Human-caused emissions of carbon dioxide are starting to harm marine life with a one-two punch of rising temperatures and stronger ocean acidity. Now a study of a reef in the Red Sea confirms the impact of rising acidity and suggests that it could eventually make reefs across the globe dissolve. "This is a very significant and important result," says Ben McNeil of the University of New South Wales in Sydney. "We are finally moving towards a more complete picture of how coral reefs will respond to a high CO2 and warmer ocean."
As the atmosphere warms, so does the ocean, and the extra heat can affect the metabolism of corals. Extreme temperatures can lead to so-called bleaching of coral. In addition, according to laboratory studies, these reef-builders and other organisms that construct skeletons out of calcium carbonate are affected by the increasing acidity created when the sea absorbs carbon dioxide.
Now, a study that began with marine geochemist Jacob Silverman's doctoral research at the Hebrew University of Jerusalem, Israel, has made a quantitative link between atmospheric levels of CO2 and the fate of coral. Currently a postdoc at the Carnegie Institution Department of Global Ecology in Stanford, California, Silverman looked at how nutrients were affecting photosynthesis, respiration, and calcification on the kilometer-long Eilat reef in the Red Sea north of Egypt.
Silverman and his colleagues determined the relationship among the rate of calcification, water temperature, and the concentration of carbonate ions. Corals use these ions to build their skeletons, and the available amount falls with stronger acidity. They also showed how the calcification rate affected the amount of live coral on the reef. The resulting equations, or "rate law," is described in the current issue of Geophysical Research Letters. The group confirmed the equations with measurements from studies done at other reefs from the 1970s to 1990s.
The equations show how rising CO2 levels will harm reefs worldwide. Working with Long Cao and Ken Caldeira of Carnegie, Silverman predicted conditions at 9733 reef locations around the world for six levels of atmospheric CO2 concentrations. Then they plugged those data into the rate to determine what the effect on the corals would be. Compared with preindustrial levels, reefs today have slowed their rates of creating skeletons. When CO2 doubles to 560 parts per million (ppm), all coral reefs will have stopped growing and begin to dissolve. "The picture is pretty plain," Silverman says. "Acidification spells doom for coral reefs."
Silverman and others note that rising temperatures may pose a more immediate threat than does acidification. By the time the atmosphere reaches 450 ppm of CO2, ocean temperatures will likely have triggered widespread bleaching. In the meantime, rising acidity is only making life harder for coral. The remedy for both problems is the same, Caldeira notes: reduce emissions of CO2 dramatically and rapidly.