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Antarctic currents aren’t a-changing

The Southern Ocean’s carbon-sponging capacity has been getting a scientific rethink lately, and a paper in Nature Geoscience this week (subscription required) offers new info on what assistance we can expect from these frigid waters. Unfortunately, the wire report on the paper garbles its bottom line.

According to one long-term prediction, the Earth’s oceans – our greatest natural ally in any war on climate change – will soak up 70 to 80% of the entire industrial era’s anthropogenic carbon dioxide emissions over the next several centuries . The Southern Ocean accounts for some 15% of that storage. Last year, however, a widely reported study (Science, subscription required) led by the University of East Anglia’s Corinne Le Quéré concluded that the Southern Ocean’s ability to absorb the gas was weakening.

The new paper, by Claus Boening of IFMGEOMAR and colleagues, has garnered a Reuters article that unsurprisingly doesn’t go into detail about past studies. But the summary from Reuters wrongly implies an about-face on worries over a weakening ocean sink. The story leads with:

The Southern Ocean has proved more resilient to global warming than previously thought and remains a major store of mankind’s planet-warming carbon dioxide, a study has found.

and it later continues:

The analysis shows the Southern Ocean has maintained its ability to soak up excess carbon despite changes to currents and wind speeds.

Actually, though, the Boening et al. paper doesn’t evaluate carbon-soaking ability – current and wind speeds are what it’s all about. In contrast, storage capacity is the primary concern of Le Quéré et al. So the direct contradiction between today’s story, Southern Ocean changing but still major CO2 sink, and the 2007 Reuters report on Le Quéré et al., Southern Ocean saturated with carbon dioxide, looks like an oversimplification.

I called Boening to clarify. “We don’t have anything to directly challenge that conclusion [of Le Quéré et al.],” he says. “We are just challenging the scenario behind that conclusion, namely changing circulation patterns.”

Le Quéré et al., to explain the Southern Ocean’s steadily declining appetite for carbon since 1981, had cited climate models showing changes in currents that carry water from the carbon dioxide-absorbing surface down to deep-ocean storage. The altered currents in the models are driven by westerly winds in the Southern Hemisphere that are known to be growing more intense. As summarized in Nature News at the time (subscription required), “These stronger winds, thought to be driven by the depletion of the ozone layer over Antarctic regions, churn up the ocean and bring more dissolved carbon up from the depths.”

The new paper says those carbon-transporting currents, in fact, are wending downwards as firmly as always. As the Reuters story notes, they have shifted their position, but not their strength. Boening et a new compilation of historical data with recent temperature and salinity measurements gathered using floats that bob between the sea surface and depths of 2,000 metres. From temperature and salinity, they calculate water densities and infer how currents should flow. Though the Southern Ocean has warmed and become less salty since the 1960s and especially since the 1980s – signs of human-caused climate change, the authors say – density gradients and therefore currents remain much the same. The results point to the importance of more detailed climate models that include small-scale eddies and don’t show the large-scale circulation changes that Le Quéré et al. invoke.

So is the Southern Ocean “more resilient to global warming than previously thought”? In a sense, yes: circulation is chugging along fine, contrary to the assumptions of Le Quéré et al. But there’s nothing here to refute their key warning about the declining carbon sink.

It may be true that the Southern Ocean’s carbon sink is weakening, Boening told me – and if so, there is some other explanation besides the wild west wind.

Anna Barnett


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