Danger beneath the surface: Coral and shellfish under threat as seas turn acidic

Shellfish, coral reefs and other marine animals with exterior skeletons will find it difficult to cope with the speed at which the oceans are turning acidic due to rising concentrations of carbon dioxide, a study has found.

Click here to view In depth: How the oceans are acidifying

Scientists have calculated that the current rate of ocean acidification today is unprecedented over the past 300 million years, when the seas experienced at least four major mass extinctions involving rising ocean acidity.

In the past 100 years, the concentration of carbon dioxide in the atmosphere has increased by about a third. This has resulted in a corresponding decrease in the natural alkaline state of the oceans, effectively making them more acidic as CO2 in the air dissolves in seawater to create carbonic acid, scientists said.

The oceans have been more acidic than they are now on several occasions in the past. But the current rate of acidification is many times faster than, for instance, at the time of the "Permian mass extinction" 252 million years ago, which wiped out 95 per cent of marine life on Earth, they said.

"We know that life during past ocean-acidification events was not wiped out – new species evolved to replace those that died off. But if industrial carbon emissions continue at the current pace, we may lose organisms we care about, such as coral reefs, oysters and salmon," said Bärbel Hönisch of Columbia University's Lamont-Doherty Earth Observatory in New York.

The study, published in the journal Science, investigated seabed sediments built up over hundreds of millions of years. These contain microscopic fossils and the physical signatures of marine organisms that lived during this period, which can be used to estimate average levels of pH, the chemical measure of acidity and alkalinity.

Shellfish, reef-building corals and microscopic organisms such as foraminifera, which are at the base of the marine food chain, all rely on high concentrations of carbonate ions in seawater, which are affected by the production of carbonic acid. As acidity increases, then these organisms will find it more difficult to make the shells and bony skeletons essential for survival, said Dr Daniela Schmidt of Bristol University, a co-author of the study.

"We looked at the past 300 million years of the Earth's history and analysed events that are associated with ocean acidification. None of these events is a perfect analogy to what is happening now because the current rate of change is so exceptional," Dr Schmidt said.

"But we estimate it is happening 10 times faster than in the past and this is important in terms of the biology because the speed puts pressure on the ecosystem. If it runs too fast, you are not giving the genetics time to cope."

The scientists found that there was only one previous mass extinction when the rate of acidification remotely matched that of today. But today's acidification is still occurring about 10 times faster than that of the Palaeocene-Eocene Thermal Maximum (PETM) 56 million years ago, they said.

Over the past 100 years, for instance, the rise in atmospheric carbon dioxide has led to a fall in the pH of the ocean of 0.1 units, which is equivalent to a 30 per cent rise in acidity. By 2100, scientists estimate that the pH of the oceans could fall by a further 0.3 units, bringing the average ocean pH down to 7.8.

This would mean that the increase in ocean acidification since the Industrial Revolution would by the end of this century match the acidity changes that occurred during the PETM, which are believed to have occurred over a relatively short geological period of 5,000 years.