Ocean acidification already slowing coral reef growth

When we burn coal, oil, or gas, the resulting carbon dioxide is released into the atmosphere where it acts as a greenhouse gas. Greenhouse gases emitted by human activity don't just affect the atmosphere; they also have a negative impact on the world's oceans. This is partially due to overall warming caused by climate change. But also, over time, most of the carbon dioxide in the atmosphere is absorbed by the ocean, where it reacts with seawater to form an acid that is corrosive to coral reefs, shellfish, and other marine life. This process is known as 'ocean acidification'.

Coral reefs are particularly vulnerable to the ocean acidification process, because reef architecture is built by the accretion of calcium carbonate, called calcification, which becomes increasingly difficult as acid concentrations increase and the surrounding water's pH decreases. Scientists predict that reefs could switch from carbonate accretion to dissolution within the century due to this acidification process.

Previous studies have demonstrated large-scale declines in coral reefs over recent decades. Work from another team led by Caldeira found that rates of reef calcification were 40 percent lower in 2008 and 2009 than they were during the same season in 1975 and 1976. But it has been hard to pinpoint exactly how much of the decline is due to acidification and how much is caused by warming, pollution, and over-fishing.

The team manipulated the alkalinity of seawater flowing over a reef flat off Australia's One Tree Island in the southern Great Barrier Reef. They brought the reef's pH closer to what it would have been in the pre-industrial period based on estimates of atmospheric carbon dioxide from the era. They then measured the reef's calcification in response to this pH increase. They found that calcification rates under these manipulated pre-industrial conditions were higher than they are today.

"Our work provides the first strong evidence from experiments on a natural ecosystem that ocean acidification is already slowing coral reef growth," Albright said. "Ocean acidification is already taking its toll on coral reef communities. This is no longer a fear for the future; it is the reality of today."

Increasing the alkalinity of ocean water around coral reefs has been proposed as a geoengineering measure to save shallow marine ecosystems. These results show that this idea could be effective. However, the practicality of implementing such measures would be almost impossible at all but the smallest scales.

"The only real, lasting way to protect coral reefs is to make deep cuts in our carbon dioxide emissions," Caldeira said. "If we don't take action on this issue very rapidly, coral reefs--and everything that depends on them, including both wildlife and local communities--will not survive into the next century."

Albright will be presenting this research Monday Feb. 22 at the 2016 Ocean Sciences Meeting co-sponsored by the Association for the Sciences of Limnology and Oceanography, The Oceanography Society and the American Geophysical Union.

Other members of the team include: Carnegie's Lilian Caldeira, Lester Kwiatkowski, Jana Maclaren (also of Stanford University), Yana Nebuchina, Julia Pongratz (now at Max Planck Institute for Meteorology), Katharine Ricke, Kenny Schneider (now at The Hebrew University of Jerusalem), Marine Sesboue, and Kai Zhu (now at RiceUniversity); as well as Jessica Hosfelt and Aaron Ninokawa of University of California Davis, Benjamin Mason of Stanford University, Tanya Rivlin of The Hebrew University of Jerusalem, Kathryn Shamberger of Woods Hole Oceanographic Institution and Texas A&M University, and Kennedy Wolfe of The University Sydney.

This work was supported by the Carnegie Institution for Science and the Fund for Innovative Climate and Energy Research.