The waters of the North Pacific have reached their warmest levels on record, according to BBC analysis of a mysterious marine heatwave that has puzzled climate scientists.
Between July and September, sea surface temperatures were more than 0.25°C higher than the previous record set in 2022 — a significant increase across an area roughly ten times the size of the Mediterranean.
While climate change is known to make marine heatwaves more frequent, scientists are struggling to explain why the North Pacific has been so hot for so long.
This accumulation of heat, often referred to as the “warm blob,” could paradoxically contribute to colder early winter conditions in the UK, researchers say.
“There’s definitely something unusual going on in the North Pacific,” said Zeke Hausfather, a climate scientist at Berkeley Earth in the US. “Such a widespread temperature jump is quite remarkable,” he added.
BBC analysis using data from the Copernicus Climate Service shows that not only has the region been warming steadily for decades, but 2025 has also been exceptionally hotter than recent years.
Globally, human-induced carbon emissions have already tripled the number of days with extreme ocean heat, yet temperatures in the Pacific have exceeded even what most climate models predicted. Berkeley Earth’s modeling suggests that the temperatures observed this August had less than a one percent chance of occurring in any given year.
Part of the explanation may lie in weaker-than-usual winds this summer, which prevented warm surface waters from mixing with cooler layers below. However, scientists say that cannot fully account for the anomaly.
One hypothesis points to new shipping fuel regulations introduced in 2020, which drastically reduced emissions of sulfur dioxide — a pollutant that, paradoxically, helped cool the planet by reflecting sunlight.
With that cooling effect removed, regions like the North Pacific may now be experiencing the full force of human-caused warming.
Efforts to cut air pollution in China may have added to the trend by cleaning up aerosols that previously reflected solar radiation away from the ocean.
The North Pacific heatwave is already influencing weather across both sides of the Pacific. Japan and South Korea saw record-breaking heat this summer, while the US West Coast experienced intensified thunderstorms.
Professor Amanda Maycock of the University of Leeds said, “Warm Pacific waters fuel what we call atmospheric rivers — streams of moist air that can bring heavy rainfall or snow when they reach land.”
She explained that these oceanic conditions can influence weather downstream into the North Atlantic and Europe, potentially favoring colder starts to winter. However, forecasting remains complex, as multiple climate patterns interact to shape European winters.
Adding another factor, a weak La Niña — characterized by unusually cool waters in the eastern tropical Pacific — has begun to form. According to the US National Oceanic and Atmospheric Administration (NOAA), these conditions are expected to persist for several months.
All else being equal, La Niña typically increases the likelihood of a cold start to winter in the UK, though it may bring milder weather later in the season.
Professor Maycock concluded, “Since this year’s La Niña is weak, the extreme warmth in the North Pacific may be the dominant influence shaping the months ahead.”
