Marine heatwave

node · earth · Global ocean

A prolonged period of anomalously warm water at a given location, conventionally defined relative to a local climatological percentile rather than an absolute temperature, so that the same event definition works in the Arctic and in the tropics. Marine heatwaves are the proximate agent of every mass bleaching event: the background warming trend raises the baseline, and the heatwave is the excursion above it that crosses the physiological threshold. The two combine multiplicatively rather than additively from the reef's point of view — a heatwave of a size that was survivable against a 1980 baseline crosses the bleaching threshold against a 2020 one, without the heatwave itself having become larger. This is why reef thermal risk rises faster than mean sea surface temperature does.