In a major review article published in Science, Pandolfi et al.
(2011) summarize what they describe as "the most recent evidence for
past, present and predicted future responses of coral reefs to
environmental change, with emphasis on rapid increases in temperature
and ocean acidification and their effects on reef-building corals."
This they do, in their words, because "many physiological responses in
present-day coral reefs to climate change are interpreted as consistent
with the imminent disappearance of modern reefs globally because of
annual mass bleaching events, carbonate dissolution and insufficient
time for substantial evolutionary responses," all of which
interpretations, they go on to demonstrate, may not be correct.
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)."
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