Tall ice cliffs are slumping and may trigger rapid sea-level rise, study finds

Scientists have found that tall ice cliffs in Greenland are slumping and this may eventually lead to a more rapid rise in sea levels.

The study, publishedthis month in the journalGeology, suggests iceon glacial cliffs in Greenland and Antarctica are acting like soil and rockby slumping which refers to when weakened sediment breaks apart from landand slidesdown a slope.

"It's sort of like a human slumping down in an easy [recliner] chair," said Richard Alley, one of the authors of the paperand a professor ofgeosciencesat Pennsylvania State University.

Alley's teamstudied the Helheim, one of Greenland's largest glaciers, and found slumping on ice cliffs as tall as 100 metres above sea level, and 900 metres below.

"What we see in Greenland is the stuff above the water slumps down as if in the easy chair. But it was holding down this giant iceberg below the water that is almost 10 times as high.

"And then that breaks up."

When the ice slumps, it rotates, then the ice below the sea will pop up, Alley said. In turn, thatcreates a crack in the ice, which weakens it and leadsto calving a huge chunk ofice breaking off.

"It does so fairly dramatically," said Alley. "There are 50-storey splashes."

Helheim Glacier calving event in June 2018

5 years ago

Duration 1:41

A sped-up video of a calving event on Greenland's Helheim Glacier on June 22, 2018. (Denise Holland/NYU Abu Dhabi Research Institute)

Alley said these big calving events happen very quickly and can cause earthquakes with a magnitude of up to 5.0 "that you could hear in Yellowknife or ... down in the States."

Researchers observed, mapped and monitored the speed and motion of the calving ice on the Helheimusing seismometers and scanning technology calledreal-aperture terrestrial radar interferometry, said Alley.

Why is slumping ice worrisome?

Alley said as the climate gets warmer, the ice cliffs could get taller.

As glaciers retreat farther inland due to melting,there may be thicker ice sometimes upward of 3,500 metres thick in parts of Antarctica which would create massive ice cliffs, said Alley.

"What we're worried about is that if things go wrong in Antarctica, that eventually it will make cliffs that are so high that they can break without much weight."

He explained that normal calving happens after the ice melts and weakens due to warming. On the other hand, tallerice cliffs can slump and break off more rapidly.

"Rather than, 'Break wait a long timebreak,' it [would] go, 'Break. Break. Break. Break. Break.' That would be bad for sea level."

"If we continue to make it hot thensea level rise might be very rapid," said Alley, adding that it could rise threemetres in a matter of decades once the super tall ice cliffs started slumping.

That could impact coastal cities, people living on islands, and even some airports that aren't much higher than sea level, said Alley.

Alley said theteam plans on studying what the anatomy of the ice is like on these glaciers that slump, and work on measuring just how much warming is needed to create the massive cliffs "at which point there'd be real trouble."

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