive predictions of sea-level rise.

Read this next: Sea level rise is accelerating every year, “like a driver merging onto a highway”

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