Where are corals bleaching?

Coral reefs grow in all the tropical ocean basins, between roughly 25°N and 25°S. They are concentrated in locations where the mean annual surface ocean temperatures are in the range of ~70-85°F, and where temperatures are relatively constant year-round. In other words, they live in places where dramatic seasonal variation is minimal. Corals are very sensitive to temperature changes, so they thrive in regions where they don’t experience much day-to-day change in warmth.


Dots show locations of stony coral reefs around the world (from NOAA)

Corals often bleach because the temperature of the water they live in gets too warm (or too cold) (Hoegh-Guldberg, 1999). Coral bleaching events in the past have occurred when waters got too warm for too long. For instance, an El Niño event in 1997-1998 warmed surface waters in the central Pacific by ~2-3°F above the long-term average temperature. During the event, corals across the Central Pacific bleached, and scientists estimate that ~16% of corals worldwide may have bleached (Wilkinson et al., 2008). In 2004-2005, an El Niño event prominently affected the Atlantic basin, and corals bleached across the tropical Atlantic. Florida, the Caribbean, and the Atlantic sides of Central and northern South America experienced heavy impacts.


Scientists can infer the surface ocean temperature all over the planet, all the time, by using satellites.  They keep track of these temperatures and can figure out when corals have been in water that’s too warm for too long. They can predict that corals within those areas have a high chance of bleaching. Every day, NOAA produces a map that shows where corals are in danger of bleaching. We don’t know, however, how well those predictions match reality, and this is where your observations can help! In the animation below,  you can see where NOAA predicts corals might be bleaching. We need your observations to tell us whether their predictions are accurate, how to make better use of these predictions, and to help improve them.


Animation of predicted coral bleaching risk over the last 90 days from NOAA’s Coral Reef Watch. Red areas indicate highest risk of bleaching.

Right now, we are experiencing one of the strongest El Niño events of the past few decades. Ocean temperatures across the Pacific are as warm or warmer than they’ve been since the massive 1997-1998 El Niño event. This means that corals are already bleaching, or might bleach in the next few months. Your observations will help us see how broad the effects of this event become.