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Study co-author Robert Hatfield (Oregon State University) prepares to sample sediment carried by icebergs that fill a Greenland fjord.

(Photo courtesy of Alberto Reyes)

A month ago, two separate reports indicated that the West Antarctic Ice Sheet has begun a long-feared collapse due to climate change, potentially raising sea level by four meters over the next few centuries.

A new study released Wednesday suggests another climate threshold is approaching -- this time for the two-mile-thick Greenland Ice Sheet -- with an even greater potential to boost sea levels.

Using sediment core evidence taken from the sea floor off Greenland's coast, the team of researchers from Oregon State University and the University of Wisconsin-Madison were able to estimate the extent of the Greenland Ice Sheet during an interglacial period 400,000 years ago, when global sea levels were much higher than today.

As massive ice sheets scoured the underlying bedrock, they delivered fine-grained sediment to the ocean, depositing age and location clues in the abundance of trace element isotopes. Lack of land-based sediments suggests lack of ice to erode the bedrock.

"Not only can we estimate how much ice there was," said Anders Carlson, associate professor at Oregon State University, and a co-author on the study, "but the isotopic signature can tell us where ice was present or from where it was missing."

The research team's results, published in the journal Nature, indicate that the southern part of Greenland experienced near total loss of glacial ice during this interglacial period. They determined that between 4.5 and 6.0 meters of sea level rise could be attributed to this ancient ice sheet collapse. Antarctic ice melt contributed as well, raising seas to somewhere between 6 and 13 meters higher than today.

"The climate 400,000 years ago was not that much different than what we see today, or at least what is predicted for the end of the century," said Carlson. Data from marine records in the North Atlantic show that the average temperatures in Greenland during that period were only about 1°C warmer than today's temperatures.

The drivers of climate change are different – changes in the Earth's orbit 400,000 years ago, increased atmospheric greenhouse gas concentration now. But the similarity in climate between then and now "suggests the threshold for ice sheet collapse is pretty low," according to Carlson. "We could be nearing the tipping point."

-- Casey O'Hara