Ocean Acidification

Ocean acidification. A national strategy to meet challenges of a changing ocean, National Research Council of the National Academies

Washington, DC, The National Academies Press, 2010. 188 pp., ISBN: 978-0-309-15359-1. £22.00

This book is the result of committee work under the auspices of the national US academies; it addresses the problem of ocean acidification. It includes recommendations for the organization and strategy of future research including collaboration and coordination with international research programmes – a part of the book with a terribly large number of acronyms. It also reviews current knowledge and identifies uncertainties and key questions regarding the progression and impacts of ocean acidification and the possibilities of mitigating adverse effects.

Acidification should not be interpreted to mean that oceanic water is becoming acid, that is, with a pH < 7; oceanic water will remain slightly alkaline. However, since the beginning of the industrial revolution the pH of oceanic water has decreased by about 0.1 pH unit from a level of about 8.2. This is generally interpreted as a result of increasing atmospheric pCO₂ that again is caused by the burning of fossil fuels and deforestation. About one-third of the anthropogenic CO₂ has been absorbed by the sea and this has caused a shift in the equilibrium CO₂+H₂O↔HCO₃ ^–+H⁺ and HCO₃ ^–↔CO₃ ^2–+H⁺. Over longer periods – in geological terms – the problem of control of seawater pH is more complicated; it is also a function of carbonate deposition in the form of calcium and magnesium carbonates and on silicate minerals, and this again depends on erosion rates on land and transport to the sea via rivers. Projections indicate that with ‘business as usual’ – that is, unchanged rates of anthropogenic CO₂ emission – oceanic water will drop by a further 0.1 pH unit by the end of this century.

There have been a large number of studies on the physiological effects of exposure to lowered pH or to increase in pCO₂. There may be a variety of physiological effects. One of the most obvious effects of decreasing pH is likely to be inhibition of calcite or aragonite formation: these minerals form skeletal elements in a wide variety of protists, algae and invertebrates. The results of such experiments have been variable and there seems to be a considerable variation in the sensitivity to lowered pH. However, there has been no documented effect of a lowering of 0.1 pH unit relative to the pre-industrial level. But, lowering pH by a further 0.1 pH unit – as predicted to happen by the end of the century – has in some cases shown adverse effects. Effects on whole communities or ecosystems are very difficult to study experimentally and to interpret the results. Some reported phenomena such as failed oyster cultivation on the American west coast and degrading coral reefs have been attributed to seawater acidification, but may well have other causes. The biota of coastal and estuarine waters that show a larger variability in pH than do oceanic waters could be worth a closer study in this respect.

The contents of calcium borate in biogenic calcified structures provide information on the pH of the water in which these structures were formed. Also, regarding the Pleistocene period, enclosed air bubbles in the Arctic and Antarctic ice sheets provide information on the past atmospheric composition of gases. During the Pleistocene, atmospheric CO₂ (and CH₄) varied with the glacial cycles with a period of about 100 000 years. During interglacial periods atmospheric pCO₂ was about similar to the present level and during glacial periods it was lower and oceanic pH followed the same patterns: higher values during glacial periods and lower values during interglacial periods. Far longer back in Earth's history, during periods with a ‘green house climate’ such as the Palaeocene–Eocene thermal maximum about 55 million years ago, atmospheric pCO₂ was much higher than the current value and seawater pH correspondingly lower. The results of palaeontologists’ studies on the effects on the biota seem equivocal. The book claims that it was another situation than that of the current ocean acidification because changes in the geological past took place over a longer period so that the organisms could adapt to the changing pH. However, warming during the Palaeocene–Eocene period apparently also took place quite rapidly in geological terms – only over some thousand years – and whether fundamental physiological changes could follow through Darwinian evolution is unknown.

Altogether, this will be a useful book for those who work on oceanic acidification or for anyone who just wishes to be oriented in the current state of ocean acidification studies. The book includes a large and comprehensive reference list covering the subject.

Tom Fenchel

Emeritus Professor

Marine Biological Laboratory, University of Copenhagen,

Strandpromenaden 5, DK-3000 Helsingør, Denmark

E-mail: [email protected]

© 2011 Tom Fenchel