San Francisco: The Antarctic Ice Sheet plays a major role in regional and global climate variability, researchers said.
The discovery may help explain why sea ice in the Southern Hemisphere has been increasing despite the warming of the rest of the Earth, Xinhua news agency quoted the researchers as saying.
Global climate models have failed to account for the amount of climate variability captured in the paleoclimate record, said Pepijn Bakker, a former post-doctoral researcher at Oregon State University and now with the MARUM Centre for Marine Environmental Studies at the University of Bremen in Germany.
“One thing we determined right off the bat was that virtually all of the climate models had the Antarctic Ice Sheet as a constant entity,” said Bakker, lead author of the study being published this week in the journal Nature.
“It was a static blob of ice, just sitting there. The ice sheet has undergone numerous pulses of variability that have had a cascading effect on the entire climate system.”
The research team’s hypothesis was that climate modellers were overlooking one crucial element in the overall system that might affect all parts of the system.
According to Andreas Schmittner, a climate scientist in OSU’s College of Earth, Ocean, and Atmospheric Sciences, the Antarctic Ice Sheet has demonstrated dynamic behaviour over the past 8,000 years.
“There is a natural variability in the deeper part of the ocean adjacent to the Antarctic Ice Sheet — similar to the Pacific Decadal Oscillation or El Nino/La Nina but on a time scale of centuries — that causes small but significant changes in temperatures,” said Schmittner, a co-author of the study.
“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.”
If the entire Antarctic Ice Sheet were to melt, global sea levels would probably rise some 200 feet or about 60 metres.
“The introduction of that cold, fresh water lessens the salinity and cools the surface temperatures, at the same time, stratifying the layers of water,” Peter Clark, a paleoclimatologist in OSU’s College of Earth, Ocean, and Atmospheric Sciences and co-author on the study, said.
“The cold, fresh water freezes more easily, creating additional sea ice despite warmer temperatures that are down hundreds of metres below the surface.”
The Antarctic Ice Sheet covers an area of more than 5 million square miles or 13 million sq km, and is estimated to hold some 60 per cent of all the fresh water on the Earth.
The east part of the ice sheet rests on a major land mass, but in West Antarctica, it rests on bedrock that extends into the ocean at depths of more than 8,000 feet or 2,500 metres making it vulnerable to disintegration.
The team analysed sediments from the last 8,000 years to reveal that many more icebergs calved off the ice sheet in some centuries than in others, and used sophisticated computer modelling to trace the variability in iceberg calving to small changes in ocean temperatures.
The discovery may help explain why sea ice has expanded in the Southern Ocean despite global warming.
The same phenomenon does not occur in the Northern Hemisphere with the Greenland Ice Sheet because it is more landlocked and not subject to the same current shifts that affect the Antarctic Ice Sheet.