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Sea life in peril as oceans turn acid
09 July 2005
From New Scientist Print Edition.
Rowan Hooper

The oceans are gradually becoming more acidic as they soak up the excess carbon dioxide released into the atmosphere. The change could be catastrophic for marine ecosystems and for economies that rely on reef tourism and fishing - and there is no way to reverse it.

That is the conclusion of the first review looking at all the studies relating to the acidification of the oceans. It was commissioned from an international group of scientists by the Royal Society, the UK's national academy of science.

The rising level of CO₂ in the atmosphere is what causes global warming. As the gas dissolves in the oceans, however, it is causing a quite different problem by forming carbonic acid.

The seas, which are naturally alkaline with an average pH of 8.2, act as a "buffer" that can soak up vast quantities of CO₂ with little change in acidity. But levels of CO₂ in the atmosphere are now rising so fast - from 280 parts per million before the industrial age to 380 ppm today - that the oceans are becoming more acidic. Models suggest that if emissions continue at present rates, then by 2100 the pH of the sea will fall by as much as 0.5 units.

Surprisingly little research has been carried out into the effects of increasing acidity on ocean chemistry and biology, and the report recommends that more is urgently carried out. But what we do know is ominous. "It would not directly kill penguins and orca and big animals like that, but it will affect the food chain, with potentially damaging effects on larger animals," says John Raven of the University of Dundee, UK, who led the review.

For starters, oxygen becomes more difficult to extract from water as the pH falls. This could affect the growth and reproduction of animals with high oxygen demands, such as squid. The proportion of dissolved forms of toxic metals will also increase as pH falls.

The greatest effect is likely to be on organisms with calcium carbonate shells, from lobsters and crabs to shellfish, certain plankton species and coral polyps. Calcium carbonate is insoluble in seawater only because the water is saturated with carbonate ions. As acidity rises, the level of carbonate ions falls. That will make it more difficult for animals to make their shells, and in some parts of the ocean such structures might even start to dissolve.

Coral reefs face a triple whammy: global warming, coastal pollution and now acidification. We can expect to see degradation of coral reefs in the tropics, Raven says, which would affect tourist industries, fisheries and also leave shorelines more vulnerable.

Some sceptics claim that corals will grow even faster as the oceans warm up, more than making up for the increased acidity. But all studies so far suggest coral calcification peaks at current temperatures, the review says.

What's more, acid oceans may be less able to mop up CO₂, making the problem even worse. At the moment, photosynthetic plankton called coccolithophores, which grow calcium carbonate shells, form giant "blooms" in spring and summer. Many sink to the bottom of the ocean, safely locking away vast amounts of carbon in ocean sediments.

The sea has absorbed about half of the CO₂ produced by humans in the last 200 years and currently soaks up 1 tonne of the gas each year for every person on the planet. But if the growth of coccolithophores is hindered, less carbon might be removed from the atmosphere and oceans, speeding up both climate change and acidification.

Acidification is effectively irreversible. "It will take many thousands of years for natural processes to return the oceans to their pre-industrial state," says Raven.

The review group considered possible ways of neutralising the acid, such as dumping chalk into the sea, but all had major problems. "It's just not practical. You would have to denude an area of pure chalk covering 60 square kilometres and 100 metres deep," says Andrew Watson of the University of East Anglia, UK. It would be like dumping the White Cliffs of Dover into the sea each year, he says.

"The only way to minimise the long-term consequences is to decrease CO₂ emissions," Raven says. Watson agrees: "We need a sharp decline in CO₂ emissions, down to half that of today." That looks unlikely. The Kyoto protocol, rejected by the US, calls for cuts of only 5 per cent by 2012 relative to 1990 emissions.

From issue 2507 of New Scientist magazine, 09 July 2005, page 15

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