Find out more about marine heatwaves, their vast impact on marine life & ecosystems, and how researchers are monitoring them
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When we think of heatwaves, we tend to think of those on land: sweltering humans and land mammals with high temperatures. However, another type of heatwave exists which has just as devastating an impact – marine heatwaves.
Since 1982, the number of marine heat wave events has doubled. And the total number of days with marine heatwaves – averaged across the globe – has increased by 50 per cent over the last century.
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Already, human-induced climate change has caused a 50 per cent increase in marine heatwaves between 2011 and 2021. By the end of the century, if current levels of fossil fuel consumption and deforestation continue, marine heatwaves could become 20 to 50 times more frequent, and up to 10 times more intense.
But what exactly are marine heatwaves? How do they work, and where are they getting worse? Read on to find out…
What are marine heatwaves?
Marine heatwaves are defined as periods of unusually warm ocean temperatures that last from several days to months. They can occur at different locations in the ocean, and their magnitude has increased across the last few decades.
Averaged across the globe, in the 1940s, the sea surface typically experienced around 15 days of extreme heat annually. Today, that number has jumped to almost 50 days per year.
Out of all the seas in the world, the Mediterranean Sea is most affected by marine heatwaves, with the area experiencing significant warming throughout the last several years.
However, much of the world’s ocean is impacted by warming temperatures – nearly ten per cent of the global ocean hit record-high temperatures during the 2023–2024 period.
Without climate change, 47 per cent of marine heatwaves observed between 2000 and 2020 would not have happened at all.
Marine heatwaves are caused by two main drivers. Firstly, surface heat flux: heating from the atmosphere which tends to occur when a high-pressure system sits above a region of water for an extended period. Secondly, a process known as advection – when warmer waters move into the region via ocean currents.
Heatwaves caused by surface heat flux tend to be shallower and shorter in length. Conversely, advection heatwaves tend to be deeper and longer.
What impact do they have?
The most direct impact of marine heatwaves is the triggering of mass mortality events for many species and habitats. In particular, organisms like coral, algae and sponges, are prone to extreme thermal stress, with events such as coral bleaching increasingly common. As well as this, some fish species migrate away to seek more temperate waters while other species – often invasive ones – arrive from other waters.
High temperatures during heatwave events can also cause stress to marine life, pushing animals past their optimal temperature range. Such a scenario could lead to reduced fitness and in more extreme cases, death. Species that are immobile, such as many benthic invertebrates, will not have the ability to escape high temperatures.
In addition to these impacts, heat waves can reduce the oxygen available in water for marine animals to breathe.
Out of all underwater environments, kelp forests, coral reefs and seagrass meadows have been identified as being at high risk from the effects of marine heatwaves.
Intertidal areas – the area that lies between the low and high tide water mark – are particularly at risk, as they can experience the impacts of both atmospheric and marine heatwaves. When these occur simultaneously, intertidal marine life can experience extreme air temperatures coupled with harmful warmer waters.
Marine heatwaves can also have a significant impact on land. They are able to fuel powerful tropical cyclones or hurricanes, and have been associated with heavy precipitation. Heavy rainfall is caused by them as warming ocean temperatures favour strong evaporation, which increases the probability of strong rainfall events across land.
Beginning in 2024, the longest, largest and most intense marine heatwave ever recorded in Western Australia killed coral throughout an area stretching 1,500 kilometres, and lasted until May 2025. The amount of bleaching and coral death on reefs ranged from medium (11 to 30 per cent) to extreme (greater than 90 per cent).
How do we know heatwaves are happening?
Scientists have been studying sea surface temperatures since the 1980s, using satellite data to monitor marine heatwaves from space. On Earth, measurements are taken onboard ships as well as permanent ocean observatories, such as the National Oceanography Centre’s Porcupine Abyssal Plain Sustained Observatory (PAP).
Located at a depth of 4,850 metres in the Northeast Atlantic, this observatory has been operational since 1985 and is used to monitor long-term changes throughout the water column.
As well as this, networks of autonomous robots and floating instruments continuously collect ocean data through the water column. For example, the ARGO network is a global array of instruments that drift with ocean currents, taking 100,000 observations of ocean temperatures and salinity every year.
Studies have also shown exactly where marine heatwaves can crop up. One such study found there are regional marine heatwave spots in the Southern North Sea and the English Channel. Events here are weaker than in other areas around the UK, but last longer.
With the number of marine heatwaves increasing, good forecasting can help to reduce their impact. In Australia, a quarter of the endangered red handfish population was safely moved into aquariums ahead of a marine heatwave, before being released again once waters had cooled.
Elsewhere, in the US, some coral and conch species were manually moved to deeper, cooler waters, helping to protect them from rising oceanic temperatures.
