In the recent geologic past, about 416,000 years ago, much of Greenland was an ice-free tundra landscape, perhaps covered by trees and woolly mammoths. This is shown by an international study conducted by the University of Vermont, Utah State University and fourteen other institutions, published in the journal Science.

The results overturn the previous view that much of the Greenland ice sheet survived for most of the past two and a half million years and show that moderate warming from 424,000 to 374,000 years ago led to significant ice melt.

At that time, despite the fact that atmospheric levels of heat-trapping carbon dioxide were much lower than they are today, 280 versus 420 ppm, there was enough ice melting in Greenland territory to cause sea levels to rise by at least three feet. This indicates that the Greenland ice sheet may be more sensitive to human-caused climate change than previously thought, and that it will become increasingly vulnerable in the coming centuries and will undergo rapid irreversible melting.

To arrive at this discovery, scientists used sediments from a long-lost ice sample collected at a secret U.S. Army base in the 1960s and applied advanced luminescence and isotope techniques-atoms of any chemical element with the same atomic number but different mass numbers-to provide direct evidence of the timing and duration of the ice-free period.

During the Cold War, a secret U.S. Army mission at Camp Century in northwest Greenland drilled 4560 feet of ice on the frozen island and then continued to the excavations to extract a twelve-foot-long pipe of soil and rock from beneath the ice. This frozen sediment was, then, forgotten in a freezer for decades. In 2017 it was accidentally rediscovered and was found to contain not only sediment, but also leaves and moss, remnants of an ice-free landscape, perhaps a boreal forest. The study presents direct evidence that sediments just below the ice sheet were deposited by flowing water in an ice-free environment during a period of moderate warming, called Marine Isotope Stage 11, from 424,000 to 374,000 years ago.

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