Scientists are grappling to fully understand an unprecedented and prolonged marine heatwave in the North Pacific, with some research suggesting a link to the unintended consequences of cleaner shipping fuels.
Between July and September 2025, surface temperatures in the North Pacific Ocean reached their highest recorded levels. The water was more than 0.25 degrees Celsius warmer than the previous record set in 2022.
This seemingly small increase is considered immense, affecting an area roughly ten times the size of the Mediterranean Sea, according to Europe’s Copernicus climate service. Scientists have dubbed the phenomenon the “Warm Blob.”
While marine heatwaves are becoming more frequent due to human-caused global warming, the current event is unusually severe and extended, defying clear explanation.
“Something is definitely wrong in the North Pacific Ocean,” said Zeke Hausfather, a climate scientist at Berkeley Earth in the U.S. He emphasized that this is not merely natural variation.
Climate models significantly underestimated the 2025 temperatures, which had less than a 1% chance of occurring naturally in any given year.
A leading hypothesis focuses on the global transition to lower-sulfur marine fuels for shipping, which began in 2020. Previously, dirtier fuels emitted significant amounts of sulfur dioxide, creating atmospheric aerosols. These aerosols reflected sunlight, contributing to a slight cooling effect on the planet.
The reduction in global sulfur emissions, particularly along the Pacific’s busy shipping routes, may have inadvertently removed this natural heat shield. This could allow the ocean to absorb more energy from the sun.
Related research also suggests that cleaner air in major Chinese cities, due to reduced pollution, might contribute to the trend. Less atmospheric reflection means more heat penetrates the surface waters.
Another potential factor is weaker-than-normal summer winds. These conditions could have prevented the mixing of surface heat with cooler deep waters, leading to greater heat accumulation. However, Dr. Hausfather noted that this alone cannot account for the full extent of the warming.
The marine heatwave has already impacted weather patterns on both sides of the Pacific. East Asian nations like Japan and South Korea experienced record-high summer temperatures. The United States, meanwhile, saw an increase in severe thunderstorms and tropical cyclones.
Professor Amanda Maycock of the University of Leeds explained that warmer sea surface temperatures can lead to “atmospheric rivers.” These phenomena transport significant moisture from the ocean inland, causing heavy rainfall or snowfall in affected areas.
The profound warming in the North Pacific stands in contrast to the tropical South Pacific, where a La Niña phenomenon is currently causing unusually cold surface waters. The U.S. NOAA forecasts a mild La Niña to persist for several more months, potentially influencing global rain and storm patterns, especially when interacting with the intense North Pacific heat.
This event underscores a global trend toward “boiling oceans.” Recent studies indicate that the number of marine heatwave days has more than tripled in just a few decades. Scientists suggest 2025 could be a critical turning point, requiring a re-evaluation of global warming models.