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— CH. 1 · DEFINING THE PHENOMENON —

Marine heatwave

~4 min read · Ch. 1 of 6
6 sections

  • In the summer of 2011, a massive warming event struck the west coast of Australia. This unprecedented heat caused kelp forests to die back rapidly along hundreds of kilometers of coastline. Scientists needed a new term for this specific type of oceanic disaster. They coined the phrase marine heatwave to describe periods when water temperature is abnormally warm for that time of year. A 2016 study defined such an event as lasting five or more days with temperatures warmer than the 90th percentile based on a 30-year historical baseline period. The Intergovernmental Panel on Climate Change later refined this definition in their Sixth Assessment Report. They stated that these phenomena can manifest at scales up to thousands of kilometres and persist from days to months. Unlike land-based heatwaves, these events extend deep into subsurface waters and cover vast areas.

  • Researchers developed a four-tier classification system to compare different events globally. Events are rated on a scale from one to four depending on severity. Category one represents a moderate event while category two indicates strong conditions. Category three describes severe situations and category four marks extreme occurrences. The Mediterranean Sea experienced a category four event in 2011 off Western Australia. That specific incident reached an intensity of 4.9 degrees Celsius above normal. Another major event hit the Northwest Atlantic in 2012 classified as category three. It lasted 132 days and reached an intensity of 4.3 degrees Celsius. Scientists also analyze symmetry, duration, onset rate, and decline rate when assigning categories. While most studies focus on sea surface data, these waves can occur at depth including the sea floor.

  • Local weather patterns often trigger these heating events through ocean advection and wind stress. Teleconnection processes connect geographically separated areas via atmospheric blocking or jet-stream positions. The El Niño Southern Oscillation contributed significantly to the Northeastern Pacific event known as The Blob. This regional climate pattern persisted for years between 2013 and 2016. Decadal oscillations like the Pacific decadal oscillation operate on Earth-wide scales. These drivers disproportionately affect Western boundary currents where warm surface air temperature accumulates. Ocean carbon sinks in mid-latitudes experience persistent heatwaves while upwelling regions release gases. Mathematical models show how mean sea surface temperature and variability characterize these complex interactions. Regional warming trends emerge from a combination of short-term fronts and long-term decadal modes.

  • Human-caused global warming has doubled the frequency of marine heatwaves since the 1980s. The IPCC Sixth Assessment Report stated that these events are very likely attributable to anthropogenic climate change since 2006. Projections indicate they will become four times more frequent by the end of the century under low emissions scenarios. Under high emission scenarios, the frequency could increase eightfold compared to the period between 1995 and 2014. Sea surface temperatures in the tropical Indian Ocean may reach a near-permanent heatwave state by 2100. Model projections suggest annual marine heatwave days rising from 20 days per year during the late 20th century to over 200 days. The western North Pacific region shows mean duration rising from about 11 days to roughly 138 days per event. These changes depend heavily on Shared Socioeconomic Pathways chosen for future greenhouse gas mitigation efforts.

  • Sea surface temperatures have been recorded continuously since 1904 at Port Erin on the Isle of Man. Major incidents affected the Great Barrier Reef in 2002 and the Mediterranean Sea in 2003. A massive event struck Western Australia again in 2011 covering an area of 0.95 million square kilometres. The Northeastern Pacific experienced The Blob from 2015 through 2016 with an intensity reaching 2.6 degrees Celsius. This specific event covered between 4.5 and 11.7 million square kilometres according to NOAA data. Another outbreak hit the Santa Barbara region in 2015 lasting 93 days. Recent records show the Northern Pacific experiencing another extreme event starting in 2025. Measurements continue today through global organizations like NASA and NOAA to track these expanding threats.

  • Mass mortality events occurred when 25 benthic species died off in the Mediterranean during 2003. Coral bleaching became a widespread consequence of rising sea surface temperatures across multiple regions. Sea star wasting disease emerged as a direct result of these thermal shifts. Widespread mass sealife die-offs happened in five consecutive years within the Mediterranean Sea between 2015 and 2019. Habitat degradation resulted in the complete loss of seagrass beds, corals, and kelp forests. Large range shifts moved many marine species away from the equator toward cooler waters. Toxic algal blooms impacted species across all taxa while food web dynamics shifted dramatically. Management of migrant species became increasingly difficult for fisheries and conservationists. Predator-prey relationships changed throughout the ecosystem leading to dramatic changes in animal abundances.

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Common questions

When was the term marine heatwave coined and what definition did scientists establish in 2016?

Scientists coined the phrase marine heatwave to describe periods when water temperature is abnormally warm for that time of year. A 2016 study defined such an event as lasting five or more days with temperatures warmer than the 90th percentile based on a 30-year historical baseline period.

What are the four categories used to classify marine heatwave severity globally?

Researchers developed a four-tier classification system where events are rated on a scale from one to four depending on severity. Category one represents a moderate event while category two indicates strong conditions, category three describes severe situations, and category four marks extreme occurrences.

How has human-caused global warming affected the frequency of marine heatwaves since the 1980s?

Human-caused global warming has doubled the frequency of marine heatwaves since the 1980s. The IPCC Sixth Assessment Report stated that these events are very likely attributable to anthropogenic climate change since 2006.

Which major marine heatwave occurred off Western Australia in 2011 and how large was it?

A massive event struck Western Australia again in 2011 covering an area of 0.95 million square kilometres. That specific incident reached an intensity of 4.9 degrees Celsius above normal during this unprecedented heat event.

What were the biological consequences of the Mediterranean Sea marine heatwave in 2003?

Mass mortality events occurred when 25 benthic species died off in the Mediterranean during 2003. Coral bleaching became a widespread consequence of rising sea surface temperatures across multiple regions.

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All sources

49 references cited across the entry

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  2. 2journalMean warming not variability drives marine heatwave trendsEric C. J. Oliver — 2019-08-01
  3. 3journalLonger and more frequent marine heatwaves over the past centuryEric C. J. Oliver et al. — 2018-04-10
  4. 4journalMarine heatwaves disrupt ecosystem structure and function via altered food webs and energy fluxDylan G. E. Gomes et al. — 13 March 2024
  5. 5journalBiological Impacts of Marine HeatwavesKathryn E. Smith et al. — 16 January 2023
  6. 6journalEffects of temperature, season and locality on wasting disease in the keystone predatory sea star Pisaster ochraceusAE Bates et al. — 2009-11-09
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  9. 9journalSubsurface intensification of marine heatwaves off southeastern Australia: The role of stratification and local winds: SUBSURFACE MARINE HEAT WAVESA. Schaeffer et al. — 2017-05-28
  10. 10journalThe Role of Natural Variability and Anthropogenic Climate Change in the 2017/18 Tasman Sea Marine HeatwaveS. E. Perkins-Kirkpatrick et al. — 2019-01-01
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  12. 12journalEmerging risks from marine heat wavesThomas L. Frölicher et al. — December 2018
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  14. 14journalEvidence for Adaptation from the 2016 Marine Heatwave in the Northwest Atlantic OceanGulf of Maine Research Institute et al. — 2018-06-01
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  16. 16journalSatellite sea surface temperatures along the West Coast of the United States during the 2014–2016 northeast Pacific marine heat wave: Coastal SSTs During "the Blob"Chelle L. Gentemann et al. — 2017-01-16
  17. 17journalMarine heatwaves threaten global biodiversity and the provision of ecosystem servicesDan A. Smale et al. — April 2019
  18. 18journalAn unprecedented coastwide toxic algal bloom linked to anomalous ocean conditionsRyan M. McCabe et al. — 2016-10-16
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  27. 27journalInterdecadal Climate Fluctuations That Depend on Exchanges Between the Tropics and ExtratropicsD. Gu — 1997-02-07
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  34. 35journalAnalysis and prediction of marine heatwaves in the Western North Pacific and Chinese coastal regionYifei Yang et al. — 5 December 2022
  35. 36journalThe rise and fall of the "marine heat wave" off Western Australia during the summer of 2010/2011Alan F. Pearce et al. — 2013-02-01
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  37. 38journalContextualizing Marine Heatwaves in the Southern California Bight Under Anthropogenic Climate ChangeJames T. Fumo et al. — May 2020
  38. 39webRecord-breaking North Atlantic Ocean temperatures contribute to extreme marine heatwavesEuropean Commission
  39. 40webPacific 'blob' heat wave now spans an area the size of the USOlivia Hebert — 23 September 2025
  40. 41webThe California Current Marine Heatwave Tracker - BlobtrackerNOAA
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  43. 44journalMarine Heat Waves Hazard 3D Maps and the Risk for Low Motility Organisms in a Warming Mediterranean SeaGiovanni Galli et al. — 2017-05-11
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  46. 47journalMarine heatwaves drive recurrent mass mortalities in the Mediterranean SeaJoaquim Garrabou et al. — 18 July 2022
  47. 48webThe blob EarthdataLaura Naranjo — 2 November 2018
  48. 49journalEvolution of marine heatwaves and air-sea interactions during the 2020 Southwest Monsoon in the Bay of BengalHitesh Gupta et al. — 2026-02-16
  49. 50journalGenesis and Trends in Marine Heatwaves Over the Tropical Indian Ocean and Their Interaction With the Indian Summer MonsoonJ. S. Saranya et al. — February 2022