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30 May 2016 16:00

Ocean conveyor key to sluggish Antarctic warming, study says

Robert McSweeney


Robert McSweeney

30.05.2016 | 4:00pm
Ocean warmingOcean conveyor key to sluggish Antarctic warming, study says

While the Arctic is warming at twice the rate of the global average, on the other side of the planet, warming of the Southern Ocean around the Antarctic has been much slower.

A new study finds the key to this lack of warming lies in an ocean conveyor system that carries warm water away from Antarctica, and pulls cold water up from a depth of thousands of metres in its place. The researchers have, for the first time, tracked where this ocean heat goes.

The results suggest that warming in the Southern Ocean will only start to catch up with the rest of the world once this deep water also warms – which could take hundreds of years.

Conveyor belt

Heat is transported around the world’s oceans via a giant conveyor belt. It’s one part of this conveyor system, for example, that brings warm water up from the Caribbean to the North Atlantic, giving Europe its relatively mild climate.  

While scientists have long known about this system, the new study, just published in Nature Geoscience, finds that it is dampening the warming in the region of the Southern Ocean around Antarctica.

Lead author Dr Kyle Armour, assistant professor of oceanography and atmospheric sciences at the University of Washington, explains to Carbon Brief how the most southerly part of the ocean conveyor carries water into and out of the Southern Ocean:

“The strong westerly winds that circle around Antarctica push surface waters northward, away from the continent, continually drawing up water from below in their place. The deep waters that are drawn up – ‘upwelled’ – to the surface of the Southern Ocean are quite old, originally sinking in the North Atlantic near Greenland and following a circuitous path through the world’s oceans before resurfacing around Antarctica.”

When the water arrives at the surface of the Southern Ocean, it is warmed by the air above it. But the ocean conveyor means that this warm water doesn’t stay put.

The researchers tracked where the heat was going using a combination of observations from a global fleet of 3,000 drifting floats, known as the Argo array, and climate model simulations.

Their findings show that the majority – approximately 68% – of the heat taken up by the most southerly parts of the Southern Ocean is then carried north, says Armour:

“When we looked at where that heat has ended up in the ocean, we find that the greatest warming has occurred far to the north [of the Southern Ocean].”

This northward flow of water in the Southern Ocean meets warm water travelling south from the Atlantic, Pacific and Indian Oceans, where collectively it is pushed down into the deep ocean.

By carrying heat away from Antarctica, the ocean conveyor slows the warming of the surrounding Southern Ocean, Armour says. As a result, it’s only warmed by 0.02C per decade since 1950, compared to 0.08C per decade for the average of the global ocean surface.

You can see this in the map below. It shows observed sea surface temperature change per decade between 1950 and 2012. The yellow and orange shading indicates areas of warming around the northern extremities of the Southern Ocean, while the white areas in Antarctica’s more immediate surroundings show little change.

Observed changes in annual sea surface temperatures around Antarctica, from 1950 to 2012, in degrees Celsius per decade. Red and yellow shading shows warming, while white shading shows no change. The grey line around Antarctica indicates the maximum winter extent of sea ice. Source: Armour et al. (2016) Supplementary information.

Observed changes in annual sea surface temperatures around Antarctica, from 1950 to 2012, in degrees Celsius per decade. Red and yellow shading shows warming, while white shading shows no change. The grey line around Antarctica indicates the maximum winter extent of sea ice. Source: Armour et al. (2016) Supplementary information.

Significant warming

So, when will warming in the Southern Ocean really kick in? It could take hundreds or thousands of years, the study suggests.

The slow speed of the ocean conveyor means the waters rising up from thousands of metres deep in the Southern Ocean originally sank from the surface of the North Atlantic before the industrial revolution began. This means they haven’t yet been exposed to the warmer atmosphere caused by our CO2 emissions.

These deep waters won’t start to warm until surface waters that have been warmed by atmosphere cycles through the ocean conveyor belt, the paper says. Other studies suggest this could take around two centuries since CO2 emissions started to rise.

And once warming of the Southern Ocean gets going, it will continue for millennia, says Armour:

“Thus, our hypothesis is that full warming around Antarctica will occur only when these deep ocean waters are warmed themselves, which likely takes thousand of years.”

The findings highlight that not all of the planet responds to rising global temperatures in the same way, says Armour:

“When we talk about ‘global warming’, there is a tendency to imagine that the warming will occur everywhere at the same rate. Yet, we’ve seen very rapid warming in the Arctic and very slow warming over the ocean around Antarctica over the last 50 years.”

Land and sea ice

Another difference between the Arctic and Antarctic is that while Arctic sea ice has been declining steadily in recent decades, sea ice has actually increased around Antarctica.

You can see the maximum winter extent in Antarctic sea ice by the grey line in the earlier figure.

The dampening impact of the ocean conveyor doesn’t explain the increase, says Armour, but it may have a role to play:

“Whatever the cause of the sea ice expansion, my view is that it could not have occurred without this upwelling mechanism in place to slow the greenhouse gas warming [of the Southern Ocean].”

And warming in the Southern Ocean as a whole is already impacting the ice shelves that spill out onto the ocean surface from Antarctica’s ice sheets, says Dr Emily Shuckburgh, deputy head of polar oceans at the British Antarctic Survey. She tells Carbon Brief:

“It should be noted that as a whole the Southern Ocean has shown considerable warming, from the surface to depths of more than 1,000m, and that changes to the circulation in the Southern Ocean have been bringing more relatively warm water up under the ice sheets, melting them from below.”

This doesn’t bode well for Antarctica’s land ice in the future, warns Prof Andy Shepherd, director of the NERC Centre for Polar Observation and Modelling at the University of Leeds. He tells the Science Media Centre:

“[W]e have seen colossal ice losses from West Antarctica over recent decades, and so the prospect that additional warming is on the way is a real concern because it edges us further towards the upper range of future sea level rise predictions.”

So, while the prospect of significant warming in the ocean around Antarctica is a long way off, it will only accelerate the loss of ice from Antarctica and its contribution to sea level rise.

Armour, K. et al. (2016) Southern Ocean warming delayed by circumpolar upwelling and equatorward transport, Nature Geoscience, doi:10.1038/ngeo2731

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