Unprecedented, Man-made Trends in Ocean’s Acidity

As part of the natural carbon cycle, atmospheric CO2 reacts with the ocean’s surface waters to become carbonate, which can be converted by marine organisms into calcium carbonate. Many marine organisms—including corals, mussels, and algae—rely on calcium carbonate to build their shells or skeletons, making the molecule an important part of marine processes.

When it reacts with seawater, CO2 lowers the ocean’s pH and raises its acidity. Recently, a team of researchers, including Dr. Nick Bates of BIOS and led by Drs. Tobias Friedrich and Axel Timmermann of the International Pacific Research Center, University of Hawaii at Manoa, concluded that anthropogenic (human-caused) CO2 has increased ocean acidity beyond the levels attributable to natural variation. The study was published online recently in the prominent science journal Nature.

Changes in ocean acidification are typically measured by studying levels of aragonite, a naturally occurring form of calcium carbonate. Ocean acidification and aragonite have a clear and easily quantified relationship: as the acidity of the ocean increases, the saturation level of aragonite decreases. Three global time series sites, including the Bermuda Atlantic Time-series Study (BATS) deep-ocean site, were integral in gathering the data utilized in this study. By combining field observations and data with computer models, researchers were able to simulate climate and ocean conditions as far back as the Last Glacial Minimum, over 21,000 years ago, and as far forward as the end of the 21st Century.

Researchers concluded that, given seasonal and annual variability, the aragonite saturation levels in key coral reef regions around the world were five times lower than their pre-industrial levels. This translates into a potential reduction in calcification rates for marine organisms, resulting in stress to individual organisms and their ecosystems.

“In some regions, the man-made rate of change in ocean acidity since the Industrial Revolution is [a] hundred times greater than the natural rate of change between the Last Glacial Minimum and pre-industrial times,” says lead author and Postdoctoral Fellow Tobias Freidrich. “When Earth started to warm 17,000 years ago…atmospheric CO2 levels rose from 190 parts per million (PPM) to 280 ppm over 6,000 years. Now, for a similar rise in CO2…to the present level of 392 ppm, the [time for marine organisms to adjust] is reduced to only 100-200 years.”

Due to a natural variability in oceanic acidity, some regions (such as the eastern tropical Pacific) will be less vulnerable to anthropogenic changes in ocean acidification than others (such as the Caribbean). However, co-author Axel Timmerman points out that, “our results suggest that severe reductions are likely to occur in coral reef diversity, structural complexity and resilience by the middle of this century,” bringing home the point that human-induced changes to ocean acidification can have real and measurable impacts in the near future.

“BIOS seeks to understand the impact this increased stressor has on marine organisms, because the health of our oceans depends on it. The BATS site is the longest continuous record of ocean acidity available and continues to provide essential data that is improving our understanding of changes in the ocean,” said Dr. Bates.

The study was funded by The Nature Conservancy, the Japan Agency for Marine-Earth Science and Technology and the National Science Foundation.