Oceans '13: Changes in circulation due to ozone thinning

According to a Johns Hopkins earth scientist, the hole in the Antarctic ozone layer has caused changes in the way that waters in those southern oceans mix—a situation that has the potential to alter the amount of CO2 in the atmosphere and eventually could have an impact on global climate change.

In a paper published in the Feb. 1 issue of the journal Science, Darryn W. Waugh and his team show that subtropical intermediate waters in the southern oceans have become "younger" as the upwelling, circumpolar waters have gotten "older," changes that are consistent with the fact that surface winds have strengthenedĀ as the ozone layer has thinned.

"This may sound entirely academic, but believe me, it's not," says Waugh, of the Morton K. Blaustein Department of Earth and Planetary Sciences in the university's Krieger School of Arts and Sciences. "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."

Waugh's team used measurements taken in the early 1990s and in 2005-2010 of the amount of a chemical compound known as chlorofluorocarbon-12 in the southern oceans. CFC-12 was first produced commercially in the 1930s, and its concentration in the atmosphere increased rapidly until the 1990s, when it was phased out by the Montreal Protocol on substances that deplete the ozone layer. (Before the Montreal Protocol, CFC-12 was used in products such as aerosol hairsprays and refrigerants, and in air conditioning systems.)

From those ocean measurements, Waugh's team was able to infer changes in how rapidly surface waters have mixed into the interior of the southern oceans. Because the scientists knew that concentrations of CFCs at the ocean surface increased in tandem with those in the atmosphere, they were able to surmise that the higher the concentration of CFC-12 in the ocean's interior, the more recently those waters were at the surface.

As the concentrations of CFCs at the ocean surface increase in parallel with those in the atmosphere, they surmised that the higher the concentration of CFC-12 in the ocean's interior, the more recently that water was at the surface.

The inferred age changes are consistent with the observed intensification of surface westerly winds, which have occurred primarily because of the Antarctic ozone hole, suggesting that stratospheric ozone depletion is the primary cause of the changes in ocean ventilation.Ā As stratospheric ozone recovers over the next 50 years, the changes in ventilation may slow or reverse. However, the impact of continued increases in greenhouse gases will need to be considered, and the integrated impact of stratospheric ozone recovery and increases in greenhouses on the southern ocean's ventilation and uptake of heat and anthropogenic carbon is an open question.

Also on the research team were François Primeau, of the University of California, Irvine; Tim Devries, of the University of California, Los Angeles; and Mark Holzer, of the University of New South Wales and Columbia University. Funding for the study was provided by the National Science Foundation and the Australian Research Council.