"Unfortunately, This Makes The Greenland Ice Sheet Look Highly Unstable" - Lamont Doherty Melt Study
Scientists have found evidence in a chunk of bedrock drilled from nearly two miles below the summit of the Greenland Ice Sheet that the ice nearly disappeared for an extended time in the last million years or so. The finding casts doubt on assumptions that Greenland has been relatively stable during the recent geological past, and implies that global warming could tip it into decline more precipitously than previously thought. Such a decline could cause rapid sea-level rise. The findings appear this week in the leading journal Nature.
The study is based on perhaps Earth’s rarest geologic sample: the only bit of bedrock yet retrieved from the ice sheet’s base, more than two decades ago. The authors say that chemical isotopes in it indicate that the surface was exposed to open sky for at least 280,000 years during the last 1.4 million years. The reason would have been natural, probably tied to cyclic natural climate changes that have caused ice ages to wax and wane. The scientists say that in the most conservative interpretation, there might have been only one ice-free period that ended 1.1 million years ago. But, more likely, they say, the ice vanished multiple times for shorter periods closer to the present. Greenland contains about 684,000 cubic miles of ice – enough to raise global sea levels about 24 feet if it were to melt completely.
“Unfortunately, this makes the Greenland ice sheet look highly unstable,” said lead author Joerg Schaefer, a paleoclimatologist at Columbia University’s Lamont-Doherty Earth Observatory. “If we lost it in periods of natural forcing, we may lose it again.” With human-induced warming now well underway, loss of the Greenland ice has roughly doubled since the 1990s; during the last four years by some estimates, it shed more than a trillion tons.
No one knows exactly what it might take to make the ice collapse, or how long that might take. Some models project that it will melt partially or completely over the next 2,500 to 10,000 years, depending on the amount of greenhouse gases humans pour into the air. Ice loss from Greenland now accounts for about a quarter of the current sea-level rise, which is about 3 millimeters a year, but this could accelerate. Projections of sea-level rise during this century hover around 3 or 4 feet, but many, including the one from the Intergovernmental Panel on Climate Change, do not take Greenland into account. More drastic models put the potential rise much higher.
Coauthor Richard Alley, a glaciologist at Pennsylvania State University, said the study “doesn’t say that tomorrow Greenland falls into the ocean. But the message is, if we keep heating up the world like we’re doing, we’re committing to a lot of sea-level rise.” This could take centuries or millennia, he said.
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http://www.ldeo.columbia.edu/news-events/most-greenland-ice-melted-bedrock-recent-geologic-past-study-says
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