During glacial periods, the sea level decreases, due to which huge amounts of water are stored in huge inland glaciers. However, computer models have not yet been able to reconcile sea level rise with thickness of glaciers. Using new and innovative calculations, a team of climate researchers led by the Alfred Wegener Institute was able to explain the discrepancy. The study was recently published in the journal Nature CommunicationsIt could significantly advance research into the history of our planet’s climate.
During transitions from glaciers to glaciers, glaciers in Greenland, North America and Europe are diminishing over tens of thousands of years. The more water is stored in powerful glaciers, the less it is in the oceans – and the lower the sea level. Climate researchers are now looking at how much glaciers could melt in the coming centuries due to anthropogenic climate change, and how much sea level will rise as a result. To do this, they look back at the past. If they can understand the growth and melting of ice during past glaciers and glaciers, they will be able to draw valuable conclusions about the future.
The lost ice problem
However, reconstructing the distant past is not easy, because the thickness of glaciers and sea level cannot be directly measured. Accordingly, climate researchers will have to painstakingly gather evidence that they can use to shape a picture of the past. Problem: Different images appear, depending on the types of evidence collected. We cannot say with certainty what the situation actually was like ten thousand years ago. The “lost ice problem” has remained unsolved for many years. Describes the contrast between two different scientific approaches that sought to reconcile sea level rise and thickness of glaciers at the height of the last glacier, California. 20,000 years ago. A team of climate experts led by Evan Joan of the Alfred Wegener Institute, Helmholtz Polar and Marine Research Center (AWI) in Bremerhaven solved the problem using a new method. “It seems like we found a new way to reconstruct the past 80,000 years ago,” says Dr. Joan, who has been investigating the problem for nearly a decade. These results are now published in the journal Nature Communications.
Sediment analysis versus global climate modeling
The “lost ice problem” is based, on the one hand, on sediment analysis from core samples collected from the sea floor in the tropics. These contain traces of coral reefs that still tell us today how far sea level has risen or fallen over thousands of years. Why? Because corals only live in well-lit waters near the ocean surface. Sediment foci indicate that 20,000 years ago, the sea level in the tropics would indicate that the sea level was about 130 meters lower than it is today. On the other hand, previous models suggested that the glaciers were not large enough 20,000 years ago to explain the sea level decline. To be more precise, for sea level to be so low, an additional volume of water on a global scale equivalent to twice the mass of the Greenland ice sheet must be frozen; Hence the “lost ice problem”.
Understanding glacial behavior
Thanks to his new method, Joan now successfully reconciled the sea level with the mass of glaciers: according to his calculations, the sea level at that time was California. 116 meters less than it is today. Based on his approach, there is no contradiction in terms of the mass of the glaciers. In contrast to the previous world model, Joan closely examined the geological conditions in the glaciers: how steep is the surface of the ice? Where do the glaciers flow? How much is the resistance of the rocks and sediments in the base of the ice to the flow of ice? His model considers all these aspects. It also takes into account the extent to which the ice sheet has pressed the earth’s crust in the respective regions. “It depends on how viscous the underlying mantle is,” explains Joan. “We base our calculations on different viscosities of the mantle, and thus arrive at different ice masses.” The resulting glaciers can now be reconciled with sea level without any contradiction.
The applicable model is defective
A recent article by Joan and his team re-critically examined the well-established scientific method used to estimate the masses of glaciers: the oxygen isotope method. Isotopes are atoms of the same element that have different numbers of neutrons and therefore different masses. Oxygen, for example, has a lighter isotope 16O and a heavier isotope 18O. According to the traditional theory, the lighter 16O evaporates from the ocean, while the heaviest 18O remains in the water. Accordingly, during glaciers, when large inland glaciers are forming and the volume of water in the oceans decreases, the concentration of 18O in the oceans must increase. However, as shown, this static model produces inconsistencies when it comes to reconciling sea level rise and glacier masses for the pre-20,000 and earlier period. “For many years, the isotope model has been used repeatedly to determine the size of ice from glaciers for several million years. Our study calls into question the reliability of this method,” says Joan. His goal now is to use his new method to improve the traditional oxygen isotope method.
Evan J. Joan Xu Zhang, Sarah Khosraoy, Alessio Rovere, Paolo Stucci, Anna LC Hughes, Richard Gilenkruetz, Jan Mangierud, John Inge Svendsen, Gerrit Loman: A New Global Reconstruction of the Ice Cover over the Past 80,000 Years. Nature Communications (2021);
DOI: 10.1038 / s41467-021-21469-w
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