Marine heatwave

A marine heatwave is a period of abnormally high seawater temperatures compared to the typical temperatures in the past for a particular season and region. Marine heatwaves are caused by a variety of drivers. These include shorter term weather events such as fronts, intraseasonal events (30 to 90 days), annual, and decadal (10-year) modes like El Niño events, and human-caused climate change. Marine heatwaves affect ecosystems in the oceans. For example, 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 over vast areas, persist for weeks to months or even years, and occur at subsurface levels.

Major marine heatwaves have occurred for example in the Great Barrier Reef in 2002, in the Mediterranean Sea in 2003, in the Northwest Atlantic in 2012, and in the Northeast Pacific during 2013–2016. These events have had drastic and long-term impacts on the oceanographic and biological conditions in those areas.

Scientists predict that the frequency, duration, scale (or area) and intensity of marine heatwaves will continue to increase. This is because sea surface temperatures will continue to increase with global warming. The IPCC Sixth Assessment Report in 2022 has summarized research findings to date and 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 confirms earlier findings in a report by the IPCC in 2019 which had found that "marine heatwaves [...] have doubled in frequency and have become longer lasting, more intense and more extensive (very likely).". The extent of ocean warming depends on greenhouse gas emission scenarios, and thus humans' climate change mitigation efforts. Scientists predict 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.

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."

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 term marine heatwave was coined following an unprecedented warming event off the west coast of Australia in the austral summer of 2011, which led to a rapid dieback of kelp forests and associated ecosystem shifts along hundreds of kilometers of coastline.

Categories
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 heatwaves have been studied at the sea surface for more than a decade, they can also occur at the sea floor.

Drivers
The drivers for marine heatwave events can be broken into local processes, teleconnection processes, and regional climate patterns. Two quantitative measurements of these drivers have been proposed to identify marine heatwave, mean sea surface temperature and sea surface temperature variability.

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 distant areas. For marine heatwave, the teleconnection process that play a dominant role are atmospheric blocking/subsidence, jet-stream position, oceanic kelvin waves, regional wind stress, warm surface air temperature, and seasonal climate oscillations. These processes contribute to regional warming trends that disproportionately effect Western boundary currents.

Regional climate patterns such as 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, like Pacific decadal oscillations (PDO), and anthropogenic ocean warming due to climate change.

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.

Climate change as an additional driver
Scientists predict that the frequency, duration, scale (or area) and intensity of marine heatwaves will continue to increase. 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.

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. 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).

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.

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, Isle of Man, 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.

On marine ecosystems
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 mortality 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' ranges northward, while temperate species have lost their southern limits. Large range shifts, along with outbreaks of toxic algal blooms, have 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.

Increases in sea surface temperature have been linked to a decline in species abundance such as the mass mortality of 25 benthic species in the Mediterranean in 2003, sea star wasting disease, and coral bleaching events. Climate change-related exceptional 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. The impact of more frequent and prolonged marine heatwave events will have drastic implications for the distribution of species.

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.

Options for reducing impacts
To address the root cause of more frequent and more intense marine heatwaves, 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.