Heatwaves are long periods of hotter-than-usual weather. They happen both on land and in the ocean. In the ocean, these hot periods are called marine heatwaves. Marine heatwaves can be deadly for marine life such as fish, seabirds, and corals. In the past decade, marine heatwaves have become more frequent, and more intense. Every marine heatwave is harmful to marine ecosystems. Marine heatwaves happen all around the world, from the Pacific Ocean to the Atlantic Ocean to the Mediterranean Sea. Climate scientists predict that, in the future, marine heatwaves will last longer and be more frequent, which will inevitably create more problems for marine life and humans. Therefore, we need to better understand why marine heatwaves occur and how they impact life in the ocean.
Heatwaves, which are long periods of hot weather, are becoming more common and intense due to global warming. When the air gets too hot, water evaporates and causes very dry weather conditions, which can trigger bushfires, droughts, and damage to crops. Heatwaves are responsible for thousands of deaths around the world each year , and they threaten many animals that cannot survive long periods of heat. Heatwaves can occur in the ocean as well, and we call these events marine heatwaves.
Marine heatwaves are long periods of warmer-than-usual temperatures, measured at the surface of the ocean [2, 3]. They can persist for days, weeks, or even months, and they can occur at any time of year. Marine heatwaves have disastrous consequences for marine animals and plants, such as the destruction of coral reefs and kelp forests. Many species of fish, marine mammals, and seabirds do not survive major marine heatwaves. Death of marine species is also a major problem for humans, as many people living along the coast rely on fish and other ocean organisms for food.
Marine heatwaves happen all around the world and are becoming more frequent (Figure 1). Notable marine heatwaves occurred along the western Australian coast in 2011, in the Tasman Sea in 2015–2016, in the Pacific Ocean in 2013–2015 (called “The Blob”), and in 2019 . In 2015–2016, an important marine heatwave happened over the northwest Pacific Ocean. The water temperature was over 6°C warmer than usual. This event forced entire fish communities such as Chinook salmon, cod, and sockeye salmon to move far away where the water temperature was more bearable . Many sea lions, whales and seabirds were found beached because of the lack of food caused by the migration of fish .
Climate scientists predict that, due to global warming, future marine heatwaves will become more frequent and last longer. Why do marine heatwaves happen? Understanding what causes these dangerous heatwaves is important if we want to protect life in the ocean.
What Causes Marine Heatwaves?
Temperatures at the surface of the ocean vary all year long. The surface water gets colder in winter and warmer in summer, due to the energy of the sun. This natural cycle benefits marine life, which migrates according to the water temperature. Certain weather conditions can increase the surface water temperature significantly. If these conditions last for a long time, we call it a marine heatwave. Marine heatwaves are often unbearable for marine life.
The main factors that influence the ocean surface temperature are ocean currents and the exchange of heat between the ocean surface and the atmosphere  as shown in Figure 2. Ocean currents can bring warm waters to a colder area. Additionally, the atmosphere controls heat transfer into and from the ocean, via the winds. When winds are strong, heat can escape easily from the ocean, just like wind blowing on your skin draws heat away from your body. The opposite is also true: if the winds are weak, the heat remains trapped at the ocean surface for longer. We can broadly classify marine heatwaves into two categories: those driven by the ocean, and those driven by the atmosphere.
An ocean-driven marine heatwave occurred in 2011 offshore of western Australia. During that time, a climate phenomenon called La Niña modified the ocean currents and brought warm water to the western Australian coast. The ocean temperatures were more than 3°C above normal. Weak winds allowed this heatwave to persist for several months .
“The Blob” in the northwest Pacific, on the other hand, was caused by the atmosphere. The atmospheric pressure (the weight of air) over the region was higher than usual, which led to very weak winds. Those weak winds prevented the heat from escaping to the atmosphere, resulting in the strongest marine heatwave ever recorded at that time .
Now that we know why marine heatwaves happen, we can make projections for the future. Marine heatwaves have already become 34% more frequent and 17% longer between 1925 and 2016 . At current rates of global warming, climate scientists predict that global ocean and atmospheric temperatures will keep increasing. As the ocean surface warms up, marine heatwaves will probably become more frequent, more intense, and last longer [3, 8].
Marine Heatwaves Are Disastrous for Marine Life
Heatwaves have disastrous impacts on marine life. Just like heatwaves on land worsen human health issues and can kill people (for example, through heat stress or bushfires), marine heatwaves affect the health of marine species . In some cases, heat stress destroys marine habitats (Figure 3). Fish, whales, and sea turtles are forced to migrate to cooler areas of the ocean. Although these animals naturally migrate to hunt for food and reproduce, marine heatwaves force them to change their migration behavior more drastically.
For example, during “The Blob,” fishermen unexpectedly saw blue sharks and thresher sharks in the Gulf of Alaska in summer, when these animals are usually found around the coast of California . Many other species of fish, such as coho salmon and Alaskan pollock, experienced impacts on their migration patterns as well. This shift in where fish are found can have an important effect on the food chain .
Other marine species, like corals, cannot change location. The 2011 western Australia marine heatwave caused extreme damage to immovable plants and animals. It wiped out kelp forests and caused coral bleaching, which happens when the algae living on corals, which give them their color, escape from the heat, leaving the corals white .
What Does the Future Hold for Marine Life?
Each time marine heatwaves occur, they alter and often destroy marine ecosystems . Some species can adapt to the extreme temperatures, such as fish that can swim away to cooler areas. Many species, like corals or kelp forests, cannot escape the heat—although some are better at resisting it. In general, marine heatwaves are destructive and their impacts on the marine ecosystem could be greater than we currently know. For example, coral reefs provide a habitat for fish, sea stars, crabs, sponges, and clams. Thus, coral bleaching due to marine heatwaves could lead to disastrous consequences for a lot of marine life.
The ocean is valuable; it provides vast economic benefits, from food (such as fish and shellfish) to tourism to all the jobs related to those industries, along with energy for our homes, and even medicines. We need to protect ocean inhabitants and resources. Actions to halt global warming can prevent marine life from dying, including reducing our emissions of carbon dioxide. Marine heatwaves are becoming increasingly more common, so we need to find ways to protect and maintain marine ecosystem systems and all the animals that live there. One solution is to create marine protected areas, which are regions of the ocean where human activities, from fishing to mining, are limited or completely banned. These areas do not prevent damage caused by marine heatwaves, but they serve as sanctuaries where marine life can develop peacefully, away from human pressure, and they allow marine life to recover from periods of intense stress such as those caused by marine heatwaves . Understanding the impacts of marine heatwaves on life in the ocean, and developing strategies to counteract their impacts, is important as healthy marine ecosystems are necessary to our livelihood and wellbeing.
Heatwaves: ↑ Long periods of abnormally hot weather that can last days, weeks, or even months.
Global Warming: ↑ Gradual increase of the Earth’s air temperature caused by excess carbon dioxide in the atmosphere from human activities, such as burning fossil fuels and deforestation.
Ocean Currents: ↑ Continuous movement of large quantity of water in the ocean, driven by winds and temperature differences.
La Niña: ↑ A weather pattern in the equatorial Pacific Ocean, causing water to become much colder than usual off the coast of Peru, and much warmer off the coast of Australia.
Atmospheric Pressure: ↑ Force exerted by the weight of the air above the Earth’s surface.
Coral Bleaching: ↑ Whitening of corals caused when the algae normally living on them, which give corals their color, leave or die. This happens when the ocean gets too warm.
Marine Protected Areas (MPAs): ↑ Protected areas of the ocean providing marine ecosystems with an environment with limited to no stress caused by human activities such as fishing, boating and leisure activities.
Conflict of Interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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 ↑ Alaska Fisheries Science Center. 2021. Most Recent Data Shows Gulf of Alaska Ecosystem Slow to Return to Pre-Heatwave State. NOAA Fisheries. Available online at: https://www.fisheries.noaa.gov/feature-story/most-recent-data-shows-gulf-alaska-marine-ecosystem-slow-return-pre-heatwave-state
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 ↑ Di Lorenzo, E., and Mantua, N. 2016. Multi-year persistence of the 2014/15 North Pacific marine heatwave. Nature Clim Change 6, 1042–1047. doi: 10.1038/nclimate3082
 ↑ Oliver, E. C. J., Donat, M. G., Burrows, M. T., Moore, P. J., Smale, D. A., Alexander, L. V., et al. 2018. Longer and more frequent marine heatwaves over the past century. Nat. Commun. 9:1324. doi: 10.1038/s41467-018-03732-9
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 ↑ Freedman, R. M., Brown, J. A., Caldow, C., and Caselle, J. E. 2020. Marine protected areas do not prevent marine heatwave-induced fish community structure changes in a temperate transition zone. Sci. Rep. 10:21081. doi: 10.1038/s41598-020-77885-3