Acidic oceans may mean mass extinctions 'almost inevitable'

The oceans are more acidic now than they have been for at least 300 million years, due to carbon dioxide emissions from burning fossil fuels, and a mass extinction of key species may be almost inevitable as a result, leading marine scientists warned yesterday.

An international audit of ocean health found that overfishing and pollution are also contributing to a deadly combination of destructive forces imperilling marine life on which billions of people depend for their nutrition and livelihood.

In the starkest warning yet of the threat to ocean health, the International Programme on the State of the Ocean (IPSO) said: "This [acidification] is unprecedented in the earth's known history. We are entering an unknown territory of marine ecosystem change and exposing organisms to intolerable evolutionary pressure. The next mass extinction may have already begun."

It published its findings yesterday in the State of the Oceans report, collated every two years from global monitoring and other research studies. The report was peer reviewed in the Marine Pollution Bulletin journal.

"The health of the ocean is spiralling downwards far more rapidly than we had thought," Oxford University biology professor Alex Rogers said. "We are seeing greater change, happening faster, and the effects are more imminent than previously anticipated."

Coral is particularly at risk. Increased acidity dissolves the calcium carbonate skeletons that form reefs, and rising temperatures lead to bleaching where the corals lose symbiotic algae they rely on. The report says that world governments' current pledges to curb carbon emissions would not go far enough or fast enough to save many reefs.

Carbon dioxide in the atmosphere is absorbed by the seas - at least a third of the carbon that humans have released has been dissolved in this way - and makes them more acidic. IPSO found the situation was even more dire than that laid out by top climate scientists in their landmark report last week.

In absorbing carbon and heat from the atmosphere, the world's oceans had shielded humans from the worst effects of global warming, the marine scientists said. This had slowed the rate of climate change on land, but its profound effects on marine life were only now being understood.

Trevor Manuel, a South African government minister and the Global Ocean Commission's co-chair, called the report "a deafening alarm bell on humanity's wider impacts on the global oceans". "Governments must respond as urgently as they do to national security threats," he said.