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June 1999

Carbon dioxide's threat to coral reefs: A warning from models

Joanie Kleypas. (Photo by Carlye Calvin.)

Tropical coral reefs could be directly threatened by the buildup of atmospheric carbon dioxide (CO2) entering the oceans, and some reefs may already be declining, say six scientists in the April 2 issue of Science. Writes lead author Joanie Kleypas (CGD): "We believe that these findings represent some of the first evidence of a direct negative impact of increased CO2 on a marine ecosystem." Joanie completed the work during a visiting-scientist appointment under former CGD researcher Starley Thompson.

The international team's findings apply primarily to coral reefs located in surface waters between 35° north and 35° south latitudes. However, the authors predict that reefs in greatest danger are those where the production and destruction of calcium carbonate are closely balanced. These include some higher-latitude reefs, such as those off Bermuda; those in areas where colder, deeper waters rise to the surface, such as those off the Galapagos Islands; and many reefs already stressed by human activity.

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

For their study, the authors used scenarios in which the preindustrial level of CO2 doubles by the year 2065, considered a moderate projection by the Intergovernmental Panel on Climate Change. As the gas builds up in the atmosphere, the tropical sea surface takes it up at a proportional rate. Scientists have so far focused on CO2 storage in the ocean. This is one of the first studies to examine how CO2 increases may affect the chemistry and biology of ocean ecosystems.

As CO2 dissolves, it produces an acid that lowers the seawater pH. The interaction of carbon dioxide with calcium carbonate in seawater decreases the level of calcium carbonate saturation. Given the rapid rise in CO2 levels expected over the coming decades, the authors project that by the year 2065, the interaction of CO2 with seawater will have reduced calcium carbonate saturation in tropical surface waters by 30% relative to preindustrial levels.

This space-shuttle photo captures the Rangiroa Atoll in the Tuamotu archipelago, located at 15 degrees S, 147.5 degrees W. (Photo courtesy NASA.)

The findings are based on ocean carbon data and computer models, and on laboratory experiments showing that coral and algal calcification declines as the saturation state declines. The coral reefs themselves have not been studied in situ. "Our work is somewhat speculative," says Joanie. "We need more studies at the ecosystem level. If the laboratory results are borne out in the oceans, I think many coral-reef species could be vulnerable."

The buildup of CO2 may also warm ocean surface temperatures. Although warmer sea-surface temperatures are being blamed for the recent increase in coral bleachings worldwide, some feel that this warming could be a boon for reefs in chilly waters. However, says Joanie, if the calcium carbonate saturation rate is as important as water temperature in reef building, warmer waters won't save higher-latitude reefs.

Joanie's coauthors on the Science paper are Robert Buddemeier (Kansas Geological Survey), David Archer (University of Chicago), Jean-Pierre Gattuso (French Oceanological Observatory), Chris Langdon (Lamont-Doherty Earth Observatory), and Bradley Opdyke (Australian National University).

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