Airy Threat to Reefs

Rising carbon dioxide levels in the atmosphere will slow the growth of coral reefs and possibly weaken their limestone skeletons during the next century, scientists predict. The finding, reported in today's Science, lengthens the list of threats faced by the world's besieged reefs. Experts note that pollution and commerce remain the immediate danger for many reefs.

Covering less than 1% of the sea floor, coral reefs support a vast diversity of life. Their rigid limestone is laid down by colonies of microscopic organisms that absorb the calcium carbonate from the water column. In the 1970s, scientists predicted that CO2 from burning fossil fuels, by dissolving into the oceans, would reduce calcium carbonate there. But they didn't expect calcium carbonate levels to drop low enough to endanger coral any time soon.

Taking a fresh look at the situation, Joan Kleypas of the National Center for Atmospheric Research and co-workers set out to estimate past and future changes in calcium carbonate based on CO2 concentrations. When they coupled CO2 data with new estimates of how much dissolved calcium carbonate is needed for coral growth, the researchers concluded that tropical coral reef growth must have fallen more precipitously than expected in the past century, declining 11% since 1880. They estimate it could drop another 17% by 2100.

The findings bode ill for coral everywhere, even reefs far removed from the threat of sewage, dredging, and overfishing, says ecologist James Porter of the University of Georgia, Athens. And the study could have even wider implications, he says, because all shelled sea creatures, from mussels to lobsters, make their outer skeletons from calcium carbonate and so might be weakened.