Scientists studying the icy depths of the sea around Antarctica have detected changes in salinity that could have profound effects on the world's climate and ocean currents.
The scientists returned to the southern Australian city of Hobart on Thursday after a one-month voyage studying the Southern Ocean to see how it is changing and what those changes might mean for global climate patterns.
Voyage leader Steve Rintoul said his team found that salty, dense water that sinks near the edge of Antarctica to the bottom of the ocean about five kilometres down was becoming fresher and more buoyant.
So-called Antarctic bottom water helps power the great ocean conveyor belt, a system of currents spanning the Southern, Pacific, Indian and Atlantic Oceans that shifts heat around the globe.
"The main reason we're paying attention to this is because it is one of the switches in the climate system and we need to know if we are about to flip that switch or not," said Mr Rintoul of Australia's government-backed research arm the CSIRO.
"If that freshening trend continues for long enough, eventually the water near Antarctica would be too light, too buoyant to sink and that limb of the global-scale circulation would shut down," he said yesterday.
Cold, salty water also sinks to the depths in the far north Atlantic Ocean near Greenland and, together with the vast amount of water that sinks off Antarctica, this drives the ocean conveyor belt.
This system brings warm water into the far north Atlantic, making Europe warmer than it would otherwise be, and also drives the large flow of upper ocean water from the tropical Pacific to the Indian Ocean through the Indonesia Archipelago.
If these currents were to slow or stop, the world's climate would eventually be thrown into chaos.
"We don't see any evidence yet that the amount of bottom water that's sinking has declined. But by becoming fresher and less dense it's moving in the direction of an ultimate shutdown."
Mr Rintoul said results of the bottom water samples in the Ross Sea directly south of New Zealand and off Antarctica's Adelie Land further to the west were a crucial finding.
"We didn't know that before we left but it's now clear that both of those regions are becoming fresher for some reason."
During the voyage, scientists from Australia, Britain, France and the United States measured salinity, carbon dioxide and iron concentrations as well as currents between Antarctica and Australia.
Mr Rintoul said his team are studying if faster melting of icesheets or sea ice is the source of the fresher water, but he said it was too early to tell if global warming was to blame.
Over the coming months, his team will study oxygen isotopes collected from water samples.
"Oxygen isotopes act as a tracer of ice melt and that information should help pin down exactly what the cause of the freshening is in the deep ocean," said Mr Rintoul, of the Antarctic Climate and Ecosystems Cooperative Research Centre.
"The leading hypothesis at the moment for why it's freshening is that the floating ice around Antarctica is melting more rapidly than in the past."