Reinvigorating Our Pulse on the Reef: Advancements in Coral Bleaching Monitoring

April 20, 2023Chiara Moore

The Allen Coral Atlas recently launched an updated version of its coral bleaching monitoring system, including nearly five years' worth of coral bleaching data. This new data allows you to identify coral reef areas that have been hit with bleaching year after year amongst those that have experienced no bleaching at all. Read on for more details of what makes the new system different from the original bleaching monitoring system on the Allen Coral Atlas.

Dr. Greg Asner doing field work in Hawai'i

New bleaching data are released every 2 weeks via a system that uses satellite data to monitor changes in the brightness of coral-containing areas after they are exposed to heat stress. Brighter pixels suggest that the corals may have undergone bleaching, but to observe this brightening accurately, it's essential to take into consideration environmental changes and water signals. The "water signal" refers to the interference caused by the presence of water when observing a specific area of the ocean's surface using satellite imagery. Water can absorb and reflect light, causing a distortion in the signals received by the satellite. To accurately observe the area of interest, it is necessary to eliminate or correct for this interference caused by the water and establish a baseline brightness threshold, or in other words, the “normal” color of the coral as seen from satellite.  

Last week, we announced that we modified the Allen Coral Atlas monitoring system to improve two factors. For one, we improved the estimation of bottom reflectance by using a local average chlorophyll-a value for each of the regions that we monitor (we split the world into 214 monitoring regions, aligned with NOAA’s Coral Reef Watch’s regions). Accurately measuring the concentration of chlorophyll in the water column is crucial to obtaining precise data, as it varies by region. Secondly, the new methodology updates the baseline determination for each potential bleaching event, making it more adaptable to changes in the dynamic ecosystem of coral reefs. The new approach assesses baseline color during the time period just before a warming event, and sets a new baseline every time there is a new potential bleaching event.

To prevent false positives in coral bleaching detection, the updated version of the bleaching system now focuses on monitoring only those regions with heat stress levels hitting "Alert 1," which is one category higher than the previous version and is reported by the NOAA-CRW program. The Atlas also has a unique feature that locates heating events and identifies their level. This means that if you observe an Alert 1 in your area of interest, you can use the bleaching tool to determine whether that heatwave is causing coral bleaching.

Check out this treasure trove of data now!