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

What does a surge in ocean temperatures, compounded with El Nino, bode for the summer?

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.