Mass bleaching events were unheard-of a half century ago, but they've been increasing in frequency and severity since the 1980s. By some estimates, the world has lost half of its coral cover since 1950.
The coral reefs of the Florida Keys have suffered a sharp decline since the 1970s, primarily because of disease and bleaching, both of which are directly linked to increasing ocean temperature, Dr. Manzello said. "You talk about canaries in the coal mine," he said. "These canaries have been dying now for 40 years."
The losses have inspired scientists and enthusiasts to intervene, propelling the field of coral restoration. Ken Nedimyer, for example, stepped away from a successful business as a tropical fish wholesaler some 20 years ago to throw himself into growing staghorn and elkhorn coral in offshore nurseries and planting them on Florida's reef.
He went on to found Coral Restoration Foundation and then a newer nonprofit group, Reef Renewal USA, which he still runs. He has dealt with bleaching and hurricanes before, but these past couple of weeks have shaken him like never before. "I don't really know how to process it," Mr. Nedimyer said.
To be clear, he hasn't stopped working. It has been a whirlwind of gathering genetic samples, finding space for coral in tanks on land, applying for emergency permits to move nurseries to deeper, cooler water.
But for the first time, he said, he's questioning whether such efforts can be successful over the long term. Last year, greenhouse gas emissions in the US went up, not down. Globally, emissions were on target to hit a record high. "I keep thinking, what's it going to take to get people's attention?" Mr. Nedimyer asked.
David Obura, a marine biologist and co-chairman of the coral specialist group for the International Union for Conservation of Nature, praised certain restoration efforts but noted that without climate action, they are all but useless.
"With the main drivers of impact continuing to rise, they may just ‘buy time' for just a few years," Dr. Obura wrote in an email. "It is of course critical to attempt this, but this must not distract focus on addressing what and who is causing the problem."
As the natural warming cycle of El Nino is compounded by climate change, he expects "several years of massive coral bleaching" around the world. Beyond Florida, bleaching is already underway in reefs off the Bahamas, Belize, Colombia, Costa Rica, Cuba, El Salvador, Mexico and Panama.
Ms. Thomasson returned to Looe Key on Friday, getting her first look at the reef where she'd hoped to one day plant the now-dead young coral from the nursery. Thickets of wild elkhorn and mounds of brain coral were bleached or already dead.
She clung to the knowledge that her group's sites in the Upper Keys were faring better, so far. Ms. Thomasson is determined to keep working on coral restoration, but she needs an ocean hospitable to corals for them to return to.
"It's up to everyone else to demand climate action right now," Ms. Thomasson said. "Not in a year, not tomorrow, but right now. Actually yesterday."