Ocean Acidification

In a paper recently published in Geophysical Research Letters, Silverman et al. (2009) created a model of coral calcification based on field observations of gross community calcification as a function of aragonite saturation state (Ωarag), sea surface temperature (SST) and live coral cover, after which they calculated calcification rates for more than 9,000 reef locations using model values of Ωarag and SST at different atmospheric CO2 concentrations, which exercise led them to conclude that “by the time atmospheric partial pressure of CO2 will reach 560 ppm, all coral reefs will cease to grow and start to dissolve.”



What’s wrong with this picture?

For starters — and as actually acknowledged by the researchers themselves — “coral reefs were exposed throughout their geological history to higher temperatures and CO2 levels than at present and yet have persisted,” which is a pretty amazing admission for them to make, in light of the fact that they have boldly declared that when the atmosphere’s CO2 concentration reaches 560 ppm in the not too distant future, “all coral reefs will cease to grow and start to dissolve.”

So how did the five modelers get things so wrong? … as we clearly believe they did.

For one thing, they say their calculations “are based on the assumption that an increase of 1°C in the maximum summer monthly average SST [relative to pre-Industrial Revolution or PIR values] will result in bleaching that will reduce the live coral cover [of a reef] by 50%.” This means that if a reef’s live coral coverage parameter (AC, which can vary from 1.0 to 0.0) was initially 0.5, it will decline to 0.25, as they describe it, “when monthly average model SST increases by >=1°C above the temperature of the warmest month during PIR,” and that “a further decrease of AC to 0.125 is invoked by the model on the next encounter with >=1°C SST increase,” so that the reef’s live coral coverage gradually dwindles away to next to nothing over the course of subsequent SST spikes.

Fortunately, real-world corals do not behave in this manner. They almost always recover from bleaching episodes, and they come back even better prepared for the next bleaching, so that equally severe — or even more severe — high temperature anomalies often have less of a negative effect on them than prior heat waves had; and this phenomenon enables earth’s corals to indefinitely maintain — and possibly even expand –their undersea structures, which end result is just the opposite of what Silverman et al. assume in their model.

In describing the work of Adjeroud et al. (2002), for example, Adjeroud et al. (2005) reported that an interannual survey of reef communities at Tiahura on the French Polynesian island of Moorea “showed that the mortality of coral colonies following a bleaching event was decreasing with successive events [our italics], even if the latter have the same intensity.”

Commenting on these and the similar observations of others, the seven French scientists additionally noted that the “spatial and temporal variability of the impacts observed at several scales during the present and previous surveys may reflect an acclimation and/or adaptation of local populations,” such that “coral colonies and/or their endosymbiotic zooxanthellae may be phenotypically and possibly genotypically resistant to bleaching events,” citing the work of Rowan et al. (1997), Hoegh-Guldberg (1999), Kinzie et al. (2001) and Coles and Brown (2003) in support of this conclusion.

Still other researchers have also confirmed the phenomenon of thermal adaptation in coral reefs. Guzman and Cortes (2007), for example, studied coral reefs of the eastern Pacific Ocean that had “suffered unprecedented mass mortality at a regional scale as a consequence of the anomalous sea warming during the 1982-1983 El Niño.” At Cocos Island, in particular, they found in a survey of three representative reefs (which they conducted in 1987) that the remaining live coral cover was only 3% of what it had been prior to the occurrence of the great 1982-1983 El Niño (Guzman and Cortes, 1992); and based on this finding and the similar observations of other scientists at other reefs, they predicted that “the recovery of the reefs’ framework would take centuries, and recovery of live coral cover, decades.” Just 15 years later, however, they found that the mean live coral cover had increased nearly five-fold — from 2.99% in 1987 to 14.87% in 2002 — at the three sites studied during both periods, while the mean live coral cover of all five sites studied in 2002 was 22.7%. In addition, they found that “most new recruits and adults belonged to the main reef building species from pre-1982 ENSO, Porites lobata, suggesting that a disturbance as outstanding as [the 1982-1983] El Niño was not sufficient to change the role or composition of the dominant species.”

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CO2 Science, Volume: 12(21), 27 May 2009, Full article.