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Old leaves preserved under a mile of Greenland ice – and lost in a freezer for years – hold lessons about climate change
Remains of the old Greenland tundra were preserved in the soil under the ice sheet. Andrew Christ and Dorothy Peteet, CC BY-ND In 1963, a team of scientists at a secret US military base in northern Greenland began drilling through the Greenland ice sheet. Piece by piece, they extracted an ice core 10 centimeters wide and nearly a mile long. At the very end, they pulled up something else – 3 meters of frozen ground. The ice told a story of Earth’s climate history. The frozen ground was examined, set aside and then forgotten. Half a century later, scientists rediscovered that soil in a Danish freezer. It now reveals its secrets. Using laboratory techniques that were unimaginable in the 1960s, when the core was drilled, we and an international team of fellow scientists were able to demonstrate that Greenland’s enormous ice sheet had melted to the ground there over the past million years. Radiocarbon dating shows it would have happened more than 50,000 years ago. It most likely happened during times when the climate was warm and sea levels were high, possibly 400,000 years ago. And there was more. While exploring the soil under a microscope, we were surprised to discover the remains of a tundra ecosystem: twigs, leaves, and moss. We looked at northern Greenland as it existed 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 are pulling up part of the 4,560-foot 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 soil. Without an ice sheet, sunlight would have warmed the ground enough to allow tundra vegetation to cover the landscape. The oceans around the world would have been more than 3 meters higher, and maybe as much as 6 meters. The land on which Boston, London and Shanghai sit today is said to have been covered in ocean waves. All of this happened before humans started heating up the Earth’s climate. The atmosphere at the time contained much less carbon dioxide than it is now, and it was not rising as quickly. The ice core and the bottom underneath are a kind of Rosetta stone to understand how durable the Greenland ice sheet has been during warm periods in the past – and how quickly it could melt again as the climate warms. Secret military bases and Danish freezers The story of the ice core begins during the Cold War with a military mission called Project Iceworm. Beginning around 1959, the United States military towed hundreds of soldiers, heavy equipment, and even a nuclear reactor across the Northwest Greenland ice sheet and dug a base of tunnels in the ice. They called it Camp Century. It was part of a secret plan to hide nuclear weapons from the Soviets. The public knew it as an Arctic research laboratory. Walter Cronkite even came by and reported it. Workers built the snow tunnels at Camp Century’s 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 military to abandon it in 1966. However, during its short life, scientists were able to extract the ice core and analyze Greenland’s climate history. As ice builds up year after year, it traps layers of volcanic ash and changes in precipitation over time, trapping air bubbles that reveal the composition of the atmosphere from the past. One of the original scientists, glaciologist Chester Langway, kept the core and soil samples frozen for years at the University of Buffalo before sending 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 glass cookie jar box with faded labels: “Camp Century Sub-Ice.” Geomorphologist Paul Bierman (right) and Columbia University geochemist Joerg Schaefer are investigating the pots of 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 soil samples arrived in our laboratory at the University of Vermont, frozen solid. We started the painstaking process of splitting the precious few grams of frozen mud and sand for different analyzes. First, we photographed the layering in the ground before it was lost forever. Then we chiseled small pieces to examine under the microscope. We melted the rest and kept the ancient water. Then came the biggest surprise. While we were washing the soil, we saw something floating in the rinse water. Paul took a pipette and some filter paper, Drew took a pair of tweezers and turned on the microscope. We were amazed when we looked through the eyepiece. Looking back were leaves, twigs and mosses. This wasn’t just earth. This was an ancient ecosystem that has been perfectly preserved in the natural freezer of Greenland. Glacial geomorphologist Andrew Christ (right), with geology student Landon Williamson, holds up the first twig seen washing a Camp Century sediment sample. Paul Bierman, CC BY-ND Dating a million years old moss How old were these plants? Over the past million years, Earth’s climate has been interrupted by relatively short warm spells, typically lasting about 10,000 years, called interglacials, when there was less ice at the poles and sea levels were higher. The Greenland ice sheet has survived all of human history during the Holocene, the current interglacial period of the last 12,000 years, and most of the interglacials in the last million years. But our research shows that at least one of these interglacial periods was warm enough long enough to melt large areas of the Greenland ice sheet, creating a tundra ecosystem in northwestern Greenland. We used two techniques to determine the age of the soil and the plants. First, we used cleanroom chemistry and a particle accelerator to count atoms that form in rocks and sediment when exposed to natural radiation bombarding Earth. Then a colleague used an ultra-sensitive method to measure light from grains of sand to determine when they were last exposed to sunlight. Maps of Greenland show the speed of the ice sheet as it flows (left) and the landscape hidden below (right). BedMachine v3; Copernicus Climate Change Service (C3S), CC BY-ND The concentration of carbon dioxide in the atmosphere today is far above past levels determined from ice cores. On March 14, 2021, the CO2 content was around 217 ppm. NASA Jet Propulsion Laboratory, CC BY-ND The time frame of one million years is important. Earlier work on another ice core, GISP2, extracted from central Greenland in the 1990s, showed that the ice was absent there over the past million years, perhaps about 400,000 years ago. Lessons for a world facing rapid climate change The loss of the Greenland ice sheet would be catastrophic for humanity today. The melted ice would cause sea levels to rise by more than 20 feet. That would redraw coastlines worldwide. About 40% of the world’s population lives within 60 miles of a coast, and 600 million people live within 30 feet of sea level. If warming continues, melting ice from Greenland and Antarctica will pour more water into the oceans. Communities will be forced to relocate, climate refugees will be more common and expensive infrastructure will be abandoned. Sea level rise has already amplified flooding from coastal storms, causing hundreds of billions of dollars in damage every year. Tundra at the Greenland ice sheet today. Is this what Camp Century looked like before the ice came back sometime in the past million years? Paul Bierman, CC BY-ND The Camp Century story spans two critical moments in modern history. An Arctic military base built in response to the existential threat of nuclear war inadvertently led us to discover another threat posed by ice cores: the threat of sea-level rise from man-made climate change. Now his legacy is helping scientists understand how Earth is responding to a changing climate. [Deep knowledge, daily. Sign up for The Conversation’s newsletter.]This article was republished from The Conversation, a nonprofit news site dedicated to sharing ideas from academic experts. It is written by: Andrew Christ, University of Vermont and Paul Bierman, University of Vermont. Read more: Shrinking glaciers have set a new standard for Greenland’s ice sheet – consistent ice loss for the foreseeable future The Arctic hasn’t been this warm for 3 million years – heralding big changes for the rest of the planet where 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 the UVM Gund Institute for Environment.