When the climate cooled during the last ice age, water was locked up in the land ice of the northern hemisphere, which resulted in an expansion of the ice sheet in Antarctica. This is because a drop in sea level in the south means that more land is available, causing the underside of the ice to rest firmly on the ground and enabling it to expand. When the climate started to warm up 19,000 to 9,000 years ago, the shrinking ice in the northern hemisphere in contrast caused the Antarctic ice sheet to retreat. The rising sea level destabilizes the base and the ice is in danger of floating up and breaking off. As a result, it shrinks overall.
"We have now discovered a very variable signal of the loss of ice mass over the last 20,000 years, left behind by icebergs that break off from Antarctica and leave debris in the surrounding oceans as they melt," says Michael Weber from the Institute for Geosciences at the University of Bonn. It was almost impossible to reconcile this evidence with existing models until the researchers took into account how the ice sheets of both hemispheres interact with each other globally over long periods of time. "All we knew from an earlier study was that the ice sheets at both poles were connected at the end of the last ice age," adds Weber. However, only with the help of new models that link the ocean, the ice sheet and vertical crustal movements were the researchers now able to consider longer periods of time and compare them with the latest geological findings.
"Ice sheets can affect each other over long distances because of the water that flows between them," says Natalya Gomez. "It's as if they're communicating with each other through changes in sea level." The results of the model calculations are consistent with the records in ocean sediments and from past coastlines. Polar ice sheets are not just large, static icebergs. They develop on different time scales and are in constant flux. "During which the ice, depending on the climate and the surrounding water levels, grows and shrinks again," the researcher continues. The ice sheet grows when snow piles up on it, and the resulting ice mass spreads outwards under its own weight like jello. There, the mass flows into the surrounding ocean and icebergs break off from the edges.
To investigate the mechanisms that drive changes in the Antarctic ice sheet over geological time scales, the study relies on numerical models and a wide range of geological records, from sediment cores of the sea floor near Antarctica to records of land areas and past coastlines. This information enabled the researchers for the first time to simultaneously simulate changes in both sea level and ice dynamics in both hemispheres over the last 40,000 years. This time frame forms the basis for a deeper understanding of how climate factors influence the ice sheets, as the period covers the peak of the last ice age between 26,000 and 20,000 years up to the present.
Water and ice sheets on the move
Drill core records and reconstruction of coastlines indicate that ice loss from the Antarctic ice sheet was significant during this period, with intermittent periods of accelerated retreat. The researchers discovered that the only mechanism that could explain this phenomenon are the changes in sea level in Antarctica. These were in turn caused by changes in the ice sheets in the northern hemisphere.
"The scale and complexity of the ice sheets and oceans, and the mysteries of the Earth's past climate are fascinating and inspiring," says Gomez. She explains that the results show how interconnected the Earth system is: Changes in one part of the planet cause changes in another part. The retreat of the large ice sheets in the continuing warming climate is unprecedented. A look at records and models of changes in the Earth's history may provide information about this.
Participating institutions and funding
In addition to McGill University, Montreal (Canada), the University of Bonn, the Oregon State University (USA), the University of Ulster (United Kingdom) and Harvard University, Cambridge (USA) were involved. The study was funded by the Natural Sciences and Engineering Research Council (NSERC), the Canada Research Chair's programme, the Canadian Foundation of Innovation, the Deutsche Forschungsgemeinschaft (DFG), NASA and Harvard University.
Publication: Natalya Gomez, Michael E. Weber, Peter U. Clark, Jerry X. Mitrovica & Holly K. Han: Antarctic ice dynamics amplified by Northern Hemisphere sea-level forcing, Nature, DOI: 10.1038/s41586-020-2916-2
Contact:
Michael Weber
Institut für Geowissenschaften
Abteilung für Geochemie und Petrologie
Universität Bonn
Tel. +49 (0160) 96635405
E-mail mike.weber@uni-bonn.de
