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Analysis finds melting ice sheet affects a second thousands of miles away

To see how deeply interconnected the planet truly is, look no further than the massive ice sheets on the Northern Hemisphere and South Pole.

Thousands of miles apart, they are hardly next-door neighbors, but according to new research from a team of international scientists — led by Natalya Gomez, Ph.D. ’14, and including Harvard Professor Jerry X. Mitrovica — what happens in one region has a surprisingly direct and outsized effect on the other, in terms of ice expanding or melting.

The analysis, published in Nature, shows for the first time that changes in the Antarctic ice sheet were caused by the melting of ice sheets in the Northern Hemisphere. The influence was driven by sea-level changes caused by the melting ice in the north during the past 40,000 years. Understanding how this works can help climate scientists grasp future changes as global warming increases the melting of major ice sheets and ice caps, researchers said.

The study models how this seesaw effect works. Scientists found that when ice on the Northern Hemisphere stayed frozen during the last peak of the Ice Age, about 20,000 to 26,000 years ago, it led to reduced sea levels in Antarctica and growth of the ice sheet there. When the climate warmed after that peak, the ice sheets in the north started melting, causing sea levels in the southern hemisphere to rise. This rising ocean triggered the ice in Antarctica to retreat to about the size it is today over thousands of years, a relatively quick response in geologic time.

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The question of what caused the Antarctic ice sheet to melt so rapidly during this warming period had been a longstanding enigma.


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“That’s the really exciting part of this,” said Mitrovica, the Frank B. Baird Jr. Professor of Science in the Department of Earth and Planetary Sciences. “What was driving these dramatic events in which the Antarctic released huge amounts of ice mass? This research shows that the events weren’t ultimately driven by anything local. They were driven by sea level rising locally but in response to the melting of ice sheets very far away. The study establishes an underappreciated connection between the stability of the Antarctic ice sheet and significant periods of melting in the Northern Hemisphere.”

The retreat was consistent with the pattern of sea level change predicted by Gomez, now an assistant professor of earth and planetary sciences at McGill University, and colleagues in earlier work on the Antarctic continent. The next step is expanding the study to see where else ice retreat in one location drives retreat in another. That can provide insight on ice sheet stability at other times in the history, and perhaps in the future.

“Looking to the past can really help us to understand how ice sheets and sea levels work,” Gomez said. “It gives us a better appreciation of how the whole Earth system works.”


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Along with Gomez and Mitrovica, the team of scientists on the project included researchers from Oregon State University and the University of Bonn in Germany. The rocks they focused on, called ice-rafted debris, were once embedded inside the Antarctic ice sheet. Fallen icebergs carried them into the Southern Ocean. Researchers determined when and where they were released from the ice sheet. They combined ice-sheet and sea-level modeling with sediment core samples from the ocean bottom near Antarctica to verify their findings. And researchers also looked at markers of past shorelines to see how the ice sheet’s edge has retreated.

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Source
Harvard Gazette

Journal Reference

Antarctic ice dynamics amplified by Northern Hemisphere sea-level forcing

Abstract


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Sea-level rise due to ice loss in the Northern Hemisphere in response to insolation and greenhouse gas forcing is thought to have caused grounding-line retreat of marine-based sectors of the Antarctic Ice Sheet (AIS).  Such interhemispheric sea-level forcing may explain the synchronous evolution of global ice sheets over ice-age cycles.

Recent studies that indicate that the AIS experienced substantial millennial-scale variability during and after the last deglaciation (roughly 20,000 to 9,000 years ago) provide further evidence of this sea-level forcing. However, global sea-level change as a result of mass loss from ice sheets is strongly nonuniform, owing to gravitational, deformational and Earth rotational effects, suggesting that the response of AIS grounding lines to Northern Hemisphere sea-level forcing is more complicated than previously modelled.

Here, using an ice-sheet model coupled to a global sea-level model, we show that AIS dynamics are amplified by Northern Hemisphere sea-level forcing. As a result of this interhemispheric interaction, a large or rapid Northern Hemisphere sea-level forcing enhances grounding-line advance and associated mass gain of the AIS during glaciation, and grounding-line retreat and mass loss during deglaciation.

Relative to models without these interactions, the inclusion of Northern Hemisphere sea-level forcing in our model increases the volume of the AIS during the Last Glacial Maximum (about 26,000 to 20,000 years ago), triggers an earlier retreat of the grounding line and leads to millennial-scale variability throughout the last deglaciation. These findings are consistent with geologic reconstructions of the extent of the AIS during the Last Glacial Maximum and subsequent ice-sheet retreat, and with relative sea-level change in Antarctica.

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