Commentary: Reconstructing Four Centuries of Temperature-Induced Coral Bleaching on the Great Barrier Reef

Mass coral bleaching events have occurred with increasing frequency over the past several decades (Hughes et al., 2018). It is generally thought that bleaching events either did not occur, or were exceedingly rare, prior to the 1980s (Glynn, 1993), which supports the attribution of recent bleaching events to increasing sea surface temperature (SST) associated with anthropogenic climate change (Hughes et al., 2017, 2018). Information preserved within the skeletons of long-lived corals is currently the only way to identify past bleaching events that were not directly observed by humans, and several studies have done so by detecting anomalous high-density “stress bands” (Carilli et al., 2009; Cantin and Lough, 2014; Barkley and Cohen, 2016; DeCarlo et al., 2017, 2019; Barkley et al., 2018; Mollica et al., 2019). A recent study (Kamenos and Hennige, 2018; hereafter“KH18”) proclaimed a new bleaching proxy based on coral annual extension rates inferred from densitometer data made publicly available by the Australian Institute of Marine Science. KH18 presented provocative results, claiming to show that the Great Barrier Reef (GBR) of Australia has a long history of bleaching events dating back to the seventeenth century. According to KH18, widespread bleaching occurred in almost every decade since 1650, with a total of 88 bleaching events over this time and as many as 6 bleaching events striking the GBR per decade during periods of the eighteenth and nineteenth centuries. If true, these results would completely re-write the history of coral bleaching on the GBR and would up-end several decades of scientific literature in coral reef ecology.