A mysterious marine heatwave in the North Pacific Ocean has broken records for the warmest summer ever recorded, with sea surface temperatures soaring to unprecedented levels. Scientists are puzzled by the intensity, which exceeds climate model predictions and may have far-reaching effects on global weather patterns.
The marine heatwave, often referred to as the ‘warm blob,’ saw sea surface temperatures between July and September 2025 rise more than 0.25 degrees Celsius above the previous record set in 2022. This increase occurred across a vast area approximately ten times the size of the Mediterranean Sea, highlighting the scale of the anomaly. Data from the European Copernicus climate service confirmed these record-breaking temperatures, which have been warming rapidly over recent decades. The 2025 figures show a marked spike compared to recent years, emphasizing the unusual nature of this event.
While climate change is a known driver of increasing marine heatwaves, the intensity in the North Pacific has exceeded most climate model predictions. Analysis by the Berkeley Earth group suggests that the observed sea temperatures in August had less than a 1% chance of occurring in any single year based on current models. Natural variability, such as weaker-than-usual winds this summer, may have contributed by allowing more heat to accumulate at the surface. However, scientists argue that this alone cannot explain the exceptional conditions, indicating other factors are at play.
One leading hypothesis points to recent changes in shipping fuels and air pollution reductions. Before 2020, sulphur dioxide from ship engines created aerosols that reflected sunlight, providing a cooling effect. The removal of this sulphur, aimed at improving human health, may have unmasked the full impact of global warming in shipping hotspots like the North Pacific. Similarly, efforts to reduce air pollution in Chinese cities could have reduced aerosol levels, inadvertently allowing more solar radiation to heat the ocean. These human activities might be amplifying the heatwave beyond natural cycles.
The heatwave has already had tangible impacts on weather in the Pacific region. It likely amplified extreme summer temperatures in Japan and South Korea, while contributing to supercharged thunderstorms and atmospheric rivers in the western United States. These phenomena bring intense rainfall and snow, affecting communities and ecosystems. For instance, warm ocean waters provide moisture that fuels storms, leading to potential flooding and other hazards. The immediate consequences underscore the interconnectedness of ocean temperatures and regional climate events.
Looking ahead, the North Pacific heatwave could influence weather in distant regions like the UK and Europe through teleconnections. Extreme heat in the Pacific can generate atmospheric waves that alter weather patterns downstream, potentially favoring high-pressure systems over Europe. This might draw colder Arctic air southward, resulting in a colder start to winter. However, such forecasts are complex and uncertain, as multiple factors, including the overall warming trend from climate change, play a role. Experts note that while a cold outcome is possible, it is not guaranteed.
Another factor is the emergence of La Niña in the tropical Pacific, where surface waters are unusually cool. La Niña typically increases the risk of a cold start to winter in the UK but may lead to milder conditions later. With weak La Niña conditions expected this year, the extreme warmth in the North Pacific could be more influential in shaping the winter forecast. The interplay between these two Pacific phenomena adds layers of complexity to seasonal predictions, requiring careful monitoring by meteorologists.
In summary, this record-breaking marine heatwave highlights the ongoing challenges in understanding and predicting climate impacts. Scientists continue to investigate the causes, while the event serves as a reminder of the far-reaching effects of ocean warming. As research progresses, it may inform better climate models and preparedness for future extremes. The situation underscores the need for global attention to marine ecosystems and weather patterns in the face of climate change.