Scientists have found evidence in a chunk of bedrock drilled from nearly two miles below the summit of the Greenland ice sheet that the sheet 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 over 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.
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