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Concept
Marine heatwave
Summary
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, annual, or decadal modes like El Niño events, and longer term changes like climate change. Marine heatwaves can lead 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 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". 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.
The IPCC Sixth Assessment Report defines marine heatwave as follows: "A period during which water temperature is abnormally warm for the time of the year relative to historical temperatures, with that extreme warmth persisting for days to months. The phenomenon can manifest in any place in the ocean and at scales of up to thousands of kilometres."
Another publication defined it as follows: an anomalously warm event is a marine heatwave "if it lasts for five or more days, with temperatures warmer than the 90th percentile based on a 30-year historical baseline period".
Official source
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