Paper Reviewed
Comeau, S., Edmunds, P.J., Spindel, N.B. and Carpenter, R.C. 2013. The responses of eight coral reef calcifiers to increasing partial pressure of CO2 do not exhibit a tipping point. Limnology and Oceanography 58: 388-398.

In a study designed to determine if a set of species-specific "tipping points" might exist in the calcification responses of coral reef calcifiers to increasing ocean acidification (OA) driven by elevated atmospheric CO2 concentrations, Comeau et al. (2013) "compared the effects of six partial pressures of CO2 from 28 Pa to 210 Pa on the net calcification of four corals (Acropora pulchra, Porites rus, Pocillopora damicornis, and Pavona cactus), and four calcified algae (Hydrolithon onkodes, Lithophyllum flavescens, Halimeda macroloba, and Halimeda minima)." This they did from August to October of 2011 in Moorea, French Polynesia, using organisms collected from the back reef of the island's north shore at 1-2 meters depth, which they placed in "a mesocosm apparatus consisting of 12 tanks that allowed for the maintenance of six PCO2 treatments in duplicate," where the calcifiers were studied in detail for several 2-week periods.

Results indicated that calcification rates in two of the eight species (P. damicornis and H. macroloba) were impervious to changes in pH across the full range of treatment. As for the remaining six species, they experienced a mean decrease in net calcification of ~10% when the ambient PCO2 (39 Pa) was doubled. However, all of these species maintained positive rates of calcification up through the highest PCO2, where the calcium carbonate aragonite saturation sate was approximately equal to 1.

In discussing the significance of their findings, Comeau et al. write that "in contrast to previous studies that have predicted rapid decreases in calcification of corals and coral reefs exposed to PCO2 ≥ 50 Pa ... our study, performed at the organismic level on eight of the main calcifiers in Moorea, suggests that tropical reefs might not be affected by OA as strongly or as rapidly as previously supposed" ... or not at all in the case of some species, we might add, in light of the nil responses of P. damicornis and H. maeroloba noted above. Further, when PCO2 was increased to 210 Pa (fully double the pessimistic value predicted by some for the end of the century), they say they "did not detect a threshold at which the effect of PCO2 on calcification became nonlinear and intensified (i.e., a tipping point)."

When it comes to coral reef responses to dreaded ocean acidification, climate alarmist projections of their demise a few short decades (or even centuries) from now are unsupported by the findings of this paper. And scare stories of tipping points in which reefs reach a point of no return beyond which calcification declines accelerate are equally unsupported.