100-year-old traces on the seabed put models of the Greenland Ice Sheet to the test

As a glacier moves across the ground, the ice picks up sand and other material. Some of this material is carried along when icebergs break off the glacier and drift out into the fjord. When the icebergs melt, the material sinks to the bottom of the fjord.

In this study, the researchers are focusing specifically on sand. The sand grains are so large that they cannot be transported far out into the fjord by meltwater. Sand on the fjord bed may therefore originate from icebergs that have carried it with them. Periods with more sand may indicate increased iceberg production and, consequently, greater ice transport from the glacier.

By analysing the amount of this so-called ice-rafted debris (IRD) in sediment cores from Sermilik Fjord, the researchers can gain an insight into how ice transport from the Helheim Glacier has varied over time. The sediment cores extend far further back in time than direct satellite measurements, thereby enabling researchers to compare the models with the glacier’s development over a longer period.

The researchers tested seven different initial conditions for the Helheim Glacier around the year 1900. They then had the model calculate the glacier’s evolution up to the present day, based on the climate changes that have actually taken place.

If the researchers had looked only at the results of the simulations, several of them might have appeared realistic at first glance. However, the sediment cores also made it possible to investigate whether the simulations accurately reflected the major changes the glacier underwent along the way.

Here, the difference was clear. The sediment cores show signs of a marked change in the glacier’s ice transport during the first half of the 20th century. Only two of the seven simulations reproduced a corresponding retreat of the glacier and change in ice transport.

“This means that it is not enough for a model to be able to reproduce the glacier’s current size and shape. It must also be able to reproduce the major changes the glacier has undergone on its way to that point. Here, the sediment cores provide us with important knowledge about how the glacier has changed over the last century,” says Signe Hillerup Larsen.

The initial conditions set for an ice sheet model have a major impact on its results. Even small differences in the initial conditions can lead to significant differences when the model is used to calculate changes many years into the future.

By testing the simulations against geological history, researchers gain a better basis for selecting the initial conditions that best reflect the glacier’s past changes. This can strengthen the basis for the models used in future projections.

“We have geological records that go back much further than our modern measurements. They allow us to test the models over the long timescales in which the ice actually reacts. In this way, we can improve our ability to select the starting point the models should use when calculating future changes,” says Camilla S. Andresen, senior researcher at GEUS and co-author of the study.

The method has so far been tested on the Helheim Glacier. The researchers will now continue to develop this approach and investigate how geological data can be used to test models across larger areas. The study is designed so that the method can be applied to larger model areas.