The waters of the North Pacific Ocean experienced their warmest summer on record between July and September 2025, according to a detailed analysis by the BBC. This marine heatwave, often referred to as the "warm blob," has baffled climate scientists due to its intensity and persistence. Sea surface temperatures in this vast expanse—stretching from the east coast of Asia to the west coast of North America—were more than 0.25°C higher than the previous record set in 2022. This increase is remarkable both in magnitude and scale, covering an area roughly ten times the size of the Mediterranean Sea.
The steady rise in ocean temperatures is consistent with broader global warming trends driven by human activities, primarily the emission of carbon dioxide and other greenhouse gases. Research published earlier this year has shown that global warming has already tripled the number of days with extreme marine heat worldwide. However, despite this general warming trend, the exceptional heat in the North Pacific has surpassed what most climate models had predicted. According to analyses by Berkeley Earth's climate scientists, the observed sea surface temperatures in August 2025 had less than a 1% chance of occurring in any given year based on current climate projections.
Scientists attribute part of this extreme warming to natural weather variability. For example, this summer saw weaker-than-usual winds over the North Pacific, which reduced the mixing of warm surface water with cooler water below. This allowed more heat from the summer sun to remain concentrated near the ocean surface. Yet, this explanation alone does not fully account for the extraordinary temperature rise. Zeke Hausfather, a climate scientist at Berkeley Earth, emphasizes that "there’s something else going on here as well," pointing to factors beyond natural variability and greenhouse gas-driven warming.
One intriguing hypothesis involves changes in the composition of shipping fuels. Before 2020, ships commonly used heavy, sulphur-rich engine oils that emitted large amounts of sulphur dioxide—a harmful pollutant. While damaging to human health, sulphur dioxide also produced tiny aerosols that reflected sunlight away from the Earth's surface, providing a cooling effect. Recent regulations have drastically reduced sulphur emissions from shipping, especially in key areas like the North Pacific. The reduction in these reflective aerosols may have inadvertently lifted a natural “sunshade,” allowing more solar heat to reach and warm the ocean surface. Hausfather suggests that this shift in shipping fuel regulations could be a primary driver behind the rapid warming observed in the region.
In addition to shipping-related changes, efforts to reduce air pollution in Chinese cities might also be contributing to the warming of the Pacific. Historically, dirty air from industrial emissions contained particles that reflected sunlight, similar to the effect of shipping aerosols. As air quality has improved, the decrease in these particles may have unintentionally allowed more solar radiation to warm the oceans.
The consequences of this marine heatwave are already apparent across both sides of the Pacific. Countries like Japan and South Korea experienced unusually high summer temperatures, while the United States saw an increase in severe weather events. In California, for instance, the warm ocean waters have intensified thunderstorms. Amanda Maycock, a professor of climate dynamics at the University of Leeds, explains that warm seas fuel atmospheric rivers—long bands of moisture-laden air that bring heavy rainfall or snowfall to coastal and inland regions. These atmospheric rivers are powered by heat and moisture from the ocean surface, meaning a warmer North Pacific can lead to wetter, stormier conditions on land.
Looking beyond the immediate Pacific region, the effects of this marine heatwave may extend as far as Europe and the UK in the coming months. This is due to a complex network of atmospheric teleconnections—weather patterns in one part of the world that influence conditions elsewhere. According to Prof. Maycock, the warm waters in the North Pacific can generate wave-like motions in the atmosphere that affect weather downstream over the North Atlantic and Europe. These atmospheric changes tend to promote high-pressure systems over the continent, which can draw cold Arctic air southward, potentially leading to a colder start to winter.
However, predicting the exact impact on European weather remains challenging. Winters in the UK and Europe have generally been getting milder over recent decades due to climate change, and other weather patterns also play influential roles. Moreover, a warm North Pacific does not produce a uniform effect throughout the entire winter season. While it may contribute to colder conditions in early winter, later periods could see milder and wetter weather in parts of Europe.
Further complicating the forecast is the state of the