Stressful Summer for Coral Reefs

For several months in 2023, global sea surface temperatures reached record-high levels, fueled by decades of human-caused climate warming and a recent boost from the natural climate phenomenon El Niño. Some areas—including the seas around Florida, Cuba, and the Bahamas—saw particularly high temperatures, with implications for the health of coral reefs.

Corals thrive within a small range of temperatures and become stressed when water is too hot or cold. Bleaching occurs when stressed corals expel the algae that live inside them, stripping corals of their color. Extreme bleaching can leave a reef susceptible to starvation, disease, and even death. Observations made by divers in the Florida Keys found that the marine heatwave in summer 2023 caused widespread bleaching.

Stress on corals can also be detected using data from satellites. This animation shows the evolution of accumulated heat stress from July through September 2023. The colors depict “degree heating weeks” (°C-weeks)—a measure that provides an estimate of the severity and duration of thermal stress. Data for the product are compiled by NOAA’s Coral Reef Watch, which blends observations from polar orbiting satellites such as the NASA-NOAA Suomi NPP, and from geostationary satellites such as GOES, with computer models.

Observations have shown that when the accumulated heat stress reaches a value of 4, significant coral bleaching can result. At values of 8, coral bleaching and widespread mortality are likely. By midway through this animation, in August, heat stress across much of the region already soared well above both of those thresholds. According to NOAA, cumulative heat stress by late September 2023 hit 22°C-weeks (40°F-weeks), nearly triple the previous record for the region.

Bleaching was already observed in some areas as early as July. Notice that areas of coral reef (gray) near the Florida Keys, Cuba, and the Bahamas, are among the first areas to show high cumulative heat stress. Hurricane Idalia in late August helped cool surface waters somewhat, but only temporarily.

Nearing mid-October, waters around the Florida Keys were under a bleaching watch. Further south, waters around parts of Cuba and the Bahamas remained at bleaching alert level 2, the highest level of the scale, signifying that severe bleaching and mortality are likely.

NASA Earth Observatory animation by Wanmei Liang, using Daily 5km Degree Heating Weeks data from Coral Reef Watch. Coral reef data from UNEP-WCMC, WorldFish Centre, WRI, TNC. Story by Kathryn Hansen.

References & Resources