In late July, a fierce ocean heat wave ratcheted up temperatures in Florida’s coastal waters to unprecedented highs. One buoy bobbing in shallow, turbid Manatee Bay logged a measurement of 38.3˚ Celsius (101˚ Fahrenheit). It may be the highest ocean temperature ever recorded. One week later, the ocean’s heat surge had subsided. But South Florida’s denizens are still in hot water.
The concern is not just that the Manatee Bay buoy recorded shockingly high, hot tub–level temperatures — actually, “close to the limit of hot tub temperatures” — for several days in a row, says Benjamin Kirtman, a climate scientist at the University of Miami’s Rosenstiel School of Marine, Atmospheric and Earth Science.
And it’s not just that June and July’s brutally hot water temperatures in the North Atlantic Ocean are linked to shockingly hot temperatures on land. This summer, Miami’s heat index, a measure of air temperature and humidity, soared to a record-breaking streak of nearly two months, reaching a daily heat index of 38° C (100° F).
It’s not even that such Ocean heat waves have become the new normalClimate change is causing the oceans to warm at a faster rate.SN: 2/1/22). Florida’s waters may have hit a record high, but The month of July was marked by widespread ocean heat wavesAround the World, from the North Atlantic Ocean through the Eastern Equatorial Pacific and the Southern Indian Ocean.
“The global oceans have warmed up so much … we’re seeing a ratcheting up that’s unprecedented in the modern instrument record, and maybe in the last 125,000 years,” Kirtman says. “It’s really quite remarkable.”
‘Way outside of the bounds of anything these corals have experienced’
The temperatures of Florida’s coastal water have, for the time being, returned to their normal summertime range. But the danger remains acute for many ocean dwellers, from corals to fish, says Andrew Baker, a coral biologist also at the University of Miami’s Rosenstiel School.
Murky Manatee Bay, swirling with sediment, isn’t home to corals — but the water temperatures in the reefs around the Florida Keys were still “incredibly hot,” perhaps reaching up to 36° C (96° F), Baker says.
Coral Restoration Foundation (a nonprofit marine conservation group based in Key Largo in Florida) found that corals had died out at Sombrero Reef, off Key West, as the sea temperatures reached their peak in July. The corals had bleached due to heat.
Bleaching occurs when corals’ symbiotic algae, the main source of their food, flee, leaving the corals colorless and essentially starving. Corals can recover after bleaching but if it is too severe or frequent, the event can cause reefs to die. U.S. National Oceanic and Atmospheric Administration data shows that the Since the 1980s, heat has become a greater burden for corals around the world. (SN: 1/4/18).
Even with the return to typical summertime water temperatures off Florida’s coasts, the impacts of July’s heat wave on the region’s corals will linger. That’s because corals have a limit to how much accumulated heat they can tolerate before bleaching. Researchers say that corals are already suffering from too much heat, and this is too early in summer.
NOAA records from sites across the Florida Keys each tell the same worrisome story — that what’s happened in 2023 so far is “way outside of the bounds of anything these corals have experienced,” Baker says. Corals will still have to deal with the expected but still extremely hot water for two more months in August and September.
Scientists are working to rescue corals from nurseries in Keys and bring them onshore to labs away from overheated waters. The cultivated corals form part of a 10-year effort to protect the two important reef species, staghorn & elkhorn, in the region from the threat of coral bleaching.
These tiny corals, which are about the size of a finger or hand, are grown in coastal waters on top of PVC tubes. They will eventually be planted into reefs. Researchers collected the corals in a hurry as water temperatures climbed, anticipating their expected spawning to occur early August. Scientists feared the “heat stress is just too much for these baby corals,” and that they might not spawn at all, Baker says. Happily, a few of the rescued staghorns corals that are now safely ensconced inside the laboratory managed to spawn in August, releasing clouds and eggs into the water. Baker and his colleagues remain cautiously optimistic about the chances of the sperm fertilizing the eggs.
It’s not just corals in trouble
Overheated water can also be bad for sponges, sea grasses and fish. “There are a lot of studies that show that Migrating species are those that experience ocean heat waves [to cooler waters],” says Regina Rodrigues, a physical oceanographer at the Federal University of Santa Catarina in Brazil (SN: 8/10/20). But in tropical regions like the Caribbean and the Gulf of Mexico, where cooler waters are prohibitively far away, “that community doesn’t have anywhere to go.”
That lack of access to an escape route to cooler waters is why the region’s cold-blooded ocean species, including fish, may be even more vulnerable to warming than their counterparts on land. Ocean ectotherms are on average larger than terrestrial ectotherms Spend more time at the upper limit of body temperatureAs marine ecologist Malin Pinsky and colleagues of Rutgers University, New Brunswick, N.J. reported in 2019, there are more ectotherms than land-based ectotherms.
Then there’s the anoxia. The water releases oxygen as it heats, just like the bubbles that escape from a pot on the stove. This leaves less oxygen for marine life. Anoxic waters resulting from heat have been linked with increased fish and seagrass deaths. In June for example, thousands upon thousands of fish werehed up along the Texas Gulf Coast south of Houston after a low-oxygen incident.
Florida’s sea grasses have been in free fall for years, with thousands of hectares of marine sea grass beds wiped out by anoxia as well as nutrient pollution, which can lead to harmful algal blooms that block out the light for underwater plants. Manatees, and other animals that depend on sea grasses as food, have been killed by the loss of these ecosystems.
‘It’s just bonkers hot’
What’s driving the brutal ocean temperatures is still uncertain — but human-caused climate change is undeniably at its core, researchers say. “Ninety-three percent of the excess heat in the atmosphere is being absorbed by the ocean,” Rodrigues says. That’s raised the average temperature of ocean waters, “and once the mean temperature is raised, the extremes are easier to achieve.”
There are likely to be other factors at play, such as this year’s onset of the global climate pattern known as the El Niño-Southern Oscillation (SN: 7/13/23). The El Niño phase of that climate pattern tends to increase the global average temperature, and this year’s El Niño is bidding to be “a strong one,” Kirtman says.
“Certainly, one of the questions that’s come up is how much [of the heat] is internal natural variability, and how much a ratcheting up of climate change,” he says.
Local extremes — such as the temporary hot tub in Manatee Bay — may also be influenced by factors such as the shallowness of the water and murkier, less-reflective waters absorbing more heat.
But, Kirtman says, the global oceans have warmed up so much that El Niño or sediment-laden waters alone can’t possibly explain what’s going on. “This is so crazy, so bonkers. It’s just bonkers hot.”
