the Editor says: Did you know that in 2020, Taiwan experienced large-scale coral bleaching, with a mortality rate of up to 40%, posing a serious threat to reef ecosystems and their economic value? In response to rising ocean temperatures, NOAA has expanded its alert system from 5 levels to 8, in the hope of giving coral reef ecosystems a better chance of recovering their former vitality. <Full article reprinted from the Taiwan Environmental Information Center, compiled from international sources; translated by Xie Mingshan; reviewed by Chen Wenzi and Lin Dali, 2024.02.07>
In the summer of 2020, Taiwan's waters experienced a large-scale coral bleaching event, killing 30 to 40 percent of corals. Many areas reached "Alert Level 2" on the bleaching warning scale — the most severe tier in the National Oceanic and Atmospheric Administration (NOAA) warning system, indicating that "severe coral bleaching and mortality are likely."
As ocean temperatures have continued to surge in recent years, the existing alert classification has proven inadequate. At the end of 2023, NOAA added three new coral bleaching alert levels in one go, expanding the system from five tiers to eight.
Daily coral bleaching alert status, as of February 3, 2024. Image source: NOAA
Coral Reef Watch (CRW) is a program under NOAA that provides a global coral bleaching early-warning system using tools such as satellite-based sea surface temperature monitoring, field measurements, and model analysis. The original five levels were: No Stress, Watch, Warning, Alert Level 1, and Alert Level 2.
Starting December 15, 2023, CRW updated its alert system by adding three new tiers. "Alert Level 3" signals a risk of multi-species mortality; "Alert Level 4" indicates a risk of severe multi-species coral mortality (>50%). The most extreme tier, "Alert Level 5," represents a risk of near-complete coral mortality (>80%).
The primary driver of coral bleaching is elevated sea water temperature, which causes coral polyps to expel the algae living in their tissues, turning the coral white. This typically occurs during sustained and abnormally high temperatures. Bleaching does not necessarily kill coral, but surviving corals may become more susceptible to disease and less able to reproduce.
The main indicator in the classification system is the Degree Heating Week (DHW), which represents the cumulative effect of thermal stress. For example, if sea water temperature is forecast to exceed the maximum sea water temperature by 2°C for 4 consecutive weeks, the DHW value is 8 (2°C × 4 weeks).
In the past, once DHW exceeded 8, it was automatically classified under the most severe Alert Level 2. In 2020, corals in southern Taiwan's waters endured a DHW of over 15 — yet were still only listed under Alert Level 2.
This classification framework, established in 2009, has gradually become unable to keep pace with a warming planet. The Guardian reported that the summer of 2023 was the hottest on record, with coral reef thermal stress in many regions far exceeding 8 DHW — and in some cases surpassing 20.
Derek Manzello, CRW's Federal Coordination Lead, told CBC (Canadian Broadcasting Corporation) that Alert Level 5 represents the most extreme and worst-case scenario. According to the scientific literature, prior to 2023 this level of thermal stress had only been recorded three times.
Environmental news site Earth.org reported that coral reefs are not only the most biodiverse marine ecosystems on the planet, but also protect coastlines from erosion. Coral reefs generate an estimated US$2.7 trillion in value for global food, tourism, and pharmaceutical industries. Yet under the pressures of global warming, ocean acidification, and pollution, coral reefs now face a critical fight for survival.
Taiwan's largest-ever coral bleaching event occurred in 2020. Pictured: bleached Acropora coral colonies at 10 m depth in Guiwan Bay, Green Island. Photo courtesy of Guo Zhaoyang.
References
- NOAA Coral Reef Watch official website
- Ecowatch (January 31, 2024), NOAA's Coral Reef Watch Adds New Alerts Due to Record-Breaking Heat
- The Guardian (January 31, 2024), 'Literally off the charts': global coral reef heat stress monitor forced to add new alerts as temperatures rise
- Earth.org (February 2, 2024), Global Coral Reef Monitor Adds New Heat Stress Alerts Amid Rise in Climate Change-Driven Mass Bleaching Events
- CBC (February 2, 2024), Coral reef monitor adds new alert levels to keep up with soaring ocean temperatures
- Science Monthly (October 1, 2022), Warning Signals from the Ocean: Stopping the Coral Bleaching Crisis Caused by Marine Heatwaves
Related Links
- Taiwan Environmental Information Center
- Original article: 熱到「破表」 NOAA珊瑚白化監測新增三個警戒級別
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