Published online 15 July 2004 | Nature | doi:10.1038/news040712-14


Sea survey measures acid increase

Oceanic carbon sinks herald bad news for wildlife.

Coral will suffer as the oceans acidify.Coral will suffer as the oceans acidify.© Photodisc

By the end of the century, rising levels of carbon dioxide in the atmosphere could halve the rate at which marine creatures such as coral and plankton build their calcareous skeletons - with potentially catastrophic effects on the rest of the ocean's wildlife.

That is the conclusion scientists are drawing from a global survey of how much of the carbon dioxide produced by human activity is being absorbed by the oceans.

Now, after ten years of collecting over 72,000 samples of water from all over the oceans, and another five years of analysing the data, a team has finally come up with an answer: nearly half of the extra carbon released into the atmosphere since the dawn of the Industrial Revolution has ended up in the sea.

That is good news from one point of view, because it means that, in the long term, the oceans are helping to buffer the effects of global warming by absorbing a lot of carbon dioxide that would otherwise end up in the atmosphere. But it could one day be catastrophic for oceanic ecosystems, as increasingly acidic waters dissolve the calcium carbonate that many marine creatures use to make their skeletons and shells.

The advantages of this ocean buffer will take thousands of years to have an effect, says Chris Sabine, an oceanographer from the National Oceanic and Atmospheric Administration in Seattle, who is a lead author on the study published in Science1 this week. Whereas, he warns, "the ecosystem response will happen almost immediately and will get worse over time."

Acid test

In a related study, also published in Science2, the team used the global survey results to calculate exactly how severe the increase in acidity is likely to be. If carbon dioxide emissions continue to rise at the levels projected by the Intergovernmental Panel on Climate Change, surface waters will become acidic enough to cut the growth rates of organisms such as corals, plankton, calcareous plants and bivalves by between 20% and 50 %, the researchers predict.

Nobody knows how that will affect other marine species, but the consequences are likely to be significant, because so many animals rely on plankton, either directly or indirectly, for food. "These organisms make up the base of the food web," explains Sabine. "How the impact will ripple through the ecosystem is not clear."

Despite the gloomy outlook, finally having an exact figure for the amount of extra carbon in the sea will, at least, allow scientists to make more accurate predictions and to inform policies on limiting greenhouse-gas emissions.

"When I started working in this field, any time we dealt with carbon, everything was speculative," says Jorge Sarmiento, a biogeochemist from Princeton University, New Jersey, who has done much of the foundation research on global carbon cycling. "Being able to pin down the size of these sources and sinks is critical to any policy about reducing emissions in the future: you need to know by how much you should reduce them."