'Strikingly warm' ocean heat wave off Florida coasts could decimate corals, other marine life, experts say

Oceans all over the world are experiencing warmer-than-normal temperatures, but waters off the southern U.S. could be on the brink of an ocean heat wave emergency as temperatures rise to unprecedented levels.

More than 40% of all global ocean temperatures are currently experiencing a heat wave, according to the National Oceanic and Atmospheric Administration.

The heat is even more acute off the coast of Florida, where ocean temperatures are currently "strikingly warm," Brian McNaldy, senior research associate at the University of Miami's Rosenstiel School of Marine, Atmospheric, and Earth Science, told ABC News.

Dangerous heat indexes blanket Florida for over a month

On Thursday, Miami reached a heat index in the triple digits for the 39th consecutive day, extending a record the city had broken the week before, according to the National Weather Service.

"When you're breaking records by such large margins, that's what makes it alarming," McNaldy said. "We're not even close to what the previous record was, let alone the average."

The waters surrounding the Sunshine State have been no different. This week, while land temperatures have been about 95 degrees in South Florida, ocean temperatures clocked in at about 94 degrees -- up to 7 degrees warmer than they should be this time of year. Water temperatures do not typically measure this high until late August or early September, experts said.

The relentless heat on both land and ocean are expected to persist through the rest of July and August -- traditionally the hottest time of the year -- forecasts show.

Days with water temperature above 90 degrees Fahrenheit have increased by 2500% in the Florida Keys since 1975, according to Ian Enochs, NOAA research ecologist and head of the Atlantic Oceanographic and Meteorological Laboratory's Coral Program.

Relative to temperatures recorded from 1991 to 2020, the waters surrounding South Florida are 5 to 7 degrees Fahrenheit above normal, McNaldy said.

One of the reasons why ocean temperatures in the Atlantic are so hot is because the Bermuda high is stuck in an abnormal position. Typically, the high sits further north and is stronger, but this year it has remained further south and is weaker, pushing those cooling trade winds south of Florida. Without those cooling winds, there is far less evaporation off the top of the water, which would bring the temperatures down.

While reliable records only go back between 40 and 80 years for water temperatures on buoys and observation docks in the Keys, inferred heat data indicates this is the hottest ocean waters have gotten since humans have been on Earth, McNaldy said.

How ocean warming will affect marine life

The warming can have "significant impacts on marine life as well as coastal communities and economies," according to NOAA, which has issued a level 2 alert for the reefs, the highest level, indicating bleaching of the coral reefs surrounding South Florida is likely to occur.

"Florida's coral reefs don't respond well to this," Abigail Clark, an assistant professor of marine science and technology at The College of the Florida Keys, told ABC News.

The effects of a mass coral die-off could be devastating, as 25% of all marine life, including dolphins, turtles and sharks, depend on the reef for survival. The reef also brings in billions in revenue to the state through the tourism and commercial fishing industries.

This bleaching event is different than anything the Florida Keys has ever seen, said Keri O'Neil, director and senior scientist of the Coral Conservation Program at The Florida Aquarium in Tampa. While some of the coral in the region has survived 3,000 years of change, several coral ecosystems have succumbed to the heat in the past 10 days, O'Neil said.

The last four decades have not been kind to the Florida Reef Tract, North America's only barrier reef that stretches about 360 miles from Dry Tortugas National Park west of the Florida Keys to the St. Lucie Inlet in Martin County, O'Neil said.

The barrier reef has lost about 90% of its live coral cover since the 1980s, O'Neil said.

The Florida Aquarium and its partners restore Florida's coral reef with a "concrete seawall" that aims to absorb storm surge and wave action. Researchers at the Florida Aquarium have also created an ocean greenhouse near Tampa where they genetically select and breed coral for strength and heat resilience.

The "babies" that were planted four years ago have thrived despite transient periods of warmth because there have also been periods of relief.

Relief likely won't come this year, as the next six weeks are forecast to be just as hot.

The bleaching events have already started, and alert levels for corals are at their highest mark, across the Caribbean.

"Our staff might come up high-fiving or they might come up in tears," O'Neil said of the monitoring.

Climate change is likely contributing to ocean warming, scientists say

The ocean warming is widespread. Elsewhere in the world, marine heat waves are currently occurring in the equatorial Pacific; the Northeast Pacific; the Northwest Pacific in the Kuroshio extension region and the Sea of Japan; the tropical North Atlantic; the Northeast Atlantic along the Iberian coast as far northward as Ireland and the U.K.; the Southwest Pacific just southeast of New Zealand; and the Western Indian Ocean southeast of Madagascar, according to NOAA.

The average sea surface temperature for June across the north Atlantic was 0.91 degrees Celsius above average, according to Copernicus, the the European Union's climate change service. This is around 0.5 degrees Celsius more than the previous warmest June, recorded in 2010.

The warming atmosphere may also be contributing to ocean warming in the region. January through June ranked as the warmest on record for air temperature in Florida, which could be causing increases in ocean temperatures as well, experts say.

The current ocean temperatures in the North Atlantic are typical of late summer, rather than early to mid-summer, experts say.

The warming could also affect the intensity of hurricanes to come later in the season.

Because of the warm temperatures on land and sea, hurricane forecasters at Colorado State University have revised their predictions for the number of named storms in the 2023 season from 13 to 18. When a tropical cyclone goes over unusually warm water, it makes them resilient to wind shear, and gives them a chance to rapidly intensify.

Anomalous warmth may not be directly tied to climate change, but it makes marine heat waves like this more likely, McNaldy said.

Climate scientists have attributed warming oceans to human-caused climate change, as oceans absorb about 90% of the heat generated by emissions. While water is much more difficult to heat than land, it is also much harder to cool.

As the excessive heat and energy warms the ocean, the change in temperature leads to "unparalleled cascading effects," including ice-melting, sea-level rise, marine heat waves and ocean acidification, according to the United Nations.

The changes will cause a "lasting impact" on marine biodiversity, as well as the 680 million people around the world who live in low-lying coastal areas and almost 2 billion people -- about one-quarter of the world's population -- who live in half of the world's coastal megacities, according to the U.N.