Can an umbrella work underwater? The answer appears to be yes, just not for the reason many might imagine. In the Florida Keys, scientists have tested floating shades to protect corals from extreme heat, offering a glimpse of a possible emergency measure—but they caution that this is far from a permanent solution.
Marine heatwaves have become a recurring threat to coral reefs worldwide. When ocean temperatures climb too high, stressed corals expel the microscopic algae living in their tissues, turning white and losing their primary food source—a process known as bleaching. The frequency of such events is alarming, and researchers are scrambling for ways to buy time for the most vulnerable reefs.
Testing underwater umbrellas in the Florida Keys
After a record-breaking marine heatwave struck the Florida Keys in 2023, a team led by Karen Neely of Nova Southeastern University set out to test a simple idea: could shading corals reduce the damage? Their findings, published in Frontiers in Marine Science, suggest that it can help on a small scale.
“Heat-related coral bleaching is probably the biggest threat to reefs worldwide,” says Neely. “Solutions like shading can offer temporary protection on small scales.”
The 2023 event was devastating. At most sites, every coral species bleached, and branching corals—such as staghorn and elkhorn—were nearly wiped out. At one inshore site, more than half of the monitored corals lost some or all of their living tissue.
“This study stemmed directly from that event: we wanted to provide proof of concept for tools to save targeted corals in similar future events,” Neely explains.
The following summer, the team selected 40 colonies of two brain coral species in the lower Florida Keys. Half were left exposed, while the other half were fitted with floating shades made from PVC frames and nylon mesh, suspended about 30 centimetres above the corals. These underwater parasols were not designed to cool the water; instead, they reduced intense sunlight and ultraviolet radiation that can worsen bleaching.
Divers began monitoring in July 2024 and installed the umbrellas in August, when summer heat intensified. They tracked the corals until November, noting changes in colour and the algae living inside their tissues. Despite 2024 seeing the second-highest level of heat stress recorded at the site since 1985, some shaded corals began to regain their colour after the umbrellas were installed. In contrast, exposed corals became paler during the hottest part of the experiment.
But the researchers stress that this is a temporary fix. Covering entire reefs would be impractical, so they see shading primarily as a way to protect vulnerable corals during extreme heat events. Ultimately, long-term protection depends on limiting the warming of the seas.
“Scaling is the big limitation of this study,” Neely says. “Shading is not a cure-all.”
Global bleaching crisis deepens
Ocean temperatures continue to break records. On 22 August, the average global sea surface temperature outside the polar regions reached 21.1°C, the highest daily temperature ever recorded. Extreme heat is hitting reefs more often: the Global Coral Reef Monitoring Network reported in August that bleaching events now occur roughly every five to six years, compared with about once a decade in the past.
Although corals can recover when sea temperatures cool, frequent marine heatwaves leave too little time for damaged reefs to repair themselves. The most recent global bleaching event began in 2023. Between 2023 and 2024, hard coral cover fell by 8.9 per cent globally—the largest one-year decline on record. By March 2025, 84 per cent of the world’s reefs had experienced temperatures hot enough to cause bleaching, while parts of Australia’s Great Barrier Reef recorded their largest annual losses of living coral in almost 40 years of monitoring.
Coral reefs cover less than 0.2 per cent of the ocean floor, yet they are vital to life both underwater and on land. They provide food and shelter for at least a quarter of all marine species, including many fish caught for food, and act as natural shields for coastlines. As new research links stronger El Niños to fossil fuel emissions, the pressure on reefs is only expected to grow.
While underwater umbrellas may offer a lifeline for individual corals, scientists agree that the only lasting solution is to address the root cause: rising greenhouse gas emissions. As Europe's storm risk rises with climate change, the continent is not immune to the broader impacts of a warming planet. For now, shading remains a stopgap measure—one that highlights the urgency of the crisis rather than solving it.
