‘Profound impacts’: record ocean heat is intensifying climate disasters, data shows. Oceans absorb 90% of global heating, making them a stark indicator of the relentless march of the climate crisis

 

The extra heat makes hurricanes and typhoons more intense, causes heavier downpours of rain and greater flooding, and results in longer marine heatwaves. Photograph: Michael Probst/AP

by Damian Carrington Environment editor

 

 

The world’s oceans absorbed colossal amounts of heat in 2025, setting yet another new record and fuelling more extreme weather, scientists have reported.

More than 90% of the heat trapped by humanity’s carbon pollution is taken up by the oceans. This makes ocean heat one of the starkest indicators of the relentless march of the climate crisis, which will only end when emissions fall to zero. Almost every year since the start of the millennium has set a new ocean heat record.

This extra heat makes the hurricanes and typhoons hitting coastal communities more intense, causes heavier downpours of rain and greater flooding, and results in longer marine heatwaves, which decimate life in the seas. The rising heat is also a major driver of sea level rise via the thermal expansion of seawater, threatening billions of people.

 

Reliable ocean temperature measurements stretch back to the mid-20th century, but it is likely the oceans are at their hottest for at least 1,000 years and heating faster than at any time in the past 2,000 years.

The atmosphere is a smaller store of heat and more affected by natural climate variations such as the El Niño-La Niña cycle. The average surface air temperature in 2025 is expected to approximately tie with 2023 as the second-hottest year since records began in 1850, with 2024 being the hottest. Last year the planet moved into the cooler La Niña phase of the Pacific Ocean cycle.

“Each year the planet is warming – setting a new record has become a broken record,” said Prof John Abraham at the University of St Thomas in Minnesota, US, and part of the team that produced the new data.

“Global warming is ocean warming,” he said. “If you want to know how much the Earth has warmed or how fast we will warm into the future, the answer is in the oceans.”

The analysis, published in the journal Advances in Atmospheric Sciences, used temperature data collected by a range of instruments across the oceans and collated by three independent teams. They used this data to determine the heat content of the top 2,000 metres of the oceans, where most of the heat is absorbed.

The amount of heat taken up by the ocean is colossal, equivalent to more than 200 times the total amount of electricity used by humans across the world. “Ocean warming continues to exert profound impacts on the Earth system,” the scientists concluded.

Ocean warming is not uniform, with some areas warming faster than others. In 2025, the hottest areas included the tropical and South Atlantic and North Pacific oceans, and the Southern Ocean. In the latter, which surrounds Antarctica, scientists are deeply concerned about a collapse in winter sea ice in recent years.

 

The North Atlantic and the Mediterranean Sea are also getting warmer, as well as saltier, more acidic and less oxygenated owing to the climate crisis. This is causing “a deep-reaching ocean state change in, making the ocean ecosystems and the life they support more fragile”, the researchers said.

“As long as the Earth’s heat continues to increase, ocean heat content will continue to rise and records will continue to fall,” said Abraham. “The biggest climate uncertainty is what humans decide to do. Together, we can reduce emissions and help safeguard a future climate where humans can thrive.”

Dramatic slowdown in melting of Arctic sea ice surprises scientists. Natural climate variation is most likely reason as global heating due to fossil fuel burning has continued

Melting is likely to start again at about double the long-term rate in the next five to 10 years, the scientists said. Photograph: Keren Su/China Span/Alamy

 

The melting of sea ice in the Arctic has slowed dramatically in the past 20 years, scientists have reported, with no statistically significant decline in its extent since 2005.

The finding is surprising, the researchers say, given that carbon emissions from fossil fuel burning have continued to rise and trap ever more heat over that time.

They said natural variations in ocean currents that limit ice melting had probably balanced out the continuing rise in global temperatures. However, they said this was only a temporary reprieve and melting was highly likely to start again at about double the long-term rate at some point in the next five to 10 years.

The findings do not mean Arctic sea ice is rebounding. Sea ice area in September, when it reaches its annual minimum, has halved since 1979, when satellite measurements began. The climate crisis remains “unequivocally real”, the scientists said, and the need for urgent action to avoid the worst impacts remains unchanged.

 The natural variation causing the slowdown is probably the multi-decadal fluctuations in currents in the Atlantic and Pacific oceans, which change the amount of warmed water flowing into the Arctic. The Arctic is still expected to see ice-free conditions later in the century, harming people and wildlife in the region and boosting global heating by exposing the dark, heat-absorbing ocean.

 

Dr Mark England, who led the study while at the University of Exeter, said: “It is surprising, when there is a current debate about whether global warming is accelerating, that we’re talking about a slowdown.

“The good news is that 10 to 15 years ago when sea ice loss was accelerating, some people were talking about an ice-free Arctic before 2020. But now the [natural] variability has switched to largely cancelling out sea ice loss. It has bought us a bit more time but it is a temporary reprieve – when it ends, it isn’t good news.”

The research, published in the journal Geophysical Research Letters, used two different datasets of Arctic sea ice levels from 1979 to the present day. The scientists analysed the sea ice area for every month of the year and the slowdown was seen in all cases.

To see if such a slowdown could be a result of natural variation, they examined the results of thousands of climate model runs. “This is not an extremely rare event – over a century, it should happen a couple of times,” said England, now at the University of California, Irvine. Furthermore, all the simulations showed sea ice loss accelerating again after the slowdown.

Prof Julienne Stroeve, of University College London, said: “We know climate records, be it in global temperatures or sea ice, can remain the same for several years in a row as a result of internal climate variability.”

Stroeve’s analysis of the long-term trend from 1979 to 2024 shows that about 2.5 sq metres of September ice is lost for every tonne of CO₂ emitted.

 

Prof Andrew Shepherd, of Northumbria University, said: “We know that the Arctic sea ice pack is also thinning, and so even if the area was not reducing, the volume still is. Our data show that since 2010 the average October thickness has fallen by 0.6cm per year.”

The rate of the rise in global surface temperature has also slowed down in the past, before resuming a rapid rise. A major El Niño event in 1998 was followed by a decade or so of similar global temperatures, which was nicknamed “the pause”. However, the planet continued to accumulate heat throughout and global temperatures have since risen rapidly.

England rejected any suggestion the sea ice slowdown suggested climate change was not real. “Climate change is unequivocally real, human-driven, and continues to pose serious threats. The fundamental science and urgency for climate action remain unchanged,” he said.

“It is good to explain to people that [the slowdown] is happening, else they are going to hear it from someone who is trying to use it in bad faith as a way to undermine our very solid understanding of what’s happening with climate change.”

Planet’s darkening oceans pose threat to marine life, scientists say. Band of water where marine life can survive has reduced in more than a fifth of global ocean between 2003 and 2022

Changes in global photic zones between 2003 and 2022 are shown with red areas to indicate ocean darkening and blue lightening. Illustration: Thomas Davies/University of Plymouth

Ian Sample Science editor

 

 

Great swathes of the planet’s oceans have become darker in the past two decades, according to researchers who fear the trend will have a severe impact on marine life around the world.

Satellite data and numerical modelling revealed that more than a fifth of the global ocean darkened between 2003 and 2022, reducing the band of water that life reliant on sunlight and moonlight can thrive in.

The effect is evident across 75m sq km (30m sq miles) of ocean, equivalent to the land area of Europe, Africa, China and North America combined, and disturbs the upper layer of water where 90% of marine species live.

Dr Thomas Davies, a marine conservationist at the University of Plymouth, said the findings were a “genuine cause for concern”, with potentially severe implications for marine ecosystems, global fisheries and the critical turnover of carbon and nutrients in the oceans.

 

Most marine life thrives in the photic zones of the world’s oceans, the surface layers that allow sufficient light through for organisms to exploit. While sunlight can reach a kilometre beneath the waves, in practice there is little below 200 metres.

This upper band of water is where microscopic plant-like organisms called phytoplankton photosynthesise. The organisms underpin virtually all marine food webs and generate nearly half the planet’s oxygen. Many fish, marine mammals and other creatures hunt, feed and reproduce in the warmer waters of the photic zones where food is most abundant.

Davies and his colleagues drew on satellite data and an algorithm used to measure light in sea water to calculate the depths of photic zones around the world. Darkening affected 21% of the global ocean in the 20 years to 2022. In 9% of the ocean, this led to photic zones being 50 metres shallower, while in 2.6% of the ocean, the zones were 100 metres shallower. Details of this study appear in Global Change Biology.

The oceans darken when light finds it harder to penetrate the water. It is often seen along coastlines where upwellings of cold, nutrient rich water rise to the surface, and where rainfall sweeps nutrients and sediments from the land into the water.

The drivers for darkening far offshore are less clear, but global heating and changes in ocean currents are thought to be involved. “The areas where there are major changes in ocean circulation, or ocean warming driven by climate change, seem to be darkening, such as the Southern Ocean and up through the Gulf Stream past Greenland,” Davies said.

 

Despite an overall darkening, about 10% of the ocean, or 37 million sq km, became lighter over the past 20 years, the study found. Off the west coast of Ireland, for example, a very large area of ocean has brightened, but further out it has darkened.

“Marine organisms use light for a whole variety of purposes. They use it for hunting, for mating, for timing reproductive events. They use it for basically every single part of their biology,” said Davies. “With ocean darkening, they have to move up the water column, and there is less space, they’re all being squished up towards the surface.”

Prof Oliver Zielinski, the director of the Leibniz Institute for Baltic Sea Research in Germany, said the darkening of vast ocean areas was a “worrying trend”.

“Such changes can disrupt marine food webs, alter species distributions, and weaken the ocean’s capacity to support biodiversity and regulate climate,” he said. “Coastal seas, being closest to human activity, are particularly vulnerable, and their resilience is crucial for both ecological health and human wellbeing.”

 

‘Catastrophic’: Great Barrier Reef hit by its most widespread coral bleaching, study finds

 

Rising water temperatures began to turn the corals of One Tree Island reef white in early 2024. Photograph: Sydney University

 

 More than 40% of individual corals monitored around One Tree Island reef bleached by heat stress and damaged by flesh-eating disease

 

More than 40% of individual corals monitored around a Great Barrier Reef island were killed last year in the most widespread coral bleaching outbreak to hit the reef system, a study has found.

Scientists tracked 462 colonies of corals at One Tree Island in the southern part of the Great Barrier Reef after heat stress began to turn the corals white in early 2024. Researchers said they encountered “catastrophic” scenes at the reef.

Only 92 coral colonies escaped bleaching entirely and by July, when the analysis for the study ended, 193 were dead and a further 113 were still showing signs of bleaching.

 

Prof Maria Byrne, a marine biologist at the University of Sydney and lead author of the study, has been researching and visiting the island for 35 years.

“Seeing those really massive colonies die was really devastating,” she said. “I have gone from being really sad to being really cranky. We have been trying to get the message across about climate change for ages.”

 

 

Shows mass coral bleaching on Great Barrier Reef amid global heat stress event 

 

In November the Australian Institute of Marine Science visited eight reefs in the same Capricorn-Bunker sector of the reef. They found the single largest annual decline in hard coral cover in that area since monitoring started in the mid-1980s, with coral cover dropping by 41%.

Similar falls in coral cover were also recorded by Aims scientists in parts of the northern section of the reef, where one government scientist has described seeing a “graveyard of corals”.

Corals at One Tree Island reef have suffered also from a flesh-eating disease known as black band. Photograph: Sydney University

Byrne and colleagues set up the study in early February last year. The team used temperature loggers, video and direct observations to track the welfare of 12 different types of coral.

The scientists wrote in the study: “As corals can recover from mild bleaching when water cools, there is a perception that while bleaching is bad, it is not necessarily catastrophic. What we observed at [One Tree Reef] was by contrast, catastrophic.”

One genus of coral – Goniopora, which is long-lived and forms large boulders covered by vibrant flower-like polyps – was observed bleached and then afflicted with a flesh-eating disease known as black band.

Byrne said it was the worst bleaching recorded at One Tree reef. The corals that were still white at the end of their study could recover, or could die, she said.

 

Dr Shawna Foo, a coral reef scientist at the University of Sydney and study co-author, has worked on the island for several years but said that after tracking the corals for five months “it was hard to recognise” many of the colonies, because they were either “covered with algae, dead or crumbling”

“It’s horrible to see this happen to somewhere I know really well but we were expecting this to happen because we have seen it in other parts of the reef, and other parts of the world,” she said.

March is usually the peak month for heat stress on the reef. The Great Barrier Reef Marine Park Authority said last week temperatures were currently up to 1.2C above average across most of the marine park.

 Great Barrier Reef suffering ‘most severe’ coral bleaching on record – video

 

The US government’s Coral Reef Watch program is projecting parts of the reef, north of Cooktown, will be subjected to heat stress and potentially more widespread bleaching by mid-February.

Richard Leck, the head of oceans at WWF-Australia, said: “We are yet to see the full data about last summer’s coral bleaching, but it’s clear there has been major mortality in areas from the north and this new research shows major mortality in the south.

“The reef is under more heat stress this summer, especially in the north, and there’s a risk we could see another back-to-back bleaching event. It’s a case of Russian roulette whether that occurs or not.

“We know the reef is under increasing pressure from climate change and its world heritage status is under increasing pressure.”

The Australian government has been asked by Unesco to report on the condition of the reef by early next month, and Leck said it was “vital an accurate representation of the reef’s health is given, and new and increased efforts to protect the reef are committed to”.