| Summary: | The increase in frequency and duration of marine heatwaves (MHWs) under global warming brings great pressure to society. The high vulnerability of ecosystems to MHWs may lead to severe ecological and socioeconomic impacts. Most studies assessed future climate responses at the Paris Agreement temperature goals based on transient, rather than stabilization, model simulations. Here, we investigate the differences between transient and stabilization responses at global warming of 1.5 °C in terms of MHWs. Concentrations of greenhouse gases and response time scales to the anthropogenic forcing differ in these two types of simulations because subsurface ocean temperatures take decades to centuries to adjust under external forcing. While global mean metrics of MHWs show little difference between the transient and stabilization responses, significant regional disparities are revealed worldwide, including many climate change hotspots. Regionally intensified MHWs at stabilized 1.5 °C are mostly due to air-sea interactions and subsurface ocean warming that contributes to the stronger SST warming in eastern boundary upwelling systems and the southern Indian Ocean, indicative of potentially greater impacts on marine ecosystems in these regions. In contrast, MHWs would be alleviated in the central equatorial Pacific and Arctic in stabilization responses. Substantial differences in regional MHWs between the transient and stabilization responses oblige us to reconsider previous assessments regarding 1.5 °C warming and future mitigation pathways.
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