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Old leaves preserved under a kilometer of Greenland ice – and lost in a freezer for years – bring lessons on climate change
Remains of the ancient Greenland tundra have been preserved in the soil below 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 to drill through the Greenland ice sheet. Piece by piece, they extracted an ice core 10 centimeters in diameter and almost a kilometer in length. In the end, they pulled something else – 3.6 meters of frozen soil. The ice told a story of Earth’s climate history. The frozen soil was examined, set aside and then forgotten. Half a century later, scientists rediscovered this soil in a Danish freezer. Now you’re revealing your 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 huge ice sheet has melted to the ground in the last million years. Radiocarbon dating shows that this would have happened more than 50,000 years ago. It probably happened during times when the climate was hot and the sea level was 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 – branches, leaves and moss. We were looking at northern Greenland as it existed the last time the region was free of ice. Our peer-reviewed study was published on March 15 in the journal Proceedings of the National Academy of Sciences. Engineers pull a section 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. Without a layer of ice, sunlight would have warmed the ground enough for the tundra’s vegetation to cover the landscape. The oceans around the globe would be more than 3 meters higher and maybe even 6 meters. The land that Boston, London and Shanghai are in today would be under the waves of the ocean. All of this happened before humans started to heat up the Earth’s climate. The atmosphere at that time contained much less carbon dioxide than it does today and was not rising as quickly. The ice core and the soil below are a kind of Rosette Stone to understand how durable the Greenland ice sheet was during the last hot periods – and how quickly it can melt again as the climate warms. Secret military bases and Danish freezers The history of the ice core begins during the Cold War with a military mission called Project Iceworm. Beginning around 1959, the US Army transported hundreds of soldiers, heavy equipment and even a nuclear reactor through the ice sheet in northwestern Greenland and dug a tunnel base inside the ice. They called it Camp Century. It was part of a secret plan to hide the Soviets’ nuclear weapons. The public knew it as an Arctic research laboratory. Walter Cronkite even paid a visit and submitted a report. Workers built the snow tunnels at the Camp Century research base in 1960. The US Army Corps of Engineers, Camp Century did not last long. Snow and ice slowly began to crush buildings inside the tunnels below, forcing the military to abandon them in 1966. During their short life, however, scientists were able to extract the ice core and begin to analyze history Greenland climate change. As the ice builds up year by year, it captures layers of volcanic ash and changes in precipitation over time, and captures air bubbles that reveal the previous composition of the atmosphere. One of the original scientists, glaciologist Chester Langway, kept the soil and core samples frozen at the University of Buffalo for years, then dispatched 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 cookie jars with faded labels: “Camp Century Sub-Ice”. Geomorphologist Paul Bierman (right) and geochemist Joerg Schaefer of Columbia University examine pots containing Camp Century sediments for the first time. They were in a Danish freezer set to -17 F. Paul Bierman, CC BY-ND A surprise under the microscope On a hot day in July 2019, two soil samples arrived at our laboratory at the University of Vermont frozen solid. We started the meticulous process of dividing the few precious ounces of frozen mud and sand for different analyzes. First, we photograph the stratification on the ground before it gets lost forever. Then, we cut small pieces to examine under the microscope. We melt the rest and save the ancestral 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 absolutely surprised to look through the eyepiece. Looking at us, there were leaves, branches and moss. This was not just solo. This was an ancient ecosystem perfectly preserved in the deep natural freeze of Greenland. Glacial geomorphologist Andrew Christ (right), with geology student Landon Williamson, holds the first branch seen while washing a sediment sample from Camp Century. Paul Bierman, CC BY-ND Dating million-year-old moss How old were these plants? Over the past million years, the Earth’s climate has been punctuated by relatively short warm periods, usually lasting about 10,000 years, called interglacials, when there was less ice at the poles and the sea level was 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 past million years. But our research shows that at least one of these interglacial periods has been hot enough for a long period of time to melt large portions of Greenland’s ice sheet, allowing a tundra ecosystem to emerge in northwestern Greenland. We use two techniques to determine the age of the soil and plants. First, we use clean room chemistry and a particle accelerator to count the atoms that form in rocks and sediments when exposed to the natural radiation that bombards the Earth. Then a colleague used an ultra-sensitive method to measure the light emitted by grains of sand to determine the last time they were exposed to sunlight. Greenland maps show the speed of the ice sheet as it flows (on the left) and the landscape hidden beneath it (on the right). BedMachine v3; Copernicus Climate Change Service (C3S), CC BY-ND The concentration of carbon dioxide in the atmosphere today is well beyond the previous levels determined from ice cores. On March 14, 2021, the CO2 level was about 217 ppm. NASA’s Jet Propulsion Laboratory, CC BY-ND The million-year period is important. Previous work on another ice core, GISP2, extracted from central Greenland in the 1990s, showed that ice had also been absent there in the past million years, perhaps some 400,000 years ago. Lessons for a world facing rapid climate change Losing the Greenland ice sheet would be catastrophic for humanity today. The melted ice would raise the sea level by more than 6 meters. This redesigns coastlines around the world. About 40% of the global population lives less than 60 miles from the coast and 600 million people live less than 30 feet from sea level. If warming continues, the melting of the Greenland and Antarctic ice will spill more water into the oceans. Communities will be forced to move, climate refugees will become more common and expensive infrastructure will be abandoned. Rising sea levels have already amplified coastal storm floods, causing hundreds of billions of dollars worth of damage each year. Tundra near the Greenland ice sheet today. Was this what Camp Century looked like before the ice returned at some point in the past million years? Paul Bierman, CC BY-ND The history of Camp Century encompasses two critical moments in modern history. An Arctic military base built in response to the existential threat of nuclear war has inadvertently led us to discover another threat from ice cores – the threat of rising sea levels due to man-made climate change. Now, its legacy is helping scientists understand how the Earth responds to climate change. [Deep knowledge, daily. Sign up for The Conversation’s newsletter.]This article was republished 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: The shrinking of glaciers has created a new standard for the Greenland ice sheet – consistent loss of ice in the foreseeable future. The Arctic hasn’t been so hot for 3 million years – and that foreshadows major 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 the UVM Gund Institute for Environment.