By Alex Kirby
The Antarctic ozone hole has changed how the seas around the Antarctic mix, scientists say, threatening their ability to absorb atmospheric carbon dioxide and potentially speeding up global warming.
The discovery is important because the Antarctic accounts for about 40% of the total carbon absorbed by the world’s seas.
Writing in the journal Science, Darryn W. Waugh, an earth scientist at Johns Hopkins University, and his team show that both the sub-tropical waters in the southern oceans and the upwelling circumpolar waters closer to the Antarctic landmass have changed, in a way they say is consistent with the changes in the westerly winds around Antarctica.
These have grown stronger and moved poleward over the past few decades as the ozone layer has thinned. The new study finds evidence that those shifting winds are speeding circulation patterns in polar waters, with the currents closer to the land pushing more deep water up to the ocean surface.
The scientists’ worry is that the increasing upwelling of that water, hundreds of years old and naturally rich in carbon dioxide, is reducing the amount of manmade carbon absorbed by sub-polar waters.
“This may sound entirely academic, but believe me, it’s not,” said Waugh. “This matters because the southern oceans play an important role in the uptake of heat and carbon dioxide, so any changes in southern ocean circulation have the potential to change the global climate.”
The team used measurements taken from the early 1990s to the mid-to-late 2000s of the amount of a chemical, chlorofluorocarbon-12 (CFC-12), in the southern oceans.
CFC-12 was first produced commercially in the 1930s and was widely used in aerosols, refrigeration systems and air conditioning. It was finally phased out by the Montreal Protocol on Substances that Deplete the Ozone Layer.
Because they knew that concentrations of CFCs at the surface increased in tandem with those in the atmosphere, the scientists were able to surmise that the higher the concentration of CFC-12 deeper in the ocean, the more recently those waters had been at the surface, and they worked out how fast the mixing had happened.
They believe north-south circulation in the deep ocean has been speeding up, sending water from the ocean surface near the pole to intermediate depths (500-1,000 metres down) more quickly.
At the same time, the currents closer to Antarctica’s shores appear to be pushing more old, deep water up to the surface.
If surface waters are already rich in carbon, “that would mean more of the carbon we’re producing would stay in the atmosphere, and that would contribute more to climate change,” Waugh says.
Michael Meredith, a British Antarctic Survey oceanographer, said the new research drove home the importance of the Southern Ocean carbon sink. “It’s doing us a very big favour, if you like, by taking carbon from the atmosphere and slowing the rate of atmospheric climate change,” he said.
He believes the question now is what will happen as the ozone layer slowly heals and human activities pump out increasing amounts of greenhouse gases. The ozone hole is expected, on present trends, to have recovered by mid-century.
This article was produced by the Climate News Network