Florida’s coral reefs are facing what could be an unprecedented threat from a marine heat wave that is heating up the Gulf of Mexico, pushing water temperatures into the 90s Fahrenheit.
The biggest concern for coral isn’t just the current sea surface temperatures in the Florida Keys, although they are the warmest on record. The daily average surface temperature of the Keys on Monday was just over 90 degrees Fahrenheit, or 32.4 Celsius, according to the National Oceanic and Atmospheric Administration.
The real concern, scientists say, is that it’s only July. Corals typically experience the most heat stress in August and September.
“We’re entering uncharted territory,” said Derek Manzello, an ecologist and the coordinator of NOAA’s Coral Reef Watch program.
Coral reefs are natural wonders that support a myriad of species and blunt damage from storms. In the United States, reefs generate economic benefits to a degree $3.4 billion annually for fisheries, tourism and coastal protection, according to NOAA.
But oceans have absorbed about 90 percent of the extra heat caused by humans as we burn fossil fuels and destroy forests. When sea temperatures rise too high, corals bleach, expelling the algae they need for nourishment. If waters do not cool fast enough, or if bleaching events occur in close succession, the corals die. For decades, scientists have warned that climate change is an existential threat to coral reefs. Already the world has lost a huge proportion of its coral reefs, maybe half since 1950.
“To be blunt, it can be very depressing,” Dr. Manzello said. “Unfortunately, I’m a scientist watching it happen.”
Sea heat doesn’t just affect the Gulf of Mexico. Globally, about 40 percent of the planet experiences ocean heat, according to Dillon Amaya, a physicist at NOAA who studies them.
“Florida is one patch in a terrible blanket right now,” Dr. Amaya said.
In part, that’s because the planet is entering a natural climate phenomenon known as El Niño, which usually brings warmer oceans. But now, El Niño is outpacing long-term warming caused by greenhouse gases.
While coral is particularly vulnerable, heat waves harm countless species, and the effects are different around the world, as species are adapted to different temperatures.
In general, fish need more oxygen when the water is warmer. That’s a problem because warmer water holds less oxygen.
“Large-scale fish kills are becoming more common as our climate changes,” said Martin Grosell, a professor of ichthyology at the University of Miami.
Coral reefs are especially important because so many species depend on them. About 25 percent of all marine life, including more than 4,000 types of fish, depend on reefs at some point in their lives, according to NOAA.
Although there are no reports of bleaching in Florida yet, it has already begun on reefs to the south, Dr. Manzello said, off Belize, Mexico, El Salvador, Costa Rica and Colombia.
The coral reef system of Florida extends approximately 350 miles, from the St. Lucie fjord on the mainland south and west beyond the end of the Keys, and is frequented by sea turtles, manta rays, flounder and lobster.
What happens in Florida will depend on conditions over the next few weeks. Storms that churn up deeper, cooler water and reduce sunlight could provide relief, scientists say. El Niño periods are usually associated with below-average Atlantic hurricane seasons, but that may not be the case this year.
Researchers who care about coral are deeply concerned.
“I do lose sleep over it,” said Andrew Baker, a professor of marine biology at the University of Miami, where he directs the Coral Reef Futures Laboratory. “But I don’t want to write the eulogy yet.”
Scientists like Dr. Baker are racing to come up with ways to help corals become more resistant to higher temperatures, for example by crossing Florida’s corals with varieties that seem to withstand more heat. But ultimately, the survival of corals and countless other species depends on humans’ ability to curb climate change.
“You have to go to the root causes,” said Lizzie McLeod, the director of global oceans at The Nature Conservancy. “We need to reduce emissions, we need to move to clean energy, we need to reduce subsidies to the fossil fuel industry.”
In Key West, beachgoers expressed surprise at the warmth of the ocean, comparing it to bathwater. Lynsi Wavra, a captain and ecotour guide, said her mother had lived there for 20 years and had seen the coral decline.
“She would come home crying,” Mrs. Wavra said.
Frances Robles contributed reporting.