SHELLFISH, crabs, lobsters and a host of other familiar species could become extinct around Britain and Europe because our seas are becoming steadily more acidic.

An official report is to warn that carbon dioxide generated by human activity, already linked to climate change, is also sharply altering the chemistry of the oceans. The gas forms carbonic acid when it dissolves into sea water.

Some species, such as corals and certain plankton, are so sensitive to the rising acidity that they could be in rapid decline within decades. Others, such as crabs, mussels and lobsters, are more resistant, but they too will be in danger by the end of the century. All the affected organisms build their shells or skeletons from calcium carbonate, a mineral they extract from sea water but which is attacked by carbonic acid.

The report is being published tomorrow by Ospar, the inter-governmental organisation set up by northern European countries, including Britain, to monitor the state of the North Sea and North Atlantic.

One of its authors, Carol Turley, of Plymouth Marine laboratory, said: “This issue is emerging as one of the most serious environmental threats humanity has faced. The oceans are acidifying very rapidly and many marine organisms are at risk.”

Turley and her colleagues have carried out experiments measuring how marine organisms cope when sea water becomes more acidic. They tried growing a range of plankton and animal species in water that had been slightly acidified with extra carbon dioxide.

Although the findings are not yet formally published, she said: “We had some very alarming results. Just a small change in acidity saw some of these creatures unable to grow or reproduce properly.”

The acidification of the oceans is directly linked to the 23 billion tonnes of carbon dioxide emitted annually by human activities such as power generation, car use and air travel. About half of these emissions are soaked up by the oceans — a reaction that has so far benefited the planet by reducing the amount of carbon dioxide in the atmosphere and so slowing climate change.

About 10 years ago, however, scientists noticed hints that the oceans could no longer cope. Measurements of pH — the basic measurement of acidity — showed it was falling sharply, meaning the oceans were becoming more acidic.

John Raven, professor of biology at Dundee University, chaired a Royal Society inquiry into ocean acidification. It concluded that a marine catastrophe could be looming.

“The pH of the oceans has fallen by 0.1 units so far and could fall by 0.5 units by 2100,” said Raven. “This pH would be lower than for hundreds of millions of years and the rate of change is so fast that marine life may be unable to adapt.”

A change of 0.1 units sounds small but represents a huge shift in ocean chemistry. Crucially, it represents a 30% decrease in the amount of dissolved carbonate — which marine creatures must extract from water to build their shells.

The further carbonate levels fall, the more sea creatures struggle first to build their shells and then to stop them dissolving back into the sea.

Turley’s work was aimed at pinpointing how this effect would impact marine life around Britain and Europe. The work was part-funded by the Department for Environment Food and Rural Affairs, and the Department for Trade and Industry.

One of the biggest questions is how rising acidity will affect plankton species because these are an essential component of the marine food chain.

If plankton levels fell, or there were a sharp change in the types of species, then the fish which feed on them and marine mammals would also be affected.

Acidification can affect other processes besides shell formation — such as altering the amount of oxygen dissolved in water and changing the pH levels in the cells of sea creatures. The Ospar report will warn that fast-swimming creatures such as squid and some fish could find it impossible to extract enough oxygen to survive.

The scale of the disaster that could be caused by ocean acidification is only just becoming clear. Scientists working on tropical coral reefs have predicted that they will start dissolving by 2050 and eventually face extinction.

More research confirming such findings is to be published at conferences shortly, including a paper by Toby Tyrrell of the National Oceanography centre at Southampton University.

He warned that crabs, lobsters, mussels and many other familiar species faced extinction unless humanity reduced its carbon emissions. He said: “There is a high probability that within 300 years there will be no life based on calcium carbonate left in the oceans.”

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