Aragonite saturation at different atmospheric CO2 concentrations. Pre-industrial concentration was ~ 280 ppm. Current concentration is >380 ppm. In the absence of strong policy, atmospheric CO2 will reach 550 ppm in several decades. Coral reefs are unlikely to survive in regions colored yellow, orange, or red. Source: Caldeira House testimony Feb 2009. Click to enlarge.

The impact on reefs is a consequence of both ocean acidification caused by the absorption of carbon dioxide into seawater and rising water temperatures. Previous studies have shown that rising carbon dioxide will slow coral growth, but this is the first study to show that coral reefs can be expected to start dissolving just about everywhere in just a few decades, unless carbon dioxide emissions are cut deeply and soon.

In testimony before the US House of Representatives Subcommittee on Insular Affairs, Oceans and Wildlife of the Committee on Natural Resources on 25 February, study co-author Ken Caldeira of the Carnegie Institution’s Department of Global Ecology said:

Globally, each second, we dump over 1000 tons of carbon dioxide into the atmosphere and, each second, about 300 tons of that carbon dioxide is going into the oceans. We can say with a high degree of certainty that all of this CO₂ will make the oceans more acidic—that is simple chemistry taught to freshman college students. It is less certain how coral reefs, and marine ecosystems generally, will respond, but there are several lines of evidence—all of them disturbing.

The study was designed determine the impact of this acidification on coral reefs. The research team, consisting of Jacob Silverman, Caldeira, and Long Cao of the Carnegie Institution as well as Boaz Lazar and Jonathan Erez from The Hebrew University of Jerusalem, used field data from coral reefs to determine the effects of temperature and water chemistry on coral calcification rates.

They input the field data to a model to calculate global seawater temperature and chemistry at different atmospheric levels of CO₂ ranging from the pre-industrial value of 280 ppm (parts per million) to 750 ppm. The current atmospheric concentration is more than 380 ppm, and is rapidly rising due to human-caused emissions, primarily through the burning of fossil fuels.

Based on the model results for more than 9,000 reef locations, the researchers determined that at the highest concentration studied, 750 ppm, acidification of seawater would reduce calcification rates of three quarters of the world’s reefs to less than 20% of pre-industrial rates. Field studies suggest that at such low rates, coral growth would not be able to keep up with dissolution and other natural as well as manmade destructive processes attacking reefs.

Prospects for reefs are even worse when the effects of coral bleaching are included in the model. Coral bleaching refers to the loss of symbiotic algae that are essential for healthy growth of coral colonies. Bleaching is already a widespread problem, and high temperatures are among the factors known to promote bleaching.

According to their model, the researchers calculated that under present conditions 30% of reefs have already undergone bleaching and that at CO₂ levels of 560 ppm (twice pre-industrial levels) the combined effects of acidification and bleaching will reduce the calcification rates of all the world’s reefs by 80% or more. This lowered calcification rate will render all reefs vulnerable to dissolution, without even considering other threats to reefs, such as pollution.

Resources

• Caldeira House Subcommittee testimony