Some warm phases are exceptional, however, marked by extraordinarily high biological activity in the lake, well above that of "regular" climate cycles.
To quantify the climate differences, the scientists studied four warm phases in detail: the two youngest, called "normal" interglacials, from 12,000 years and 125,000 years ago; and two older phases, called "super" interglacials, from 400,000 and 1.1 million years ago.
According to climate reconstructions based on pollen found in sediment cores, summer temperatures and annual precipitation during the super interglacials were about 4 to 5 degrees C warmer, and about 12 inches wetter, than during normal interglacials.
The super interglacial climates suggest that it's nearly impossible for Greenland's ice sheet to have existed in its present form at those times.
Simulations using a state-of-the-art climate model show that the high temperature and precipitation during the super interglacials can't be explained by Earth's orbital parameters or variations in atmospheric greenhouse gases alone, which geologists usually see as driving the glacial/interglacial pattern during ice ages.
That suggests that additional climate feedbacks are at work.
"Improving climate models means that they will better match the data that has been collected," says Paul Filmer, program director in NSF's Division of Earth Sciences, which funded the "Lake E" project along with NSF's Office of Polar Programs.
"The results of this collaboration among scientists in the U.S., Austria, Germany and Russia are providing a challenge for researchers working on climate models: they now need to match results from Antarctica, Greenland--and Lake El'gygytgyn."
Adds Simon Stephenson, director of the Division of Arctic Sciences in NSF's Office of Polar Programs, "This is a significant
|Contact: Cheryl Dybas|
National Science Foundation