Assessing Future Projections of Warm-Season Marine Heatwave Characteristics With Three CMIP6 Models

Marine heatwaves in the summertime when temperatures may exceed organisms' thermal thresholds ("warm-season marine heatwaves (MHWs)") have huge impacts on the health and function of ecosystems like kelp forests and coral reefs. While previous studies showed that MHWs are likely to become more frequent and severe under future climate change, there has been less analysis of the thermal properties of warm-season MHWs or on the effects of climate model biases on these projections. In this study, we examine CMIP6 model ability to simulate five key thermal properties of warm-season MHWs, and evaluate the global pattern of future projections for coral reef and kelp systems. The results show that the duration, accumulated heat stress and peak intensity are projected to increase by >60 days, 160 degrees C center dot day and 1 degrees C, respectively, across most of the ocean by the end of the 21st century. In contrast, the duration of "priming" (a period of sub-lethal heat stress prior to substantial MHW heat stress) is projected to decrease by >30 days in the tropics, potentially reducing organisms' ability to acclimate to heat stress. The projected increases in MHW duration and accumulated heat stress in some coral reef and kelp forest locations, however, are likely overestimated due to model limitations in simulating surface winds, deep convection and some other processes that influence MHW evolution. The findings point to some possible processes to target in model development and regional biases to be considered when projecting the impacts of MHWs on marine ecosystems. Plain Language Summary Periods of extremely high ocean temperatures that persist for days to months, known as marine heatwaves (MHWs), can cause the loss of marine life and impact coastal communities and economies. Climate change is expected to drive substantial increases in the length, strength and frequency of MHWs this century. There has been less analysis, however, of the characteristics of individual MHWs, like the rate at which they develop. In this research, we examine how well climate models can simulate these characteristics and the implication for future projections. We find considerable biases in the simulation of some key MHW characteristics in parts of the ocean due to model limitations in capturing physical processes like surface winds along the equator. Most MHW characteristics like duration and total heat stress are projected to increase sharply this century, particularly for coral reefs and kelp forests, although the increases in some regions are likely overestimated due to model biases. Conversely, we project decreases in "priming"-periods of sub-lethal heat stress that help marine life prepare for heatwaves. These findings identify regional errors to consider when interpreting MHW projections and can help researchers identify areas for improving model performance.