Probabilistic and Threshold-Based Marine Heat Indicators Show Different Signatures of Near-Perpetual Heat Emergence around Mesoamerican Reef under Climate Change

Abstract We analyze a combination of probabilistic marine heatwave (pMHW; based on comparison with climatology) and fixed threshold-based marine heatwave [degree heating week (DHW; based on accumulated heat] metrics for the Mesoamerican and Caribbean regions to characterize projected coral reef system risks. We use 10 Earth system models (ESMs), biasadjusted using satellite-derived observations and future projections under intermediate [shared socioeconomic pathway (SSP) 2-4.5] and very high (SSP5-8.5) emission scenarios. Climate change causes an increase in marine heat across all seasons with events intensifying and increasingly combining into prolonged events. We calculate a time of emergence (ToE) by noting the projected date in which the typical projected year’s marine heat is more extreme than the mean most extreme year in the 1985–2014 baseline. ToE for pMHW events is 2027 for the intermediate scenario and 2028 for the very high scenario, and ToE for maximum DHW conditions is 2037 and 2034, respectively. Many regions exhibit pMHW events commonly lasting the entire year, notably beginning in the 2060s under the very high scenario and in the 2090s under the intermediate scenario. DHW projections indicate that events once considered rare would become nearly perpetual in the summertime and expand beyond current seasonal constraints across a broad region by 2040 in the very high emission scenario and by 2050 under the intermediate scenario. Characteristics of change for each type of indicator across a range of climate models provide unique insight into the urgency of potential adaptation and risk management strategies as the Mesoamerican Reef enters a new era of marine heat. Significance Statement Climate change is a present and growing threat to marine ecosystems and communities around the Mesoamerican Reef, with marine heatwaves, in particular, having direct and persistent impacts. This study combines an ensemble of climate models with observed sea surface temperatures and local coral bleaching data to provide actionable climate risk information for local communities and ecosystem protection. Climate risk and adaptation planning in the region benefit from examination of both probabilistic marine heatwave indices (which tell us how abnormal future conditions will be) and fixed marine heatwave metrics (which tell us when conditions will affect specific biophysical processes in exposed ecosystems). Both show a substantial shift toward more intense and pervasive marine heat hazards, increasing urgency for mitigation and adaptation in the context of specific biophysical and engineered tolerance thresholds that may be surpassed in the next decade.