New research by Australian scientists suggests that up to 60 percent of "Antarctic Bottom Water", the dense water formed around the edges of Antarctica that seeps into the deep sea and spreads out through the world's oceans, has disappeared since 1970, reports Agence France-Presse. The research, by the Commonwealth Scientific and Industrial Research Organisation (CSIRO), suggests that the phenomenon is a symptom of man-made climate change, but admitted the feedback effect it might have on ocean temperatures and Arctic climate is difficult to predict.
"It's a clear signal to us that the oceans are responding rapidly to variations in climate in polar regions. The sinking of dense water around Antarctica is part of a global pattern of ocean currents that has a strong influence on climate, so evidence that these waters are changing is important," said Dr Steve Rintoul, of CSIRO and the Antarctic Climate and Ecosystems CRC.
"It's not driving changes in climate," Rintoul told AFP, but "it's responding to changes in climate. So it's a signal to us that things are changing around Antarctica."
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Agence France-Presse: Antarctic waters changing due to climate
Scientists are not sure what is causing the phenomenon but Rintoul said the leading hypothesis is that as more of the ice on Antarctica melts around the edges of that continent, it adds fresh water to the ocean.
He said this could be causing the "sinking" of the dense water at high latitudes, a process that has been linked to major changes in climate in the past.
"We're tracking these water masses to see if changes like have happened in past climates might be coming again in the future," he said. [...]
"We don't see them yet, but this... contraction of the dense water around Antarctica might be the first indication that we're on the way to do that."
Rintoul said the change was "likely reflecting both human impact on the planet as well as natural cycles".
"And the human impact includes both the increase in greenhouse gases but also the ozone hole over Antarctica," he said, adding that this hole had caused winds of the Southern Ocean to strengthen.
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Rintoul said it was important to resolve why the changes were occurring because it was relevant to how fast sea levels may rise in the future.
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"It's a clear signal to us that the oceans are responding rapidly to variations in climate in polar regions. The sinking of dense water around Antarctica is part of a global pattern of ocean currents that has a strong influence on climate, so evidence that these waters are changing is important," Dr Rintoul said.
The research was carried out by more than 50 scientists on the Australian Antarctic Division's research and resupply vessel Aurora Australis, which sailed to Commonwealth Bay, west along the Antarctic coast, and returned into Fremantle.
The Australian Antarctic Division's Chief Scientist, Dr Nick Gales, said the findings of the oceanographic study are profoundly important.
"Not only will this research improve our understanding of ocean currents, but will also feed into our knowledge of how the Southern Ocean and the Antarctic continent drives the world's climate processes," Dr Gales said.
Dr Rintoul was Chief Scientist on the recent voyage and has made a dozen voyages to the Southern Ocean. "When we speak of global warming, we really mean ocean warming: more than 90 per cent of the extra heat energy stored by the earth over the last 50 years has gone into warming up the ocean."
"The Southern Ocean is particularly important because it stores more heat and carbon dioxide released by human activities than any other region, and so helps to slow the rate of climate change," Dr Rintoul said. "A key goal of our work is to determine if the Southern Ocean will continue to play this role in the future."
The causes of the observed changes in the Southern Ocean are not yet fully understood. Changes in winds, sea ice, precipitation, or melt of floating glacial ice around the edge of Antarctica may be responsible. Data collected on the latest voyage will help unravel this mystery.
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