"Off The Charts" - Global Coral Reef Monitoring System Adds New Levels To Denote Coral Loss

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. The changes introduced by the US government’s Coral Reef Watch program come after reefs across the Americas were hit by unprecedented levels of heat stress last year that bleached and killed corals en masse. “We are entering a new world in terms of heat stress where the impacts are becoming so pervasive that we had to rethink how we were doing things,” the Coral Reef Watch director, Dr Derek Manzello, told the Guardian.

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Coral Reef Watch, hosted by the National Oceanic and Atmospheric Administration, previously issued warnings for coral reefs in four stages, with the highest, alert level 2, suggesting that “severe bleaching and significant mortality [is] likely”. Now the program has added three further alert levels to a program used by scientists, conservationists, marine park managers and citizen scientists around the world to understand the conditions being faced by coral reefs in their areas.

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. For example, 1 DHW is accumulated if corals are subjected to temperatures that are 1C 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.

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“When you exceed a DHW value of 20 it is analogous to a Category Five cyclone, with unbelievably severe, drastic damage. It’s the worst case scenario.” 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. The text attached to the warning levels has also changed to reflect how the severity of bleaching and the risk of corals dying can differ depending on their sensitivity to heat. At alert level 5, the warning says there is a risk of “near complete mortality” of all corals. Until the changes were introduced just before Christmas, the warning system had remained unchanged since it launched in 2009. The underlying assumptions around the risks from different levels of heat stress had been in place since 1997.

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https://www.theguardian.com/environment/2024/feb/01/literally-off-the-charts-global-coral-reef-heat-stress-monitor-forced-to-add-new-alerts-as-temperatures-rise