They measured several features of the ice core, including changes in the levels of deuterium, a heavy version of the element hydrogen, and dust trapped in the ice core.
Using new measurement techniques, the team could distinguish not only individual years of climate data, but seasonal fluctuations.
For some of the measurements, "we measure basically month to month," Dahl-Jensen said. "We can see the seasonsis it winter, summer?"
"We were very surprised, because one of the parameterscalled the deuterium excesscan switch modes basically from year to year," Dahl-Jensen said.
This shift in deuterium reflects a "dramatic change" in the source of moisture that creates snowfall in Greenland.
It appears that this moisture suddenly started coming from another Atlantic Ocean region that was about 4 to 7 degrees Fahrenheit (2 to 4 degrees Celsius) warmer.
The researchers argue that this change points to a major shift in atmospheric circulation, mostly likely in the so-called Intertropical Convergence Zone, a low-pressure area of the atmosphere that controls tropical storms and monsoons.
These shifts seem to have triggered rapid warming that was more gradual, which heated Greenland by 18 to 22 degrees Fahrenheit (10 to 12 degrees Celsius) over 50 years.
Some previous studies have found evidence of fast changes in the climate, says Richard Alley, a geoscientist at Pennsylvania State University. He notes that Greenland ice cores have shown abrupt shifts in snowfall occurring within as little as three years.
The new findings bolster "the strong evidence for very fast changes having occurred," Alley said.
Jeffrey Severinghaus, a geoscientist at the Scripps Institution of Oceanography in La Jolla, California, said the quick shift in Greenland's ancient climate shows "it really has to be something about the atmosphere that's causing the changes."
However, it's not clear what exactly is causing those changes, he noted, adding that realistic computer models that simulate Earth's climate don't show such abrupt changes.
"There's something missing from the models," Severinghaus said. "It's important to know that, especially in the light of ongoing human changes to the climate."
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