Why an immense marine heatwave off the US west coast has alarmed scientists

A large marine heatwave off the US west coast has persisted since it peaked in September 2025, with warm water stretching thousands of miles from California. The affected area forms a triangle-shaped region of ocean habitats extending from Hawaii to British Columbia and southward to Mexico. Early April expectations that the heatwave might weaken have been replaced by NOAA projections indicating it will expand and strengthen in coming months. Scientists link a potential surge to El Niño formation in the tropical Pacific, which could disrupt marine food chains and contribute to record-breaking land temperatures. New observations show ocean temperatures in the eastern Pacific have risen to unusually high levels, prompting climate scientists to reassess how ocean-atmosphere interactions accelerate human-caused climate impacts.

"The unusual area of warm water has persisted since peaking in size during September 2025 and still stretches thousands of miles from the California coastline more than halfway across the Pacific affecting a vast triangle-shaped region of oceanic habitats from Hawaii to British Columbia and southward to Mexico."

"As recently as early April, marine scientists had hoped that the heatwave might diminish and the worst of its effects may be avoided. However, new projections released last week by the National Oceanic and Atmospheric Administration (Noaa) show it is now expected to expand and strengthen in the months to come."

"A surge in the marine heatwave would accompany the formation of El Nino in the tropical Pacific resulting in an atmospheric and oceanic melange that could influence everything from record-breaking temperatures on land to disrupted marine food chains."

"Wood was reacting to data showing ocean temperatures in the eastern Pacific had recently surged to a level warmer than typically seen during peak hurricane season. Risks in a hot summer Climate scientists said the persistent marine heatwave has contributed to shockingly extreme temperatures downstream across most of the United States."

Read at www.theguardian.com