Nearly every coral reef could be dying by 2100 if current carbon dioxide emission trends continue, according to a new review of major climate models from around the world.
The only way to maintain the current chemical environment in which reefs now live, the study suggests, would be to deeply cut emissions as soon as possible. It may even become necessary to actively remove carbon dioxide from the atmosphere, such as with massive tree-planting efforts or machines.
The world's open-ocean reefs are already under attack by the combined stresses of acidifying and warming water, overfishing and coastal pollution. Carbon emissions have already lowered the pH of the ocean a full 0.1 unit, which has harmed reefs and hindered bivalves' ability to grow. The historical record of previous mass extinctions suggests that acidified seas were accompanied by widespread die-offs, but not total extinction.
To study how the world's slowly souring seas would affect reefs in the future, scientists with the Carnegie Institution for Science in Palo Alto, Calif., analyzed the results of computer simulations performed by 13 teams around the world. The models include simulations of how ocean chemistry would interact with an atmosphere in the future that has higher carbon dioxide levels. This so-called "active biogeochemistry" is a new feature mostly absent in the previous generation of global climate models.
Using the models' predictions for future physical traits, such as pH and temperature in different sections of the ocean, the scientists were able to calculate a key chemical measurement that affects coral. Corals make their shells out of the dissolved carbonate mineral known as aragonite. But as carbon dioxide pollution steadily acidifies the ocean, chemical reactions change the extent to which the carbonate is available in the water for coral. That availability is known as its saturation, and is generally thought to be a number between 3 and 3.5.
No precise rule of thumb exists to link that figure and the health of reefs. But the Carnegie scientists say paleoclimate data suggests that the saturation level during preindustrial times -- before carbon pollution began to accumulate in the sky and seas -- was greater than 3.5.
The models that the Carnegie scientists analyzed were prepared for the major global climate report coming out next year: the Intergovernmental Panel on Climate Change report. The team compared the results of those simulations with the locations of 6,000 reefs for which there is data, two-thirds of the world total. That allowed them to do what amounted to a chemical analysis of future reef habitats.
In a talk reviewing the study at the American Geophysical Union's fall meeting earlier this month, senior author and Carnegie geochemist Ken Caldeira showed how the amount of carbon emitted in the coming decades could have huge impacts on reefs' fates. In a low-emissions trajectory in which carbon pollution rates were slashed and carbon actively removed from the air by trees or machines, between 77 percent and 87 percent of reefs they analyzed stay in the safe zone, with aragonite saturation above 3.
"If we are on the [business as usual] emissions trajectory, then the reefs are toast," Mr. Caldeira said. In that case, all the reefs in the study were surrounded by water with aragonite saturation below 3, dooming them. In that scenario, he said, "details about sensitivity of corals are just arguments about when they will die."
Adapted from ScienceNOW, the online daily news service of the journal Science.