Marine heatwave

A marine heatwave is a period of abnormally high sea surface temperatures (SST) compared to typical temperatures for a particular season and locale. Marine heatwaves are caused by a variety of drivers. These include short term weather events such as fronts, intraseasonal events (30 to 90 days), annual, and decadal (10-year) modes like El Niño events, and human-caused climate change. Such heatwaves affect marine ecosystems. For example, heatwaves can lead to events such as coral bleaching, sea star wasting disease, harmful algal blooms, and mass mortality of benthic communities. Unlike heatwaves on land, marine heatwaves can extend over vast areas, persist for weeks to months to years, and extend to subsurface levels.

Major marine heatwaves affected the Great Barrier Reef in 2002, the Mediterranean Sea in 2003, the Northwest Atlantic in 2012, and the Northeast Pacific during 2013–2016. These events had drastic, long-term impacts.

Scientists predict that the frequency, duration, scale (area), and intensity of marine heatwaves will increase. This is because sea surface temperatures will continue to increase with global warming. The IPCC Sixth Assessment Report (SAR6) in 2022 stated that "marine heatwaves are more frequent [...], more intense and longer [...] since the 1980s, and since at least 2006 very likely attributable to anthropogenic climate change". This confirmed earlier findings in a 2019 IPCC report that "Marine heatwaves [...] have doubled in frequency and have become longer lasting, more intense and more extensive (very likely)." The 2022 report predicted that marine heatwaves will become "four times more frequent in 2081–2100 compared to 1995–2014" under the lower greenhouse gas emissions scenario, or eight times more frequent under the higher emissions scenario.