Record-breaking temperatures are not only being felt on land but also in the sea, with a buoy off the coast of Florida recording a potentially record-high seawater temperature of 37.8C (100F) in late July. One meteorologist compared the conditions to a 'hot tub'. The extreme heat has led to widespread coral bleaching and could cause the death of some reef species.
Global ocean sea surface temperatures in June were the highest on record, according to the Copernicus Climate Change Service, with the North Atlantic particularly 'off the charts'. The average daily global sea surface temperature reached 20.96C (69.73F) in early August, beating a previous record from 2016. The US National Oceanic and Atmospheric Administration (NOAA) also reported the biggest gap between expected and actual sea surface temperatures in June 2023.
In Europe, a marine heatwave is intensifying in the western Mediterranean, especially around the Strait of Gibraltar. Sea surface temperatures along the coasts of Southern Spain and North Africa are 2-4C (3.6-7.2F) higher than normal, with some spots 5C (9F) above the long-term average. Extreme marine temperatures have also been observed around Ireland, the UK, the Baltic Sea, New Zealand, and Australia.
Scientists attribute these heatwaves to a combination of short-term atmospheric and ocean circulation patterns and long-term ocean warming driven by greenhouse gas emissions. About 90% of excess heat from anthropogenic climate change has been absorbed by the ocean, and the rate of heat accumulation has doubled in the past two decades. A 2021 IPCC report found that marine heatwaves have doubled in frequency since 1982 and have become more intense and longer-lasting.
Additional factors include a reduction in cooling aerosols from shipping and an unusual lack of dust from the Sahara. The World Meteorological Organization warns that the ongoing El Niño weather pattern could further exacerbate ocean heating, potentially making the current marine heatwaves even worse.
