With respect to the past, the four researchers report that shallow water tropical reef organisms existed throughout the entire 540 million years of the Phanerozoic, which included times when sea surface temperatures (SSTs) were more than 7°C higher than those of today and the air's CO2 concentration was as much as 6000 ppm higher. And with respect to what they call "the most recent reef crisis," they say that "the Paleocene-Eocene Thermal Maximum (PETM; 55.8 million years ago), was characterized by rapid SST rise and a similar order of magnitude of CO2 increase as present," yet they state there is evidence that "reef assemblages in at least one oceanic setting were unaffected (Robinson, 2011)," while noting that other reefs have also shown "greater resilience to past rapid warming and acidification than previously thought."

More recently, during the Holocene, Pandolfi et al. say that "evidence from high-resolution proxy records suggests that tropical SSTs had the potential to repeatedly warm over centennial to millennial time scales (Rosenthal et al., 2003; Schmidt et al., 2004)." And in one location, they say that SSTs rose "at rates comparable to those projected for the coming century (Lea et al., 2003)," yet they add that "none of these post-Last Glacial Maximum warming episodes appear to have interrupted reef growth."

As for current coral responses to SST increases, the four scientists note that "numerous characteristics of coral hosts have the potential to confer differences in bleaching susceptibility," and they report that "these characteristics vary substantially within and among coral species (Baird et al., 2009a; Csaszar et al., 2010)." In addition, they note that "some coral species also harbor multiple strains of zooxanthellae, which confer differential susceptibility of their hosts to bleaching (Rowan, 2004)." And they say there is also "substantial variation in reef recovery in the aftermath of bleaching events (Baker et al., 2008)."

The story is much the same with respect to coral responses to ocean acidification. Pandolfi et al. note, for example, that there have been studies where calcification actually increased under moderately elevated partial pressures of CO2 (Rodolfo-Metalpa et al., 2010; Jury et al., 2010; Reynaud et al., 2003), as has also been observed for some coralline algae, crustacea and echinoderms (Ries et al., 2009)." And they add that sensitivity of calcification to ocean acidification "appears to be reduced when (i) studies are conducted over weeks or months (Ries et al., 2009; Rodolfo-Metalpa et al., 2010; Marubini et al., 2001; Reynaud et al., 2003) as opposed to less than one day (Langdon and Atkinson, 2005; Ohde and Hossain, 2004) or (ii) corals are reared under nutritionally replete conditions by feeding or elevating inorganic nutrient concentrations (Langdon and Atkinson, 2005; Ries et al., 2009)."

As for the future, the four researchers write that "because bleaching-susceptible species often have faster rates of recovery from disturbances, their relative abundances will not necessarily decline." In fact, they say that "such species could potentially increase in abundance, depending on how demographic characteristics and competitive ability are correlated with thermal tolerance and on the response of other benthic taxa, such as algae," while they further note that "the shorter generation times typical of more-susceptible species (Baird et al., 2009b) may also confer faster rates of evolution of bleaching thresholds, which would further facilitate maintenance of, or increases to, the relative abundance of thermally sensitive but faster-evolving species (Baskett et al., 2009)."

In summing up their analysis of the subject, Pandolfi et al. thus state that emerging evidence for (1) variability in the coral calcification response to acidification, (2) geographical variation in bleaching susceptibility and recovery, (3) responses to past climate change, and (4) potential rates of adaptation to rapid warming "supports an alternative scenario in which reef degradation occurs with greater temporal and spatial heterogeneity than current projections suggest." And further noting that "non-climate-related threats already confronting coral reefs are likely to reduce the capacity of coral reefs to cope with climate change," they conclude that "the best and most achievable thing we can do for coral reefs currently to deal with climate change is to seek to manage them well," by reducing more direct anthropogenic impacts such as fishing, pollution, and habitat destruction, which fragment populations or decrease population sizes and reduce the potential of coral reefs to adapt to warmer, more acidic conditions.

Sherwood, Keith and Craig Idso

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