NOAA’s Coral Reef Watch Adds New Alerts Due to Record-Breaking Heat

The NOAA Coral Reef Watch daily global 5km 7-day Sea Surface Temperature (SST) Trend, updated daily, provides information on the pace and direction of the SST variation, and thus coral bleaching heat stress, if present, over the past seven days. NOAA

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The National Oceanic and Atmospheric Administration’s (NOAA) Coral Reef Watch has added three new alert levels to track potential risks to coral reefs and other marine life following the hottest year on record.

Previously, Coral Reef Watch’s Bleaching Alert Area product stopped at a Bleaching Alert Level 2, which warned of a “risk of reef-wide bleaching with mortality of heat-sensitive corals.” Now, the product features a Bleaching Alert Level 3, Alert Level 4 and Alert Level 5 to account for additional heat stress risks.

Bleaching Alert Level 3 is linked to a risk of death for multiple species. Level 4 refers to risk of severe multi-species mortality and the now highest alert, level 5, refers to a risk of near-complete mortality, or losing over 80% of corals.

The alert levels also reference DHW, or degree heating weeks. As NOAA explained, this metric measures how long the ocean temperatures remained above a bleaching threshold. The former Bleaching Alert Level product gave the highest alert, level 2, for areas with 8 or more DHWs. 

The updated alerts range from 8 to 12 DHWs for a level 2; 12 to 16 DHWs for level 3; 16 to 20 DHWs for level 4; and over 20 DHWs for level 5.

“We started to ask what this meant for the ecosystems at these extreme levels. Did reef managers need more information for when things got so extreme? Our old product was missing all that information and it wasn’t reflecting just how extreme it was getting,” Derek Manzello, director of Coral Reef Watch, told The Guardian. “We know that coral mortality starts at about eight degree heating weeks and we know that now things are getting catastrophic — greater than 20 degree heating weeks. 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.”

The Bleaching Alert Area global map for January 31, 2024 has already shown parts of the world’s oceans under heat stress, including some parts listed under the newly added Bleaching Alert Levels 4 and 5.

A NOAA- and the University of Queensland-led study published in 2023 found that marine heatwaves were having severe impacts on coral, leading to not just bleaching events but rapid deaths from acute heat shock.

Bleaching is already detrimental to coral reefs and marine ecosystems. While bleaching can lead to death of corals, they do have the chance to recover. But the updated Bleaching Alert Area product and the recent coral reef study reveal that extreme heat may kill off corals before they even begin showing signs of bleaching.

“What this new system shows is that ocean temperatures and the risks to coral reefs are literally off the charts,” said Rickard Leck, head of oceans for World Wildlife Fund-Australia, as reported by The Guardian. “It’s also an incredibly powerful reminder that global heating is impacting our oceans in the here and now in ways unimaginable only a decade ago.”

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