Ozone hole trims polar water’s CO2-absorbing power

Simulations also suggest that the dearth of ozone over Antarctica leads to ocean acidification

The ozone hole over Antarctica does more than let a little extra ultraviolet light reach ground level: It boosts ocean acidification in the waters surrounding the icy continent and reduces the amount of carbon dioxide emissions those waters can absorb.

OZONE EFFECT Overall, the ozone hole over Antarctica boosts wind speeds over the surrounding oceans, according to new model simulations. Yellow indicates where wind speeds increased the most in a scenario with an ozone hole compared to one without the hole. Bright blue shows where wind speeds declined the most. Lenton et al./Geophysical Research Letters

Recent research has indicated that the oceans surrounding Antarctica aren’t absorbing nearly as much planet-warming CO2 from the atmosphere as they did in previous decades (SN: 5/26/07, p. 333). In one of those studies, scientists speculated that meteorological effects of the high-altitude ozone hole over Antarctica, including strengthening of winds at sea level, might be to blame. Now, results of computer simulations bolster that notion, researchers report online June 20 in Geophysical Research Letters.

Francis Codron, an atmospheric scientist at the French national center for scientific research, CNRS, in Paris, and his colleagues used climate models to compare two scenarios: one in which the stratosphere over Antarctica had no ozone hole from 1975 to 2004 and one in which the stratosphere had a hole like the one that has actually developed. The researchers ran five simulations for each of the two scenarios, Codron says.

“This is a nice study,” says Jorge Sarmiento, a biogeochemical oceanographer at Princeton University. This team’s ocean-atmosphere simulations are the first to include effects of the ozone hole, he notes.

The average results of the two scenarios differ little from 1975 to 1986.. From 1987 onward, however, wind speeds over the high-latitude southern oceans were higher in the ozone-hole scenario than in the simulations that lacked an ozone hole. Differences between the scenarios became larger with every passing year, the researchers report. In the ozone-hole scenario, wind speeds in some areas were 60 percent higher in 2004 than they had been in 1975.

That increase in wind speeds has triggered a series of real-world effects, Codron suggests. First, the stronger winds stir the surface waters more effectively and boost the upwelling of waters from the deep — waters that include large amounts of dissolved CO2 from the decomposition of ocean life that died and sank to the depths. As the surface waters become more enriched in  CO2, they can absorb less carbon dioxide from the atmosphere, explaining the reduced uptake of that gas previously noted by scientists. The simulation suggests that between 1987 and 2004, the southern oceans absorbed about 9 billion metric tons less  CO2 than they would have without the ozone hole.

The increased concentrations of dissolved  CO2 also boosted the ocean’s acidity in the ozone-hole scenario. Surface water pH dropped — meaning the water became more acidic — by about 0.02 units, about 10 percent of the change measured in oceans since the beginning of the Industrial Revolution.

For the next half century — the period that scientists estimate it will take for the ozone hole to heal itself after banning ozone-destroying chemicals (SN: 12/24/05, p. 418) — reduced CO2 uptake in the southern oceans could exacerbate or speed the effects of climate change globally.

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