For several months in 2023, global sea surface temperatures reached record highs. Decades of human-caused warming and the recent natural climate phenomenon El Niño have contributed to soaring temperatures. Satellite data shows that some areas - including waters off Florida, Cuba and the Bahamas - are experiencing particularly hot temperatures, affecting the health of coral reefs.
Corals grow within a narrow temperature range and become stressed when water temperatures are too high or too low. Stressed corals expel algae from their bodies, causing the coral to lose color and cause bleaching. Severe bleaching can leave coral reefs vulnerable to threats from starvation, disease and even death. Observations by divers in the Florida Keys found that the marine heat wave in the summer of 2023 caused widespread bleaching.
This animation shows the evolution of accumulated thermal stress between July and September 2023.
Satellite data can provide insights into stress levels in corals. The animation above shows the evolution of cumulative thermal stress from July to September 2023. The colors represent "warmth weeks" (°C-weeks) - a metric that estimates the severity and duration of thermal stress. Data for the product were compiled by the National Oceanic and Atmospheric Administration's (NOAA) Coral Reef Watch, which combines observations from polar-orbiting satellites such as NASA-NOAA's SuomiNPP and geostationary satellites such as GOES with computer models.
Observation results show that when the cumulative thermal stress value reaches 4, severe coral bleaching will occur. When the value reaches 8, coral bleaching and widespread mortality are likely to occur. By the middle of this animation, in August, heat stress in much of the region has spiked well beyond both critical values. According to the National Oceanic and Atmospheric Administration (NOAA), cumulative heat stress reached 22°C-weeks (40°F-weeks) by the end of September 2023, nearly three times the area's previous record.
As early as July, bleaching has already occurred in some areas. Note that the coral reef areas (gray) near the Florida Keys, Cuba, and the Bahamas are among the first areas to experience high cumulative heat stress. Hurricane Idalia in late August helped cool surface waters somewhat, but only temporarily.
As mid-October approaches, the waters near the Florida Keys enter a bleaching observation period. Further south, waters around parts of Cuba and the Bahamas remain at bleaching alert level 2, the highest level on the scale, indicating the potential for severe bleaching and mortality.