First Direct Evidence of Ocean Acidification

A vast region of the northeastern Pacific Ocean shows signs of increased acidity, brought on by CO2.

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TAMPA, Fla.—Seawater in a vast and deep section of the northeastern Pacific Ocean shows signs of increased acidity brought on by manmade carbon dioxide in the atmosphere--a phenomenon that carries with it far-reaching ecological effects--reports a team of researchers led by a University of South Florida College of Marine Science chemist.

The scientists, whose results are published in the American Geophysical Union's journal Geophysical Research Letters, analyzed Pacific seawater between Oahu, Hawaii, and Kodiak, Alaska by comparing pH readings from 1991 and from 2006. This study provides the first direct measurements of basin-wide pH changes in the ocean's depths and at its surface and has produced the first direct evidence of acidification across an entire ocean basin, the investigators said.

Principal investigator Robert Byrne, a USF seawater physical chemistry professor, said the study leaves no doubt that growing CO2 levels in the atmosphere are exerting major impacts on the world's oceans.

"If this happens in a piece of ocean as big as a whole ocean basin, then this is a global phenomenon," Byrne said. 

Adding carbon dioxide to seawater makes it more acidic, and each year the world's oceans absorb about one-third of the atmospheric CO2produced by human activities.

Using pH-sensitive dyes that turn from purple to yellow in more acidic waters, the scientists were able to track changes produced by 15 years of CO2 uptake near the ocean's surface, Byrne said. In deeper waters, down to about half a mile, both anthropogenic and naturally occurring changes in CO2 and pH were seen. In the very deepest waters, no significant pH changes were seen. 

The results verify earlier model projections that the oceans are becoming more acidic because of the uptake of carbon dioxide released as a result of fossil fuel burning, said Richard Feely, a member of the research team and chief scientist of the cruise and NOAA researcher from the Pacific Marine Environmental Laboratory in Seattle.

Byrne and colleagues at USF's College of Marine Science developed the methods for precise pH measurements and the project was the first time a team of researchers employed those methods in the field.

Byrne led a team of scientists that made pH measurements aboard the NOAA-National Science Foundation-sponsored cruise R/V Thomas G. Thompson in the spring of 2006 using state-of-the-art techniques developed at USF's College of Marine Science. The researchers found that upper-ocean pH had, over the preceding one-and-a-half decades, decreased by approximately 0.026 units, equivalent to an average annual pH change of -0.0017, over a large section of the northeastern Pacific. Similar recent pH trends have been found at isolated time-series stations in the North Atlantic and Pacific Oceans, and corroborating observations have also been reported by scientists who study other CO2-related substances in seawater.

"The pH decrease is direct evidence for ocean acidification of a large portion of the North Pacific Ocean," said Richard Feely. "These dramatic changes can be attributed, in most part, to anthropogenic CO2uptake by the ocean over a 15-year period."

The implications for sea life and the world's food web are serious, Byrne said. When seawater becomes more acidic, lower concentrations of carbonate result. Because the protective shells of sea organisms are made of calcium and carbonate, more acidic waters make it more difficult for many organisms to make their shells and thrive.

That affects not only the food web, but also many important processes essential for healthy marine ecosystems, such as coral reef formation, Byrne said.

The cruise was part of a decade-long series of repeat hydrographic sections jointly funded by NOAA-Office of Global Programs (now the Climate Program Office) and NSF-Division of Ocean Sciences as part of the Climate Variability and Predictability/CO2 Repeat Hydrography Program.

The program focuses on the need to monitor inventories of CO2 and heat in the ocean. Earlier programs under the World Ocean Circulation Experiment and U.S. Joint Ocean Global Flux Study have provided baseline observational fields.