Scientists and conservationists have long relied on the alert system set up by the Coral Reef Watch in 2009 to monitor the conditions of corals around the world.
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The world’s main coral reef heat warning monitor has added three new bleaching alert categories to reflect the new threats that rising marine temperatures are posing to these ecosystems.
Scientists, conservationists, and marine park managers have long relied on the alert system set up by the Coral Reef Watch (CRW) – a monitoring system of global coral reef environments hosted by the US National Oceanic and Atmospheric Administration (NOAA) – to monitor the conditions of corals around the world. Until last year, the warning system comprised four stages, with the highest (level 2) indicating likeliness of “reef-wide bleaching with mortality of heath-sensitive corals.”
The three new alert levels were added in December 2023 to reflect new developments in global temperatures and climate systems. More specifically, the system relies on how extreme heat accumulating over a week – also referred to as degree heating week (DHW) – affects bleaching and the risk and extent of coral bleaching. For instance, 4 DHW means that temperatures are 4C higher than the usual maximum temperature for seven days consecutively.
As several parts of the world experienced temperatures much higher than 12C above usual maximum levels – the previous DHW threshold – the new alert categories now go all the way up to 20 DHWs. Under these conditions, there is a risk of “near complete mortality.” In other words, 80% or more of the corals are at risk of dying.
| Stress Level | Definition | Potential Bleaching/Mortality |
| No Stress | HotSpot <= 0 | No bleaching |
| Bleach Watch | 0 < HotSpot < 1 | |
| Bleach Warning | 1 <= HotSpot and 0 < DHW < 4 | Risk of Possible Bleaching |
| Bleaching Alert Level 1 | 1 <= HotSpot and 0 < DHW < 4 | Risk of Reef-Wide Bleaching |
| Bleaching Alert Level 2 | 1 <= HotSpot and 4 <= DHW < 8 | Risk of Reef-Wide Bleaching with Mortality of Heat-Sensitive Corals |
| Bleaching Alert Level 3 | 1 <= HotSpot and 8 <= DHW < 12 | Risk of Multi-Species Mortality |
| Bleaching Alert Level 4 | 1 <= HotSpot and 12 <= DHW < 16 | Risk of Severe, Multi-Species Mortality (> 50% of corals) |
| Bleaching Alert Level 5 | 1 <= HotSpot and 20 <= DHW | Risk of Near Complete Mortality (> 80% of corals) |
“Nearly all coral reef regions in the wider Caribbean, including Florida, were exposed to more than double the amount of heat stress that is expected to elicit mortality,” said Derek Manzello, coordinator of NOAA’s Coral Reef Watch program in College Park, Maryland. “The full ecological impacts of this event will not be fully realized for months-to-years, but preliminary reports have been alarming, as high levels of mortality in staghorn and elkhorn corals have already been reported in Florida, Puerto Rico and Mexico.”
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The move followed last year’s “extreme accumulation” of coral bleaching heat stress in several regions across the world and particularly in the eastern tropical Pacific Ocean and Greater Caribbean, where reefs experienced historically high heat stress much earlier than usual and for much longer.
Coral reefs are the world’s most diverse marine ecosystems. Despite covering only 2% of the ocean area across more than 100 countries, they are home to 25% of all marine species. Aside from their rich biodiversity, coral reefs are vital in protecting coasts from erosion and slow storm surges. They are economically significant, too, generating US$2.7 trillion per year across the food and tourism industries as well as the pharmaceutical sector. Yet the health and survival of our precious coral reefs are teetering as global temperature rises and oceans continue to become more acidic, causing widespread coral bleaching across the world’s oceans.
Coral bleaching is a phenomenon in which coral reefs expel the microscopic marine algae called zooxanthellae that live in their tissues when under stress – be it heat, ocean acidification, or human activity – and as a result, causes corals’ tissues to become transparent and lose its signature vibrant colours, exposing its white exoskeleton underneath. Bleaching itself does not kill corals but it leaves them significantly more at risk of starvation and more vulnerable to marine disease. The longer corals are bleached under various stresses, the more difficult it will be for algae to return, making it impossible for reefs to be revived.
“Corals are literally dying before they even have a chance to bleach,” said Sophie Dove, a coral reef ecologist at the University of Queensland. “Of course, this amplifies the seriousness of the escalating change on our precious coral reefs.”
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