KEY LARGO, FL — If global warming summons images of polar bears clinging to shrinking ice floes, this is its face in the Florida Keys: a sun-dappled stretch of shallows along the turquoise reef line, where scientists painstakingly attach russet polyps of regenerated coral to damaged reefs.
"When I first came here snorkeling, in 1985, it was amazing, the forest of coral was so thick," said Bill Goodwin, a resource manager for the Florida Keys National Marine Sanctuary. "Just look now," he said, gesturing to the few small brown patches amid an elephant's boneyard of skeletal remains at the foot of the Carysfort light tower in the roiling Atlantic waters seven miles off Key Largo.
On May 9, for the first time, two species of Caribbean coral — acropora palmata, or elkhorn, and acropora cervicornis, or staghorn — were added to the list of threatened species under the federal Endangered Species Act. It was a needed step, say marine biologists and environmentalists who focus on coral, and probably overdue, but just one narrow glimpse at the universe of woes affecting the undersea invertebrates in the Keys, throughout the Caribbean and across the globe.
"Elkhorn and staghorn used to be the dominant species on the Caribbean reef as recently as the early 80's," said Jennifer Moore, a natural resource specialist for the protected resources division of the National Marine Fisheries Service, which placed coral on the threatened list after prompting from the Center for Biological Diversity, based in Arizona. "But the species has declined 97 percent since the late 70's."
There is no one answer to what is killing these coral. The greatest culprit seems to be disease, especially "white diseases," which fleck the coral with pox and bands of deathly white. But there are other stresses, including degraded water quality, nutrient runoff from agriculture, human poaching and boating accidents.
Of perhaps greater impact are instances of coral bleaching affecting these and other corals that have occurred with increasing frequency in recent years. In these outbreaks, which are directly tied to rising ocean temperatures and reach their height in the warmest months, vast fields of coral shed their gaudy colors, turn bone-white and die.
"Last year was a particularly bad outbreak," said Tyler Smith, a coral biologist at the University of the Virgin Islands. "We lost anywhere from 70 to 100 percent of our coral to bleaching."
Florida  was somewhat spared in that outbreak, only because the mighty swirl of Hurricane Wilma dragged cooler, life-saving water into the region.
"Who would have thought that corals would make it onto a threatened species list, but here they are," said Andrew Baker, a marine biologist and coral specialist at the University of Miami.
While rising ocean temperatures are clearly behind coral bleaching, the link to the various diseases affecting the endangered coral is not so clear.
"We do not know the answer," Ms. Moore said. "Some say it might be linked to warmer temperatures, but the science isn't perfect yet."
And while water temperatures have gone up only a little in recent decades — just a degree or two — it is enough to affect these fragile organisms. "They were already living near their temperature maximum, so just a tiny rise can push them over the edge," Ms. Moore said.
Scott Donahue, associate science coordinator at the Keys marine sanctuary, snorkeled above one of the last thick stands of coral left at Carysfort, studying the spot where a severed arm of elkhorn had been reattached with concrete after a recent boating accident. With their thick flat ears, just like the elk horns for which they were named, the coral form a distinctive and imposing structure, covering pretty much the entire spectrum of browns from light tan to chocolate. Much harder hit in the Keys are the staghorns, which, like their namesake, are tall and finger-shaped, when you can find them.
"The importance of getting these coral on the threatened list is, most of all, that it raises awareness of the problem," Mr. Donahue said.
In four meetings with residents around southern Florida in recent weeks, and in three more planned for this week in St. Croix, St. Thomas and Puerto Rico, Ms. Moore and other federal officials will begin the process of putting together new regulations governing the use of coral and access to the reef.
That, she said, will take most of this year. At the same time, scientists will spend the next year or two putting together a recovery plan to help save the species. "It's a road map for what we should do," Ms. Moore said.
Richard Curry, science and research coordinator at Biscayne National Park, has been operating a team of volunteers since 1992 figuring out ways to grow coral polyps in the laboratory and even in the field, in underwater laboratories.
"Some of the bolder corals seem to be more amenable to a laboratory environment," Mr. Curry said. "The two that were listed, though, are the most resistant. We've been able to keep them alive in a laboratory, but not out in the field."
Others, convinced that ocean temperatures will inevitably rise, are trying to figure out which species will be hit hardest and what can be done to save them, which will flourish and what can be learned from them, and just what the reefs of the future will look like.
Not all the news is bad. Mr. Donahue said that, with rising temperatures, scientists have seen fresh stands of elkhorn and staghorn off Broward County, Fla., far north of the Keys, in waters where such coral would have never been seen two decades ago.
"Right now, we're trying to understand how corals are going to respond to sea waters' warming," said Mr. Smith of the University of the Virgin Islands. "We know it's coming. We know the oceans are going to get warmer."
At the oceanside facility of Mote Marine Laboratory in Summerland Key, David Lackland gingerly lowered a plastic syringe into one of his carefully tended tanks.
He sent out a smoky puff of particulates. Caught in the artificial current he has created, the puffs wrap around the coral polyps, some no bigger than a nailhead, others the size of a golf ball. "Ah, beautiful, they're getting a nice dinner," he said.
Mr. Lackland has 22 species in his handmade tanks, and is growing more at his home on a nearby key and on an underwater acre he cultivates offshore. It is painstaking work. His "poster boy" is a polyp of elkhorn that he has successfully cloned, though it has only grown a fraction of an inch in two years.
"I'm not going to claim to know all about global warming, but it seems clear that something is going on," Mr. Lackland said. "I am going to remain optimistic, no matter what. But I must say, the signs are all pointing to a conclusion that something must be done soon."