Abstract
Coral reefs are rapidly declining due to local environmental degradation
and global climate change. In particular, corals are vulnerable to ocean
heating. Anomalously hot sea surface temperatures (SSTs) create
conditions for severe bleaching or direct thermal death. We use SST
observations and CMIP6 model SST to project thermal conditions at reef
locations at a resolution of 1 km, a 16-fold improvement over prior
studies, under four climate emissions scenarios. We use a novel
statistical downscaling method which is significantly more skillful than
the standard method, especially at near-coastal pixels where many reefs
are found. For each location we present projections of thermal departure
(TD, the date after which a location with steadily increasing heat
exceeds a given thermal metric) for severe bleaching recurs every 5
years (TD5Y) and every 10 years (TD10Y), accounting for a range of
post-bleaching reef recovery/degradation. As of 2021, we find that over
91% and 79% of 1 km reefs have exceeded TD10Y and TD5Y, respectively,
suggesting that widespread long-term coral degradation is no longer
avoidable. We project 99% of reefs to exceed TD5Y by 2034, 2036, and
2040 under SSP5-8.5, SSP3-7.0, and SSP2-4.5 respectively. We project
that 2%-5% of reef locations remain below TD5Y at 1.5 degrees Celsius
of mean global heating, but 0% remain at 2.0 degrees Celsius. These
results demonstrate the importance of further improving ecological
projection capacity for climate-vulnerable marine and terrestrial
species and ecosystems, including identifying refugia and guiding
conservation efforts. Ultimately, saving coral reefs will require
rapidly reducing and eliminating greenhouse gas emissions.