by About 400,000 years ago, much of Greenland was ice-free. The overgrown tundra basked in the sun’s rays in the island’s northwestern highlands. Evidence suggests that a spruce forest, teeming with insects, covered the southern part of Greenland. Global sea level was much higher then, between 20 and 40 feet above current levels. Around the world, lands that are now home to hundreds of millions of people were underwater.
Scientists have long known that the Greenland ice sheet had mostly disappeared at some point in the last million years, but not exactly when.
In a new study published in the journal Science, we determined the date using frozen soil extracted during the Cold War from beneath a nearly kilometer-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 important. At the time, Earth and its first humans were going through one of the longest interglacial periods since ice sheets first covered high latitudes 2.5 million years ago.
The duration, magnitude and effects of that natural warming can help us understand the Earth that modern humans are creating for the future.
A world preserved under ice
In July 1966, American scientists and army engineers completed a six-year effort to drill into the Greenland ice sheet. The drilling took place at Camp Century, one of the most unusual military bases: it was nuclear powered and consisted of a series of tunnels dug into the Greenland ice sheet.
The drilling site in northwest Greenland was 138 miles offshore and underlain by 4,560 feet of ice. Once they reached the bottom of the ice, the team continued drilling 12 more feet into the rocky, frozen ground below.
In 1969, geophysicist Willi Dansgaard’s analysis of the Camp Century ice core revealed for the first time the details of how Earth’s climate had changed dramatically over the past 125,000 years. Long cold glacial periods, in which ice expanded rapidly, gave way to warm interglacial periods, in which ice melted and sea level rose, flooding coastal areas around the world.
For nearly 30 years, scientists paid little attention to Camp Century’s 12 feet of frozen ground. One study analyzed 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 frozen ground core had disappeared.
Several years ago, our Danish colleagues found missing soil buried deep in a Copenhagen freezer and we formed an international team to analyze this unique frozen climate archive.
In the sample above, we found perfectly preserved fossil plants, proof positive that the land far below Camp Century had been ice-free at some point in the past, but when?
Dating ancient rocks, twigs and soil.
Using samples cut from the center of the sediment core and prepared and analyzed in the dark so that the material retains an accurate memory of its last exposure to sunlight, we now know that the ice sheet covering northwest Greenland (now almost a mile thick) disappeared. during the long natural warm period known to climate scientists as MIS 11, between 424,000 and 374,000 years ago.
To more precisely determine when the ice sheet melted, one of us, Tammy Rittenour, used a technique known as luminescence dating.
Over time, minerals build up energy as radioactive elements such as uranium, thorium, and potassium decay and release radiation. The longer the sediment remains buried, the more radiation accumulates in the form of trapped electrons.
In the laboratory, specialized instruments measure small amounts of energy, released in the form of light by these minerals. That signal can be used to calculate how long the grains were buried, since the last exposure to sunlight would have released the trapped energy.
Paul Bierman’s lab at the University of Vermont dated the last time the sample was near the surface in a different way, using rare radioactive isotopes of aluminum and beryllium.
These isotopes form when cosmic rays, originating far from our solar system, collide with rocks on Earth. Each isotope has a different half-life, meaning it decays at a different rate when buried.
By measuring both isotopes in the same sample, glacial geologist Drew Christ was able to determine that melting ice had exposed sediment on the Earth’s surface for less than 14,000 years.
Ice sheet models by Benjamin Keisling, now incorporating our new knowledge that Camp Century was ice-free 416,000 years ago, show that the Greenland ice sheet must have shrunk significantly then.
At a minimum, the ice edge retreated tens to hundreds of kilometers around much of the island during that period. The water from that melting ice raised global sea levels by at least 5 feet and perhaps as much as 20 feet compared to today’s level.
Warnings for the future
Ancient frozen soil beneath the Greenland ice sheet warns of trouble ahead.
During the MIS 11 interglacial, the Earth was warm and ice sheets were restricted to high latitudes, much like today. Carbon dioxide levels in the atmosphere remained between 265 and 280 parts per million for about 30,000 years. MIS 11 lasted longer than most interglacials due to the impact of the shape of Earth’s orbit around the sun on solar radiation reaching the Arctic. Over these 30 millennia, that level of carbon dioxide caused enough warming to melt much of the Greenland ice.
Today, our atmosphere contains 1.5 times more carbon dioxide than in MIS 11, about 420 parts per million, a concentration that has increased every year. Carbon dioxide traps heat, warming the planet. Too much in the atmosphere raises global temperatures, as the world is now seeing.
Over the past decade, as greenhouse gas emissions continued to rise, humans experienced the eight warmest years on record. According to preliminary data, July 2023 was the hottest week on record. That heat melts ice sheets, and the loss of ice further warms the planet as dark rocks absorb sunlight once reflected by bright white ice and snow.
Even if everyone stopped burning fossil fuels tomorrow, carbon dioxide levels in the atmosphere would remain elevated for thousands or tens of thousands of years. This is because it takes a long time for carbon dioxide to reach soils, plants, the ocean and rocks. We are creating conditions conducive to a very long period of warmth, just like MIS 11.
Unless people dramatically reduce the concentration of carbon dioxide in the atmosphere, the evidence we found from Greenland’s past suggests a largely ice-free future for the island.
Anything we can do to reduce carbon emissions and sequester carbon already in the atmosphere will increase the chances that more of Greenland’s ice will survive.
The alternative is a world that could look a lot like MIS 11, or even more extreme: a warm Earth, shrinking ice sheets, rising sea levels, and waves over Miami, Mumbai, India, and Venice, Italy.
This article is republished from The Conversation, an independent, nonprofit news organization bringing you trusted data and analysis to help you understand our complex world. If you found it interesting, you can subscribe to our weekly newsletter.
It was written by: Paul Bierman, University of Vermont and Tammy Rittenour, Utah State University.
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Paul Bierman receives funding from the US National Science Foundation.
Tammy Rittenour receives funding from the US National Science Foundation.
