The shells of tiny sea snails called pteropods, or “sea butterflies”, are dissolving thanks to the acidification of sea water brought about to increasing levels of CO 2 in the ocean, according to researchers from the British Antarctic Survey BAS).

A letter in Nature Geoscience titled Extensive dissolution of live pteropods in the Southern Ocean details the research, which according to BAS saw researchers examine “... an area of upwelling, where winds cause cold water to be pushed upwards from the deep to the surface of the ocean.” Such areas are of interest because their waters are “... usually more corrosive to a particular type of calcium carbonate (aragonite) that pteropods use to build their shells.”

A “saturation horizon” of 1000 metres is usually the depth at which ocean water becomes sufficiently corrosive to damage pteropod shells, but the sample taken by researchers came from just 200 metres down reached the same level of acidity.

BAS says “as a result of the additional influence of ocean acidification, this corrosive water severely dissolved the shells of pteropods.”

The paper and BAS go on to point the finger at anthropogenic CO 2 as the reason for the extra acidity at 200 metres down.

BAS science cruise leader and a co-author of the paper, Dr Geraint Tarling, says “upwelling sites are natural phenomena that occur throughout the Southern Ocean” but adds his belief that “instances where they bring the ‘saturation horizon’ above 200m will become more frequent as ocean acidification intensifies in the coming years.”

Quite how he can make that prediction isn't made clear, but another co-author, The University of East Anglia's Dr Dorothee Bakker, says “Climate models project a continued intensification in Southern Ocean winds throughout the 21st century if atmospheric carbon dioxide continues to increase. In turn, this will increase wind-driven upwelling and potentially make instances of deep water — which is under-saturated in aragonite – penetrating into the upper ocean more frequent.” That could mean the saturation horizon rising even further in the future.

If that comes to pass, it's potentially bad news for pteropods and the other creatures that depend on their niche in the food chain. Of course there's plenty of room in what's been revealed about this study for sceptics to work with, too. A single upwelling does not a global threat make, and we all know how contentious climate models are. ®