How does rising atmospheric CO2 affect marine organisms?

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Calcification Response of Porites Corals to Global Warming
Lough, J.B. and Barnes, D.J. 2000. Environmental controls on growth of the massive coral Porites. Journal of Experimental Marine Biology and Ecology 245: 225-243.

Based on theoretical calculations, which we believe to be incomplete, Kleypas et al. (1999) suggested that the rising CO2 content of earth's atmosphere could lower the saturation state of the carbonate mineral aragonite in the surface waters of the world's oceans, warning that this phenomenon could result in reduced calcification rates in coral reefs that could lead to weaker coral skeletons, reduced coral extension rates, increased coral susceptibility to erosion and "a host of secondary changes in community structure, reproduction, and overall community functions." They further noted that aragonite calcite precipitation in the tropics should have already decreased by 6 to 11% since 1880, as a result of the historical increase in the air's CO2 content, and that these reductions could reach 17 to 35% by 2100, as a result of expected increases in the air's CO2 content over the next century.

In the wake of the media attention following the publication of Kleypas et al.'s paper, we were treated to headlines that trumpeted "CO2 Could Kill Coral" and "Great Barrier Reef Faces Death Knell," and we have ever since been bombarded with likeminded reports in both the popular press and the scientific literature. As a recent example of a mix of the two, Buddemeier et al. (2004), in a paper prepared for The Pew Center on Global Climate Change, continue to claim that the projected increase in the air's CO2 content, together with the concurrent decline in surface ocean-water pH calculated by Caldeira and Wickett (2003), will dramatically decrease coral calcification rates, which they say could lead to "a slow-down or reversal of reef-building and the potential loss of reef structures in the future."

What was done
In a program designed to see if there is any truth to these coral catastrophe claims, Lough and Barnes (2000) assembled and analyzed the calcification characteristics of 245 similar-sized massive colonies of Porites corals obtained from 29 reef sites located along the length, and across the breadth, of Australia's Great Barrier Reef (GBR), which data spanned a latitudinal range of approximately 9 and an annual average sea surface temperature (SST) range of 25-27C. To these data they then added other published data from the Hawaiian Archipelago (Grigg, 1981, 1997) and Phuket, Thailand (Scoffin et al., 1992), thereby extending the latitudinal range of the expanded data set to 20 and the annual average SST range to 23-29C.

What was learned
The GBR calcification data were linearly related to the average annual SST data, such that "a 1C rise in average annual SST increased average annual calcification by 0.39 g cm-2 year-1." Results were much the same for the extended data set: Lough and Barnes report that "the regression equation [calcification = 0.33(SST) - 7.07] explained 83.6% of the variance in average annual calcification (F = 213.59, p less than 0.00)," noting that "this equation provides for a change in calcification rate of 0.33 g cm-2 year-1 for each 1C change in average annual SST."

What it means
Lough and Barnes write that their results "allow assessment of possible impacts of global climate change on coral reef ecosystems," reporting that between the two 50-year periods 1880-1929 and 1930-1979 they calculated a mean calcification increase of 0.06 g cm-2 year-1. They note that "this increase [our italics] of ~4% in calcification rate conflicts with the estimated decrease [our italics] in coral calcification rate of 6-14% over the same time period suggested by Kleypas et al. (1999) as a response to changes in ocean chemistry." Even more stunning is their observation that between the two 20-year periods 1903-1922 and 1979-1998, "the SST-associated increase in calcification is estimated to be less than 5% in the northern GBR, ~12% in the central GBR, ~20% in the southern GBR and to increase dramatically (up to ~50%) to the south of the GBR."

In light of these real-world empirical-based calculations, and in stark contrast to the doom-and-gloom prognostications of the world's climate alarmists, Lough and Barnes thus conclude that coral calcification rates "may have already significantly increased along the GBR in response to global climate change." And they are likely to increase even more, we would add, if the air's CO2 content and temperature continue to rise in the years ahead.

Buddemeier, R.W., Keypas, J.A. and Aronson, R.B. 2004. Coral Reefs & Global Climate Change: Potential Contributions of Climate Change to Stresses on Coral Reef Ecosystems. The Pew Center on Global Climate Change, Arlington, VA, USA.

Caldeira, K. and Wickett, M.E. 2003. Anthropogenic carbon and ocean pH. Nature 425: 365.

Grigg, R.W. 1981. Coral reef development at high latitudes in Hawaii. In: Proceedings of the Fourth International Coral Reef Symposium, Manila, Vol. 1: 687-693.

Grigg, R.W. 1997. Paleoceanography of coral reefs in the Hawaiian-Emperor Chain - revisited. Coral Reefs 16: S33-S38.

Kleypas, J.A., Buddemeier, R.W., Archer, D., Gattuso, J-P., Langdon, C., and Opdyke, B.N. 1999. Geochemical consequences of increased atmospheric carbon dioxide on coral reefs. Science 284: 118-120.

Scoffin, T.P., Tudhope, A.W., Brown, B.E., Chansang, H. and Cheeney, R.F. 1992. Patterns and possible environmental controls of skeletogenesis of Porites lutea, South Thailand. Coral Reefs 11: 1-11.

Reviewed 30 June 2004