Coral bleaching under unconventional scenarios of climate warming and ocean acidification

Elevated sea surface temperatures have been shown to cause mass coral bleaching(1-3). Widespread bleaching, affecting >90% of global coral reefs and causing coral degradation, has been projected to occur by 2050 under all climate forcing pathways adopted by the IPCC for use within the Fifth Assessment Report4,5. These pathways include an extremely ambitious pathway aimed to limit global mean temperature rise to 2 degrees C (ref. 6; Representative Concentration Pathway 2.6-RCP2.6), which assumes full participation in emissions reductions by all countries, and even the possibility of negative emissions(7). The conclusions drawn from this body of work, which applied widely used algorithms to estimate coral bleaching(8), are that we must either accept that the loss of a large percentage of the world's coral reefs is inevitable, or consider technological solutions to buy those reefs time until atmospheric CO2 concentrations can be reduced. Here we analyse the potential for geoengineering, through stratospheric aerosol-based solar radiation management (SRM), to reduce the extent of global coral bleaching relative to ambitious climate mitigation. Exploring the common criticism of geoengineering-that ocean acidification and its impacts will continue unabated-we focus on the sensitivity of results to the aragonite saturation state dependence of bleaching. We do not, however, address the additional detrimental impacts of ocean acidification on processes such as coral calcification(9,10) that will further determine the benefit to corals of any SRM-based scenario. Despite the sensitivity of thermal bleaching thresholds to ocean acidification being uncertain(11,12), stabilizing radiative forcing at 2020 levels through SRM reduces the risk of global bleaching relative to RCP2.6 under all acidification-bleaching relationships analysed.