A long-forgotten frozen soil sample offers a warning for the future

Enlarge / Water and sediment pour over the melting margin of the Greenland Ice Sheet. Jason Edwards/Getty Images

About 400,000 years ago, large parts of Greenland were free of ice. The brushy tundra bathed in the rays of the sun on the highlands of the northwest of the island. Evidence suggests that a spruce forest, buzzing with insects, covered the southern part of Greenland. Global sea levels were much higher then, between 20 and 40 feet above present levels. Around the world, lands that are now home to hundreds of millions of people were under water.

Scientists have known for some time that the Greenland Ice Sheet all but disappeared at some point in the last million years, but not precisely when.

In a new study published in the journal Science, we pinpointed the date, using frozen soil extracted during the Cold War beneath a nearly mile-thick section of the Greenland Ice Sheet.

[embedded content] A brief overview of the evidence beneath the Greenland Ice Sheet and the lessons it holds.

The timing - about 416,000 years ago, with largely ice-free conditions lasting up to 14,000 years - is important. At this time, Earth and its first humans were going through one of the longest interglacial periods since ice sheets covered high latitudes 2.5 million years ago.

The duration, magnitude, and effects of this natural warming can help us understand the Earth that modern humans are creating for the future.

A world preserved under the ice

In July 1966, US scientists and US Army engineers completed a six-year effort to drill through the Greenland Ice Sheet. The drilling took place at Camp Century, one of the more unusual military bases. It was powered by nuclear energy and made up of a series of tunnels dug into the Greenland ice cap.

The drill site in northwest Greenland was 138 miles from the coast and rested on 4,560 feet of ice. Once they reached the bottom of the ice, the team continued to drill 12 more feet into the frozen, rocky ground below.

Enlarge / George Linkletter, working for the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory, examines a piece of ice core in the Camp Century Science Trench....