Coral Reef Bleaching Thresholds Under Projected Sea Surface Temperature Rise
DOI:
https://doi.org/10.5281/zenodo.22794810Keywords:
coral bleaching, degree heating weeks, thermal tolerance, sea surface temperature projection, CMIP6, Great Barrier Reef, Caribbean, Coral Triangle, reef vulnerability, adaptive capacityAbstract
Mass coral bleaching is triggered when accumulated thermal stress, commonly expressed as degree heating weeks
(DHW), exceeds a reef's bleaching threshold, yet published thresholds vary considerably among regions and the
pace at which projected sea surface temperature (SST) rise will push different reef systems into a regime of annual
severe bleaching (ASB) remains incompletely quantified on a directly comparable basis. We compiled in-situ
bleaching-onset DHW thresholds from 42 reef locations across five regions, the Great Barrier Reef, the Caribbean,
the Coral Triangle, the Red Sea, and the eastern tropical Pacific, spanning documented bleaching events from 1998
to 2020, and combined these thresholds with a 23-member CMIP6 ensemble of bias-corrected SST projections
under SSP2-4.5 and SSP5-8.5 to project, region by region, the year in which accumulated thermal stress is
expected to exceed each region's empirical severe-bleaching threshold (defined as 8 DHW) in every year. Historical
bleaching-onset thresholds ranged more than twofold across regions, from a median of 4.6 DHW in the Caribbean
to 7.8 DHW in the Red Sea, and regional SST warming rates over 1985-2020 ranged from 0.15 to 0.24 degrees
Celsius per decade. Under SSP5-8.5, the ensemble median projects onset of annual severe bleaching in the
eastern tropical Pacific by 2027 and the Caribbean by 2029, more than a decade before the Coral Triangle (2040)
and the Red Sea (2045); under SSP2-4.5 these onset years are delayed by 7-13 years depending on region, but no
region avoids annual severe bleaching before 2060 in either scenario absent thermal adaptation. Incorporating a
conservative adaptive thermal tolerance shift of 0.1 degrees Celsius per decade delayed onset by 9-15 years, and a
more optimistic 0.2 degrees Celsius per decade shift delayed onset by 19-31 years, underscoring how strongly
projected outcomes depend on an assumption that remains empirically poorly constrained. These results indicate
that current emission trajectories place even the most thermally tolerant reef regions on a path to annual severe
bleaching within the century, and that the timing gap between scenarios and between regions is large enough to be
directly relevant to conservation prioritisation and adaptive management planning.
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