Nature Reports Climate Change

Research Highlights

Published online: 13 August 2009 | doi:10.1038/climate.2009.77

Proc. Natl Acad. Sci. USA 106, 12235–12240 (2009)

The acid test by Alicia Newton

It's well recognized that absorption of atmospheric CO₂ is making the ocean more acidic, but there are few long-term data sets that document this trend.

Now research by John Dore of Montana State University and colleagues shows a significant increase in the acidity of surface waters over nearly 20 years at Station ALOHA in the central North Pacific Ocean. The group found that since 1988 surface pH at ALOHA has been falling by 0.0019 0.0002 each year, a value in line with that expected from increasing atmospheric CO₂ entering the ocean. However, they also found that seasonal peaks in biological productivity and temperature — as well as year-to-year fluctuations in ocean mixing — affect the surface pH. Below the mixed layer — that portion of the ocean influenced strongly by the atmosphere — pH has decreased more rapidly, most markedly at a depth of 250 metres. This may be due to the invasion of acidic waters from the north or to the transport of CO₂ from surface waters as organisms die, sink to these depths and degrade.

The team concludes that sparse or short-term records could miss the long-term trends in ocean acidification caused by rising atmospheric CO₂ concentrations.