Extreme underwater heat is killing coral, fish and seagrass, threatening fisheries, coastal economies and communities worldwide.
Mass fish deaths and haunting stretches of bleached coral are among the most visible signs of a growing problem beneath the ocean’s surface: marine heatwaves.
These extreme events occur when ocean temperatures remain abnormally high for an extended period. They may be largely invisible from shore, but underwater they can wreak havoc, killing coral, seagrass and fish and reshaping marine ecosystems.
As climate change continues to warm the world’s oceans, marine heatwaves are becoming more frequent and severe. They are also lasting longer.
The events cannot be stopped entirely, but researchers say understanding how and why they develop, reducing greenhouse gas emissions and preparing vulnerable ecosystems for extreme heat can help limit the damage.
The stakes extend far beyond marine wildlife. Marine heatwaves can cost fisheries, aquaculture businesses and tourism operators billions of dollars, damage culturally important marine environments and destroy habitats that naturally absorb carbon.
They are increasingly a threat to human health and well-being as well.
RELEVANT SUSTAINABLE GOALS
What Is a Marine Heatwave?
Marine heatwaves are naturally occurring weather events that can form when hotter water near the ocean surface does not mix with cooler water below.
Such conditions are most common during hot, dry and windless weather.
Marine heatwaves can also develop when unusually strong currents transport additional heat from tropical waters into normally colder parts of the ocean.
Scientists identify these events by measuring ocean temperatures and comparing them with historical averages, establishing what would normally be expected at a particular location and time of year.
A marine heatwave occurs when ocean temperatures are hotter than 90 per cent of historical temperature records. Those abnormally high temperatures must persist for at least five consecutive days for the event to be classified as a marine heatwave.
Because the threshold is based on normal seasonal conditions for individual regions, marine heatwaves can occur anywhere and during any season.
A water temperature of 25 degrees Celsius at Western Australia’s Ningaloo Reef, for example, may not appear unusually warm for the tropical location. A temperature of 25 degrees Celsius in the waters surrounding Tasmania, however, would be considered extreme.
Terrestrial heatwaves operate on a similar principle, although a heatwave on land needs to persist for only three days to receive the classification. Minimum nighttime and maximum daytime temperatures must also be unusually high.
Some Marine Species Can Escape. Others Cannot
Every marine ecosystem is potentially exposed because marine heatwaves can occur in any part of the ocean.
What happens to individual species depends partly on whether they can escape the heat.
Mobile species such as fish may swim toward cooler waters. Coral, kelp and bottom-dwelling creatures such as sea urchins do not have that option.
They must endure the extreme temperatures where they are.
Heat stress can interfere with their ability to reproduce. Marine organisms may also suffer indirectly if species lower in the food chain are wiped out, leaving less food available throughout an ecosystem.
The result is an underwater disaster that can extend from individual species through entire marine food webs.
The Economic Damage Reaches Coastal Communities
The effects do not remain underwater.
Marine heatwaves can have severe economic and cultural consequences for coastal communities, causing billions of dollars in lost income for fisheries, aquaculture operations and tourism businesses.
They can also damage culturally significant locations.
One example is Shark Bay, a World Heritage-listed marine ecosystem off the coast of Western Australia.
Marine heatwaves can also destroy blue carbon habitats. These include ocean-based ecosystems such as mangroves and seagrass that absorb carbon and help cool the planet.
Their loss therefore threatens marine biodiversity while weakening natural systems that store carbon.
Western Australia’s 2011 Heatwave Showed the Damage on Land and Sea
Western Australia experienced a particularly severe example in February 2011, when a massive compound heatwave struck the region.
A compound heatwave occurs when marine and terrestrial heatwaves happen together.
The event decimated ecosystems across Western Australia.
On land, trees died and the endangered cockatoo population crashed. Underwater, widespread coral bleaching occurred across the Pilbara, Ningaloo and Houtman Abrolhos Islands regions.
The event demonstrated how extreme heat can simultaneously disrupt ecosystems above and below the water.
Record Marine Heatwave Killed About Half the Coral on the North West Shelf
Another major marine heatwave began off Western Australia’s coast in September 2024 and persisted for several months.
It became the state’s longest, largest and most severe marine heatwave ever recorded.
About half of the coral in the North West Shelf region was killed.
The scale of the event offered another indication of the vulnerability of marine ecosystems when abnormally high ocean temperatures persist over long periods.
European Seas Face Temperatures Up to 5°C Above Average
More recently, sea surface temperatures have surged during the 2026 European summer.
In some locations, temperatures have risen as much as 5 degrees Celsius above average.
The extreme conditions threaten marine ecosystems across the Mediterranean Sea, North Sea and Baltic Sea.
These events underline the global nature of marine heatwave risks. Because the definition is based on what is historically normal for a particular location and season, ecosystems in both tropical and cooler waters can experience dangerous heat.
Climate Change Is Raising the Ocean’s Thermal Baseline
Marine heatwaves have always formed as part of the planet’s naturally variable climate system.
Climate change, however, is altering the conditions in which they occur.
The ocean’s thermal baseline, its underlying natural temperature, is rising because of climate change. Temperature fluctuations that occur on top of this increasingly warm baseline compound the effects of background warming.
As a result, marine heatwaves are becoming more common, more severe and longer-lasting.
Climate patterns including El Niño and La Niña can further intensify the events.
In Australia, El Niño typically warms waters along the east coast, increasing the likelihood of severe marine heatwaves there.
La Niña tends to push warm water toward Australia’s west coast, contributing to extreme ocean temperatures off Western Australia.
But climate change means large-scale patterns such as El Niño and La Niña are no longer required for a marine heatwave to develop.
Even relatively small changes in these climate drivers may now bring extreme ocean heat to either coast.
Some of the Worst Heatwaves May Be Hidden Beneath the Surface
Understanding the scale of the problem presents another challenge.
More research is needed into changes in air and ocean currents that contribute to marine heatwaves.
Researchers also need to pay greater attention to the ocean below the surface.
Satellites monitor sea surface temperatures, but some severe marine heatwaves remain concealed underwater. These events can devastate marine life without being detected from surface observations.
Better knowledge of the subsurface ocean could therefore improve understanding of where marine ecosystems face the greatest risks.
Cutting Greenhouse Gas Emissions Remains the Most Important Step
The most important measure for limiting the impact of marine heatwaves is reducing greenhouse gas emissions and preventing further ocean warming.
At the same time, researchers are investigating ways to help vulnerable ecosystems adapt to increasingly extreme conditions.
One approach involves introducing thermally tolerant species into vulnerable regions.
Researchers are also examining marine cloud brightening, a technique intended to temporarily reduce ocean heating by making clouds more reflective.
Neither eliminates the need to address the underlying warming of the ocean.
The consequences extend beyond coral reefs, seagrass beds and fish populations.
Researchers warn that unusually warm oceans can contribute to stronger storms, harmful algal blooms, reduced seafood supplies and disrupted livelihoods.
For people living in coastal communities, those disruptions can also contribute to anxiety, grief and depression.
Marine heatwaves may begin beneath the surface, largely unseen from land. But as the oceans continue warming, their consequences increasingly reach far beyond the water, affecting ecosystems, economies and the well-being of the communities that depend on them.
You may also be interested in :
Major Blind in Ocean Carbon Research Could Undermine Global Climate Predictions
