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

The Coral Reef Watch program, the primary global system for heat stress alerts on coral reefs, has expanded its warning categories in response to increasing temperature extremes. This adjustment comes after unprecedented heat stress levels last year led to widespread coral bleaching and mortality across the Americas.

Dr. Derek Manzello, the director of Coral Reef Watch, pointed out that we are entering a new era of heat stress impact, necessitating a reevaluation of their approach. Coral reefs, which are home to a quarter of marine species, are highly vulnerable to global heating caused by activities such as fossil fuel burning and deforestation.

Excessive heat can lead to coral bleaching, where corals separate from the algae providing color and nutrients. Even surviving corals become more susceptible to diseases and struggle with reproduction. The Coral Reef Watch program, hosted by the National Oceanic and Atmospheric Administration, previously issued warnings in four stages. However, due to extreme heat events exceeding previous thresholds, three new alert levels have been added.

The warning system relies on “degree heating weeks” (DHW), measuring accumulated heat stress on corals. For instance, 1 DHW accumulates if corals endure temperatures 1°C above the usual maximum for seven days. The old system’s highest rating was 8 DHWs, but last year, some areas experienced heat stress beyond 20 DHWs, prompting the need for a more detailed system.

The new alert levels include level 3 for DHWs between 12 and 16, level 4 from 16 to 20, and level 5 for anything above 20. The severity of bleaching and the risk of coral death are now differentiated based on their heat sensitivity.

The changes in the Coral Reef Watch warning system, in place since 2009, reflect the escalating threats to coral reefs due to climate change. Dr. David Wachenfeld from the Australian Institute of Marine Science praised the quick action taken by NOAA, emphasizing the unprecedented nature of last year’s thermal stress.

Richard Leck, head of oceans at WWF-Australia, highlighted the significance of the new system in illustrating the off-the-charts impact of ocean temperatures on coral reefs. Professor Tracy Ainsworth, vice-president of the International Coral Reef Society, emphasized that the alterations represent a shift from discussing bleaching events to mortality events, underscoring the profound impact of ongoing heat stress on reefs.

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