A climate-disrupted ocean is pushing sharks, rays and other species to flee ever-hotter water in the tropics, only for them to be killed by increasingly intense upwells of cold water from the depths, a study has found.

One of the authors of the paper described the “eerie” aftermath of a mass die-off of more than 260 marine organisms from 81 species in a singular event of extreme cold upwelling off the coast of South Africa in 2021.

The paper, published in Nature Climate Change on Monday, found that shifts in ocean currents and pressure systems driven by climate breakdown were increasing the frequency and intensity of upwellings, which may in turn increase the vulnerability of migratory species such as bull sharks.

Scientists focused on the mass die-off event in 2021, which they were able to track in unusually precise detail because one of affected creature that survived was a bull shark that had been satellite tagged. They found it had been caught in water that fell more than 10C below the temperature that such tropical species were used to.

The paper details how the shark changed its behaviour in an attempt to avoid the cold areas. It swam much closer to the surface than normal and moved outside its normal migration pattern.

Many of the affected sea creatures’ carcasses washed up on the shore of South Africa, including the pup of a big manta ray that had been aborted by its traumatised mother.

“It was eerie to see so many species washed up dead,” said Ryan Daly, one of the authors of the paper. He said he was surprised that even the very mobile species, such as manta rays and bull sharks, were caught in the upwelling. “You’d think they would have swum away but they got squeezed. They couldn’t escape,” he said.

To understand the broader trends behind the die-off, the scientists tagged other sharks and used 41 years of sea surface temperature data and 33 years of wind records to investigate the frequency and intensity of cold “killer events” inshore of the Indian Ocean’s Agulhas current and the east Australian current in the past 30 years.

They found cold upwelling events had increased in frequency and intensity in these regions between 1981 and 2022. Other species killed in such events include whale sharks, convict surgeonfish, bigeye trevallies and common blacktip sharks.

Tagged bull sharks appeared to change their behaviour to avoid sudden temperature drops by swimming closer to the surface, sheltering in bays and estuaries and only moving to the extent of their poleward distribution during warm seasons.

The marine biologists worked with oceanographers to map upwelling trends and forecast what could happen in the future as climate disruption becomes ever more pronounced and extreme cold upwellings increase in frequency along with periods of extreme surface heat.

Within individual species, certain groups tend to live on the edge of their population range. Daly said these groups were most vulnerable to the sudden and protracted temperature shifts. “There are bull sharks that run the gauntlet of a cool area to get to thermal refuges. Upwellings could kill off this unique genetic diversity.”

He said the findings suggested the need for a new approach to marine conservation that incorporated knowledge about the increasingly complex ways that climate chaos is affecting marine species. “We need to think about expanding conservation areas and prioritising different species,” he said. “We need to think out of the box.”

Global heating has pushed the world’s coral reefs to a fourth planet-wide mass bleaching event that is on track to be the most extensive on record, US government scientists have confirmed.

Some 54% of ocean waters containing coral reefs have experienced heat stress high enough to cause bleaching, the National Oceanic and Atmospheric Administration’s Coral Reef Watch said.

A global bleaching event is declared when at least 12% of corals in each of the main ocean basins – Pacific, Atlantic and Indian – experience bleaching-level heat stress within a 12-month period. The declaration also requires confirmed reports of bleaching.

Coral Reef Watch also confirmed the world’s largest coral reef system – Australia’s Great Barrier Reef – had been through its most widespread heat stress event on record in 2024.

The first global bleaching event happened in 1998 with 20% of the ocean’s reef corals exposed to a level of heat stress high enough to cause bleaching. The second event, in 2010, saw 35% reaching that threshold, and the third from 2014 to 2017 peaked at 56%.

Dr Derek Manzello, the Coral Reef Watch director, told the Guardian the current bleaching was likely to surpass the previous most widespread event soon “because the percentage of reef areas experiencing bleaching-level heat stress has been increasing by roughly 1% per week”.

NOAA’s threshold for the onset of bleaching relates to the amount of accumulated heat corals are facing at any given time, known as degree heating weeks.

For example, a 1 DHW is accumulated if corals are subjected to temperatures 1C above the usual maximum for seven days. Coral Reef Watch considers 4 DHWs as a bleaching threshold.

Coral reefs are rich in biodiversity and provide habitat to a quarter of all marine species while covering less than 1% of ocean area. Reefs provide food and tourism income to millions of people and protect coastlines, but are considered to be one of the most vulnerable ecosystems to global heating.

The current global event started in early 2023 and in the northern hemisphere summer reefs across the Americas bleached from record levels of heat stress.

Mass bleaching has been confirmed throughout the tropics, NOAA said, including Florida, the Caribbean, Brazil, many countries across the south Pacific, the Middle East and in parts of the Indian Ocean from Indonesia’s west coast to reefs off east Africa.

Quick Guide

What is coral bleaching?

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Coral bleaching describes a process where the coral animal expels the algae that live in their tissues and give them their colour and much of their nutrients.

Without their algae, a coral’s white skeleton can be seen through their translucent flesh, giving a bleached appearance.

Mass coral bleaching over large areas, first noticed in the 1980s around the Caribbean, is caused by rising ocean temperatures.

Some corals also display fluorescent colours under stress when they release a pigment that filters light. Sunlight also plays a role in triggering bleaching.

Corals can survive bleaching if temperatures are not too extreme or prolonged.  But extreme marine heatwaves can kill corals outright.

Coral bleaching can also have sub-lethal effects, including increased susceptibility to disease and reduced rates of growth and reproduction.

Scientists say the gaps between bleaching events are becoming too short to allow reefs to recover.

Coral reefs are considered one of the planet’s ecosystems most at risk from global heating. Reefs support fisheries that feed hundreds of millions of people, as well as supporting major tourism industries.

The world’s biggest coral reef system – Australia’s Great Barrier Reef – has suffered seven mass bleaching events since 1998, of which five were in the past decade. 

Prolonged and severe bleaching can kill corals, but if temperatures fall quickly enough the animals can recover. Research has found previously bleached corals find it harder to reproduce and can be more susceptible to disease after bleaching.

Manzello said global heating had combined with a global El Niño to push up sea surface temperatures. He said predictions made by scientists decades ago about the fate of corals in a warming world were now coming to pass.

“The bottom line is that as coral reefs experience more frequent and severe bleaching events, the time they have to recover is becoming shorter and shorter. Current climate models suggest that every reef on planet Earth will experience severe, annual bleaching sometime between 2040 and 2050.”

Prof Ove Hoegh-Guldberg, a pioneer of coral research who was among the first to link bleaching to global heating, said: “It’s a shock. We clearly have to prevent governments from investing in fossil fuels, or we won’t have a chance in hell [to save reefs].”

Earlier this year, Coral Reef Watch was forced to add three new alert levels to its global coral bleaching warning system to represent ever-increasing extremes.

Prof Tracy Ainsworth, the vice-president of the International Coral Reef Society, said the bleaching had extended to some of the most remote places on Earth.

“Globally we are failing to protect coral reefs and the communities that rely upon them. This is neglect on a global scale,” Ainsworth said.

The Great Barrier Reef is now suffering its fifth mass bleaching in eight years. Coral Reef Watch data shows 80% of the reef was subjected to bleaching-level heat stress in 2024, the highest extent on record and above the previous high of 60% seen in 2017.

Dr Roger Beeden, the chief scientist at the Australian government’s Great Barrier Reef Marine Park Authority, said it was more important than ever to see global action on climate change.

“But the prognosis is not good for coral reefs as we know them, and the GBR is not immune. We are certainly not giving up on reefs, but they’re under serious pressure.”

Squat lobsters, bright red sea toads and deep-sea dragon fish were among more than 160 species never previously seen in the region that were spotted on a recent expedition exploring an underwater mountain range off the coast of South America. Researchers from the California-based Schmidt Ocean Institute believe that at least 50 of those species are likely to be new to science.

Underwater mountain ranges are oases of life and biodiversity, where communities of different organisms band together: some creatures make the most of the elevation and unique currents that the peaks provide, while others find refuge in the nooks and crannies of the rocky slopes to build intricate structures.

Erin Easton, of the University of Texas Rio Grande Valley, was the chief scientist on the 40-day research voyage along the Salas y Gómez ridge, which spans the waters from Chile to Rapa Nui, also known as Easter Island. “We’re still astounded by what we observed,” she says.