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Coral Calcification Will Not be Reduced in a CO2-Enriched Warmer World
Volume 8, Number 4: 26 January 2005

A few years back, Kleypas et al. (1999) calculated 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, stating that this phenomenon could result in reduced calcification rates in coral reefs, which could lead to weaker coral skeletons, reduced coral extension rates, and increased coral susceptibility to erosion, and noting that these primary effects could lead to "a host of secondary changes in community structure, reproduction, and overall community functions."  They further noted that aragonite precipitation in the tropics should have already decreased by 6 to 11% since 1880, as a result of the increase in atmospheric CO2 concentration experienced to the time of the writing of their paper, 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 21st century.

In the wake of the media attention that followed the publication of the Kleypas et al. paper, the world was treated to headlines that trumpeted "CO2 Could Kill Coral" and "Great Barrier Reef Faces Death Knell."  We immediately responded with an Editorial (15 Apr 1999) that described the extreme tenuousness of these claims.  Other scientists, however, joined the doom-and-gloom chorus with papers that kept the theoretical threat continually before the eyes of the public.  In return, we continued to respond to their unrealistic contentions with Journal Reviews and Editorials that noted the serious shortcomings of their thinking and that reported real-world observations that refuted their conclusions (see essentially everything listed under Coral Reefs (Calcification) in our Subject Index).  Now, at long last, we have the luxury of reviewing a paper by three Australian scientists that clearly demonstrates, once and for all, the fallacy of all those earlier wrongheaded studies with which we have had to contend over the past half-decade.

McNeil et al. (2004) begin their important paper by referencing some of the studies that form the nucleus of the CO2-will-harm-corals contention.  Then, as we typically did time after time over the past several years, they cite a host of papers that establish -- beyond doubt, because they are based on real-world observations -- that coral calcification rates increase with increasing sea surface temperature, which listing of pertinent papers includes the studies of Bessat and Buigues (2001), Carricart-Ganivet (2004), Clausen and Roth (1975), Coles and Coles (1977), Kajiwara et al. (1995), Lough and Barnes (2000), Nie et al. (1997) and Reynaud-Vaganay et al. (1999).  As to why this is so, they go on to state that "these observed increases in coral reef calcification with ocean warming are most likely due to an enhancement in coral metabolism and/or increases in photosynthetic rates of their symbiotic algae," just as we did when noting over and over that coral calcification is a biologically-driven process that can overcome physical-chemical limitations that in the absence of life would appear to be insurmountable.

With these preliminaries out of the way, McNeil et al. used a coupled atmosphere-ice-ocean carbon cycle model to calculate annual mean sea surface temperature (SST) within the world's current coral reef habitat from 1995 to 2100 for increases in the air's CO2 concentration specified by the IPCC's IS92a scenario, after which concomitant changes in coral reef calcification were estimated by combining the output of the climate model with empirical relationships between coral calcification rate and (1) aragonite saturation state (the negative CO2 effect) and (2) annual SST (the positive temperature effect).

McNeil et al.'s choice for the first of these two relationships was one that had been derived by Langdon et al. (2000), which leads to an even greater reduction in calcification than was predicted in the original study of Kleypas et al.  Their choice for the second relationship was derived by Lough and Barnes (2000), which leads to an increase in calcification that is only half as large as that derived by Carricart-Ganivet (2000).  Consequently, it can be appreciated that the net result of the two phenomena was doubly weighted in favor of reduced calcification.  Nevertheless, McNeil et al. found that the increase in coral reef calcification associated with ocean warming far outweighed the decrease associated with the decrease in aragonite saturate state.  In fact, they calculated that coral calcification in 2100 would be 35% higher than what it was in pre-industrial times at the very least.  And, of course, they found that the area of coral reef habitat expands in association with the projected ocean warming.

If there is a lesson to be learned from this study, it is that people should be paying much more attention to real-world observations than to theoretical predictions, both in the case of biology, as demonstrated by this study, and in the case of climate, as demonstrated by the many materials archived on our website that deal with global warming and what does and does not accompany it.  Too many predictions of CO2-induced catastrophes in both realms are being treated as sure-to-occur when real-world observations show them to be false.

Sherwood, Keith and Craig Idso

Reference
Bessat, F. and Buigues, D.  2001.  Two centuries of variation in coral growth in a massive Porites colony from Moorea (French Polynesia): a response of ocean-atmosphere variability from south central Pacific.  Palaeogeography, Palaeoclimatology, Palaeoecology 175: 381-392.

Carricart-Ganivet, J.P.  2004.  Sea surface temperature and the growth of the West Atlantic reef-building coral Montastraea annularisJournal of Experimental Marine Biology and Ecology 302: 249-260.

Clausen, C.D. and Roth, A.A.  1975.  Effect of temperature and temperature adaptation on calcification rate in the hematypic Pocillopora damicornisMarine Biology 33: 93-100.

Coles, S.L. and Coles. P.L.  1977.  Effects of temperature on photosynthesis and respiration in hermatypic corals.  Marine Biology 43: 209-216.

Kajiwara, K., Nagai, A. and Ueno, S.  1995.  Examination of the effect of temperature, light intensity and zooxanthellae concentration on calcification and photosynthesis of scleractinian coral Acropora pulchraJ. School Mar. Sci. Technol. 40: 95-103.

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.

Langdon, C., Takahashi, T., Sweeney, C., Chipman, D., Goddard, J., Marubini, F., Aceves, H., Barnett, H. and Atkinson, M.J.  2000.  Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef.  Global Biogeochemical Cycles 14: 639-654.

Lough, J.M. and Barnes, D.J.  2000.  Environmental controls on growth of the massive coral PoritesJournal of Experimental Marine Biology and Ecology 245: 225-243.

McNeil, B.I., Matear, R.J. and Barnes, D.J.  2004.  Coral reef calcification and climate change: The effect of ocean warming.  Geophysical Research Letters 31: 10.1029/2004GL021541.

Nie, B., Chen, T., Liang, M., Wang, Y., Zhong, J. and Zhu, Y.  1997.  Relationship between coral growth rate and sea surface temperature in the northern part of South China Sea.  Sci. China Ser. D 40: 173-182.

Reynaud-Vaganay, S., Gattuso, J.P., Cuif, J.P., Jaubert, J. and Juillet-Leclerc, A.  1999.  A novel culture technique for scleractinian corals: Application to investigate changes in skeletal δ18O as a function of temperature.  Marine Ecology Progress Series 180: 121-130.