Projecting Impacts of Marine Heatwaves and Their Effects on Long-Term Changes in Marine Ecosystems Over the 21st Century.

Marine heatwaves (MHWs) are increasing in frequency, duration and intensity under climate change. Although current MHWs trigger broad ecological impacts, such as coral bleaching, mass mortality of seagrass meadows, and declines in important fish stocks, the cascading effects of MHWs on ecosystem structure and functions in the 21st century remain unclear. In this study, we utilise a global biomass flow modelling framework to analyse these impacts. Leveraging climate projections from two Earth system models (ESMs), we project marine food webs dynamics at a spatial resolution of 1° latitude and longitude for the global ocean under a high-emission 'no mitigation' scenario, two MHW baseline definitions (fixed vs. moving baseline), and five organismal acclimation capacity scenarios to MHWs. We project that MHWs exacerbate declines in total consumer biomass under intensifying Global Warming Level (GWL), with spatial variations. Globally, under a scenario assuming a fixed baseline, MHWs' additional biomass decline increases with GWL at a rate ranging from 1.3% to 6.7% biomass loss per Celsius degree of warming, according to best-to-worst acclimation capacity scenarios to MHWs. Furthermore, the projected impacts of MHWs on marine biomass vary across the food web, with higher trophic levels being impacted by 0.8% more than lower trophic level compartments. Moreover, under a scenario that assumes a moving baseline, reflecting marine communities' capacity to adapt or acclimatise to climate change, the projected biomass loss from MHWs is reduced by an average of 3.7% per °C of global warming compared to fixed baseline simulations. We conclude that it is essential to incorporate extreme temperature events into climate change impact and risk assessments and examine the potential role of adaptive capacities of marine communities to inform robust climate responses and adaptations for biodiversity and fisheries. We emphasise that limiting global warming to below +2°C would significantly reduce MHWs' impacts.