Ozone hole weakens oceanic carbon sink

The hole in the ozone layer over Antarctica may be impairing the Southern Ocean's ability to mop up carbon dioxide from Earth's atmosphere, according to work presented at a meeting in France today.

Earth's oceans are the largest sink of carbon dioxide, with the Southern Ocean accounting for more than 40% of the annual oceanic uptake of the greenhouse gas, says Andrew Lenton, a marine biochemist at the Pierre and Marie Curie University in Paris. In theory, seas should soak up more carbon dioxide as levels of the gas in the atmosphere rise.

But recent measurements have bucked simulations1–3 and shown that the Southern Ocean's surface waters have higher carbon levels than expected, which also makes them more acidic. As a result, the amount of CO2 that the ocean absorbs each year has also flattened out.

Missing link

What was missing from the models, says Lenton, was stratospheric ozone damage — which, along with the climatic effects of greenhouse-gas emissions, is thought to be behind the observed strengthening of southern winds. These winds, he says, may be stirring up ocean currents that bring carbon stored in the deep ocean up to the surface. As part of the five-year CARBOOCEAN project, a research consortium on marine impacts of carbon-dioxide emissions that is meeting in Dourdan from 8–12 December, Lenton and his colleagues built Southern Ocean simulations that coupled the ozone's effects on winds to ocean currents and marine carbon levels.

Until now, these connections have only been studied piecemeal, says Christoph Heinze, a biogeochemical modeller at the University of Bergen in Norway, who was not involved in the study. "It's one of the rare examples where somebody has really looked at several components of the Earth system together," he says.

By running the models both with and without ozone depletion since 1975, the researchers "isolated the signal from ozone depletion", says Lenton's co-worker Francis Codron, an atmospheric scientist at the Dynamic Meteorology Laboratory in Paris.

Including the ozone hole reproduced the unexpectedly feeble carbon sink observed by oceanographers. "These sound like very different parts of the system, and yet one affects the other," says Codron.

The signal from ozone, the researchers found, drove a drop in Southern Ocean surface pH of 0.01 units from 1994 to 2004 — half the total pH decline in that period, and one-tenth of the change since the pre-industrial era.