Paper Reviewed
Sully, S., Burkepile, D.E., Donovan, M.K., Hodgson, G. and van Woesik, R. 2019. A global analysis of coral bleaching over the past two decades. Nature Communications 10: 1264, doi: 10.1038/s41467-019-09238-2.

In a very revealing paper, Sully et al. (2019) write as background for their work that "compared with coarse-grained global models that predict minimal coral survival in the tropical oceans within the next 100 years, recent field work shows considerable geographic variability in both temperature stress and coral survival." And they add that "this mismatch between global models and field results underscores the urgent need to develop better models that accurately predict the geographical heterogeneity of coral bleaching as corals respond to ocean warming."

Think about that for a second, there is an "urgent need" to develop "better models" that can "accurately predict" how corals respond to ocean warming because there is a "mismatch between global models and field results." Stunning!

It is very rare these days for scientists to admit (and in Nature Communications no less!) that a global warming-related model has been invalidated by observational data, yet that is exactly what Sully et al. have done in introducing their paper. And because of that failure, the team of five researchers set out to "synthesize field observations of coral bleaching at 3351 sites in 81 countries from 1998 to 2017 and use a suite of environmental covariates and temperature metrics to analyze bleaching patterns," so that their findings could presumably be used to improve the next generation of models predicting coral bleaching.

And what did their synthesis reveal?

Sully et al. report that "significantly more coral bleaching occurred at mid-tropical latitude sites, between 15 and 20° north and south of the Equator than in the equatorial regions, where coral diversity is highest." And they add that this clustering of coral bleaching both north and south of the Equator "was not, however, a consequence of higher thermal anomalies at those latitudes than elsewhere," as the equatorial region experienced "similar thermal stress levels."

With respect to why bleaching levels were higher in the mid-tropical latitudes (between 15-20°N and S) than the equatorial region (between 15°N and S), Sully et al. opine it is "because: (i) of the geographical differences in species composition, (ii) of the higher genotypic diversity at low latitudes, which include genotypes less susceptible to thermal stress, and (iii) some corals were preadapted to thermal stress because of consistently warmer temperatures at low latitude prior to thermal stress events," which hypotheses, they say, "are not mutually exclusive."

Another interesting finding was the observation that in the past decade, "the onset of coral bleaching has occurred at significantly higher sea surface temperatures (~0.5 °C) than in the previous decade." And in this regard, Sully et al. write that "at the thousands of sites surveyed, the mean sea surface temperature recorded during coral bleaching in the first decade of the dataset, from 1998 to 2006, was 28.1 °C, whereas the mean sea surface temperature recorded during coral bleaching in the second decade, from 2007 to 2017, was 28.7 °C."

Commenting on this key observation, the researchers say it "suggests that past bleaching events may have culled the thermally susceptible individuals, resulting in a recent adjustment of the remaining coral populations to higher thresholds of bleaching temperatures," or that "coral communities also may have acclimatized to increasing sea surface temperatures." Whatever the mechanism, it is clear that model projections of future coral reef demise are likely way off the mark, having been invalidated by observational data showing that present coral populations currently have a higher thermal threshold for bleaching giving them an increased likelihood of persisting well into the future.