Why extreme heat kills corals<\/strong><\/p>\n Corals are colonies of living animals, known as polyps, that have a symbiotic relationship with a kind of algae that lives inside their cells. The algae give coral food \u2014 and their color \u2014 in exchange for nutrients and a place to absorb sunlight.<\/p>\n When the ocean gets too hot, however, this symbiotic relationship breaks down, and the polyps expel the algae and turn white. This is bleaching. When a coral is bleached, it\u2019s essentially weak and starving, and if the heat persists, it can die.<\/p>\n During extreme marine heat waves \u2014 like what Florida saw in summer 2023 \u2014 corals can die in a matter of days, sometimes without bleaching. Heat shock kills the polyps and causes their soft tissue to slough off their skeleton.<\/p>\n Starting in July 2023, water temperatures in Southeast Florida, home to the only barrier reef in the continental US, started rising to record-breaking levels, partly due to climate change<\/a>. Sensors recorded temperatures above 93 degrees in some parts of the reef. And corals were ultimately exposed to heat that was as much as four times greater than \u201call prior years on record,\u201d the authors write. That wiped out 97.8 percent to 100 percent of staghorn and elkhorn corals in the Florida Keys, where most of them were found, according to the study, which was led by Derek Manzello, a coral researcher at the National Oceanic and Atmospheric Administration.<\/p>\n \u201cWhat we saw happen was an extreme heat wave where conditions surpassed the thresholds of survival of a whole, entire species \u2014 two species \u2014 across all of Florida\u2019s coral reef,\u201d said Ross Cunning<\/a>, a coral biologist at Chicago\u2019s Shedd Aquarium, who was closely involved in the research. \u201cThat\u2019s something we haven\u2019t seen before. We were in shock.\u201d<\/p>\n These results should alarm anyone living in coastal Florida. Staghorn and elkhorn corals \u2014 native to Florida and the Caribbean, where their populations have also plummeted \u2014 are not only pretty to look at but help sustain human life.<\/p>\n Olivia M. Williamson\/Science<\/p>\n Olivia M. Williamson\/Science<\/p>\n Elkhorn coral in the Florida Keys progressed from healthy (top left) to bleached (top right) to dead (bottom) in a matter of months in the summer of 2023. Dana E. Williams\/Science<\/p>\n These species create complex structures that attract fish by providing them with a place to hide from predators and food to eat. Those include fish such as snappers and groupers that people catch and consume in Florida, where one analysis shows fishing is a whopping $24.6 billion industry<\/a>. Then there\u2019s the value that corals provide for nature-related tourism. Diving and snorkeling generate some $900 million<\/a> a year in Southeast Florida. When you go snorkeling, there\u2019s typically an expectation that you\u2019ll see coral or the animals it supports, from eels to octopuses.<\/p>\n But perhaps most importantly, these two coral species safeguard coastlines from flooding during hurricanes. Or at least they did. Staghorn and elkhorn grow in the shallows, where their many large branches help reduce wave energy, much like a seawall. Waves with less energy are smaller and slower and don\u2019t deal as much damage when they reach the shore. A 2014 meta-analysis<\/a> found that coral reefs \u2014 which comprise other species beyond staghorn and elkhorn \u2014 can reduce wave energy by an average of 97 percent. That translates to money: A 2019 government study found that Florida\u2019s coral reefs avert $675 million<\/a> worth of flood damage each year.<\/p>\n Now that these coral species are mostly dead, staghorn and elkhorn skeletons in Florida will eventually wither away, leaving the coastline less attractive to fish and tourists, and more vulnerable to storms.<\/p>\n What happened in Florida is not a one-off event. Top scientists are confident that extreme marine heat waves are becoming more frequent<\/a>. And while staghorn and elkhorn are especially sensitive to warming, they certainly aren\u2019t the only species that heat puts at risk. The planet has already lost roughly half of its live coral cover<\/a>, and climate change \u2014 and the bleaching of corals it causes \u2014 has emerged as the top threat. In Australia, for example, a 2024 bleaching event killed off<\/a> roughly a quarter of the corals in the northern Great Barrier Reef, a record decline.<\/p>\n The state of coral reefs is so bleak, in fact, that a large, international team of scientists recently announced that these ecosystems worldwide have surpassed a climate tipping point<\/a>, beyond which they can no longer survive.<\/p>\n That means that unless wealthy economies stop burning fossil fuels, it\u2019s unlikely that Florida will ever be able to bring back abundant colonies of elkhorn and staghorn corals, even with aggressive restoration efforts. Indeed, many of the corals that died in the 2023 heat wave were planted on the reef by conservation groups. (I detailed that in a separate story here<\/a>.)<\/p>\n![\"\"](\"https:\/\/www.europesays.com\/us\/wp-content\/uploads\/2025\/10\/Fig-2D.jpg\"\/)
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