In early June 2023, the coral reefs in the lower Florida Keys and the Dry Tortugas were stunning. We were in diving gear, checking up on hundreds of corals we had transplanted as part of our experiments. The corals’ classic orange-brown colors showed they were thriving.
In early June 2023, the coral reefs in the lower Florida Keys and the Dry Tortugas were stunning. We were in diving gear, checking up on hundreds of corals we had transplanted as part of our experiments. The corals’ classic orange-brown colors showed they were thriving.
Just three weeks later, we got a call—a marine heat wave was building, and water temperatures on the reef were dangerously high. Our transplanted corals were bleaching under the heat stress, turning bone white. Some were already dead.
That was the start of a global mass bleaching event. As ocean temperatures rose, rescuers scrambled to relocate surviving corals to land-based tanks, but the heat wave, extending over 2023 and 2024, was lethal.
In a study published Oct. 23, 2025, in the journal Science, we and colleagues from NOAA, the Shedd Aquarium and other institutions found that two of Florida’s most important and iconic reef-building coral species had become functionally extinct across Florida’s coral reef, meaning too few of them remain to serve their previous ecological role.
In summer 2023, the average sea-surface temperature across Florida’s reef was above 87°F (31°C) for weeks. We found that the accumulated heat stress on the corals was 2.2 to 4 times higher than it had ever been since modern satellite sea-surface temperature recordings began in the 1980s, a time when those two species—branching staghorn and elkhorn corals—were the dominant reef-builders in the region.
The temperatures were so high in the middle and lower Florida Keys that some corals died within days from acute heat shock.
Everywhere on the reef, corals were bleaching. That occurs when temperatures rise high enough that the coral expels its symbiotic algae, turning stark white. The corals rely on these algae for food, a solar-powered energy supply that allows them to build their massive calcium carbonate skeletons, which we know as coral reefs.
