History Reveals Greenland Ice Might Melt Much Faster than Believed

The huge ice sheet melted several times between ice ages, leading to belief it will melt faster with global warming, with dire implications for sea level rise.

Greenland rocks now buried under 10,000 feet of ice were ice-free for long stretches during the past 1.4 million years, leading scientists to conclude the Greenland Ice Sheet could melt more suddenly than previously believed.

That could raise global sea level far beyond current projections over the next few centuries, including past recent estimates from the Intergovernmental Panel on Climate Change, according to a new study published Wednesday in the journal Nature.  The research challenges the prevailing idea that the ice sheet remained relatively intact during the recent geological past, showing even the thickest ice had vanished during warm periods between ice ages.

Columbia University paleoclimatologist Joerg Schaefer, who co-authored the study, said the findings show the Greenland Ice Sheet may be much less stable than scientists thought.

"Over the last 15 years, it looked like we wouldn't have to worry too much about the Greenland ice sheet melting too fast," Schaefer said.  "Most of the climate models treated it as a solid ice cube sitting on bedrock.  With climate warming, it melts off the top, but it takes a long time. Compared to other ice sheets like West Antarctica, is looked pretty strong and resilient to warming."

New, direct samples of the bedrock pierced that belief.

"These might be the most precious rocks since the moon rocks," he said.  "We got the huge privilege of working with these rocks, and cutting-edge isotopic tools, and the surface of Greenland gave us a very clear answer.  Unfortunately, the answer is it was ice-free during an important period."

If the ice sheet had persisted across most of Greenland during those warm periods, there was hope that it could stay mostly intact despite the Earth's thickening blanket of greenhouse gases.  The new study suggests that, because it mostly melted, there's little doubt the planet's warming trajectory will melt the Greenland Ice Sheet in the centuries ahead.

"We lost the Greenland Ice Sheet during periods of natural forcing.  We lost it several times, so it's not reasonable to believe we'll keep that ice sheet in the future with human-caused global warming," he said.

The paper doesn't project a specific timeline for melting, but the findings could have an effect in the coming century.  The models currently used to project sea level rise indicate a stable ice sheet.  But other factors, including accelerating glaciers or more water at the base of the ice, could push the ice sheet past a tipping point toward a rapid meltdown, Schaeffer said.

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