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Underside of Antarctic Ice Sheet Melting, Despite Gains on Top

Scientists have underrated the major role the Antarctic ice sheet plays in global climate variability, which helps explain why sea ice in the Southern Hemisphere has increased even while the Earth warms.

(CN) – Scientists have underrated the major role the Antarctic ice sheet plays in global climate variability, which helps explain why sea ice in the Southern Hemisphere has increased even while the Earth warms.

That’s the main finding of a study published Monday in the journal Nature, which examines global climate models of the past several thousand years that fail to account for climate variability in the paleoclimate record.

The researchers first analyzed the Scotia Sea, since most icebergs breaking off the Antarctica ice sheet pass through this region due to oceanic and atmospheric circulation, according to co-author Michael Weber.

“The icebergs contain gravel that drop into the sediment on the ocean floor – and analysis and dating of such deposits that shows that for the last 8,000 years, there were centuries with more gravel and those with less,” he said.

The team’s study was based on a theory that climate modelers have historically overlooked how centuries-long phases of reduced and enhanced Antarctic ice-mass loss, documented over the several thousand years, have affected the planet’s overall climate system.

Using advanced computer modeling, the researchers traced variabilities in ice breaking away from glaciers – known as iceberg calving – to small changes in ocean temperatures.

“There is natural variability in the deeper part of the ocean adjacent to the Antarctic ice sheet that causes small but significant changes in temperatures,” said co-author Andreas Schmittner, a climate modeler from Oregon State University. “When the ocean temperatures warm, it causes more direct melting of the ice sheet below the surface, and it increases the number of icebergs that calve off the ice sheet.”

These factors combine to introduce an influx of freshwater into the Antarctic Ocean during warm regimes, according to co-author Peter Clark, a paleoclimatologist from Oregon State University.

“The introduction of that cold fresh water lessens the salinity and cools the surface temperatures, at the same time, stratifying the layers of water,” he said. “The cold, fresh water freezes more easily, creating additional sea ice despite warmer temperatures that are down hundreds of meters below the surface.”

The finding may also explain why sea ice is currently expanding in the Antarctic Ocean despite global warming, the team said.

“This response is well-known, but what is less-known is that the input of fresh water also leads to changes far away in the Northern Hemisphere, because it disrupts part of the global ocean circulation,” said co-author Nick Golledge. “Meltwater from the Antarctic won’t just raise global sea levels, it might also amplify climate changes around the world. Some parts of the North Atlantic may end up with warmer temperatures as a consequence of part of Antarctica melting.”

Weber also pointed how computer models provide more evidence that the Antarctica ice sheet has experienced more natural variability in the past than previously expected, a scary concept given the increasing effects of climate change.

“We should therefore be concerned that it will possibly act very dynamically in the future, too, specifically when it comes to projecting future sea-level rise,” he said.

In a separate study also published Monday, scientists found a significant amount of ice-sheet melting in East Antarctica over the summer, a surprising trend as the area is supposed to be too cold for noticeable ice loss.

However, the team found that large pockets of the ice shelf present weakness throughout its structure, suggesting a greater potential vulnerability to collapse – which could have a devastating effect on global sea levels.

“Tens of meters of rising sea levels are locked away in Antarctica,” said lead author Jan Lenaerts.

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