Sour times

Every silver lining has its cloud. At the moment, the world’s oceans absorb a million tonnes of carbon dioxide an hour. Admittedly that is only a third of the rate at which humanity dumps the stuff into the atmosphere by burning fossil fuels, but it certainly helps to slow down global warming. However, what is a blessing for the atmosphere turns out to be a curse for the oceans. When carbon dioxide dissolves in water it forms carbonic acid. At the moment, seawater is naturally alkaline—but it is becoming less so all the time.

The biological significance of this acidification was a topic of debate at the American Association for the Advancement of Science meeting in Boston. Many species of invertebrate have shells or skeletons made of calcium carbonate. It is these, fossilised, that form rocks such as chalk and limestone. And, as anyone who has studied chemistry at school knows, if you drop chalk into acid it fizzes away to nothing. Many marine biologists therefore worry that some species will soon be unable to make their protective homes. According to Andrew Knoll, of Harvard University, many of the species most at risk are corals.

The acid test

Dr Knoll drew this conclusion by studying the fossil record. The end of the Permian period, 252m years ago, was marked by the biggest extinction of life known to have happened on Earth. At least part of the cause of this extinction seems to have been huge volcanic eruptions that poured carbon dioxide into the atmosphere. But some groups of animals became more extinct than others. Sponges, corals and brachiopods (a once-widespread group that look a bit like bivalve molluscs) were particularly badly hit.

Rather than counting individual species of fossils, which vary over time, palaeontologists who study extinction usually count entire groups of related species, called genera. More than 90% of Permian genera of sponges, corals and brachiopods vanished in the extinction. By contrast, only half of the genera of molluscs (the real ones) and arthropods disappeared.

Dr Knoll reckons this is because molluscs and arthropods are able to buffer the chemistry of the internal fluids from which they create their shells. This keeps the acidity of those fluids constant. Sponges, corals and brachiopods, however, cannot do this.

The situation at the moment is not as bad as it was at the end of the Permian. Nevertheless, calculations suggest that if today’s trends continue, the alkalinity of the ocean will have fallen by half a pH unit by 2100. That would make some places, such as the Southern Ocean, uninhabitable for corals. Since corals provide habitat and food sources for many other denizens of the deep, this could have a profound effect on the marine food web.

Gretchen Hofmann of the University of California, Santa Barbara, has brought some experimental evidence to bear on the question. She is investigating the effects of changing acidity and temperature in the sea on a creature called the purple sea urchin. This animal is a scientists’ favourite for embryological experiments, and has thus had its genome sequenced (in part by Dr Hofmann, as it happens), so it is well understood. Dr Hofmann’s work suggests that a combination of heat and acidity is more deadly than either alone. When she and her team reproduced the conditions which are predicted to prevail in 2100 if carbon-dioxide emissions are not curbed, they found that the genes of larval sea urchins had to work up to three times harder than normal to form the animals’ skeletons. On top of that, those skeletons were often deformed.

No corals, no sea urchins and no who-knows-what-else would be bad news indeed for the sea. Those who blithely factor oceanic uptake into the equations of what people can get away with when it comes to greenhouse-gas pollution should, perhaps, have second thoughts.

The Economist, 21 February 2008. Article.

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