Image from World Meteorological Organization
Forty percent: That is the share of annual oceanic carbon dioxide uptake accounted for by the Southern Ocean. Given that oceans comprise Earth's largest carbon sink, that is not an insignificant figure; indeed, this sole region is responsible for absorbing a staggering amount of anthropogenic emissions -- close to 1 petagram (1 Pg = 1,000,000,000,000,000 grams) per year. Which makes it all the more worrisome that ocean acidification is rapidly weakening its capacity to take up our excess emissions, something I discussed last month. Now we hear that ozone depletion, already a long-standing problem in itself, could be worsening the situation. Nature's Anna Barnett (sub. required) reports on the findings of new research linking the hole in the ozone layer over Antarctica to a deterioration of the Southern Ocean's buffering capacity. According to Andrew Lenton, a marine biochemist at the Pierre and Marie Curie University in Paris, ozone damage and the associated climatic effects of rising GHG emissions are strengthening winds in the region, intensifying the Southern Ocean's upwelling system (the movement of cold, nutrient-rich deep waters to the surface).
Though upwelling stimulates primary productivity by making more nutrients available to phytoplankton, it also has the nasty tendency of causing more carbon dioxide "outgassing" -- basically, letting more CO2 escape to the atmosphere. Lenton and his colleagues built several simulations that coupled the ozone hole's effects on winds to currents and dissolved carbon levels.
Comparing the results of simulations run both with and without ozone depletion since 1975, Lenton found that the presence of the ozone hole helped explain the weakening of the Southern Ocean's buffering capacity; from 1994 to 2004, it contributed to a drop in surface seawater pH of 0.01 units, which is worse than it sounds (this seemingly infinitesimal drop represents one-tenth of the change in seawater pH since pre-industrial times).
Not all oceanographers agree with Lenton's findings, however. While acknowledging that his model may accurately reflect the sink's deterioration, critics dispute the notion that ocean currents in the region are changing -- saying that local observations and measurements simply don't bear the theory out. Better models, which include physical processes like eddy formation and current movement, could elucidate the role of currents in ocean acidification, they argue.
To be helpful, Lenton counters, they would have to scrutinize CO2 levels -- something which most of them fail to consider. Either way you look at it, it's a bad situation. Whether caused by current alteration or a combination of processes, it is clear that more CO2-rich deep waters are rising to the surface, releasing copious amounts of the greenhouse gas to the atmosphere in the process. As ocean acidification proceeds apace, further weakening the Southern Ocean's declining buffering capacity (and thus its ability to store CO2), we can expect to see a major surge in atmospheric CO2 levels.
Via: Nature News: Ozone hole weakens oceanic carbon sink
More about ozone depletion
Ozone Hinders Plants' Ability to Absorb Carbon Dioxide