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. 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. Marine heatwaves can have biological impacts on ecosystems at individual, population, and community levels. MHWs have led to severe biodiversity changes 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 for millions of square kilometers, persist for weeks to months or even years, and occur at subsurface levels.

Major marine heatwave events such as Great Barrier Reef 2002, Mediterranean 2003, Northwest Atlantic 2012, and Northeast Pacific 2013-2016 have had drastic and long-term impacts on the oceanographic and biological conditions in those areas. "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."

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".^: 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.