Before any technical details or data, the first thing that divers mention is that it feels off. After the boat ride and the sun on your shoulders, you anticipate a momentary rush of coolness as you plunge into the water. It doesn’t appear. This summer, the water off the southern tip of Florida has been as warm as a bath and, in certain shallow areas, hotter than the hot tub at a roadside motel. Something about that is confusing. It’s not supposed to feel like indoor plumbing in the ocean.
Katey Lesneski has been witnessing it firsthand down on the reefs. She has been getting in the water as frequently as her schedule permits while working for NOAA at the Florida Keys National Marine Sanctuary. She claims that the reef itself is already in the low 90s. Manatee Bay, a shallow, partially enclosed area where the water has nowhere to go and the sun has all day to work on it, produced the 101-degree reading that made headlines. The corals are still going ghost white. Once that bleaching, almost fluorescent pallor sets in, time begins to run out.
| Detail | Information |
|---|---|
| Location of record | Manatee Bay, Florida Keys |
| Highest reading recorded | 101.1°F (Manatee Bay buoy) |
| Date of peak reading | July 24, 2023 |
| Surrounding buoy readings | 99.3°F at Murray Key; 98.4°F at Johnson Key |
| Average hot tub temperature | 100–102°F |
| Reef water temperature observed | Low 90s°F across Florida Keys reef tract |
| Phenomenon triggered | Mass coral bleaching event |
| Lead researcher cited | Katey Lesneski, NOAA |
| Previous unofficial world record | 99.7°F, Kuwait Bay |
| Ocean warming trend since 1991 | Doubled the projected scale of marine heat waves per NOAA data |
| Forecast for September 2023 | Up to 50% of global oceans in heat wave conditions |
In simple terms, bleaching is the process by which a stressed coral expels the algae from its tissue. In addition to providing the color, those algae serve as a source of food. A bleached coral is hungry, worn out, and waiting for its surroundings to improve, but it is not yet dead. Some of them come back when the water cools. If it doesn’t, they perish within a few days or weeks, and something else—typically a fuzz of opportunistic algae—moves onto the skeleton and remains there. The coral doesn’t regrow. That area of the reef has simply vanished.
This summer is peculiar because so many little things came together at once. The trade winds that typically push cooler water around and create the southeasterly sea breeze that residents plan their afternoons around were killed off when a stubborn high-pressure dome parked itself over Florida.
The temperature of the air reached the upper 90s. Like asphalt in a parking lot, the water absorbed sunlight, particularly in shallow, silty bays. Some scientists have drawn a helpful analogy: murky water is like asphalt, and clear water is like concrete. While one absorbs, the other reflects. The shallows along Florida’s coast have been absorbing a lot.
It’s difficult to ignore the fact that, if confirmed, the 101-degree reading would surpass a long-standing Kuwait Bay measurement as the highest sea surface temperature ever noted. That type of claim requires verification, and the silt and close proximity to land make matters more difficult. But even putting the record aside, what’s unsettling is the bigger picture. According to NOAA’s own analysis, the size of the marine heat wave predicted for this fall has nearly doubled due to ocean warming since 1991. Models indicated that perhaps 25% of the world’s oceans would experience heat wave conditions in September if that warming wasn’t factored in. The figure rises to half with it.
Speaking with those who have worked underwater for their entire careers gives me the impression that while this isn’t exactly the disaster everyone feared ten years ago, it’s also not far off. After a poor year, the corals can recover. They’ve already done it. Nobody really wants to respond aloud to the question of whether they can recover from a series of terrible years piled on top of one another in water that keeps getting warmer. Lesneski and her team are still working, counting, and hoping that the wind will change while they watch the reef bleach in real time. That’s all there is to do sometimes.
