A vast marine heatwave off the US west coast is causing alarm among scientists as new data shows it is intensifying and expected to expand. The warm water mass, which peaked in September 2025, stretches thousands of miles from California across the Pacific, affecting a triangle-shaped region from Hawaii to British Columbia and Mexico.
Despite hopes in early April that the heatwave might diminish, projections from the National Oceanic and Atmospheric Administration (NOAA) indicate it will strengthen in coming months. The heatwave coincides with El Niño formation in the tropical Pacific, potentially leading to record land temperatures and disrupted marine food chains.
Scientists have been stunned by the data. Kim Wood, an atmospheric scientist at the University of Arizona, said she was “out of superlatives” after seeing ocean temperatures in the eastern Pacific surge above typical hurricane season levels. The heatwave has contributed to extreme temperatures across the US, with March seeing a remarkable land-based heatwave that pushed temperatures more than 30°F above seasonal norms in states like Minnesota, Colorado and Idaho.
Robert Rohde of Berkeley Earth noted that over a third of US weather stations set new March temperature records, with some places like Phoenix recording temperatures higher than any previous April. He said the event “would have been impossible without a boost from climate change”. The heatwave capped the warmest winter on record in the west, with snowpack at record lows in several states.
Larry O’Neill, an Oregon State University climatologist, described the marine heatwave as “incredibly long lasting” and warned it could bring higher humidity onshore, triggering dry thunderstorms that may spark wildfires. “There’s real concern right now that even if this marine heatwave didn’t persist, we’re heading into a bad wildfire season with poor water supply conditions,” he said.
