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.
Since the 1980s, satellites have revolutionised ocean science by making it possible to take daily measurements of ocean temperatures. But satellites watch from above. They can’t see what’s happening below the surface.
Our new research explores what’s happening in deeper waters. It turns out, marine heatwaves aren’t just on the surface. In the most devastating marine heatwaves, heat can penetrate right down to the sea bed. Remarkably, some heatwaves only affect the seafloor.
Why do deep marine heatwaves matter?
While we usually only see sea creatures at the surface of the ocean, there’s life all the way down. In the shallower seafloors of the continental shelf – the sunken parts of our continents – live fish, kelp beds, sponges, cold water corals, shellfish and crustaceans.
These shallow oceans are, on average, less than 100 metres deep. When the shelf ends, there’s usually an abrupt slope into the deep ocean, where there are kilometres of water between surface and seabed.
Marine heatwaves are damaging to life in the seas covering the continental shelf. Creatures here are sensitive to extreme temperatures, just like those at the surface. But “extreme” to them is different to what we think of as extreme. If you’re used to water at 12℃, a heatwave of 15℃ can be devastating.
When marine heatwaves strike, they can kill. More than a billion sea creatures died during a single heatwave off the coast of the western United States and Canada in 2021. This year, extreme heatwaves have hit large parts of the oceans during the northern summer.
Fish and other creatures that can move do so, heading towards the poles or down deeper in search of cooler water. Those that can’t have to endure it or die. Heatwaves can trigger migration. New species arrive, seeking refuge and can alter the ecosystem.