Researchers have found that carbon dioxide, the gas already blamed for causing global warming, is also raising the acid levels in the sea. The shells of coral and other marine life dissolve in acid. The process is happening so fast that many such species, including coral, crabs, oysters and mussels, may become unable to build and repair their shells and will die out, say the researchers.

“Increased carbon dioxide emissions are making the world’s oceans more acidic and could cause a mass extinction of marine life similar to the one that occurred on land when the dinosaurs disappeared,” said Professor Ken Caldeira of the Carnegie Institution’s global ecology department.

When CO2 produced by burning fossil fuels dissolves in the ocean, it forms carbonic acid. A little of this can benefit marine life by providing carbonate ions — a vital constituent in the biochemical process by which sea creatures such as corals and molluscs build their shells.

Caldeira found, however, that the huge volumes of carbon dioxide being released by humans are dissolving into the oceans so fast that sea creatures can no longer absorb it. Consequently, the levels of carbonic acid are rising and the oceans are “turning sour”.

Speaking at the American Geophysical Union’s ocean sciences conference in Hawaii last week, Caldeira said: “The current rate of carbon dioxide input is nearly 50 times higher than normal. In less than 100 years, the pH (measure of alkalinity) of the oceans could drop by as much as half a unit from its natural 8.2 to about 7.7.”

This would mark a huge change in ocean chemistry. The shells of marine creatures are made of calcium carbonate, the same substance as chalk, which is vulnerable to acidity. Even a slight increase in acidity would mean many creatures would dissolve. Others might be able to rebuild their shells but would be unable to reproduce.

Nature, the scientific journal, recently published a study by Jim Orr, of the Laboratory for Science of the Climate and Environment, Paris. It said that by 2050 the Southern Ocean and subarctic regions of the Pacific might be so acidic that the shells of smaller marine creatures would start eroding.

Such a loss would have disastrous consequences for larger marine animals such as salmon, mackerel, herring, cod and baleen whales. These all feed on pteropods, or sea butterflies, one of the species most threatened by rising acidity.

Last week another warning was issued about the threat of acidity to sea life at the annual meeting in St Louis of the American Association for the Advancement of Science.

Katherine Richardson, professor of biological oceanography at Aarhus University in Denmark, said: “These marine creatures do humanity a great service by absorbing half the carbon dioxide we create. If we wipe them out, that process will stop. We are altering the entire chemistry of the oceans without any idea of the consequences.”