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During 2015-2016, record temperatures triggered a pan-tropical episode of coral bleaching, the third global-scale event since mass bleaching was first documented in the 1980s. Here we examine how and why the severity of recurrent major bleaching events has varied at multiple scales, using aerial and underwater surveys of Australian reefs combined with satellite-derived sea surface temperatures. The distinctive geographic footprints of recurrent bleaching on the Great Barrier Reef in 1998, 2002 and 2016 were determined by the spatial pattern of sea temperatures in each year. Water quality and fishing pressure had minimal effect on the unprecedented bleaching in 2016, suggesting that local protection of reefs affords little or no resistance to extreme heat. Similarly, past exposure to bleaching in 1998 and 2002 did not lessen the severity of bleaching in 2016. Consequently, immediate global action to curb future warming is essential to secure a future for coral reefs.
The world's tropical reef ecosystems, and the people who depend on them, are increasingly affected by climate change1-7. Since the 1980s, rising sea surface temperatures owing to global warming have triggered unprecedented mass bleaching of corals, including three pan-tropical events in 1998, 2010 and 2015/16 (ref. 1). Thermal stress during marine heatwaves disrupts the symbiotic relationship between corals and their algal symbionts (Symbiodinium spp.), causing the corals to lose their colour2,3. Bleached corals are physiologically damaged, and prolonged bleaching often leads to high levels of coral mortality5-8. Increasingly, individual reefs are experiencing multiple bouts of bleaching, as well as the effects of more chronic local stressors such as pollution and over-fishing1-4. Our study represents a fundamental shift away from viewing bleaching events as individual disturbances to reefs, by focusing on three recurrent bleachings over the past 18 years along the 2,300 km length of the Great Barrier Reef, as well as the potential influence of water quality and fishing pressure on the severity of bleaching.
The geographic footprints of mass bleaching of corals on the Great Barrier Reef have varied markedly during three major events in 1998, 2002 and 2016 (Fig. 1a). In 1998, bleaching was primarily coastal and most severe in the central and southern regions. In 2002, bleaching was more widespread, and affected offshore reefs in the central region that had escaped in 1998...