The last, largest stands of ancient elkhorn coral survive in shallow waters off North Key Largo, where rough seas sometimes expose thick golden branches reaching toward the sunlit surface.

Forty years ago, elkhorn grew in dense forests that would cover parking lots. Now, the biggest clump would barely fill one space.

In another 40 years, elkhorn could disappear altogether -- along with just about every other hard coral forming South Florida's once-vibrant barrier reefs.

Federal regulators last week designated a 1,329-square-mile strip of sea bottom stretching from southern Palm Beach County to the Dry Tortugas as critical habitat for elkhorn and staghorn corals, two species that have long formed the foundation of barrier reefs off Florida and in the Caribbean.

But a new report by the Environmental Defense Fund and co-authored by two University of Miami scientists argues localized protections will do little to address the biggest threat to reefs.

Global warming is not only accelerating problems that already have sickened and shrunken coral reefs, it has created a new, potentially more lethal threat: Increasingly acidic ocean waters that can reduce living coral to dead rubble.

The report, ''Corals and Climate Change: Florida's Natural Treasures at Risk,'' concludes that 5,000-year-old reefs, which support an array of marine life, will be among the first ecosystems to collapse if greenhouse gas levels continue to rise in the atmosphere.

''All of the forecasts show that at the rate we're going that somewhere at the middle or the end of the century, it's going to be very challenging for corals,'' said Harold Wanless, UM's chairman of geological sciences.

Wanless, who has studied rising sea levels in South Florida for decades, is one of the report's six co-authors, along with department colleague James Klaus, a UM assistant professor. The others: Terry Gibson, longtime environmental journalist in Florida; Patricia Foster-Turley, wildlife biologist based in Fernandina Beach; and Karen Florini and Thomas Olson, attorneys with the Environmental Defense Fund.

GREENHOUSE EMISSIONS

Jerry Karnas, director of Environmental Defense's Florida climate project, said the report bolsters the case for dramatic state, federal and international steps to reduce greenhouse emissions -- particularly of carbon dioxide -- largely produced by cars and power plants burning fossil fuels.

''We're ground zero for impacts,'' said Karnas, who served on Gov. Charlie Crist's climate action team.

The report doesn't break new ground but it does compile the latest studies of reef decline, threats and ripple effects on both the environment and economy.

It estimates the loss of reefs, which are a magnet for tourists, divers and anglers, would cost 70,000 jobs and $5.5 billion in sales annually in five southeast counties alone: Miami-Dade, Broward, Monroe, Palm Beach and Martin.

Staghorn and elkhorn corals, the first coral species to be listed as threatened under the federal Endangered Species Act in May 2006, came under new federal protection issued last week.

Once the major reef builders in South Florida's shallow barrier reefs, the large corals, whose stalks resemble the horns of their animal namesakes, have declined by as much as 97 percent off the Florida Keys and the Caribbean. Scattered colonies also have been found off Broward and Palm Beach counties.

Last month, the National Marine Fisheries Service issued new regulations focused on direct human damage from anchoring, groundings, fishing, pollution and collection -- similar to protections already in place in the Florida Keys National Marine Keys sanctuary.

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