It's a curious fact that should you find yourself at the bottom of the ocean almost anywhere on the planet, the water temperature will be uniformly near zero degrees.

That hasn't always been the case. Tropical seabeds 65 million years ago were about 15 degrees when temperatures were higher, says Ian Simmonds, a professor at Melbourne University's School of Earth Sciences.

During that earlier epoch, the Earth was probably free of sea ice and it was also missing the remarkable processes driven by what's known as Antarctic Bottom Water and the global conveyor belt that distributes it. 

Record-low Antarctic sea ice levels reached this week are drawing renewed international attention to climate change at a time when Arctic ice extent is also reaching minimums not recorded before.

But scientists say the potential exists for much bigger shifts than demonstrated by sea ice totals alone.

"Sea ice is a canary in the coal mine," Simmonds says. "We are running this huge experiment on the planet."

'Holy grail'

The experiment goes something like this: we dig up and burn carbon trapped for millions of years, changing the chemistry of the atmosphere and the oceans. The Earth traps more of the sun's heat, and temperatures rise – so far about 1 degree. We hope other processes we set in train that we only partially understand don't lift sea levels too much or don't disrupt global-scale processes.

Antarctica looms as a critical place to watch not least because existing data is relatively scant.

Accurate satellite coverage of sea ice, for instance, only goes back to 1979 and knowledge of ice volume remains a "holy grail" for researchers, says Jan Lieser, a sea-ice scientist at the Antarctic Climate & Ecosystems Cooperative Research Centre. 

At the surface, extreme winds and cold don't make it easy to conduct science (and Australia's Mawson Station is unhelpfully located in a type of wind tunnel). And unlike the Arctic, where submariners needed to keep accurate readings of sea-ice thickness, Antarctic waters have had little such traffic and logbooks.

Still, the overall workings of the global engine that runs from Antarctica are well understood.

Every winter as sea ice forms around Antarctica, salt is rejected from the salty seas, creating a pulse of very cold, briny water. That water flows to the bottom of the sea, eventually finding its way through to much of the world's oceans, equalising temperatures even in the tropics.

The return path, sometimes taking centuries or longer for the complete cycle, transports warmth to the poles, helping to spread equatorial heat to the higher latitudes. 

This over-turning cycle, also known as the thermohaline circulation, also transports oxygenated water, helping to ventilate the oceans. It draws carbon dioxide out of the atmosphere into the oceans too.

Slowing down 

Unfortunately, changes are afoot.

"Certainly there's very strong evidence that this conveyor belt is slowing down," Simmonds says. "It's reinforcing what the models say ... it's one of the consequences of global warming."

Andy Hogg, an associate professor at the Australian National University's Research School of Earth Sciences, works on the models that attempt to capture and project what's going on in Antarctica.

"Even though we know the bottom water is becoming warmer and slightly fresher, we still don't have reliable models that can really describe why that is happening," he says. Less saline water, though, will sink slower and create less of that pulse effect.

"The signals are small but compared with the natural variability, it's fairly clear that it's happening," Hogg says. 

One source of the fresher water around Antarctica is that glacier melt is increasing, making seas near the coast less saline. 

Record low

What's happening with the sea ice itself will also play a role.

Lieser likens the annual increase and retreat in sea ices to "a giant heaving monster [and] the breathing is getting more hectic and more rapid". 

In a typical year, sea ice swells to about the size of Antarctica itself and then melts, losing as much as 90 per cent of its cover.

At the end of the freezing season in August 2014, sea ice extent reached record levels, a result highlighted by climate change sceptics struggling to explain a more consistent reduction in Arctic sea ice.

Two and a half years later, Antarctic sea ice has dived to more than 10 per cent below the previous record minimum, touching 2.075 million square kilometres on Thursday. It might yet sink below the 2-million mark, Lieser says.

A normal range would see coverage swell from about 4 million to 18 million square kilometres, Simmonds says.

"If that range decreased, you'd have a lot less salt rejected" and therefore a weaker pulse for the conveyor belt, he says.

Albedo effect

Matthew England, a professor of oceanography at the Climate Change Research Centre at the University of NSW, says the loss of polar sea ice at either end of the planet will have an amplifying effect on what are already among the fastest warming regions.

"The sea ice provides a nice reflective surface," he says. "If that was to go, it would have a huge impact on the climate because of the ice-albedo effect."

Absent the ice, more of the sun's energy is absorbed by the oceans, potentially curtailing the ice season at both ends.

"The Arctic has seen really rapid warming because of this loss of sea ice," England adds. "If the Antarctic were to follow suit – and 2016 is a bit of a wake-up call – we would also see an added impact on warming."

Among the concerns is that a slowing overturning mechanism will mean there is less uptake of carbon from the atmosphere but also increased risks for another of Antarctica's vulnerabilities: the ice shelves that face melting from the air above but also from warming waters below.

"The overturning circulation, when it shows down, you stop cool water invading the interior [of the ice shelves]," England says. That means waters at depths of 300-600 metres tend to get warmer.

Trigger point

If the ice shelves don't stay intact, glaciers behind them will accelerate their slide, adding yet more fresh water to surrounding seas, but also raising sea levels globally. 

"We're going in the direction of a trigger point, but in a broad sense, we don't really know what the value is," Simmonds says.

The scientists caution that a range of negative feedback loops will go some way to counterbalance the forces being unleased by climate change.

If ice shelf waters become fresher, for instance, sea ice can form faster the following year. That process, though, doesn't produce the same salt rejection that drives the thermohaline system.

A slowing in the sinking of briny water also means the transport of warmer waters southwards would also be weaker, a process that may reduce the warm water impact on ice shelves.

"We probably lack the observations and models to make the call which ones are going to dominate," Hogg says.

Lieser says the potential for a slowing or changing direction of the Gulf Stream, which could bring sharply colder conditions for northern Europe, has gained a lot more public attention than what might be happening in the south.

He says it is hard to know when a tipping point might be reached, after which huge global processes will settle into new equilibria.

"There are really cascading consequences in that," Lieser says. "It's a scary thought if you really think it through."