How do ‘marine heatwaves’ affect the ocean – and what can be done?

As we enter El Niño, periods of surging temperatures at sea are predicted to grow more frequent and intense. What do they do to marine life, and can we adapt?
<p>A bed of Neptune grass on a beach in Mallorca, Spain. Sudden rises in ocean temperature, known as marine heatwaves, can wipe out sensitive marine ecosystems like seagrass meadows, which are vital to stemming dangerous climate change and nature collapse. (Image: Clara Margais / Alamy)</p>

A bed of Neptune grass on a beach in Mallorca, Spain. Sudden rises in ocean temperature, known as marine heatwaves, can wipe out sensitive marine ecosystems like seagrass meadows, which are vital to stemming dangerous climate change and nature collapse. (Image: Clara Margais / Alamy)

On 4 July 2023, the World Meteorological Organization declared the beginning of an El Niño phase, a climate pattern that drives up temperatures across land and sea.

Past weather events provide clues about what extreme temperatures could mean for the ocean. For instance, in December 2010, a wave of unusually warm water swept across the luxuriant and biodiverse seagrass meadows of Australia’s Shark Bay. In a matter of days, it destroyed a third of the habitat, unthreading the delicate seagrass quilt and, over the next three years, releasing between 2 and 9 billion tonnes of carbon into the atmosphere. “The losses there were phenomenal,” says Kathryn Smith, a researcher with the UK’s Marine Biological Association.

Scientists call the event a “marine heatwave”, meaning a period of exceptionally high water temperature that starts suddenly and continues for days to months, distinguishing it from long-term warming trends. Like heatwaves on land that threaten terrestrial ecosystems, heatwaves at sea harm marine life, posing “a clear and present threat to the systems we depend on,” says Sarah Cooley, director of climate science at the Ocean Conservancy.

These impacts are expected to grow. The UN’s climate science body, the IPCC, projects that by 2100 marine heatwaves will be up to 50-fold more frequent, and 10-fold more intense compared to pre-industrial times. Scientists are now developing ways to forecast these events. Their research can feed into measures that mitigate the threats for vulnerable habitats, species, and the coastal communities that depend on them.

What are the impacts of heatwaves?

The ocean absorbs 90% of the excess heat caused by greenhouse gas emissions in the atmosphere. On top of this, climate change is amping up weather conditions that inject even more heat into the planet’s largest ecosystem.

One of these is high-pressure systems. While low-pressure systems bring cool, cloudy air that draws heat out of the ocean, attracting colder, nutrient-rich waters to the surface, high-pressure systems disrupt these. They bring warmer, windless, cloudless conditions in which the sun can heat the water unimpeded. Fewer nutrients rise to the water’s surface to support the base of the marine food chain, such as krill and phytoplankton.

In 2014, these factors combined to create a gigantic sauna in the North Pacific. Hundreds of kilometres wide and 6C warmer than average, it was known as “the Blob.” The sudden displacement of nutrients spelled disaster for fish, seabirds, and cetaceans who had to migrate elsewhere to find food, or simply starved.

By the time the Blob had dissipated, an estimated one million seabirds had died, along with countless crustaceans, fish and seals. A record number of whales became entangled in fishing gear when searching for food closer to shore.

humpback whale trapped in a net
Divers attempt to rescue a humpback whale trapped in a fishing net in Spanish seas, May 2023 (Image: Alamy)
scuba diver holds underwater camera to photograph bleached coral
Documenting bleached coral in Australia’s Great Barrier Reef (Image: The Ocean Agency / The Ocean Image Bank)

Warm water surges can also make conditions unbearable in sensitive hubs of biodiversity like seagrass meadows, coral reefs, and kelp beds. Compared to general ocean warming, heatwaves lead to faster bleaching, more coral death, and the decimation of kelp forests. Higher water temperatures also trigger suffocating algal blooms that drive large scale die-offs of marine life.

For instance, kelp forests along a 100km stretch of western Australia were erased by the same heatwave that destroyed the Shark Bay seagrass fields in 2010. “That was 12 years ago, and the kelp hasn’t returned,” says Smith, whose primary research focus is marine heatwaves.

What causes these events?

Scientists are still unpicking the meteorological triggers behind these phenomena, but they are linked to anthropogenic climate change. One hypothesis is that as the Arctic warms – three times faster than the planetary average – its temperature difference with the tropics is reduced. Svenja Ryan, a physical oceanographer at the Woods Hole Oceanographic Institute, explains that this reduction might weaken the jet stream that usually pushes a band of rain and wind around the centre of the planet. The air current then becomes more vulnerable to intrusion by high pressure systems, which block usual atmospheric processes and form hovering hot air islands over land and sea.

The changes in the strength and direction of ocean currents could also trigger marine heatwaves, as some of the currents are transporting warm water to regions that they didn’t before.

World map showing marine heatwaves, in areas of red, orange and yellow, on June 19 2023
Sea surface temperature anomalies recorded on 19 June 2023. The cluster of red and orange at top right shows the record-breaking marine heatwave that hit the north-east Atlantic in late June. (Image: NOAA Marine Heatwave Watch)

What are the costs of hotter seas?

Comparisons against historical datasets show that the number of marine heatwave days has increased by 54% since 1925, and eight of the hottest 10 on record have occurred in the last 13 years. “They are happening, they are really intense in certain regions, and they can really impact local communities and also economies,” Ryan says.

This socioeconomic toll is increasingly visible. Smith of the UK’s Marine Biological Association says: “There are lots of coastal communities who are losing their entire income in one hit from marine heatwaves.”

After the Blob, the Gulf of Alaska’s cod fishery, worth $100 million annually, collapsed. A 2016 heatwave off Chile’s coast triggered a toxic algal bloom that destroyed 20% of the country’s salmon farming production for the year, and cost the industry $800 million. Meanwhile, a survey shows that if general ocean warming continues and coral bleaching worsens in Australia’s Great Barrier Reef, the country will suffer $1 billion in lost tourism.

Increasingly popular carbon credit schemes that depend on intact seagrass meadows and kelp forests may also be undermined by marine heatwaves, Smith explains. Her research shows that 34 ocean heatwaves over the last 25 years have in fact cost several billion dollars annually in lost fisheries, tourism, and carbon storage. Harder to measure is the lost cultural value caused by heatwaves, and their contribution to extreme weather events like hurricanes. 

How to adapt to heatwaves?

To stop excessive heat warming our oceans at root, we need to cut greenhouse gas emissions. But Cooley says it will take a while for the effects of decarbonisation to show. “We have so much heating that’s sort of ‘locked in’, and [in the meantime] the heatwaves will be getting worse and more extreme,” she explains. “But we know that we can decrease the other things under our control.”

Strengthening ecosystem resilience gives environments and their inhabitants the best chance of surviving warmer seas, Cooley says. Therefore, measures like reducing pollution and protecting more habitats like kelp beds and seagrass meadows can help mitigate the risks of marine heatwaves. “Having a large bank of a healthy ecosystem is still an insurance policy for us,” she adds.

a person dressed in blue holds a piece of burlap sack, which has seagrass seedling tied to it
Seagrass shoots, tied to a square of burlap, ready to be planted in Florida’s Indian River Lagoon, March 2023. Ecosystem restoration projects like this can help to make the ocean more resilient to future heatwaves. (Image: Kim Shiflett / NASA)

Meanwhile, there’s a growing effort to forecast marine heatwaves, as we do those on land. The nascent science still requires more research looking into how these phenomena influence ocean currents, chemistry and food availability, and even how deep into the sea heatwaves reach. This knowledge is essential if we are to match the predictive precision we have on land.

But research suggests that under certain meteorological conditions, we could forecast some marine heatwaves up to a year in advance, says Michael Jacox, an oceanographer at the US National Oceanic and Atmospheric Administration and lead author on the study that developed this estimate.

We can’t save it all, so what do we try and protect?
Kathryn Smith, Marine Biological Association

“I think the real crux of [forecasting] is in the translation to impacts, and then decision-making,” Jacox adds. More detailed information on when and where marine heatwaves will roll in creates room to adapt. Conservationists could pinpoint habitats in their path, and perhaps even introduce measures to shield them against the worst of the warming. Fishers could switch to less-threatened species, or move aquaculture pens out of harm’s way. Where livelihoods are threatened, forecasts could trigger financial support for communities most affected. Ideally, proactive national policies that recognise the reality of marine heatwaves would guide and support the interventions.

Today, Shark Bay’s seagrass meadows still bear the scars of the 2010 heatwave. Many ecosystems will face a similar threat from an increasingly febrile ocean, especially if events like El Niño heap more heat pressure onto it. Smith of the UK’s Marine Biological Association says: “We can’t save it all, so what do we try and protect? It’s about learning what the change is, and where the focus needs to be.”