How climate change has pushed our oceans to the brink of catastrophe

This time last year, Dean Grubbs and his colleagues were celebrating a conservation success story. The star was the smalltooth sawfish, a type of large ray with a saw-like snout trimmed with tiny teeth. A victim of coastal development and bycatch, in 2003 it became the first sea fish to get US federal protection under the Endangered Species Act. By 2023, Florida’s population of sawfish, the last in the US, was on the rise. “We were excited. We were seeing the population start to recover,” says Grubbs, a marine ecologist at Florida State University.

Then disaster struck. In January, sawfish were spotted thrashing in the shallows, spinning in frenzied circles and turning up dead. This came after months of smaller fish showing similar behaviour. Suddenly, Grubbs and his team were spending their days pulling dead sawfish out of the water. Only after months of investigations and tests is a culprit emerging: ocean heat. A record-breaking heatwave brought “hot tub” water temperatures to Florida’s coast in 2023, triggering a chain reaction that looks to have decimated the fragile sawfish population.

This is just one salutary tale. Something isn’t right in the world’s oceans. From orange algal blooms in the North Sea and a boom in gelatinous Bombay duck fish off China to disappearing “bottom water” in the Antarctic, there is growing evidence that extreme temperatures are wreaking havoc in our waters. After years of the oceans acting as silent sinks for excess human-caused heat, they are starting to creak under pressure – and we are finally waking up to how worried we should be.

Around 90 per cent of the excess heat trapped in the atmosphere since the industrial revolution has been absorbed by Earth’s oceans. They have, in effect, been doing us a huge favour by shielding life on land from its effects. “This means it’s not in the atmosphere, it’s not melting the ice sheets and it’s not warming the soil,” says Till Kuhlbrodt at the Met Office, the UK’s national weather service. This heat-storing service comes with a cost, though. For decades, sea temperatures have been steadily climbing. Between 1993 and 2022, global average sea surface temperatures rose by about 0.42°C per decade, says Gregory Johnson at the US National Oceanic and Atmospheric Administration (NOAA). But in March 2023, ocean temperatures began to rocket. “Global sea surface temperatures rose by about 0.28°C in just five months,” he says.

Climate change must share a large portion of the blame, but climate variability has compounded the problem. In 2023, an El Niño weather pattern developed, bringing warmer sea surface temperatures to parts of the tropical Pacific. This followed an unusual “triple dip” La Niña – three consecutive years of cooler waters in the Pacific, during which heat would have continued to build up deep below the surface. “You have three years of La Niña, you follow up on that with an El Niño and bang! The temperatures warm up,” says Johnson. “That’s something that caused this jump to be larger than normal.”

The effects have been dramatic, too. From January 2023 to May 2024, mass coral reef bleaching was confirmed in at least 62 countries and territories around the world. Such bleaching is a direct result of warming waters, with corals known to be at risk if temperatures climb by 1°C or more above their summer maximum.

Other changes – just as worrying – are less predictable. In Florida, researchers spent months trying to understand what was behind the mysterious sawfish deaths. The answer lay in the silty seabed. Down in that muddy layer lie bottom-dwelling, or benthic, algae, generally “minding their own business”, says Alison Robertson at the University of South Alabama. Some of these algal species produce toxins that can affect marine life, but their numbers are usually kept in check by other resident algae. However, when Robertson was called to Florida to investigate the sawfish deaths, sampling of the water yielded surprising results. “The level of these microalgae was elevated, not only at the benthos but also in the water column, which is very, very strange,” she says. The samples also revealed multiple different kinds of toxin-producing algae. “What we seem to have is a multi-stressor problem, with lots of different algae producing lots of different toxins,” she says. “The combined exposure of all those things causes the behaviour.”

Roberston and her colleagues think Florida’s marine heatwave in 2023 acted as a trigger event, disrupting the ecosystem and giving toxin-producing algae an opportunity to become dominant. That would explain why many different fish species – more than 50 – were seen displaying symptoms of neurotoxin poisoning. Sawfish may have been the hardest hit of larger marine species because they feed on prey buried below the seafloor, stirred up by their snout, and were therefore exposed to extremely high concentrations of toxins. Although sawfish have now stopped dying, it will take at least a year before Grubbs knows how devastating the event has been. With a likely adult population of below 1000 before the disaster, he fears it could be a “significant” blow to the species.

Such algal blooms are becoming more common around the world. Between 2003 and 2020, they increased their cover by 13.2 per cent and increased in frequency by almost 60 per cent, according to one study. Not all blooms are toxic. But they can still be a warning sign of an ecosystem under stress. “There’s definitely change afoot,” says Andrew Turner at the UK’s Centre for Environment, Fisheries and Aquaculture Science. “We are seeing the emergence of harmful algae in certain areas where it has never been seen before.”

In June 2023, a bloom of benign Noctiluca phytoplankton in the North Sea turned waters a scummy orange: the slick was so large it could be seen from space. This part of the North Atlantic has suffered some of the most extreme heat in the previous 18 months, with temperatures up to 5°C above normal in June this year. Things are so bad, NOAA scientists have described the Atlantic as “running a fever”. And colourful blooms aren’t the only symptom.

Ocean waters are naturally stratified into layers, with warmer, fresher, oxygen-rich water near the surface and cool, salty, nutrient-dense water further down. As the oceans heat up, there is less mixing between these layers, preventing nutrients from travelling to the surface and oxygen from sinking to the deep ocean. As well as setting the stage for algal blooms, this causes problems for phytoplankton, tiny, plant-like organisms that turn sunlight and carbon dioxide into sugars and oxygen. Increased stratification “prevents nutrients from coming up into the surface layer, where the phytoplankton need them,” says Angus Atkinson at Plymouth Marine Laboratory, UK. As a result, phytoplankton numbers in the English Channel during the northern hemisphere summer have halved over the past 35 years, he says.

The extreme ocean heat of 2023 may have turbocharged the problem. A 2024 preprint paper indicates that the production rate of phytoplankton, algae and bacteria slumped by 22 per cent globally in April 2023, when global sea surface temperatures were at record highs. The drop in production was particularly pronounced in the North Atlantic. Not only are there fewer phytoplankton, but the ones that are there are smaller. “That’s double trouble if you’re a grazer, because these tiny cells are too small for the traditional crustacean zooplankton to eat,” says Atkinson. This has a knock-on effect for commercially important fish such as mackerel, cod and herring, which feed on crustacean zooplankton. Meanwhile, jelly-like creatures such as salps thrive on smaller plankton. Last year saw a surge in them along the UK’s south-west coastline, highlighting the problem.

As ocean mixing declines, this could also cause havoc with the biological carbon pump, says Corinne Le Quéré at the University of East Anglia, UK. This is the process by which phytoplankton remove carbon dioxide from the atmosphere and ultimately transfer it as biomass from surface waters to the deep ocean. “This is a flux of carbon that is as big as the anthropogenic CO₂ emissions from human activities today,” says Le Quéré. But researchers fear that if biomass particles can’t sink as deeply into the ocean depths, the carbon they contain won’t be locked away for as long. Changes to this nutrient mixing will have immediate consequences for marine ecosystems, says Le Quéré, and longer-term impacts on global heating – with huge implications for humanity.

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Even in the coldest ocean waters, things aren’t normal. In Antarctica, the past two years have been a “rollercoaster”, says Ted Scambos at the University of Colorado Boulder. Until 2015, sea ice had been growing in extent each winter, but 2016 saw a sharp drop that largely stabilised until 2023, when another sharp drop saw sea ice coverage fall to a record-low maximum of 16.96 million square kilometres. Emerging data from the UK Met Office suggests 2024 will again be a near-record low year for sea ice extent in Antarctica.

Last year was “off the charts” for both low summer and winter sea ice, according to Scambos, with signs that warming waters around Antarctica are to blame. “It appears that the uppermost ocean is holding more heat than it used to. So, as the sea ice starts to extend northward, it hits ocean that still has residual heat left in it,” he says. This has sparked fears that the Antarctic is undergoing a “regime shift” that means sea ice may never recover to its previous levels.

It gets worse, though, because Antarctic “bottom water” is also disappearing. This cold, dense, oxygen-rich water cascades off the Antarctic continental shelf and sinks to great depths, driving a global network of ocean currents transporting oxygen into the deep Indian, Pacific and Atlantic Oceans. But increased meltwater and reduced sea ice formation means Antarctic waters are less salty, making it harder for them to sink. In the Weddell Sea, bottom water volume has shrunk by more than 20 per cent in the past 30 years. The result is a slowdown in Antarctic currents, with circulation down by almost a third since 1992, according to research published last year. “That’s a big change to that ocean circulation,” says co-author Matthew England at the University of New South Wales, Australia.

Changing currents

Slowing currents mean even less oxygen entering the deep ocean – with global implications for marine ecosystems. In summer 2021, another season of record-breaking heat, half of deep coastal waters from northern California to the Canadian border had oxygen levels so low they were harmful to marine life. Modelling shows that even if CO₂ emissions were stopped today, oceans would still lose more than 10 per cent of their pre-industrial oxygen content and life-sustaining deep ocean environments would decline by at least 25 per cent.

What’s more, warmer surface waters hold less oxygen, which is already causing disruption. In the East China Sea, numbers of Bombay duck – a gelatinous fish with a gaping jaw – are booming as warming waters and nutrient pollution have depleted oxygen levels. Bombay ducks thrive in a low-oxygen environment and are outcompeting native fish as oxygen levels drop.

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Warming ocean waters pose immediate danger for life on land, too. For one, hotter water takes up more space, which leads to sea level rise. In 2023, the global average sea level reached its highest point since satellite records began in 1993. Following the current trend, it could rise another 20 centimetres by 2050, increasing the frequency and severity of coastal floods.

As oceans get hotter, the risk of more powerful storms also increases. “If the sea surface temperature is now more than 1°C warmer over the North Atlantic, that means the air over the North Atlantic will also be warmer,” says Kuhlbrodt. For every degree Celsius that the atmosphere is warmer, it can hold 7 per cent more water vapour. “This means there’s a higher likelihood for extreme precipitation events and higher likelihood for particularly strong storms,” he says.

In July, Hurricane Beryl hit the Caribbean, the Gulf of Mexico and the US, causing widespread devastation and killing dozens. Beryl was the earliest Category 5 hurricane ever to develop and formed rapidly, strengthening from a tropical storm to a major hurricane in less than a day. Exceptionally high sea surface temperatures were a key factor in Beryl’s power and explain why the storm developed so early in the season. Usually, more powerful storms come later, after the Atlantic has warmed up over the summer months.

How worried should we be about this ocean chaos? Very, says Johnson. “This is probably the biggest crisis that humanity is facing.” Le Quéré feels similarly, warning that we are taking “big risks” by pushing ocean ecosystems into crisis. It is a sentiment shared by most oceanographers, who fear that we tend to forget just how much humanity depends on the oceans.

How to avoid catastrophe

But there are things we can do to reduce the stress on them. These include limiting fishing, chemical pollution and other harmful human activities. Better conservation efforts would benefit fragile marine populations, such as Florida’s sawfish. More information will help us understand what is happening in the oceans. For example, researchers at Plymouth Marine Laboratory have just deployed a new system to monitor plankton hourly, providing greater insight into how the plankton mix is changing in response to global heating. And detailed data collection, backed by artificial intelligence, promises more accurate forecasting techniques for everything from algal blooms to hurricanes, not to mention more sophisticated climate models. But oceanographers agree that immediate, rapid cuts to greenhouse gas emissions are the only real way to avoid catastrophe over the long term.

The El Niño weather pattern officially ended in June, and an expected return of La Niña conditions later this year may bring some cooling relief to the world’s oceans. However, although global sea surface temperatures seem to be slowly subsiding, they are still well above the long-term average. Scientists are watching closely: if temperatures remain high, it could be a sign that global heating has entered a new phase of acceleration.

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Madeleine Cuff is an environment reporter for New Scientist