Anyone who has ever dissolved a piece of chalk in vinegar knows that ocean acidification—the result of seawater absorbing carbon dioxide (CO2) pollution from the air—has to be bad for calcareous creatures like coral. But just how big a role does ocean acidification play in the sickly state of many reefs? A new study borrows from a common after-dinner practice: popping a Tums to neutralize the acid and thereby isolate its effect on reef health.
Near shores across the planet, the health of reefs is in decline. Global studies have shown that major reef systems are calcifying—building their stony skeletons—more slowly, with one study suggesting growth rates of the Great Barrier Reef off Australia have plummeted 40% in just 3 decades.
But lots of factors contribute to this harm: diseases, warming water, pollution, and sediment runoff, along with the 0.1 unit drop in the pH of the global ocean since the preindustrial era.
To tease out the effect of acidification, scientists led by Rebecca Albright of the Carnegie Institution for Science in Stanford, California, set out to restore ocean chemistry to its preindustrial state at the One Tree Reef in the southern Great Barrier system. They released an antacidlike solution of sodium hydroxide into a lagoon in the reef, reversing acidification. After this alkaline treatment, they monitored the chemistry of the water flowing out of the lagoon. By comparing its levels of alkalinity to how much sodium hydroxide they had added, they could get an accurate measurement of how the reef had calcified during the experiment, since that process itself affects the pH of the water. To account for loss of the solution because of mixing, the team also released a dye into the lagoon, and measured its outflow.
Their calculations, published today in Nature, suggest that human alteration of ocean pH since the preindustrial era has reduced reefs' ability to calcify by 12%. The study “provides new field-based evidence that ocean acidification-induced changes in coral reef growth are already underway,” Albright's team said in a press release.
Biological oceanographer Chris Langdon of the University of Miami in Coral Gables, Florida, who wasn't part of the research team, says the “marvelous” study “helps us establish what the effect of ocean acidification is, which has been very hard to isolate.”
Ocean acidification harms not only reefs but also shelled animals like clams and ctenophores, known as sea butterflies. To help reverse ocean acidification on a global scale, some researchers have proposed a massive effort to scale up the One Tree Reef experiment: Dump huge quantities of basic solution into the global ocean. But Carnegie geochemist Ken Caldeira, who oversaw the new study, says that approach is infeasible. “Just to keep the oceans where they are and compensate for ongoing CO2 uptake,” he says, “you would need to add over 20 billion tons of limestone to the ocean each year. Not impossible, but a huge job. To undo the acidification that has already occurred you would need to add over a trillion tons of dissolved limestone. Not impossible, but highly unlikely.”
The best way to protect reefs, he says, is to “stop treating the atmosphere like a sewer” by dumping CO2 into it, where the pollutant alters the climate and acidifies the oceans.