Seesaw current links north and south poles
Antarctica and Greenland may be at opposite ends of the planet but their climate systems appear to be linked by a remarkable ocean current, a study shows.
The results, published today in the journal Nature, suggest that Antarctica's ice could eventually start to melt because of localised warming in the far North Atlantic.
The evidence comes from a 2500 metre ice core, drilled by European scientists at Dronning Maud Land, on the part of Antarctica that faces the South Atlantic.
With its compacted layers of ice and telltale concentrations of methane in trapped air bubbles, the core yields a picture of snowfall and atmospheric temperatures going back 150,000 years.
Even better than that, it can be matched with cores of similar amplitude drilled in the Greenland icesheet.
Put together, the cores provide the first solid evidence to back a theory that millennial scale climate changes that have unfolded in the far north and south of the Atlantic are not isolated, local events, but linked.
The glacial climate in the Northern Atlantic can swing extraordinarily rapidly, with temperatures rising by 8-16°C within the space of a few decades at the end of each Ice Age and falling back, albeit more slowly, when the next Ice Age beckons.
Antarctica, though, has far smaller temperature shifts, of between 1-3°C, and these unfold over millennia.
But the two sets of ice cores point to what the European Project for Ice Coring in Antarctica scientists describe as a bipolar seesaw
In short, what happens at one end of the Atlantic has a huge effect on the other, although at different time scales and in different ways.
The cause appears to be a conveyor-belt system of ocean flows.
On the conveyor belt
Relative heat from the Southern Ocean around Antarctica is picked up by a complex system called the meridional overturning circulation (MOC), of which the Gulf Stream is the best-known component.
The MOC channels warm surface water up to the North Atlantic, coincidentally enabling countries in northwestern Europe to have a balmy climate despite their northerly latitude.
When this warm water reaches the far north, it cools and sinks, and the MOC sends it back south, back down towards Antarctica, at depths far below the ocean's surface.
"Our data shows that the degree of warming in the south is linearly related to the duration of cold periods in the North Atlantic," says lead author Dr Hubertus Fischer of Germany's Alfred Wegener Institute for Polar and Marine Research in Bremerhaven.
Understanding this link also sheds light on what the researchers say is a troubling aspect about human-induced climate change: the fate of Antarctica, where the world's biggest store of frozen water is held.
"Today, Antarctica is still a reservoir of cold. We don't see any contribution to global sea-level change because of Antarctica, it's not melting yet. In fact there has been more precipitation and some models suggest that Antarctica actually will grow a little," Fischer says.
That reassuring scenario could change if, as some studies are now tentatively suggesting, the MOC is beginning to falter, says Fischer.
The causes for this slowing of the Atlantic conveyor belt could be a run-off of cold water from melting Siberian permafrost or the Greenland icesheet, triggered by rising atmospheric temperatures.
But any disruption would lead to a build-up of warmer water off Antarctica, according to the conveyor-belt theory.
"If the thermohaline [ocean convection] circulation in the Atlantic slows down just a little, it would cause a warming in the Southern Ocean," Fischer says.
"And if you have warming around Antarctica, at a certain point, the fringes of Antarctica will even warm over the melting point. Then we could start to see melting at the borders and run-off and that would contribute to sea-level rise."