"The amount of dense Antarctic Bottom Water has contracted each time we've measured it since the 1970s," said Dr Steve Rintoul, of CSIRO and the Antarctic Climate and Ecosystems CRC. "There is now only about 40 per cent as much dense water present as observed in 1970."
The ocean profiles also show that the dense water formed around Antarctica has become less saline since 1970.
"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.
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."