Increasing Carbon Dioxide Threatens Tropical Coral Reefs

The team's findings apply primarily to coral reefs located in surfacewaters between 35 degrees north and 35 degrees south of the equator.However, the authors predict that reefs in greatest danger are thosewhere the production and destruction of calcium carbonate are closelybalanced. These include some higher-latitude reefs, such as those offBermuda; those in areas where colder, deeper waters rise to the surface,such as those off the Galapagos Islands; and many reefs already stressedby human activity.

A coral reef is the accumulation of calcium carbonate produced by thecorals and other calcium-secreting organisms, such as coralline algae.If calcium production declines, coral and algal skeletons will weakenand reef building may slow or stop. The reef then becomes morevulnerable to erosion. Ongoing calcium production depends on thesaturation state of calcium carbonate in surrounding surface waters.This saturation state declines as CO2 enters tropical surface waters.

Carbon dioxide is an important greenhouse gas produced by fossil-fueluse. For their study, the authors used future scenarios in which thepreindustrial level of CO2 doubles by the year 2065--considered amoderate projection by the Intergovernmental Panel on Climate Change, aninternational group of 2,500 scientists. As the gas builds up in theatmosphere, the tropical sea surface takes it up at a proportional rate.Scientists have so far focused on CO2 storage in the ocean. This is oneof the first studies to examine how CO2 increases may affect thechemistry and biology of ocean ecosystems.

As CO2 dissolves, it produces an acid that lowers the seawater pH. Theinteraction of carbon dioxide with calcium carbonate in seawaterdecreases the level of calcium carbonate saturation. Given the rapidrise in CO2 levels expected over the coming decades, the authors projectthat by the year 2065, the interaction of CO2 with seawater will havereduced calcium carbonate saturation in tropical surface waters by 30%relative to preindustrial levels.

The findings are based on ocean carbon data and computer models, and onlaboratory experiments which show that coral and algal calcificationdeclines as the saturation state declines. The coral reefs themselveshave not been studied in situ. "Our work is somewhat speculative," saysKleypas. "We need more studies at the ecosystem level. If the laboratoryresults bear out in the oceans, I think many species of coral reefscould be vulnerable."

The buildup of CO2 may also warm ocean surface temperatures. Althoughwarmer sea-surface temperatures are being blamed for the recent increasein coral bleachings worldwide, some feel that this warming could be aboon for reefs in chilly waters. However, says Kleypas, if the calciumcarbonate saturation rate is as important as water temperature in reefbuilding, warmer waters won't save higher-latitude reefs.

NCAR is managed by the University Corporation for Atmospheric Research,a consortium of more than 60 universities offering Ph.D.s in theatmospheric and related sciences.

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