Abstract: "Marine heatwaves (MHWs) cause significant harm to marine life, ecosystem services, and can intensify hurricanes. Global warming has increased MHW duration and intensity over the last century and permanent MHWs are predicted in many areas of the ocean by the end of the 21st century. Climate interventions such as stratospheric aerosol injection (SAI) have been proposed to reduce mean global temperature, yet the potential impact on MHWs is unclear. Here, we used output from the Community Earth System Model to quantify MHWs under multiple timeframes and climate change scenarios. We evaluated MHW properties including duration and maximum intensity over the historical (1990-2009), present (2015-2034), and future (2050-2069) periods. We analyzed output from two SAI scenarios aimed to maintain global mean surface temperatures at ∼1.5 and ∼1.0°C above pre-industrial levels (ARISE-SAI-1.0 and ARISE-SAI-1.5) and one non-SAI scenario (SSP2-4.5). Our results show that despite SAI reducing the global average maximum intensity and duration of MHWs relative to SSP2-4.5, the magnitude of effects vary spatially. Compared with the present climate, SAI scenarios would reduce MHW intensity in 25-76% of the ocean, and MHW duration in 21-80% of the ocean. The largest future reductions in maximum intensity and duration occur in the coastal regions, the Tropical Atlantic, Indian, Arctic, and South Atlantic oceans. Even with a more aggressive mitigation scenario (ARISE-SAI-1.0), nearly 25% of the ocean would remain unaffected,with areas like the North Atlantic, Tropical Pacific and parts of the Southern Oceans still experiencing more intense and longer MHWs, posing risks to marine life."