Reef Alert System Now Considers 'Worst Case Scenario'

The US government program that informs coral bleaching response plans around the world has added three new alert levels to its heat stress category system after stress on coral reefs spun off the charts. Since 2009, the NOAA's Coral Reef Watch (CRW) has issued bleaching alerts up to Level 2, marked by eight or more Degree Heating Weeks (DHW), or weeks with water temperatures at least 1 degree Celsius above the usual maximum. That indicated "severe bleaching and significant mortality [is] likely," per the Guardian. But "now things are getting catastrophic," CRW Director Dr. Derek Manzello tells the outlet. Coral mortality starts around eight DHWs, yet reefs in the Southwestern Atlantic and other areas have recently experienced around 20 DHWs.

"When you exceed a DHW value of 20, it is analogous to a Category Five cyclone, with unbelievably severe, drastic damage," says Manzello. "It's the worst case scenario." More than 20 DHWs now constitute Bleaching Alert Level 5, with a risk of mortality in more than 80% of corals, according to CRW's website. Alert Level 4, for 16 to fewer than 20 DHWs, could mean the loss of more than 50% of corals, while Alert Level 3, for 12 to fewer than 16 DHWs, brings a risk of multi-species mortality, as opposed to mortality for heat-sensitive corals with Alert Level 2, which now marks eight to fewer than 12 DHWs.

It's "an incredibly powerful reminder that global heating is impacting our oceans in the here and now in ways unimaginable only a decade ago," Rickard Leck, head of oceans at WWF Australia, tells the Guardian. "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," adds Manzello, noting these are the first changes to the Bleaching Alert system since it was introduced in 2009. The goal is to "provide important, added detail" for cases of extreme heat stress, according to CRW. It adds "discussions may need to occur now, regarding appropriate actions" in the case of Bleaching Alert Levels 3-5. (More coral reefs stories.)