Global Warming

Rising ocean acidity will exacerbate global warming

German scientists say they've identified a previously overlooked factor in climate change.

In a study published today, they say one effect of global warming is that the oceans will release less of a gas which helps shield the Earth from the sun's radiation. Carbon dioxide soaked up by seawater will cause plankton to release less cloud-forming compounds back into atmosphere.

The slow and inexorable increase in the oceans’ acidity as they soak up carbon dioxide from the atmosphere could itself have an effect on climate and amplify global warming, according to a new study. Acidification would lead certain marine organisms to emit less of the sulphur compounds that help to seed the formation of clouds and so keep the planet cool.

Atmospheric sulphur, most of which comes from the sea, is a check against global warming. Phytoplankton — photosynthetic microbes that drift in sunlit water — produces a compound called dimethylsulphide (DMS). Some of this enters the atmosphere and reacts to make sulphuric acid, which clumps into aerosols, or microscopic airborne particles. Aerosols seed the formation of clouds, which help cool the Earth by reflecting sunlight.

As more CO2 enters the atmosphere, some dissolves in seawater, forming carbonic acid. This is decreasing the pH of the oceans, which is already down by 0.1 pH units on pre-industrial times, and could be down by another 0.5 in some places by 2100. And studies using 'mesocosms' — enclosed volumes of seawater — show that seawater with a lower pH produces less DMS2. On a global scale, a fall in DMS emissions due to acidification could have a major effect on climate, creating a positive-feedback loop and enhancing warming.

Researchers agree that despite the dangers of extrapolating data accumulated over a relatively short time to changes that occur over decades, it remains important to recognize that marine organisms will be affected by environmental changes and that this may impact the climate in return. CO2 that is absorbed by the ocean should be considered to be climate-relevant.