Ocean temperatures in Florida reach 90 degrees Fahrenheit, which poses a danger to coral

Florida’s coral reefs are facing what could be an unprecedented threat from a marine heat wave that is warming the Gulf of Mexico, sending water temperatures as hot as 90 degrees Fahrenheit.

The biggest concern for corals isn’t just the current sea surface temperatures in the Florida Keys, even though they are the hottest on record. According to the National Oceanic and Atmospheric Administration, the average daily surface temperature off the Keys on Monday was just over 90 degrees Fahrenheit, or 32.4 degrees Celsius.

Scientists say the real concern is that it’s only in July. Corals typically experience the greatest amount of heat stress in August and September.

“We are entering uncharted territory,” said Derek Manzilo, an ecologist and coordinator of NOAA’s Coral Reef Monitoring Program.

Coral reefs are natural wonders that support countless species and cause severe storm damage. In the United States, coral reefs generate economic benefits commensurate with 3.4 billion dollars annually for fisheries, tourism, and coast protection, according to the National Oceanic and Atmospheric Administration (NOAA).

But the oceans have absorbed about 90 percent of the extra heat that humans create as we burn fossil fuels and destroy forests. When sea temperatures get too high, corals bleach, expelling the algae they need for sustenance. If the waters don’t cool fast enough, or if bleaching events occur in close succession, the corals die. For decades, scientists have been warning that climate change is an existential threat to coral reefs. The world has already lost a large percentage of its coral reefs. Maybe half Since 1950.

“To be honest, it can be very frustrating,” said Dr. Manzello. “Unfortunately, I am a scientist who watches what happens.”

Marine heat doesn’t just affect the Gulf of Mexico. Globally, about 40 percent of the planet is experiencing a marine heat wave, according to Dillon Amaya, a NOAA physicist who studies it.

“Florida is one spot in a terrible quilt right now,” said Dr. Amaya.

This is partly because the planet is entering a natural weather phenomenon known as El Niño, which usually brings warmer oceans. But now, El Niño is coming on top of long-term warming caused by greenhouse gas emissions.

While coral reefs are particularly vulnerable, heat waves are harming countless species, and the effects vary across the world, as species adapt to different temperature ranges.

In general, fish need more oxygen when the water is warmer. This is a problem, because warm water contains less oxygen.

“Large-scale fish deaths are becoming more frequent as our climate changes,” said Martin Grosell, a professor of ichthyology at the University of Miami.

Coral reefs are of particular importance because many species depend on them. About 25 percent of marine life, including more than 4,000 species of fish, depend on coral reefs at some point in their lives, according to NOAA.

Dr. Manzello said that while there have been no reports yet of bleaching in Florida, it has already begun in coral reefs further south, off Belize, Mexico, El Salvador, Costa Rica and Colombia.

Florida’s coral reef system stretches about 350 miles, from the entrance to St. Lucie on the mainland south and west past the end of the Keys River, and is frequented by sea turtles, manta rays, flounder and lobsters.

What happens in Florida will depend on conditions over the next few weeks. Scientists say storm surges, which spew deeper, cooler waters and reduce sunlight, can provide relief. El Niño periods are usually associated with a below average hurricane season in the Atlantic Ocean, but that may not be true this year.

Researchers interested in coral reefs are deeply disturbed.

“I lose sleep over it,” said Andrew Baker, a professor of marine biology at the University of Miami, where he directs the Coral Reef Futures Lab. “But I don’t want to write the eulogy now.”

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 coral reefs with species that appear to be more heat tolerant. But ultimately, the survival of coral reefs and countless other species depends on humans’ ability to rein in climate change.

“You have to go to the root causes,” said Lizzie MacLeod, director of global oceans at The Nature Conservancy. “We have to cut emissions, we have to transition to clean energy, we have to cut subsidies for the fossil fuel industry.”

In Key West, beachgoers have expressed amazement at the warmth of the ocean, comparing it to bathing water. Lynsey Wavra, the captain and ecotourism guide, said her mother lived there for 20 years and witnessed the decline of the reef.

“She came home crying,” said Mrs. Lavra.

Francis Robles Contribute to the preparation of reports.