The waters of the north Pacific have had their warmest summer on record, according to BBC analysis of a mysterious marine heatwave that has confounded climate scientists.

Sea surface temperatures between July and September were more than 0.25C above the previous high of 2022 - a significant increase across an area roughly ten times the size of the Mediterranean.

While climate change is known to make marine heatwaves more likely, scientists are struggling to explain why the north Pacific has been so hot for so long. The extra heat in the so-called 'warm blob' may have the opposite effect in the UK, possibly making a colder start to winter more likely.

There's definitely something unusual going on in the north Pacific, said Zeke Hausfather, a climate scientist at Berkeley Earth. The BBC analysed data from the European Copernicus climate service to calculate average temperatures and found that the current conditions are remarkable compared to the past two decades.

Natural weather variability is thought to play a part, but experts believe that human activity is also increasingly influencing these patterns. Changes in shipping fuel regulations have reduced sulfur emissions, which previously had a cooling effect on temperatures.

As the marine heatwave continues to affect weather on both sides of the Pacific, it has led to exceptionally high summer temperatures in Japan and South Korea, as well as storm patterns in the U.S. Experts warn that this unprecedented heatwave could also alter weather patterns in the UK, highlighting the complex interdependencies of global climate systems.