 Marine heatwaves, MHWs, are becoming more frequent and intense due to climate change. By the end of the 21st century, many areas of the ocean will experience a near-permanent MHW state. These MHWs will cause drastic changes to marine ecosystems, resulting in significant and long-lasting effects. This article was authored by Eric C. J. Oliver, Michael T. Burroughs, Marcus G. Donut and others.