N. E. Cantin et al. (“Ocean warming slows coral growth in the central Red Sea,” Reports, 16 July 2010, p. 322) claim to have shown that “steadily rising SSTs [sea surface temperatures], not ocean acidification,” have decreased calcification rates in the reef-building coral, Diploastrea heliopora, in the central Red Sea. This may be true, but Cantin et al. have not provided adequate evidence to support the claim that ocean acidification has played no role in the observed decrease in coral calcification. There is ample evidence that many coral species are very sensitive to changes in ocean chemistry (1), and Cantin et al. have provided no evidence that Diploastrea heliopora is not similarly sensitive. Were it not for anthropogenic CO2 in the ocean, aragonite saturation levels would have been higher, which would likely lead to higher coral calcifications rates (1, 2). Thus, the results of Cantin et al. do not show that Red Sea corals are not sensitive to ocean acidification nor do they give us any cause to believe that these corals have not been and will not be adversely affected by ocean acidification.
Dissolved inorganic carbon and alkalinity exhibit variability on many time scales: daily, seasonal, and interannual (1, 2). However, Cantin et al. estimate 3-decade trends in aragonite saturation from measurements made on only three different days. From these disparate observations, they conclude that there was no detectable trend in aragonite saturation in the central Red Sea. They may be right, but these observations are not sufficient to establish stationarity of a multi-decadal aragonite saturation time series under conditions of changing temperature and atmospheric CO2 content. Observations made in different months or years could yield significantly different results.
More importantly, even if aragonite saturation has not changed significantly in the central Red Sea over the past decades, we expect to see significant changes during this century if atmospheric CO2 concentrations continue to rise. Therefore, their results are consistent with the hypothesis that corals in the central Red Sea are threatened by ocean acidification. More experimentation and observations are required to test this hypothesis.
Ken Caldeira, Kenneth Schneider
Department of Global Ecology, Carnegie Institution, 260 Panama Street, Stanford, CA 94305, USA.
David I. Kline
Coral Reef Ecosystems Laboratory, Global Change Institute, The University of Queensland, St. Lucia, QLD 4072, Australia.
References
1. J. Kleypas, C. Langdon, in Coral Reefs and Climate Change: Science and Management J. T. Phinney, A. Strong, W. Skrving, J. Kleypas, O. Hoegh-Guldberg, Eds. (AGU, Washington, DC, 2006), vol. 61, pp. 73-110.
2. J. Silverman, B. Lazar, J. Erez, J. Geophys. Res. 112, C05004 (2007).
Ken Caldeira, Kenneth Schneider & David I. Kline, http://www.sciencemag.org, 10 February 2011. Article.
