Marine heatwaves  are periods of extreme seawater temperature relative to the long-term mean seasonal cycle, that persist for days to months, and that may carry severe consequences for marine ecosystems and their services . MHWs became more frequent over the 20th century  and into the beginning of the 21st century, approximately doubling in frequency  and becoming more intense and longer since the 1980s  . These trends in MHWs are explained by an increase in ocean mean temperatures , and human influence has very likely contributed to 84- 90% of them since at least 2006 . The probability of occurrence  of the largest and most impactful MHWs that have occurred in the past 30 years has increased more than 20-fold due to anthropogenic climate change .
IPCC
Climate Change 2022: Impacts, Adaptation and Vulnerability
IPCC Report