An enormous marine heatwave off the US west coast is ringing alarm bells among ocean and atmospheric scientists as new data shows its ecological and environmental effects are intensifying.

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 might 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.