‘Lacking Ice Drawback’ Lastly Solved by Transferring the Earth’s Crust – Watts Up With That?
A new global reconstruction of the ice sheet over the past 80,000 years
ROYAL NETHERLANDS INSTITUTE FOR MARINE EXPLORATION
Research news
IMAGE: GREENLAND GLACIERS 2018 show more CREDIT: NIOZ, KIM SAUTER
During the ice ages, the global mean sea level falls because large amounts of seawater are stored in the form of huge continental glaciers. So far, mathematical models of the last ice age could not reconcile the height of the sea level and the thickness of the glacier masses: the so-called missing ice problem. With new calculations that take into account crustal, gravitational and rotational perturbations of the solid earth, an international team of climate researchers has succeeded in solving the discrepancy, including Dr. Paolo Stocchi from the Royal Netherlands Institute for Sea Research (NIOZ). The study, now published in the journal Nature Communications, could significantly advance the study of past climates and help make better predictions about sea levels in the future.
Paolo Stocchi: “Our new reconstruction revolutionizes what we thought about the global continental ice mass during the last ice age. The total mass of the glaciers of the last Ice Age was 20% smaller and accumulated faster than previously assumed. “
Growing and melting glaciers
With the alternation of ice ages and warm times, the glaciers on Greenland, North America and Europe grow and shrink over tens of thousands of years. The more water that is stored in the form of ice, the less water there is in the oceans – and the lower the sea level. Climate researchers want to find out how much the glaciers could melt in the course of man-made climate change in the next few centuries and how much the sea level will rise as a result. To do this, they look into the past. If one can understand the growth and melting of glaciers during the last ice and warm periods, conclusions can be drawn for the future.
The “missing ice problem”
However, this look into the past is difficult because the thickness of the glaciers and the height of the sea level can no longer be measured directly afterwards. Climate researchers therefore have to laboriously collect clues with which the past can be reconstructed. However, depending on what clues you gather, the results will vary and appear to contradict each other. Earlier models and calculations led to the so-called “missing ice” puzzle. Geological evidence from marine areas suggests that sea levels may have been 120 to 140 m lower during the last ice age 20,000 years ago than they are today. However, the uncertainty of this data is quite great. To take this low sea level into account, twice the mass of the Greenland ice sheet would have to be frozen worldwide. According to climate models, however, these glacier masses could not possibly have been that large at this point in time. There is also no geological evidence at higher latitudes for such a large ice mass. How can you then explain that the water was not in the sea and at the same time was not stored in the freezer on land?
80,000 years of ice cover and sea level changes precisely reconstructed
This problem has now been solved with a new method by an international team of scientists led by Dr. Evan Gowan (Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research in Bremerhaven) solved. Among them the geophysicist Dr. Paolo Stocchi from the Royal Netherlands Institute for Sea Research. “We have found a way to accurately reconstruct the last 80,000 years of ice sheets and sea level changes,” says Dr. Paolo Stocchi, who contributed to the creation of the novel global ice sheet model by analyzing crustal, gravitational and rotational perturbations from the solid earth. Their new model explains past local sea levels, which are lower than they are today, by taking into account the relative movement of the sea surface and the earth’s crust. In this way, past local sea levels, which are much lower than they are today, can be modeled without the need for an unrealistically large global ice mass. The solid earth movements would do the trick!
Understand the behavior of glaciers using the earth’s mantle
With the new method, the scientists finally reconciled sea level and glacier mass: According to their calculations, the sea level at that time must have been around 116 meters lower than it is today. There is no discrepancy in terms of glacier mass. In contrast to the previous global model, the team examined the geological conditions in the vicinity and below the formerly glaciated areas and not in the vast ocean areas: How steep were the mountain slopes? Where did the glaciers reach the sea? Has the friction affected the speed of the ice flow? And how much? The new model includes all of these local factors. It also takes into account crust deformations caused by ice and water load. The latter are important because they change the topography of the country and thus affect the flow of ice and ultimately the volume of the glaciers. “Deformations of the crust are regulated by physical parameters of the solid earth such as viscosity,” says Paolo Stocchi. In fact, on geological time scales, the Earth’s mantle behaves like a highly viscous liquid and deforms under the weight of a fluctuating mass of ice. “By assuming different viscosities of the earth’s mantle, we model different developments in land topography, which then lead to different scenarios for the ice masses.” These can now be reconciled with the marine geological evidence from the marine areas without additional mass.
The established isotope model needs to be revised
The technical article by Evan Gowan and his team takes a critical look at the method used in science for estimating glacier masses for many years: the method of measuring oxygen isotopes. Isotopes are atoms of the same element that differ in the number of their neutrons and therefore have different weights. For example, there is the lighter 16O isotope and the heavier 18O isotope of oxygen. The theory is that the light 16O evaporates from the ocean and the heavy 18O stays in the water. Accordingly, during the ice ages, when large glaciers form on the mainland and the amount of water in the sea decreases, the 18O concentration in the oceans must increase. As it turns out, however, this established method leads to inconsistencies in the coordination of sea level and glacier mass for the time 20,000 years ago and earlier.
“The isotope model has been used extensively for years to determine the volume of ice in glaciers up to many millions of years before our time. Our work now raises doubts about the reliability of this method, ”says Paolo Stocchi. His goal is now to use the new model to quantify the current crust deformation rates in the North Sea and the Wadden Sea and thus uncover the actual contribution of current climate change to the regional relative sea level changes.
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From EurekAlert!
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