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 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".
The quantitative and qualitative categorization of marine heatwaves establishes a naming system, typology, and characteristics for marine heatwave events. The naming system is applied by location and year: for example Mediterranean 2003. This allows researchers to compare the drivers and characteristics of each event, geographical and historical trends of marine heatwaves, and easily communicate marine heatwave events as they occur in real-time.
The categorization system is on a scale from 1 to 4. Category 1 is a moderate event, Category 2 is a strong event, Category 3 is a severe event, and Category 4 is an extreme event. The category applied to each event in real-time is defined primarily by sea surface temperature anomalies (SSTA), but over time it comes to include typology and characteristics.
The types of marine heatwaves are symmetric, slow onset, fast onset, low intensity, and high intensity. Marine heatwave events may have multiple categories such as slow onset, high intensity. The characteristics of marine heatwave events include duration, intensity (max, average, cumulative), onset rate, decline rate, region, and frequency.
While marine heat waves have been studied at the sea surface for more than a decade, they can also occur at the sea floor.
Scientists predict that the frequency, duration, scale (or area) and intensity of marine heatwaves will continue to increase.: 1227 This is because sea surface temperatures will continue to increase with global warming, and therefore the frequency and intensity of marine heatwaves will also increase. The extent of ocean warming depends on emission scenarios, and thus humans' climate change mitigation efforts. Simply put, the more greenhouse gas emissions (or the less mitigation), the more the sea surface temperature will rise. Scientists have calculated this as follows: there would be a relatively small (but still significant) increase of 0.86°C in the average sea surface temperature for the low emissions scenario (called SSP1-2.6). But for the high emissions scenario (called SSP5-8.5) the temperature increase would be as high as 2.89°C.: 393
The prediction for marine heatwaves is that they may become "four times more frequent in 2081–2100 compared to 1995–2014" under the lower emissions scenario, or eight times more frequent under the higher emissions scenario.: 1214 The emissions scenarios are called SSP for Shared Socioeconomic Pathways. A mathematical model called CMIP6 is used for these predictions. The predictions are for the average of the future period (years 2081 to 2100) compared to the average of the past period (years 1995 to 2014). : 1227
Many species already experience these temperature shifts during the course of marine heatwave events. There are many increased risk factors and health impacts to coastal and inland communities as global average temperature and extreme heat events increase.
List of events
Sea surface temperatures have been recorded since 1904 in Port Erin, UK and measurements continue through global organizations such as NOAA, NASA, and many more. Events can be identified from 1925 till present day. The list below is not a complete representation of all marine heatwave events that have ever been recorded.
Changes in the thermal environment of terrestrial and marine organisms can have drastic effects on their health and well-being. marine heatwave events have been shown to increase habitat degradation, change species range dispersion, complicate management of environmentally and economically important fisheries, contribute to mass mortalities of species, and in general reshape ecosystems.
Habitat degradation occurs through alterations of the thermal environment and subsequent restructuring and sometimes complete loss of biogenic habitats such as seagrass beds, corals, and kelp forests. These habitats contain a significant proportion of the oceans biodiversity. Changes in ocean current systems and local thermal environments have shifted many tropical species' range northward while temperate species have lost their southern limits. Large range shifts along with outbreaks of toxic algal blooms has impacted many species across taxa. Management of these affected species becomes increasingly difficult as they migrate across management boundaries and the food web dynamics shift.
This section needs to be updated. The reason given is: 6th IPCC report. Please help update this article to reflect recent events or newly available information. (April 2022)
The warming ocean surface waters can lead to bleaching of corals which can cause serious damage and coral death. The IPCC Sixth Assessment Report in 2022 found that: "Since the early 1980s, the frequency and severity of mass coral bleaching events have increased sharply worldwide".: 416 Coral reefs, as well as other shelf-sea ecosystems, such as rocky shores, kelp forests, seagrasses and mangroves have recently undergone mass mortalities from marine heatwaves.: 381 It is expected that many coral reefs will "undergo irreversible phase shifts due to marine heatwaves with global warming levels >1.5°C".: 382
The Great Barrier Reef experienced its first major bleaching event in 1998. Since then, bleaching events have increased in frequency, with three events occurring in the years 2016–2020. Bleaching is predicted to occur three times a decade on the Great Barrier Reef if warming is kept to 1.5°C, increasing every other year to 2°C.
With the increase of coral bleaching events worldwide, National Geographic noted in 2017, "In the past three years, 25 reefs—which comprise three-fourths of the world's reef systems—experienced severe bleaching events in what scientists concluded was the worst-ever sequence of bleachings to date."
On weather patterns
Research on how marine heatwaves influence atmospheric conditions is emerging. Marine heatwaves in the tropical Indian Ocean are found to result in dry conditions over the central Indian subcontinent. At the same time, there is an increase in rainfall over south peninsular India in response to marine heatwaves in the northern Bay of Bengal. These changes are in response to the modulation of the monsoon winds by the marine heatwaves.
^Schwing, Franklin B.; Mendelssohn, Roy; Bograd, Steven J.; Overland, James E.; Wang, Muyin; Ito, Shin-ichi (2010-02-10). "Climate change, teleconnection patterns, and regional processes forcing marine populations in the Pacific". Journal of Marine Systems. Impact of climate variability on marine ecosystems: A comparative approach. 79 (3): 245–257. Bibcode:2010JMS....79..245S. doi:10.1016/j.jmarsys.2008.11.027. ISSN0924-7963.
^Mignot, A., von Schuckmann, K., Landschützer, P. et al. Decrease in air-sea CO2 fluxes caused by persistent marine heatwaves. Nature Communications 13, 4300 (2022). Nature website Retrieved 21 September 2022.
^IPCC (2007). "Summary for policymakers"(PDF). In Parry ML, Canziani OF, Palutikof JP, van der Linden PJ, Hanson CE (eds.). Climate Change 2007: impacts, adaptation and vulnerability: contribution of Working Group II to the fourth assessment report of the Intergovernmental Panel on Climate Change. Cambridge, UK: Cambridge University Press. pp. 7–22. ISBN978-0-521-70597-4. Archived(PDF) from the original on 13 January 2018. Retrieved 8 July 2009.
^Fischlin A, Midgley GF, Price JT, Leemans R, Gopal B, Turley C, Rounsevell MD, Dube OP, Tarazona J, Velichko AA (2007). "Ch 4. Ecosystems, their properties, goods and services"(PDF). In Parry ML, Canziani OF, Palutikof JP, van der Linden PJ, Hanson CE (eds.). Climate Change 2007: impacts, adaptation and vulnerability: contribution of Working Group II to the fourth assessment report of the Intergovernmental Panel on Climate Change. Cambridge, UK: Cambridge University Press. pp. 211–72. ISBN978-0-521-70597-4. Archived(PDF) from the original on 11 October 2017. Retrieved 8 July 2009.