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Scientists estimate lower ancient sea-level rise, narrowing future predictions

by CIFAR Mar 19 / 12

In the quest to find a consensus on how high sea levels may rise in the future, CIFAR researcher and Program Director Jerry Mitrovica (Harvard University), with colleague Maureen Raymo, concluded that ancient sea levels did not rise as high as researchers thought during the warm interglacial period 400,000 years ago. These new findings will narrow future estimates of sea-level rise.

An essential way to assess the stability of ice sheets is to study their behaviour in the distant past when the Earth faced extended periods of warming. The islands of Bermuda and the Bahamas have served as excellent sites for scientists to investigate global sea-level change because of ancient deposits in shorelines and coral reefs. Geological observations from these two islands has suggested that sea level during this warming period, 400,000 years ago, was 20 metres higher than at present, implying that both the Greenland Ice Sheet (GIS) and the West Antarctic Ice Sheet (WAIS), as well as some sections of the East Antarctic Ice Sheet (EAIS) had collapsed. “The collapse of the latter would be of deep concern, given that most scientists believe that the EAIS will remain stable over the next millennium despite global warming,” says Dr. Mitrovica.

Rather than rely solely on the geological record, Dr. Mitrovica and colleague took a different approach to understand the effect of ice-sheet melting on sea levels. Using computer models, the team factored in the fluctuations of the ice sheets that covered North America proceeding this ancient warm period. The team postulated that as these continental ice sheets grew, their weight pushed down the land causing the land on the periphery – Bermuda and the Bahamas – to bulge. When the ice pulled back, the continent rebounded and the islands sank. By correcting for island sinking, the team came up with a new and lower estimate for sea-level rise – one that was a third less than previous estimates. Their findings were published in Nature.

Due to their lower estimate, the team believes that the massive EAIS did not melt and contribute to the sea-level rise as previous studies suggested – a view that has important implications for climate and ice-sheet modelling.

“The team’s findings help narrow the projections for sea-level rise into the future and give us a better understanding of how we will be impacted in the coming decades,” adds Dr. Mitrovica. “The good news is that this implies moderate melting of the EAIS. The bad news is that it confirms that the WAIS and GIS collapsed during this ancient warm period.”