Marine heatwave
A marine heatwave (abbreviated as MHW) is a period of abnormally high
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
Definition
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."[23]
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".[1]
Categories
The quantitative and qualitative categorization of marine heatwaves establishes a naming system, typology, and characteristics for marine heatwave events.[1][24] The naming system is applied by location and year: for example Mediterranean 2003.[24][10] 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.[24]
The categorization system is on a scale from 1 to 4.[24] 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.[24]
The types of marine heatwaves are symmetric, slow onset, fast onset, low intensity, and high intensity.[1] 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.[1]
While marine heat waves have been studied at the sea surface for more than a decade, they can also occur at the sea floor.[25]
Drivers
The drivers for marine heatwave events can be broken into local processes,
At the local level marine heatwave events are dominated by ocean
Regional climate patterns such as interdecadal oscillations like
Drivers that operate on the scale of biogeographical realms or the Earth as a whole are Decadal oscillations, like Pacific Decadal Oscillations (PDO), and anthropogenic ocean warming due to climate change.[2][4][22]
Ocean areas of carbon sinks in the mid-latitudes of both hemispheres and carbon outgassing areas in upwelling regions of the tropical Pacific have been identified as places where persistent marine heatwaves occur; the air-sea gas exchange is being studied in these areas.[28]
Climate change as an additional driver
Scientists predict that the frequency, duration, scale (or area) and intensity of marine heatwaves will continue to increase.[29]: 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.[29]: 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.
Global warming is projected to push the tropical Indian Ocean into a basin-wide near-permanent heatwave state by the end of the 21st century, where marine heatwaves are projected to increase from 20 days per year (during 1970–2000) to 220–250 days per year[30].
Many species already experience these temperature shifts during the course of marine heatwave events.[1][24] There are many increased risk factors and health impacts to coastal and inland communities as global average temperature and extreme heat events increase.[31]
List of events
Sea surface temperatures have been recorded since 1904 in Port Erin, UK[4] and measurements continue through global organizations such as NOAA, NASA, and many more. Events can be identified from 1925 till present day.[4] The list below is not a complete representation of all marine heatwave events that have ever been recorded.
Name | Category | Duration (days) | Intensity (°C) | Area(Mkm2) | Ref. |
---|---|---|---|---|---|
Mediterranean 1999 | 1 | 8 | 1.9 | NA | [24][2][10] |
Mediterranean 2003 | 2 | 10 | 5.5 | 0.5 | [24][2][10] |
Mediterranean 2003 | 2 | 28 | 4.6 | 1.2 | [24][2][10] |
Mediterranean 2006 | 2 | 33 | 4.0 | NA | [24][2][10] |
Western Australia 1999 | 3 | 132 | 2.1 | NA | [24][2][32] |
Western Australia 2011 | 4 | 66 | 4.9 | 0.95 | [24][2][32] |
Great Barrier Reef 2016 | 2 | 55 | 4.0 | 2.6 | [24][2][15] |
Tasman Sea 2015 | 2 | 252 | 2.7 | NA | [24][2] |
Northwest Atlantic 2012
|
3 | 132 | 4.3 | 0.1–0.3 | [24][2][16][33] |
Northeast Pacific 2015 ("The Blob ")
|
3 | 711 | 2.6 | 4.5–11.7 | [5][17][18] |
Santa Barbara 2015 | 3 | 93 | 5.1 | NA | |
Southern California Bight 2018 | 3 | 44 | 3.9 | NA | [34] |
Northeastern Atlantic 2023 | 5 | 30 | 4.0-5.0 | NA | [35] |
Impacts
On marine ecosystems
Changes in the thermal environment of terrestrial and marine organisms can have drastic effects on their health and well-being.[19][31] Marine heatwave events have been shown to increase habitat degradation,[36][37] change species range dispersion,[19] complicate management of environmentally and economically important fisheries,[17] contribute to mass mortalities of species,[10][9][7] and in general reshape ecosystems.[5][15][38]
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.[36][37] These habitats contain a significant proportion of the oceans biodiversity.[19] 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.[9] Management of these affected species becomes increasingly difficult as they migrate across management boundaries and the food web dynamics shift.
Increases in sea surface temperature have been linked to a decline in
Coral bleaching
This section needs to be updated. The reason given is: 6th IPCC report.(April 2022) |
Extreme bleaching events are directly linked with climate-induced phenomena that increase ocean temperature, such as El Nino-Southern Oscillation (ENSO).[40] 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".[41]: 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.[41]: 381 It is expected that many coral reefs will "undergo irreversible phase shifts due to marine heatwaves with global warming levels >1.5°C".[41]: 382
This problem was already identified in 2007 by the Intergovernmental Panel on Climate Change (IPCC) as the greatest threat to the world's reef systems.[42][43]
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.[44] 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.[45]
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."[46]
In a study conducted on the Hawaiian mushroom coralOn 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.[49] 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.
Options for reducing impacts
To address the root cause of more frequent and more intense marine heatwaves,[21]: 416 climate change mitigation methods are needed to curb the increase in global temperature and in ocean temperatures.
Better forecasts of marine heatwaves and improved monitoring can also help to reduce impacts of these heatwaves.[21]: 417
See also
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