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.

It’s a political earthquake! The common earthworm, the soil-maker, food provider and grand recycler, is the landslide winner of the inaugural UK invertebrate of the year competition.

Lumbricus terrestris, also known as the lob worm, dew worm and nightcrawler, took a mighty 38% of the popular vote after readers nominated it to be added to the shortlist for the Guardian contest.

The rare and endangered shrill carder bee demonstrated the popular affection for bumblebees by coming second with 15% of the vote while the romantics’ choice, the glowworm, narrowly beat the unexpectedly popular distinguished jumping spider into bronze medal position with 9% of the vote.

Chris Packham, whose plea for the disrupter of the shortlist, the Asian or yellow-legged hornet made the front page of the Daily Star, said: “Through constant wriggling and extraordinary ecological commitment it’s great to see the earthworm take top spot. Although I suspect vote rigging by blackbirds, badgers and moles.”

The nature writer and campaigner Dr Amy-Jane Beer said: “Stop the press, here’s an award that really means something! In a media obsessed with the rare and beautiful, for the overwhelming importance of a cold, slimy, mostly unseen creature long associated with a kind of mute, spineless, humility to receive public recognition is a big deal.”

The invertebrate charity Buglife also welcomed the earthworm’s triumph. “It’s great news,” said David Smith, Buglife’s advocacy and social change officer. “These ecosystem engineers go about their lives often unnoticed yet are vital for producing the food we eat and easing the impacts of flooding. Unfortunately, earthworms are under threat from multiple sources including chemical use, invasive species, and intensive land use, hopefully with its new title, the public will support these vital invertebrates and take action to help them thrive.”

“A man may fish with the worm that hath eat of a king, and eat of the fish that hath fed of that worm,” declared Hamlet, baffling Claudius the King in Shakespeare’s play but hailing the recycling prowess of the earthworm.

Earthworms can bring 40 tonnes of soil to the surface per hectare a year in Britain. Their usefulness is increasingly hailed in an era where regenerative farmers and many others are paying new attention to soil health.

Worms make soils less prone to flooding in winter and less baking hard in summer, they boost microbial activity and, of course, are vital in supporting plant growth, including the crops that feed us.

As well as being important, earthworms lead long (up to six years in captivity) and wondrous lives, and their charisma is well appreciated by those great connoisseurs of the living world, children. One of many readers who nominated the earthworm was Lily, four, who appreciated their “soft” feel in the hand and their general wiggly appeal.

The common earthworm appears on the surface – especially during damp and wet times, hence its other names, dew worm and rain worm – and is one of those precious invertebrates who we can see almost daily, and help too. I always feel better about myself if I stop mid-stride and rescue a stranded worm from the pavement or road.

Unfortunately, like so many other common invertebrates, earthworms are disappearing: the UK’s first national assessment, in 2023, found that populations are estimated to have declined by a third over the past 25 years.

It is not just on farmland where declines are occurring, probably due to pesticides and intensive ploughing, but in broadleaved woodlands – suggesting that wider factors such as climate change and pollution from animal worming including treatments for pets are driving losses.

Surprisingly the most traditionally beautiful animals on the shortlist, the swallowtail butterfly and the Clifden nonpareil moth, polled modestly, finishing in sixth and 10th place respectively.

The naturalist and author Dominic Couzens said: “There’s no doubt that the earthworm is a worthy winner, but perhaps that reflects the grey sky of our troubled times? Perhaps one day we will go with a feelgood selection, such as the sumptuously glamorous Clifton nonpareil or swallowtail?”

Bringing up the rear on the shortlist with just 0.8% of the vote was the Asian or yellow-legged hornet, an invasive species known for attacking honeybee colonies which was nevertheless championed by Packham, who made the case for tackling the biodiversity crisis rather than scapegoating one species, which humans are to blame for spreading.