Marine heatwaves (MHW) are prolonged extreme oceanic warm water events that can have devastating impacts on marine ecosystems. A relatively new area of research, MHWs occur when ocean water temperatures are much warmer than normal (usually above the 90th percentile) for at least 5 consecutive days.
The number of annual MHW days has increased substantially since the early twentieth century. This number is projected to grow even more as the planet continues to warm, with MHW days extending for most of the year.
Sudden environmental extremes can have stronger impacts on species than the slower changes over longer time periods, both on land and in marine environments. Documented impacts of marine heatwaves include large scale shifts in location in fish stocks, collapses of kelp forests, coral bleaching, and unprecedented die-offs of sea birds.