‘Literally off the charts’: global coral reef heat stress monitor forced to add new alerts as temperatures rise

The world’s main system for warning about heat stress on the planet’s coral reefs has been forced to add three new alert categories to represent ever-increasing temperature extremes.

Underlying the warning system is a measure of the amount of accumulated heat stress that corals are facing at any given time, known as Degree Heating Weeks (DHW).

For example, 1 DHW is accumulated if corals are subjected to temperatures that are 1˚C above the usual maximum for seven days.

Coral Reef Watch’s old system gave the highest rating at 8 DHWs or above, but last year in the northern hemisphere summer, large areas of reef across several countries experienced heat stress well beyond that highest rating, surpassing more than 20 DHWs in some areas.

Coral Reef Watch’s new alert level 3 represents DHWs of between 12 and 16; alert level 4 from 16 to 20, and alert level 5 for anything above 20.