Global assessment of coral bleaching and required rates of adaptation under climate change

TLDR

Elevated ocean temperatures trigger coral bleaching by disrupting the Symbiodinium symbiosis, and climate‑change projections indicate that increased bleaching frequency threatens reef viability worldwide. The study aims to perform the first global assessment of bleaching under climate change by adapting the NOAA Coral Reef Watch prediction method to low‑ and high‑sensitivity GCM outputs. Algorithms were developed and validated to predict mass bleaching from GCM‑resolution sea‑surface temperatures for thousands of reefs, then applied to estimate bleaching frequency and the thermal adaptation needed under two emissions scenarios. The assessment shows that, without a 0.2–1.0 °C per decade increase in thermal tolerance, most reefs will experience annual or biannual bleaching within 30–50 years, with regions like Micronesia and western Polynesia especially vulnerable, and that stabilizing greenhouse gases is essential to alter this outlook.

Abstract

Elevated ocean temperatures can cause coral bleaching, the loss of colour from reef-building corals because of a breakdown of the symbiosis with the dinoflagellate Symbiodinium. Recent studies have warned that global climate change could increase the frequency of coral bleaching and threaten the long-term viability of coral reefs. These assertions are based on projecting the coarse output from atmosphere-ocean general circulation models (GCMs) to the local conditions around representative coral reefs. Here, we conduct the first comprehensive global assessment of coral bleaching under climate change by adapting the NOAA Coral Reef Watch bleaching prediction method to the output of a low- and high-climate sensitivity GCM. First, we develop and test algorithms for predicting mass coral bleaching with GCM-resolution sea surface temperatures for thousands of coral reefs, using a global coral reef map and 1985-2002 bleaching prediction data. We then use the algorithms to determine the frequency of coral bleaching and required thermal adaptation by corals and their endosymbionts under two different emissions scenarios. The results indicate that bleaching could become an annual or biannual event for the vast majority of the world's coral reefs in the next 30-50 years without an increase in thermal tolerance of 0.2-1.0°C per decade. The geographic variability in required thermal adaptation found in each model and emissions scenario suggests that coral reefs in some regions, like Micronesia and western Polynesia, may be particularly vulnerable to climate change. Advances in modelling and monitoring will refine the forecast for individual reefs, but this assessment concludes that the global prognosis is unlikely to change without an accelerated effort to stabilize atmospheric greenhouse gas concentrations.