Most of us know what a heatwave feels like on land – sweltering heat for days. But oceans get heatwaves too. When water temperature goes over a seasonal threshold for five days or more, that’s a marine heatwave. They do their worst damage in summer, when the ocean is already at its warmest, but they can occur any time of year.
Over 90% of the heat trapped by greenhouse gases has gone into our oceans. So it’s no surprise marine heatwaves are getting much more intense and more frequent. This year has been off the charts. From April this year, the world’s average ocean temperature has been the highest ever recorded.
