101°F in the Ocean Off Florida: Was It a World Record?

The reading from a buoy off Florida this week was stunning: 101.1 degrees Fahrenheit, or just over 38 Celsius, a possible world record for sea surface temperatures and a stark indication of the brutal marine heat wave that’s threatening the region’s sea life.

But determining whether that reading was in fact a world record is complicated.

First things first: The buoy’s reading is so off-the-charts, could it have been malfunctioning?

Allyson Gantt of the National Park Service, which monitors and maintains the buoy, said there was no reason to doubt the measurement. The data was consistent with high water temperatures seen in the area, Florida Bay, between the southern end of the Florida mainland and the Florida Keys, in recent weeks, she said.

Then, there’s the fact that there’s the fact that there is no official keeper of ocean temperature records. The World Meteorological Organization tracks land surface temperature records, but not ones set at sea.

Experts have pointed to a reading of 99.7 degrees Fahrenheit, recorded in the middle of Kuwait Bay in 2020 and reported in a 2020 research paper, as the world record to date.

Still, comparing the Kuwait reading, taken in the open sea, to a reading in shallow waters off the coast of Florida could be tricky.

Just like it’s easier to heat up a shallow bath than a deep one, the depth of the water is going to affect temperatures, said Jeff Masters, a former hurricane scientist with the National Oceanic and Atmospheric Administration and a co-founder of Weather Underground, a Web-based weather service.

“And maybe your buoy is near a river and there is a discharge from the river,” he said. “Or, in the case of the Everglades, there’s a lot of seaweed in the water.” Organic matter like seaweed means more dark surfaces, which are going to absorb more heat. “So a lot of complications when you’re talking near the shore,” Dr. Masters said.

Out in the open ocean, it’s rare for surface temperatures to rise above roughly 90 degrees Fahrenheit, beyond which the water usually evaporates, said Frank Edgar Muller-Karger, a professor at the College of Marine Science at the University of South Florida.

“But when you get to very, very shallow water, like over a coral reef or in a puddle of water, you can get up over that,” he said. “The sunlight is heating the water so much that it just can’t evaporate quickly enough.”

Whether or not temperatures off Florida broke a world record, 100 degrees is an alarming reading for seawater. Many parts of the country have broken or tied temperature records this month, and a warm front that’s making its way across the country is expected bring more dangerously hot weather in coming days.

Scientists said on Tuesday that extreme temperatures recorded this month in the Southwestern United States, as well as southern Europe and northern Mexico, would have been “virtually impossible” without the influence of human-caused climate change.

Oceans have absorbed about 90 percent of the additional heat caused by humans as the world burns fossil fuels and destroy forests. When sea temperatures rise too high, it causes corals to expel the algae they need for sustenance, a process known as bleaching.

Corals typically experience the most heat stress in August and September. But the recent heat means they’re now becoming stressed much earlier in the year. If waters don’t cool quickly enough, or if bleaching events happen in rapid succession, the corals can die. By some estimates, the world has already lost half of its coral reefs since 1950.

Globally, about 40 percent of the planet is experiencing a marine heat wave, experts say, including areas off the East Coast of Canada and in the Mediterranean. And in Florida, as Dr. Masters put it: “If you took a dip in Manatee Bay, it would feel like a hot tub.”

Ms. Gantt of the National Park Service pointed to some good news: Sea temperatures are still decreasing by as much as 10 degrees at night, and salinity levels are lower at this time of year than they have been in many previous years. Both help to relieve some of the stress on local marine life.

That was a reminder of the important role the Everglades played in sending clean, fresh water into Florida Bay, she said.