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Ancient leaves preserved under a kilometer of Greenland’s ice – and lost in a freezer for many years – hold lessons on climate change
Remains of ancient Greenland tundra have been preserved in the ground under the ice sheet. Andrew Christ and Dorothy Peteet, CC BY-ND In 1963, inside a secret US military base in northern Greenland, a team of scientists began drilling through the Greenland ice sheet. Piece by piece, they extracted an ice core of 4 centimeters and almost a kilometer long. At the very end, they picked up something else – 12 meters of frozen ground. The ice told a story of the Earth’s climatic history. The frozen soil was examined, set aside and then forgotten. Half a century later, scientists discovered that soil in a Danish freezer. It now reveals its secrets. Using laboratory techniques unimaginable in the 1960s when the core was drilled, we and an international team of fellow scientists were able to show that Greenland’s massive ice sheet had melted to the ground over the past million years. Radiocarbon dating shows that this would have happened more than 50,000 years ago. This most likely happened during times when the climate was hot and the sea level was high, possibly 400,000 years ago. And there was more. When we examined the soil under a microscope, we were amazed to discover the remnants of a tundra ecosystem – twigs, leaves and moss. We looked at North Greenland as this was the last time the region was ice free. Our peer-reviewed study was published March 15 in Proceedings of the National Academy of Sciences. Engineers pulled up a portion of the 4,560-foot-long ice core at Camp Century in the 1960s. US Army Corps of Engineers Paul Bierman, a geomorphologist and geochemist, describes what he and his colleagues found in the ground. With no ice sheet, sunlight would have warmed the soil enough for tundra vegetation to cover the landscape. The oceans around the world would have been more than 10 feet higher, and maybe even 20 feet. The country Boston, London and Shanghai sit on today would have been under the waves. All of this happened before humans began to warm the earth’s climate. The atmosphere at that time contained much less carbon dioxide than it does today, and it does not rise as fast. The ice core and the ground below are something of a Rosetta Stone to understand how durable the Greenland ice has been during the recent hot periods – and how fast it can melt again as the climate gets hotter. Secret Military Bases and Danish Freezers The story of the ice core begins during the Cold War with a military mission called Project Iceworm. From 1959 onwards, the U.S. Army dragged hundreds of soldiers, heavy equipment, and even a nuclear reactor across the ice sheet in northwest Greenland and dug a base of tunnels in the ice. They called it Camp Century. It was part of a secret plan to conceal nuclear weapons from the Soviets. The public knew it as an Arctic research laboratory. Walter Cronkite even visited and submitted a report. Workers built the snow tunnels on the Camp Century research base in 1960. The US Army Corps of Engineers Camp Century did not last long. The snow and ice slowly began to crush the buildings in the tunnels below, forcing the army to abandon them in 1966. During his short life, however, scientists were able to extract the ice core and begin to analyze the climatic history of Greenland. As ice builds up year after year, it picks up low volcanic ash and changes the precipitation over time, trapping air bubbles that reveal the composition of the atmosphere in the past. One of the original scientists, glaciologist Chester Langway, kept the nuclear and soil samples frozen at the University of Buffalo for many years and then sent them to a Danish archive in the 1990s, where the soil was soon forgotten. A few years ago, our Danish colleagues found the soil samples in a box of glass cookies with faded labels: ‘Camp Century Sub-Ice’. Geomorphologist Paul Bierman (right) and geochemist Joerg Schaefer of Columbia University are investigating the pots containing Camp Century sediment for the first time. They were in a Danish freezer at -17 F. Paul Bierman, CC BY-ND A surprise under the microscope On a hot July day in 2019, two samples of soil arrived at our laboratory at the University of Vermont frozen solid . We started the meticulous process of dividing the precious few ounces of frozen mud and sand for different analyzes. First, we took away the layer in the ground before it was lost forever. After that, we cut down small pieces to examine them. We melted the rest and saved the ancient water. Then came the biggest surprise. While we were washing the ground, we saw something floating in the rinsing water. Paul grabs a pipette and some filter paper, Drew grabs a pair of tweezers and turns on the microscope. We were absolutely stunned when we looked through the eyepiece. We looked back at leaves, twigs and mosses. It was not just land. It was an ancient ecosystem that was perfectly preserved in the natural deep freezing of Greenland. Glacier morphologist Andrew Christ (right) and geology student Landon Williamson are holding up the first twig that was spotted when they were a sediment sample from Camp Century. Paul Bierman, CC BY-ND Dating million year old moss How old were these plants? During the last million years, the Earth’s climate was suppressed by relatively short warm periods, which usually lasted about 10,000 years, called interglacial, when there was less ice at the poles and sea levels were higher. The Greenland ice sheet has survived throughout human history during the Holocene, the current interglacial period of the past 12,000 years, and most interglacials in the last million years. But our research shows that at least one of these interglacial periods was hot enough long enough to melt large parts of the Greenland ice sheet, so that a tundra ecosystem could form in northwestern Greenland. We used two techniques to determine the age of the soil and the plants. First, we used chemical beauty chambers and a particle accelerator to count atoms that form in rocks and sediment when we are exposed to natural radiation that is bombarding the earth. Subsequently, a colleague used an ultrasensitive method to measure the light released from sand grains to determine when they were last exposed to sunlight. Maps of Greenland show the velocity of the ice sheet as it flows (left) and the landscape hidden beneath it (right). BedMachine v3; Copernicus Service for Climate Change (C3S), CC BY-ND The concentration of carbon dioxide in today’s atmosphere is far beyond the levels of the past determined from ice cores. On March 14, 2021, the CO2 level was about 417 ppm. NASA Jet Propulsion Laboratory, CC BY-ND The million-year time frame is important. Previous work on another ice core, GISP2, which was extracted from Central Greenland in the 1990s, has shown that the ice was also absent there in the last million years, perhaps about 400,000 years ago. Lessons for a world facing rapid climate change would be catastrophic for humanity today to lose the Greenland ice sheet. The molten ice would increase sea level by more than 20 feet. It will re-establish the coastlines worldwide. About 40% of the world’s population lives within 60 kilometers of a coast, and 600 million people live within 30 feet of sea level. If warming continues, ice melting from Greenland and Antarctica will dump more water into the oceans. Communities will be forced to relocate, climate refugees will become more common and expensive infrastructure will be abandoned. The increase in sea level has already intensified the floods of coastal storms, damaging hundreds of billions of dollars annually. Tundra near the Greenland ice sheet today. Is this what Camp Century looked like before the ice returned in the last million years? Paul Bierman, CC BY-ND The story of Camp Century spans two critical moments in modern history. An Arctic military base built following the existential threat of nuclear war led us to accidentally discover another threat to ice cores – the threat of rising sea levels due to human-caused climate change. Its legacy helps scientists understand how the earth responds to a changing climate. This article has been updated to correct the caption at 417 ppm. [Deep knowledge, daily. Sign up for The Conversation’s newsletter.]This article was published from The Conversation, a non-profit news site dedicated to sharing ideas from academic experts. It was written by: Andrew Christ, University of Vermont and Paul Bierman, University of Vermont. Read more: Shrinking glaciers have created a new normal for the ice of Greenland – a constant loss of ice for the foreseeable future. The Arctic has not been warm for three million years – and it predicts big changes for the rest of the planet Andrew Christ receives funding from the Gund Institute for Environment and the National Science Foundation. Paul Bierman receives funding from the US National Science Foundation and UVM Gund Institute for Environment.