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.
Carbon dioxide is an important greenhouse gas produced by fossil-fuel use.
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. The findings are based on ocean carbon data and computer models, and on laboratory experiments which show that coral and algal calcification declines as the saturation state declines.