Worldwide marine heatwave map for 17th June 2023. Blue areas are not experiencing a heatwave. Yellow areas are experiencing a moderate heatwave (1), orange areas a strong (2) to severe (3) heatwave and red areas an extreme (4) to beyond extreme (5) heatwave. A marine heatwave is a prolonged period of abnormally high sea surface temperatures for that day and location as compared to the average temperatures recorded since 1st January 1985. The North Sea and North Atlantic around the British Isles and Ireland had been experiencing record-breaking temperatures since April 2023. In June 2023 they were up to 5 degrees Celsius hotter than usual. This warming is thought to be driven by the El Nino weather phenomenon and human-induced climate change. Marine heatwaves have harmful impacts on the marine ecosystem and threaten marine biodiversity.