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Soaring Temps Put Reefs at Risk

Summer’s skyrocketing temperatures have disrupted communities around the world, fueling unprecedented wildfires, storm events, and health concerns.  Beyond the shoreline, temperatures are also soaring. In July of 2023, South Florida’s surface water temperatures averaged about 91 degrees – several degrees above the water’s typical 85F – and on July 24, scientists measured a record-breaking 101.19F  at the Florida Keys Manatee Bay buoy. Even before this summer, the ocean had gained 396 zettajoules of heat between 1971 and 2018, according to a recent United Nations report.  Extended periods of extreme ocean heat have stressed the world’s coral reefs – endangering these critical ecosystems that first appeared on Earth nearly half a billion years ago. Coral reefs cover just 1 percent of the ocean floor but are home to over a quarter of marine species.


Corals consist of often genetically identical, tiny polyps that measure a few centimeters in length. Categorized as zoophyta, or “plant-animals”, due to their plant-like appearance, corals are grouped with cnidarians, including sea anemones and jellyfish. Coral polyps have a mouth surrounded by tentacles and grow upon the skeletons of their predecessors, fusing together to form a reef.  Healthy corals may appear blue, purple, green, red, or pink, to the symbiotic zooxanthellae algae that occupy each polyp and use their chlorophyll-containing cells to provide the coral’s food. The color may vary based on local water conditions and gene expressions of the algae.


Florida’s Coral Reef extends over 350 nautical miles along the Southeast coast, spanning the coastlines of five counties, from the Dry Tortugas islands to the St. Lucie Inlet in Martin County.  The reef provides habitat for important commercial and recreational fish species, protects shorelines from wave action, and increases overall resilience to hurricanes and storms. However, over the last half-century, Florida’s reefs have lost 95% of the coral that once covered them due to poor water quality, human activity, and climate change – the driving force behind ocean acidification and extreme heat.


According to NOAA, many corals grow optimally in temperatures between 73F and 84F and may show signs of stress outside of this range. A key indicator of stressed coral is bleaching – seen when the zooanthellae algae evacuate from a coral polyp, causing the polyp and corals to lose their brilliant coloring. Bleaching eventually causes coral to starve because without algae, there is no chlorophyll production, and therefore no food to sustain the coral polyps.


The Guardian recently reported of reefs in Panama, Colombia, El Salvador, Costa Rica, Mexico, and six countries in the Caribbean, including the Bahamas and Cuba, undergoing “significant bleaching, alongside corals in Florida that began turning white almost a month ago”. “Florida is just the tip of the iceberg,” said Derek Manzello, coordinator of NOAA’s Coral Reef Watch Program in an Associated Press interview. “The scale of this event is very alarming. We are talking about thousands upon thousands of miles of coral reefs undergoing bleaching from severe heat stress.”


To provide a snapshot of current coral heat stress around the globe, the nonprofit organization Coral Reef Watch has developed the  50km Coral Bleaching HotSpot product.  The product uses data showing how far the temperature is above the bleaching threshold, and how long it has stayed above that point. It uses Degree Heating Weeks (DHW), units that measure local heat stress by adding up temperatures that exceed the bleaching threshold over a twelve-week period. When DHW reaches 7.2°F-weeks, significant coral bleaching is likely. At eight degree C-weeks or higher, experts predict severe, widespread bleaching and significant mortality.


Ocean Temperature & El Nino


A report by the Guardian states that this year’s El Niño is not regarded as a sufficient explanation of the ocean temperature rises recorded since April. El Nino, a natural warming of the central Pacific, impacts global weather patterns, typically shifting the jet stream over North America farther south. That shift tends to block moisture from the Gulf of Mexico, reducing the chance of hurricanes in the Southeast. However, while El Nino is strong in the Pacific, its effects in the Gulf of Mexico and Atlantic have yet to appear, leading NOAA to increase its original prediction of below-average hurricane activity to a 60% chance for an above-normal hurricane season, significantly altering the agency’s May forecast.


Hurricanes are typically viewed as adverse, unwelcome disasters, but for overheated corals they can be lifesaving, churning cooler ocean waters up to the surface. However, relying on unpredictable weather to bring cooler waters to relieve coral reefs of elevated, ongoing heat is risky.  Fortunately,  the fate of coral reef ecosystems will also be influenced by dozens of individuals and groups conducting extensive research and restoration, working to ensure their continued viability. Examples of this work include:


  • NOAA’s program, Mission Iconic Reefs, which grows corals and outplants them to sites in the Florida Keys. A consortium of other organizations is also focused on an extensive, 20-year effort to rebuild Florida’s coral back to 90% of where it was 50 years ago.


  • Mote Marine Laboratory in Sarasota’s efforts to breed more heat-resistant corals and support reef survival with tools like shade covers and underwater fans.


  • The Hawaii Institute of Marine Biology, which is analyzing the effects of heat exposure on metabolites produced by heat-resistant and heat-sensitive coral species, Montipora capitata and Pocillopora acuta, and comparing them to various un-stressed corals.


  • Work on a new way to identify heat-stressed corals by monitoring reef health before visible bleaching occurs, essentially buying conservationists more time to collect and relocate them, reported by the World Economic Forum.


Earth’s treasured coral reef ecosystems are living illustrations of the intricate connections found in nature.  While their fragile nature adds to their allure, it also makes them highly susceptible to climate-related impacts and water quality issues. Now is the time to act on climate change, preventing its cascading effects – and allowing life on land, and in water, to flourish.


By Bridget Washburn



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