About 400,000 years ago, most of Greenland was ice-free. The scrubby tundra in the mountainous regions of the island’s northwest is basking in the Sun’s rays. Evidence suggests that an insect-filled spruce forest covers the southern part of Greenland. Global sea level was much higher at the time; It was between 20 and 40 feet above today’s levels. Lands that today host hundreds of millions of people around the world were under water.
Scientists have known for some time that the Greenland ice sheet mostly disappeared at some point in the past million years, but they didn’t know exactly when.
In a new study in the journal Science, we determined the date using permafrost extracted from beneath a nearly mile-thick section of the Greenland ice sheet during the Cold War.
The timing (about 416,000 years ago, when largely ice-free conditions lasted about 14,000 years) is important. At the time, Earth and its early humans were experiencing one of the longest interglacial periods since ice sheets first covered the high latitudes 2.5 million years ago.
The length, magnitude, and effects of this natural warming may help us understand the Earth modern humans are creating for the future.
A world protected under ice
In July 1966, American scientists and U.S. Army engineers completed a six-year effort to drill through the Greenland ice sheet. The drilling 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 drilling site in northwestern Greenland was 140 miles from shore and was under 4,560 feet of ice. Once they reached the bottom of the ice, the team continued digging another 12 feet into the frozen, rocky soil below.
In 1969, geophysicist Willi Dansgaard’s analysis of the ice core at Camp Century revealed for the first time details of how Earth’s climate had changed dramatically over the past 125,000 years. Long cold glacial periods during which ice expanded rapidly gave way to warm interglacial periods when ice melted and sea levels rose, inundating coastal areas around the world.
For almost 30 years, scientists paid little attention to the 12 meters of permafrost from Camp Century. One study analyzed pebbles to understand the bedrock beneath the ice sheet. Another, intriguingly, suggested that permafrost preserved evidence of a time warmer than today. However, since there was no way to date the material, few people showed interest in these studies. By the 1990s the permafrost core had disappeared.
A few years ago our Danish colleagues found lost soil buried deep in a freezer in Copenhagen, and we assembled an international team to analyze this unique frozen climate archive.
In the top example, we found perfectly preserved fossil plants; Evidence that the land far below Camp Century was ice-free at some time in the past – but when?
Dating with ancient rocks, twigs and soil
Using samples cut from the center of the sediment core and prepared and analyzed in the dark to preserve an accurate memory of the material’s last exposure to sunlight, we now know that the ice sheet that now covers northwestern Greenland, almost a mile thick, has disappeared. Between 424,000 and 374,000 years ago, during the long natural warm period known to climate scientists as MIS 11.
To determine more precisely when the ice sheet melted, one of us, Tammy Rittenour, used a technique known as luminescence dating.
Over time, minerals accumulate energy as radioactive elements such as uranium, thorium, and potassium decay and release radiation. The longer the sediment remains buried, the more radiation accumulates as trapped electrons.
In the laboratory, special instruments measure tiny bits of energy released as light from these minerals. This signal can be used to calculate how long grains remained buried; because recent exposure to sunlight will release trapped energy.
Paul Bierman’s laboratory at the University of Vermont used rare radioactive isotopes of aluminum and beryllium to differentially date the last time the sample was near the surface.
These isotopes are formed when cosmic rays originating far from our solar system hit 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 released land surface sediment over less than 14,000 years.
Ice sheet models conducted by Benjamin Keisling now integrate our new knowledge that Camp Century was ice-free 416,000 years ago and show that Greenland’s ice sheet must have shrunk significantly by that time.
During this period the ice edge retreated tens to hundreds of kilometers, at least across most of the island. Water from melting ice raised global sea levels by at least 1.5 meters, perhaps as much as 20 meters, compared to today.
Forward-looking warnings
Ancient permafrost emerging from beneath Greenland’s ice sheet warns of impending trouble.
During the MIS 11 interglacial period, the Earth was warm and ice sheets were limited to high latitudes, as they are today. Carbon dioxide levels in the atmosphere remained between 265 and 280 parts per million for approximately 30,000 years. MIS 11 lasted longer than most interglacial periods due to the influence of the shape of the Earth’s orbit around the sun on solar radiation reaching the Arctic. During those 30 thousand years, this level of carbon dioxide triggered enough warming to melt most of Greenland’s ice.
Our atmosphere today contains 420 parts per million of carbon dioxide, 1.5 times more than in MIS 11; This concentration increases every year. Carbon dioxide warms the planet by trapping heat. Too much stuff in the atmosphere is increasing global temperature, as the world is now seeing.
Over the past decade, humans experienced the eight hottest years in history as greenhouse gas emissions continued to rise. According to preliminary data, July 2023 saw the hottest week in history. Such heat melts ice sheets, and ice loss further warms the planet as dark rocks absorb sunlight once reflected by bright white ice and snow.
Even if everyone stopped using fossil fuels tomorrow, carbon dioxide levels in the atmosphere would remain high for thousands to tens of thousands of years. This is because it takes a long time for carbon dioxide to move into the soil, plants, oceans and rocks. Just like in MIS 11, we create conditions that allow for a very long warm-up period.
Unless humans significantly reduce the concentration of carbon dioxide in the atmosphere, the evidence we found about Greenland’s past suggests a largely ice-free future for the island.
Anything we can do to reduce carbon emissions and sequester the carbon already in the atmosphere will increase the chances of more of Greenland’s ice surviving.
The alternative would be a world much like MIS 11, if not 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 providing facts and authoritative analysis to help you understand our complex world. If you found it interesting, you can subscribe to our weekly newsletter.
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.