Background
The authors write that "observations indicating that mass bleaching events have recently become more common, combined with projected increases in heat stress, have prompted dire predictions for the future of coral reefs under unabated greenhouse gas emissions scenarios," citing Hoegh-Guldberg (1999) and Donner et al. (2005). However, they say "there is evidence that corals may adapt to better withstand heat stress via a number of mechanisms," noting that "corals might acquire more thermally-resistant symbionts (Buddemeier and Fautin, 1993; Rowan, 2004), or might increase their own physiological mechanisms to reduce bleaching susceptibility by producing oxidative enzymes (Coles and Brown, 2003) or photoprotective compounds (Salih et al., 2000)." And they make particular mention of the fact that there is evidence to suggest that the susceptibility of a given coral or reef to bleaching depends on the thermal history of that coral or reef (Thompson and Van Woesik, 2009; Donner, 2011; Brown et al., 2002), which is the phenomenon they explored in their study.

What was done
Carilli et al., as they describe it, "collected cores from massive Porites sp. corals in the Gilbert Islands of Kiribati to investigate how corals along a natural gradient in temperature variability responded to recent heat stress events," in the course of which activity they "examined changes in coral skeletal growth rates and partial mortality scars (Carilli et al., 2010) to investigate the impact of the bleaching event in 2004 (Donner, 2011) on corals from different temperature variability regimes."

What was learned
The three researchers - one each from Australia, Canada and the United States - discovered that the spatial patterns in skeletal growth rates and partial mortality scars found in corals from the central and northern islands suggest that "corals subject to larger year-to-year fluctuations in maximum ocean temperature were more resistant to a 2004 warm-water event," and that "a subsequent 2009 warm event had a disproportionately larger impact on those corals from the island with lower historical heat stress."

What it means
In the concluding sentence of their paper's abstract, Carilli et al. say their study indicates that "coral reefs in locations with more frequent warm events may be more resilient to future warming," which suggests that taking a little extra heat every once in a while prepares corals to better deal with less frequent but more extreme heat waves when they inevitably do occur.

References
Brown, N., Dunne, R., Goodson, M. and Douglas, A. 2002. Experience shapes the susceptibility of a reef coral to bleaching. Coral Reefs 21: 119-126.

Buddemeier, R.W. and Fautin, D.G. 1993. Coral bleaching as an adaptive mechanism. BioScience 43: 320-326.

Carilli, J., Norris, R.D., Black, B., Walsh, S.W. and McField, M. 2010. Century-scale records of coral growth rates indicate that local stressors reduce coral thermal tolerance threshold. Global Change Biology 16: 1247-1257.

Coles, S.L. and Brown, B.E. 2003. Coral bleaching - capacity for acclimatization and adaptation. Advances in Marine Biology 46: 183-223.

Donner, S.D. 2011. An evaluation of the effect of recent temperature variability on the prediction of coral bleaching events. Ecological Applications 21: 1718-1730.

Donner, S.D., Skirving, W.J., Little, C.M., Oppenheimer, M. and Hoegh-Guldberg, O. 2005. Global assessment of coral bleaching and required rates of adaptation under climate change. Global Change Biology 11: 2251-2265.

Hoegh-Guldberg, O. 1999. Climate change, coral bleaching, and the future of the world's coral reefs. Marine and Freshwater Research 50: 839-866.

Rowan, R. 2004. Coral bleaching: Thermal adaptation in reef coral symbionts. Nature 430: 742.

Salih, A., Larkum, A., Cox, G., Kuhl, M. and Hoegh-Guldberg, O. 2000. Fluorescent pigments in corals are photoprotective. Nature 408: 850-853.

Thompson, D.M. and Van Woesik, R. 2009. Corals escape bleaching in regions that recently and historically experienced frequent thermal stress. Proceedings of the Royal Society B 276: 2893.