Product Code Database
A marine heatwave is a period of abnormally high sea surface temperatures (SST) compared to typical temperatures for a particular season and locale. Marine heatwaves are caused by a variety of drivers. These include short term weather events such as weather front, intraseasonal events (30 to 90 days) , annual, and decadal (10-year) modes like El Niño events, and human-caused climate change. Such heatwaves affect Marine ecosystem. For example, heatwaves can lead to events such as coral bleaching, sea star wasting disease, harmful algal blooms, and mass mortality of benthic communities. Unlike on land, marine heatwaves can extend over vast areas, persist for weeks to months to years, and extend to subsurface levels.
Major marine heatwaves affected the Great Barrier Reef in 2002, the Mediterranean Sea in 2003, the Northwest Atlantic in 2012, and the Northeast Pacific during 2013–2016. These events had drastic, long-term impacts.
Scientists predict that the frequency, duration, scale (area), and intensity of marine heatwaves will increase. This is because sea surface temperatures will continue to increase with global warming. The IPCC Sixth Assessment Report (SAR6) in 2022 stated 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 confirmed earlier findings in a 2019 IPCC report that "Marine heatwaves ... have doubled in frequency and have become longer lasting, more intense and more extensive (very likely)." The 2022 report predicted that marine heatwaves will become "four times more frequent in 2081–2100 compared to 1995–2014" under the lower greenhouse gas emissions scenario, or eight times more frequent under the higher emissions scenario.
A 2016 study defined it as: an anomalously warm event that "lasts for five or more days, with temperatures warmer than the 90th percentile based on a 30-year historical baseline period".
The term was coined following an unprecedented warming event off the west coast of Australia in summer 2011 that led to a rapid dieback of Kelp forest and associated ecosystem shifts along hundreds of kilometers of coastline.
Marine heatwaves are classified via a naming system, typology, and characteristics. The naming system involves locale and year: for example Mediterranean 2003.
Events are rated on a scale from 1 to 4. Category 1 is a moderate event, Category 2 strong, Category 3 severe, and Category 4 extreme. The category is defined primarily by sea surface temperature anomalies (SSTA), later adjusted to include typology and characteristics.
They are further classified according to the degree of symmetry, duration, intensity (max, average, cumulative), onset rate (slow/fast), decline rate, locale, and frequency. Various combinations have been observed.
While marine heatwaves have mostly been studied at the sea surface, they can also occur at depth, including at the seabed.
At the local level marine heatwave events are dominated by ocean advection, air-sea fluxes, thermocline stability, and wind stress. Teleconnection processes refer to climate and weather patterns that connect geographically separated areas. The teleconnection processes that play a dominant role are atmospheric blocking/subsidence, Jet stream position, oceanic , regional wind stress, warm surface air temperature, and seasonal climate oscillations. These processes contribute to regional warming trends that disproportionately affect Western boundary currents.
Regional climate patterns including interdecadal oscillations like El Niño Southern Oscillation (ENSO) have contributed to marine heatwave events such as "The Blob" in the Northeastern Pacific.
Drivers that operate on the scale of biogeographical realms or the Earth as a whole are decadal oscillations, including Pacific decadal oscillations (PDO), and climate change.
Carbon sink in the mid-latitudes of both hemispheres and Outgassing in upwelling regions of the tropical Pacific experience persistent marine heatwaves.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.
Marine heatwaves are predicted to become "four times more frequent in 2081–2100 compared to 1995–2014" under the low emissions scenario, or eight times more frequent under the high emissions scenario. Mathematical model CMIP6 is used for these predictions. The predictions are for the period (2081-2100) compared to the average of (1995- 2014).
Ocean 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 (1970–2000) to 220–250 days per year.
| +List of some marine heatwaves 1999–2025
!Region and date
!Category
!Duration (days) !Intensity (°C) !Area (millions of km2) !Ref. | |||||
| Mediterranean 1999 | 1 | 8 | 1.9 | NA | |
| Mediterranean 2003 | 2 | 10 | 5.5 | 0.5 | |
| Mediterranean 2003 | 2 | 28 | 4.6 | 1.2 | |
| Mediterranean 2006 | 2 | 33 | 4.0 | NA | |
| Western Australia 1999 | 3 | 132 | 2.1 | NA | |
| Western Australia 2011 | 4 | 66 | 4.9 | 0.95 | |
| Great Barrier Reef 2016 | 2 | 55 | 4.0 | 2.6 | |
| Tasman Sea 2015 | 2 | 252 | 2.7 | NA | |
| Northwest Atlantic 2012 | 3 | 132 | 4.3 | 0.1–0.3 | |
| Northeast Pacific 2015 ("The Blob") | 3 | 711 | 2.6 | 4.5–11.7 | |
| Santa Barbara 2015 | 3 | 93 | 5.1 | NA | |
| Southern California Bight 2018 | 3 | 44 | 3.9 | NA | |
| Northeastern Atlantic 2023 | 5 | 30 | 4.0–5.0 | NA | |
| Northern Pacific 2025 | 5 | 107+ | 1.0-5.0 | ~8 |
Habitat degradation occurs through restructuring and sometimes complete loss of habitats such as seagrass beds, , and . These habitats contain a significant proportion of the oceans' biodiversity. Changes in currents and thermal environments have already shifted many marine species' ranges away from the equator. Large range shifts, along with outbreaks of toxic algal blooms, have impacted species across taxa. Management of migrant species becomes increasingly difficult and food web dynamics shift.
SST increases are linked to a decline in species abundance such as the mass mortality of 25 Benthic zone species in the Mediterranean in 2003, sea star wasting disease, and coral bleaching events. Marine heatwaves in the Mediterranean Sea during 2015–2019 resulted in widespread mass sealife die-offs in five consecutive years.
Repeated marine heatwaves in the Northest Pacific led to dramatic changes in animal abundances, predator-prey relationships, and energy flux throughout the ecosystem. Marine heatwave events were expected to impact species distribution.
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