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WHAT goes around comes around. Our greenhouse gas emissions don't just warm the planet, they also acidify the oceans. Now it turns out that the change in ocean chemistry they cause will feed back into the climate, further driving up temperatures.

Ocean acidification poses a threat to many marine organisms such as corals – the shells of some marine snails are already dissolving. Until now it seemed like this was strictly a problem for marine organisms and the people who depend on them: 'climate scientists consider the carbon dioxide that is absorbed by the ocean to be stored and unable to affect the climate.

But research now suggests that the acidification it causes will rebound on the entire planet, by acting on tiny marine plants called phytoplankton. These produce a chemical called dimethyl sulphide (DMS) that drifts up into the air and reflects sunlight back into space, cooling the planet. DMS also makes clouds brighter, with the same effect.

Katharina Six of the Max Planck Institute for Meteorology in Hamburg, Germany, and her team gathered experimental data showing that phytoplankton produce less DMS as seawater becomes less alkaline. After feeding these figures into climate models, they estimate that 18 per cent less DMS will be released from the oceans in 2100, compared with preindustrial times.

If the concentration of CO₂ in the atmosphere doubles – which is likely to happen later this century – temperatures are expected to rise between 2 and 4.5 °C. Ocean acidification will add between 0.23 and 0.48 °C to that figure, Six estimates. "We were surprised that the effect was so large," she says. "It certainly speeds up the warming."

The finding adds to a roster of unexpected positive feedback effects that could amplify global warming.

These effects could make a big difference, says Paul Pearson of Cardiff University in the UK. But he adds that Six's estimate of temperature change is based on processes that are poorly understood, like cloud physics. "These sorts of changes are possible, but very difficult to predict with certainty."

Journal reference: Nature Climate Change, DOI: 10.1038/NCLIMATE1981

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