New Alert: Global Tracking of Coral Bleaching Events

June 20, 2023Dr. Andrea Rivera-Sosa

Coral bleaching events are becoming more severe and frequent due to the impacts of climate change. As marine heatwaves increase, so do severe threats to coral reef ecosystems and the livelihoods of the people who depend on them. As of June 2023, we’re approaching the development of an El Niño climate event, which means a high probability of coral bleaching worldwide.

Although this is bad news, we now have a way of detecting bleaching events using technology and local monitoring. This gives us the opportunity to respond appropriately and in a timely manner.

Enter the Allen Coral Atlas

With its full suite of mapping and monitoring tools, the Atlas provides critically important data urgently needed for rapid response and conservation of the world's coral reefs. Collaborating alongside different data platforms, the Atlas provides free data sets of reef habitats, marine protected areas and marine geo political boundaries. Users are also able to track Reef threats, such as coral reef bleaching and ocean water turbidity.

Announcing a New Bleaching Alert System

The Atlas team is excited to launch bleaching alerts. These email alerts allow reef managers, decision makers, scientists, dive operators, environmental journalists and the general public to discover where corals are bleaching in near real-time. They can be assigned to a user’s priority areas to mobilize monitoring groups and prompt communication with stakeholders. Ultimately, these alerts are in place to help respond to a bleaching event.

Watch as the Atlas reports coral bleaching events from 2019 - 2023 in this video:

Credit: Allen Coral Atlas

Sign up for a Bleaching Alert Today

It’s easy to sign up for bleaching alerts across any region. 

        1 - Sign up for an account and/or log in at

        2 - Select a country's maritime boundary, a marine protected area or draw a custom reef area of interest

        3 - Sign up for an alert (see video below for instructions)

Atlas alerts will notify users when an area shows apparent bleaching, or whitening, of the seafloor, picked up by Sentinel-2 satellites. Past bleaching events are visible since 2019.

Learn how to sign up in this short video:

Credit: Allen Coral Atlas

Let’s Get Specific

How exactly does the bleaching alert work?

The Atlas starts monitoring bleaching when a region goes under thermal stress, cued by the NOAA Coral Reef Watch (Bleaching Alert Level 1). Using Google Earth Engine (GEE), the Sentinel-2 satellite data is processed. The pixels with clouds and shadows are masked out, and the interference of the water column on the seafloor signal is corrected. The data is available in bi-weekly periods, at ~10m spatial resolution, and bleaching pixels are clustered in three classes of low, moderate, and severe. This data is generated and released within three days of the end of a 2-week period, triggering email alerts to those whose areas are bleaching in that period.

Learn more about the updates to the bleaching monitoring in this article or check out the Coral Reef Monitoring section of the Science and Methods page.

Global Ground-Truthing

The Atlas, led by Arizona State University, is working with the Coral Reef Alliance and key partners worldwide to support ground-truthing efforts of the bleaching layer across different methods and scales. We are closely collaborating with the Wildlife Conservation Society and MERMAID, who recently led a monitoring group in Fiji during the February-May 2023 bleaching event. The Atlas is also coordinating with global teams led by Reef Check Worldwide and Reef Cloud to help monitor coral bleaching in remote coral reef locations.

To learn more about ground truthing, please contact

How You Can Help

It takes a global effort to protect coral reefs. Sign up for an Atlas login and select bleaching alerts in your area. Check the NOAA CORIS webpage for ideas on how to be prepared to mobilize before, during, and after a bleaching event strikes.