Marine heatwave

A marine heatwave (abbreviated as MHW) is a period of abnormally high ocean temperatures relative to the average seasonal temperature in a particular marine region.^[1] Marine heatwaves are caused by a variety of factors, including shorter term weather phenomena such as fronts, intraseasonal events (30- to 90-days) , annual, or decadal (10-year) modes like El Niño events, and longer term changes like climate change.^[2][3][4] Marine heatwaves can have biological impacts on ecosystems^[5] at individual, population, and community levels.^[6] MHWs have led to severe biodiversity changes such as coral bleaching, sea star wasting disease,^[7][8] harmful algal blooms,^[9] and mass mortality of benthic communities.^[10] Unlike heatwaves on land, marine heatwaves can extend for millions of square kilometers, persist for weeks to months or even years, and occur at subsurface levels.^{[11][12][13][14]}

Major marine heatwave events such as Great Barrier Reef 2002,^[15] Mediterranean 2003,^[10] Northwest Atlantic 2012,^[2][16] and Northeast Pacific 2013-2016^[17][18] have had drastic and long-term impacts on the oceanographic and biological conditions in those areas.^[10][19][9] "The term marine heatwave, referring to a discrete period of unusually high seawater temperatures, was coined following an unprecedented warming event off the west coast of Australia in the austral summer of 2011."^[20]

The IPCC Sixth Assessment Report stated in 2022 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".^[21]: 381 This confirms earlier findings, for example in the Special Report on the Ocean and Cryosphere in a Changing Climate from 2019 which stated that it is "virtually certain" that the global ocean has absorbed more than 90% of the excess heat in our climate systems, the rate of ocean warming has doubled, and marine heatwave events have doubled in frequency since 1982.^[22]