Signs of very rapid ice melt in the past

From Inside Climate News:

A study of seafloor ripples suggests that ice shelves can retreat six miles per year, a quantum increase over today’s rates.

A study released today suggests that some of the continent's floating ice shelves can, during eras of rapid warming, melt back by six miles per year, far faster than any ice retreat observed by satellites.

As global warming speeds up the Antarctic meltdown, the findings "set a new upper limit for what the worst-case might be," said lead author Julian Dowdeswell, director of the Scott Polar Research Institute at the University of Cambridge. 

The estimate of ice shelf retreat is based on a pattern of ridges discovered on the seafloor near the Larsen Ice Shelf. The spacing and size of the ridges suggest they were created as the floating ice shelves rose and fell with the tides while rapidly shrinking back from the ocean. In findings published today in Science, the researchers estimate that to corrugate the seafloor in this way, the ice would have retreated by more than 150 feet per day for at least 90 days.

Ice shelves float on the ocean but they are fastened to land and act as stoppers that prevent Antarctic ice sheets that are as big as the U.S. and Mexico combined from sliding into the sea. The shelves are frozen to outcrops on the seafloor, but when they melt away from those anchor points, the flow of ice into the ocean speeds up, accelerating sea level rise.

If the rate of retreat estimated by the new study extended across an 18-mile-wide and half-mile-thick ice shelf, as found in the closely watched Pine Island Glacier in West Antarctica, the researchers calculated it would release 138-gigatons of ice per year— three to five times more ice than is currently lost annually from that glacier system.

Sea level rise can't be directly extrapolated from the rate of ice shelf retreat, said University of Liège ice researcher Xavier Fettweis, but the results of the new study suggest it could accelerate in coming decades.

"With such retreat rates, the sea level rise contribution from Antarctica could be a lot higher and quicker than expected, as the models are tuned to represent the current observed retreat rates," he said.

Currently, the fastest retreat rates are more than half a mile per year for the ice shelves extending from the Pine Island and Thwaites glaciers, and 1.2 miles per year for the one below the Smith Glacier, which is also in West Antarctica.

There's still no way to know exactly how fast the meltdown will happen with increasing human-caused warming, Fettweis said. But between 12,000 and 15,000 years ago, global sea level rose about 1.5 to 2.3 inches per year for several centuries, raising sea level by 82 feet over a 500-year period.  There's enough ice left on Antarctica to raise sea level by about 200 feet if it all melted.

Ice retreating six miles per year is "unheard of," said glaciologist Eric Rignot, with the University of California, Irvine and Caltech's Jet Propulsion Laboratory. It's "a wow moment," he said of the findings.

"Glaciers in Greenland retreat at 500 meters (1,640 feet) per year," he said. "Glaciers in alpine landscapes retreat more like 100 meters (328 feet), and that's already something overwhelming for people visiting glacier parks," said Rignot, who was not involved in the new research.