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.

Scientists say the effects may already be far reaching. A surge in the marine heatwave would accompany the formation of El Niño in the tropical Pacific – resulting in an atmospheric and oceanic mélange that could influence everything from record-breaking temperatures on land to disrupted marine food chains.

Additional data acquired in recent weeks has left climate scientists gobsmacked and re-examining their assumptions of how the complex interplay between the ocean and the atmosphere could accelerate the effects of human-caused climate crisis.

“I’m out of superlatives,” Kim Wood, a University of Arizona atmospheric scientist, wrote on social media last month. 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 had contributed to shockingly extreme temperatures downstream across most of the United States. Abnormally warm Pacific waters tend to retain atmospheric heat from the warmer summer months and re-release it during relatively cooler winter months – significantly altering weather patterns.

In March, a remarkable land-based heatwave – what one meteorologist called “one of the most astounding global weather events of the century thus far” – sent late winter temperatures soaring more than 30F above seasonal norms to 88F (31C) or warmer in relatively temperate places such as Minnesota, Colorado and Idaho.

In a series of posts on social media, Robert Rohde, lead scientist for the climate data non-profit Berkeley Earth wrote that the March heatwave “would have been impossible without a boost from climate change”.

Rohde said more than one-third of US weather stations – including hundreds of cities from San Francisco to New York City – set new all-time temperature records for March. A few places, such as Phoenix, even recorded temperatures higher than any previous April temperatures – a feat with little precedent in modern record-keeping.

“The only other time such a large fraction of stations simultaneously set new monthly records was during the Dust Bowl,” wrote Rohde, referencing a time when comprehensive weather monitoring was still in its infancy and all-time records were easier to set.

That event capped off the warmest winter on record across the west. As of mid-May, Noaa data shows that what little snow fell had completely melted even at higher elevations across much of Oregon, California and Colorado.

“This year’s peak snowpack will be the new benchmark low for Wyoming, Utah, Colorado, and New Mexico,” according to the latest federal drought status report. “There are no comparable years.”

The low snowpack is worrying water managers across the west as warm and dry conditions have sparked a rapid intensification of drought in recent weeks.

Larry O’Neill, an Oregon State University climatologist, described the marine heatwave as “incredibly long lasting” and said he was worried that the increased ocean temperatures could bring higher humidity onshore, kicking off dry thunderstorms across California and the Pacific north-west. Given the worsening drought, those thunderstorms may act to spark wildfires rather than bring beneficial rain.

“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,” said O’Neill. “Our summer is going to be much warmer than normal.”

Concerns for marine life and ecosystems

Beyond summer heat and drought, scientists also expressed alarm about the heatwave’s effects on vast networks of marine life such as whales, seabirds and seals and the food webs they depend on.

In 2015, a similarly strong and long-lived marine heatwave known as “the Blob” kicked off a surge of environmental and ecological effects up and down the Pacific coast, including in Oregon.

“Back in 2015, we had drought that year and poor ocean conditions,” O’Neill said. “Whatever salmon went out to the ocean and then came back to spawn, they were returning to rivers that were running really low and really warm. We had really big increased pre-spawn mortality and a lot fewer salmon coming back over the following years.”

This time, O’Neill is especially concerned with a similarly devastating impact of the marine heatwave on salmon.

“This is going to be a big hit on our fisheries for a couple of years.”

Elizabeth Phillips, a research scientist with Noaa Fisheries, is preparing for the annual coast-wide offshore ecosystem assessment that will take place between June and September.

“The last six months have been really concerning,” said Phillips. “As a scientist, I’m really curious to know what the ecosystem impacts are going to be.”

Already, concern is growing that species are shifting their behaviors in profound ways. A few weeks ago, scientists tracked the first-ever evidence of a great white shark in British Columbia waters. Subtropical species – from plankton to pelicans to great whites – are shifting their range further north and closer to shore in search of cooler water and more food.

Starving seabirds washing ashore are among the first signs that something’s going wrong with the ocean. During the 2015 Blob event, scientists estimate more than a million seabirds died.

“We often monitor seabird populations to better understand what’s going on in the ocean,” said Don Lyons, a conservation biologist and director of the National Audubon Society’s Seabird Institute.

California brown pelicans are themselves an early warning species of seabirds, and Lyons said this year many of them left their nesting grounds in Mexico more than two months early. The birds feed on sardines, anchovies and other small fish that may be temporarily moving farther north to find cooler waters – and early evidence is that many abandoned their nests due to lack of food.

“We’ve seen this kind of poor success at other times. When we have an El Niño, that’s often associated with nesting failures of many species of seabirds on the west coast, including brown pelicans,” said Lyons. “This year, it was a little bit different in that the early start [to migration] was striking.”

As the summer progresses, scientists such as O’Neill and Phillips are ready with increased understanding of how intense marine heatwaves might progress and have improved monitoring methods to track them as they develop. The RV Reuben Lasker, the Noaa survey vessel Phillips will embark on, uses advanced sonar to map out the abundance of integral parts of the marine ecosystem, including organisms as small as krill and plankton.

“We saw a significant change in the amount of krill that were out on the west coast during the Blob years, 2014 to 2016,” said Phillips, which had a major effect on animals such as seabirds, mammals and salmon that depend on krill. “If there is another drop in krill this year, then I would anticipate there’s probably going to be a lot of those same impacts to the rest of the food web.”

Phillips said that, unlike other parts of Noaa which have experienced sharp budget cuts, her research activities had not been adversely affected by the Trump administration’s reprioritizations of resources among federal science staff. The consistent funding stream has helped her team prepare for the potential lingering effects of the marine heatwave.

The interplay with El Niño

Meanwhile, as the effects of the current marine heatwave play out, there’s now a greater than two in three chance of either a “strong” or “extreme” El Niño later this year. El Niño is the warm phase of the El Niño-Southern Oscillation, a periodic natural warming of the tropical Pacific Ocean that also shifts weather patterns for half the planet.

Emerging research has revealed that the rapid-fire combination of both marine heatwaves and El Niño are a symptom of human-caused climate crisis. In 2015, the Blob also merged with a strong El Niño with catastrophic effects for millions of people.

“One big problem is that these events are happening seemingly more frequently and more intensely,” said Hilary Hayford, a marine ecologist with the Puget Sound Restoration Fund.

“We’re trying to plan for these conservation or restoration actions, and we don’t have the tools to overcome these problems,” said Hayford. “A hope is to have time in between these events for systems to recover. We’re only just now starting to see recovery of sea stars and bull kelp and other species that really suffered cascading effects of the last one.”