Oceans absorb at least a quarter of the carbon dioxide we put into the atmosphere and, as a result, they've become steadily more acidic over time. It's estimated that, since the Industrial Revolution, the pH of the world's waters has dropped by 30%.
That's bad news for corals, oysters, and other shellfish. They use a process called carbonate calcification to make their shells and reefs. More acidic water is corrosive and lab experiments have shown how calcification—the accretion of a chalk-like substance—is impaired.
Scientists think acidification is already leading to thinner shells among shellfish catches in the North West. And now, in a first-of-its-kind experiment at the Great Barrier Reef, in Australia, they've shown how it affects coral, too.
To do that, researchers chose a particular set of lagoons that are segregated from the rest of the ocean at low tide. Then they introduced an alkaline solution (sodium hydroxide) to bring the water's pH back to pre-industrial levels, and tested calcification levels. Over a 22-day period, the reef saw a 7% increase in calcification compared to a control.
Led by Rebecca Albright and Ken Caldeira from Stanford University, the experiment is said to be the first to show the effects of acidification on coral in a natural setting. And it also points to the possibility of geo-engineering to improve the life-chances of coral reefs—though probably only on a small-scale.
Without net reductions in acidification—either through artificial means or, ideally, by putting less CO2 into atmosphere—coral reefs are set to deplete, studies show. And that's before you consider all the other threats to coral as well.