About 400,000 years ago, large parts of Greenland were ice-free. The wet tundra basked in the sunlight in the northwestern highlands of the island. Evidence suggests that a spruce forest, buzzing with insects, covered southern Greenland. Global sea levels were much higher then, between 20 and 40 feet above today’s levels. Around the world, the land that is now home to hundreds of millions of people was under water.
Scientists have known for a while that the Greenland ice sheet was largely gone at some point in the last million years, but not exactly when.
In a new study in the journal Science, we determined the date, using frozen soil dug up during the Cold War from beneath a nearly mile-thick section of the Greenland ice sheet.
The timing—about 416,000 years ago, with largely ice-free conditions lasting up to 14,000 years—is significant. At the time, Earth and its early humans were going through one of the longest interglacial periods since ice sheets first covered high latitudes 2.5 million years ago.
The length, magnitude, and effects of that natural warming can help us understand the Earth that modern humans are now creating for the future.
A world preserved under ice
In July 1966, American scientists and US Army engineers completed a six-year effort to drill through the Greenland ice sheet. The drill took place at Camp Century, one of the military’s most unusual bases – it was nuclear powered and consisted of a series of tunnels dug into the Greenland ice sheet.
The training site in northwest Greenland was 138 miles from the coast and underlain by 4,560 feet of ice. After reaching the bottom of the ice, the team continued to drill another 12 feet into the frozen rocky ground below.
In 1969, geophysicist Willi Dansgaard’s analysis of ice cores from Camp Century revealed for the first time the details of how Earth’s climate had changed dramatically over the past 125,000 years. Prolonged cold glacial periods when the ice quickly expanded gave way to warm interglacial periods when the ice melted and sea levels rose, inundating coastal areas around the world.
For nearly 30 years, scientists paid little attention to the 12 feet of frozen soil from Camp Century. One study analyzed the pebbles to understand the bedrock beneath the ice sheet. Another intriguingly suggested that the frozen ground preserved evidence of a warmer time than today. But with no way to date the material, few people paid attention to these studies. By the 1990s, the core of the frozen ground was gone.
A few years ago, our Danish colleagues found the lost soil buried deep in a Copenhagen freezer, and we formed an international team to analyze this unique archive of the frozen climate.
In the highest sample, we found perfectly preserved fossil plants – proof positive that the land far below Camp Century had been ice-free at some time in the past – but when?