Rising surface water temperatures and pollution place coral reefs at risk worldwide.

Slipping beneath the surface of the clear Caribbean, divers see a vast landscape of colorful coral boulders that spread across the sandy shallows 30 feet below.

This offshore island is far from the sewage outfalls of Belize City and the crowded dive sites of the northern barrier reef. Unlike most of the Caribbean these days, its corals are considered pristine. They spread out on the lagoon floor and over the cliff-like reef wall beyond: corals shaped like giant cauliflower heads, thickets of staghorn and finger corals, and occasional antler-like colonies of elkhorn.

Only there's something wrong. The elkhorn corals are dead and covered in algae - victims of a regionwide die-off. Other corals have bleached completely white, dotting the bottom conspicuously all the way out to the edge of the lagoon.

"It's amazing how much coral bleaching we're seeing today," says Jonathan Kelsey, a Peace Corps volunteer working at the University College of Belize's marine research station here. "A few weeks ago when we dove here, all of these corals looked completely normal. I'm concerned they won't recover."

Coral reefs are under severe threat in much of the world from a variety of stresses - many of them the result of human activity: fishing, pollution, silt from the land, and damage from anchors and careless divers. And according to marine biologists, possibly the greatest threat is global climate change causing sea temperatures to rise. Warm water has degraded large areas of these so-called rain forests of the seas. By one estimate, 10 percent of the world's reefs are dead or beyond recovery and another 30 percent may follow over the next 20 years.

This year brought the hottest surface temperatures since 1982, according to National Oceanic and Atmospheric Administration satellite data.

Southeast Asian reefs are believed to be the most threatened because of intense population pressures on their resources. In the Philippines, desperate fishermen have for decades dropped dynamite and cyanide on the reefs to stun or kill the fish within. That, combined with pollution from the land, is estimated to have damaged or killed 90 percent of the country's 34,000 square kilometers of reef. In Malaysia's portion of the island of Borneo, a 1997 reef survey found that 99 percent of the reefs had been damaged by blast fishing.

Half the coral reef systems of the United States, American Samoa, and the Virgin Islands are at risk, according to the National Oceanic and Atmospheric Administration. Tourists flock to many Caribbean island nations to dive and snorkel on the reefs, prompting seaside developments that increase erosion and pollution stress on the corals.

The declines are troubling, and not merely for aesthetic reasons. Millions of people depend on reefs for food, livelihood, and protection from storms. Many Caribbean and Pacific island nation economies depend on reef-based tourism; in Belize it's the largest foreign-exchange earner and central to future development plans."We're seeing the collapse of reef systems on a local scale like Jamaica or the East Coast of Panama," says Jeremy Jackson, a coral specialist now at the Scripps Institution of Oceanography in La Jolla, Calif. "Its hard not to be cynical when over the course of your career you've seen so many beautiful reef systems deteriorate and die all over the Caribbean."

Coral reefs are created through a cooperative alliance of coral polyps - a tiny animal that filters detritus from the surrounding water - and single-celled plants that live within the polyps. The plants generate many of the nutrients the corals need to live and capture calcium from the water, which the polyps use to build their limestone shells.

Over decades and centuries, succeeding generations of corals build layer upon layer of limestone skeletons, creating vast reef systems that can run for tens, even hundreds of miles. They're the most diverse marine ecosystems, harboring a myriad fish, invertebrates, sponges, and other creatures.

But corals thrive only in clear, clean, shallow tropical waters. In recent decades, such conditions are scarcer. Human population and development pressures have replaced coastal mangrove forests with cities, golf courses, fruit plantations, and aquaculture ponds. Soil erosion, sewage outfalls, and fertilizer run-off from the land have smothered corals with sediments and algal growth in much of the Caribbean.

Overfishing compounds the problem by removing creatures that feed on algae and clean the reef. Careless boaters and divers damage corals by touching, breaking, or collecting them. In the Philippines, desperate fishermen have destroyed entire reefs by using dynamite to capture remaining fish.

Staghorn and elkhorn corals are taking the brunt of the damage in the northern hemisphere, according to Jacques Carter, a coral-reef researcher at the University of New England in Biddeford, Maine. "The general pattern is that you first lose these branching corals. They become covered in fleshy brown algae, and later the algae begins overtaking the massive boulder corals."

But Belize - a small country of 200,000 at the base of the Yucatn - has taken impressive measures over the past decade to protect its coral atolls and extensive barrier reef, the longest in the hemisphere.

Marine reserves have been established to protect popular dive sites (which generate considerable revenue) and important fish and reef habitats.

"People traditionally thought about protecting a single species. Then people realized you couldn't do that without protecting their habitats," says Janet Gibson of the United Nations Development Program's Belize Coastal Zone Management Project. "But to protect habitats you have to take an even broader, ecosystem approach. That's what we're trying here in Belize." Recognizing the importance of the reef to Belize's development, UNDP funded a $3 million project to create an integrated management plan for Belize's entire coast. By regulating coastal development with an eye to minimizing impacts to the reef, the government hopes to put growth on a sustainable footing.

"Belize has an enormous potential for tourism, but it has to be balanced with environmental protection," says acting director of tourism Valerie Woods. While Belize has earned praise for its conservation policies, there are plenty of obstacles to carrying them out. Although clearing coastal mangrove forests is now illegal (it often results in the smothering of nearby reefs), the law is poorly enforced. Critics say developers' money seems to buy its way around zoning plans. Belize is also finding that there are threats to its reefs beyond its control. Pollution crosses national boundaries from Mexico or Honduras. Hurricanes can strike at any time, damaging reefs that may no longer be resilient enough to rebound as they once did.

Then there's widespread bleaching, a phenomenon believed to be caused by unusually high water temperatures and possibly linked to global warming. A small increase in water temperature - and Belize is having one of the hottest years on record - causes the algae in coral polyps to die. The weakened polyps often follow.

"We can't do anything about coral bleaching, but we can ease other stresses to the reefs to keep them healthy," says Ms. Gibson. "We must do it, because the reefs are part of what it means to live in Belize."