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What are 'acid oceans' and why is acidification important?

Research into ocean acidification is in its infancy, but scientists at NERC's collaborative centre, Plymouth Marine Laboratory, have already found that climate change is making the oceans more acidic. Higher concentrations of carbon dioxide in the atmosphere mean more dissolves into seawater, forming a very dilute acid. The pH of surface water has reduced by 0·1 units since the start of the industrial revolution. This could affect the ocean's ability to remove carbon dioxide, an important greenhouse gas, from the atmosphere. The oceans have probably not been this acidic for hundreds of thousands of years.

The oceans take up about 50% of the carbon dioxide society has been producing over the last 200 years. Tiny floating ocean plants (phytoplankton) absorb carbon dioxide when they photosynthesise. When these plants, or their minute predators (zooplankton), die, some fall to the seafloor. This effectively locks carbon up in deep sea sediments, helping mitigate climate change.

But many of these organisms have calcium carbonate-based structural plates or shells, and these might be damaged by more acid water. This could mean the oceans absorb less carbon dioxide in future.

Changes in climate will also affect the world's carbon balance directly. Warmer oceans will absorb less carbon dioxide simply because the gas becomes less soluble in warmer water. Warmer soils may emit more carbon dioxide as plant material breaks down more rapidly.

We don't know what the overall effects will be.