Acidification of the oceans, seen until recently as a side-effect of man-made climate change, is an even worse threat to marine life than warming waters, scientists believe.

The oceans have absorbed one-third of the 500bn tonnes of carbon dioxide that humanity has added to the atmosphere since the industrial revolution by burning fossil fuels. That has already increased the acidity of the sea surface on average by about a third, Gretchen Hofmann, professor of marine biology at the University of California, Santa Barbara, told the American Association for the Advancement of Science annual meeting in Vancouver.

Although scientists have been aware for several years of the threat posed by ocean acidification, they are only now becoming aware of how serious it is. Studies of marine life – from the tropics to the poles – and laboratory experiments are showing the impact of rising carbon dioxide levels across whole ecosystems.

“With ocean warming, many species can migrate to cooler waters, but acidification affects the whole ocean so there is no place to hide,” said Christopher Harley, a marine biologist at the University of British Columbia. “I am much more worried about acidification than I am about warming.”

But Jason Hall-Spencer of Plymouth University in the UK said it was hard to separate the two effects: “The combination of warming and rising carbon dioxide levels in the water is deadly, toxic mixture.”

While all oceans are absorbing carbon dioxide, the impact will vary from place to place, Professor Hofmann said: “The worst effects will be local and intense.”

For example some oyster hatcheries in the US Pacific North-west recently failed because the water had become too acidic for the spat (oyster larvae) to form viable shells, she said. “If we lose the US west coast oysters, we lose an industry worth many millions of dollars a year.”

Marine creatures that make body structures from calcium carbonate are most vulnerable to acidification because this dissolves in more acidic water. These range from oysters and corals to microscopic animals such as terapods that are a vital part of the ocean food chain.

Although most publicity about ocean acidification has focused on the threat to coral reefs, cold-water organisms – particularly the plankton that form the base of the food chain in the polar oceans – may be more at risk. “Cold water can absorb more carbon dioxide than warmer water,” said Professor Hofmann.

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