Coral Crisis

Global warming is the latest threat to the world's biologically rich and besieged coral reef ecosystem

  • Joe Dupree
  • May 01, 2007

WHEN CORAL REEFS BEGAN mysteriously dying ten years ago on a beloved reef just off Key West, local scientists and environmentalists launched a determined effort to find out why. They cordoned off parts of the reef with metal stakes and dispatched a steady stream of divers with cameras to monitor corals for signs of encroaching pests or diseases.

A decade later, the metal stakes are still in place at Eastern Dry Rocks Reef, but the corals themselves are gone. Virtually all of the reef's majestic elkhorn corals vanished in less than six years, along with purple sea fans, fingerlike pillar corals, and many of the fish and other creatures that lived among them. Their stunning decline bewildered local activists and cut deeply into the profits of coral-dependent businessmen like Peter Wassylenko, a Key West dive instructor who suddenly could no longer promise customers a dazzling underwater tour. During a swim over the reef on a late summer morning, Wassylenko could only point out the gray hulks of dead corals that once blazed with living color. "A few years ago, we'd look for diseased corals out here, and we'd report any that we found," says the sandy-haired skipper of the dive boat Bonsai. "Now, if you find a living, healthy coral, you get excited."

The corals of Eastern Dry Rocks Reef succumbed to the same forces that are devastating reefs across a wide swath of the globe, from the Florida Keys to Jamaica's Montego Bay, from the Seychelles in the western Indian Ocean to Indonesia's Komodo National Park. Coral experts say the world's biologically richest marine ecosystems are failing at an unprecedented rate because of a combination of human-induced threats, including pollution, erosion, invasive species and destructive fishing practices. And the biggest threat of all, they say--global warming--will also be the most difficult to reverse.

While for most terrestrial creatures, the worst effects of global warming are decades away, for coral reefs the future is already here. Over the past century, ocean temperatures have increased an average of 1 degree F, with frequent and prolonged higher temperature spikes in many tropical regions. Warmer oceans have triggered widespread episodes of coral "bleaching," a condition linked to environmental stress. Higher levels of carbon dioxide and other greenhouse gases have also altered the chemistry of the oceans, undermining corals' ability to build reefs and fight off predators and disease. And warmer sea currents are believed to be fueling more powerful storms, including Category 5 hurricanes that can wipe out centuries of coral growth in a single day. Battered by climate change, on top of multiple other perils, some coral species are facing the greatest threat to their survival in at least 500,000 years. According to a 2004 report for the Pew Center on Global Climate Change, a quarter of the world's reefs are already gone.

"Imagine going to your favorite national park and finding that 90 percent of the trees are dead," says Phillip Dustan, a University of Charleston biology professor and science advisor to the Cousteau Society. "There's a spectacularly beautiful world down there that is just being lost. If this were happening on land, people would be screaming about it."

As corals disappear, the impact on humans is not likely to be subtle. Coral reefs, which cover less than 1 percent of the ocean floor, are the principal habitat for 25 percent of the ocean's species, from spiny lobsters and sea urchins to important food sources such as groupers and snappers. Many other marine creatures use reefs as nurseries or feeding grounds. And both people and wildlife benefit from the protection they provide against storm surges and tsunamis. Globally, coral reefs are estimated to contribute more than $30 billion a year in direct net benefits to human economies.

Vital as they are, coral reefs are surprisingly fragile. The living part of a reef is made up of colonies of tiny animals, coral polyps, that form a thin veneer of life on rocklike structures made of the skeletons of generations of their ancestors. Most corals require shallow water that is clear and largely free of sediment and nutrients. They need warm tropical waters to thrive, but not too warm: The majority of coral species exist at the upper end of their maximum temperature range.

A recent report by the IUCN–World Conservation Union warned that half of the world's corals could disappear by 2045, when average water temperatures are expected to be as much as 3 degrees F higher than they are today. "We've reached the point where even small changes can yield big consequences," notes NWF Global Warming Specialist Patty Glick. "That's why it's urgent that we stop this global warming train now and turn it around." 

For scientists who study coral reefs, the year 1998 was a global wake-up call: the most devastating year for corals since record-keeping began. In a year that already was unusually warm, an El Niño weather pattern cranked up the heat still further in Pacific waters near the equator. Corals throughout the world responded as they normally do to environmental stress: They bleached, or rid themselves of the microscopic algae that live inside them and give coral polyps their distinctive colors. For millions of years, corals have shared a symbiotic existence with these tiny plants: The corals draw oxygen from the algae in exchange for nutrients and carbon dioxide in the corals' waste. When corals expel their algae in times of stress, they lose their color and turn transparent or white. If high temperatures persist, the corals will die.

The 1998 water temperatures triggered a rare mass bleaching that left few reefs unscathed. An estimated 16 percent of the world's corals died in a single year, and local casualty rates were far higher. Among Indian Ocean reefs, losses of up to 75 percent were common. In the Seychelles Islands, more than 90 percent of the corals died.

Since then, some of the hardest-hit reefs have mounted an impressive recovery. Yet the legacy of 1998's bleaching event remains a troubling one. A pair of worldwide surveys by the Australian Institute of Marine Science (AIMS) and the U.S. National Oceanic and Atmospheric Administration found that large numbers of damaged corals were recovering slowly, if at all.

More worrisome are signs that large-scale bleaching events are on the rise. After the devastation of 1998, some scientists initially concluded that reefs had been hit by a rare calamity that might strike as infrequently as once every 1,000 years. But just six years later, another El Niño triggered widespread bleaching in 2004, devastating reefs that had not been affected previously. An increase in these disruptive weather patterns are consistent with computer models of climate change, which predict more powerful El Niños in the future. A 2004 AIMS study warns that in a warmer world, widespread bleaching could occur as frequently as once a decade. "The extreme events of 1998 will become more common in the next 50 years," the report says.

Some scientists have predicted that corals will eventually adapt and perhaps even thrive as the Earth warms. But undermining that optimism are new findings about the effects of greenhouse gases on the chemistry of the ocean. As carbon dioxide, the primary greenhouse gas, reaches higher concentrations in the atmosphere, even greater quantities of the gas are being absorbed by the ocean. Over the past century, this additional carbon dioxide has gradually made seawater more acidic--a change scientists say is interfering with corals' ability to build the calcium-carbonate skeletons that protect the animals and serve as building blocks for new reefs.

Recently, researchers demonstrated the likely impact of higher carbon dioxide levels on the world's oceans by adding large quantities of the gas to a 700,000-gallon seawater aquarium at Biosphere 2 in Arizona. When the carbon dioxide level doubled--as scientists predict will happen by the middle of this century--the reef-building ability of corals plummeted by 60 percent. "The last time carbon dioxide levels were that high, corals disappeared from the fossil record for a million years," says Chris Langdon, a University of Miami marine biologist who led the experiment. "The lesson is, if we wipe them out, we're not going to have them around for a very long time." 

Key West's Bonsai Diving Company is as lean as its wiry, no-nonsense skipper, Wassylenko, who worries that it still is not lean enough. His World Cat dive boat holds just six paying passengers and a crew of two, which usually consists only of himself and his 12-year-old son. Yet these days Wassylenko has to supplement his income by working at a gift shop. Eco-tours of the reefs just aren't drawing the customers they used to. "There isn't a diving outfit down here that can't be bought for a song," he says.

On this late summer morning as he eases the Bonsai out of Key West's city dock, Wassylenko brings along three paying passengers, including two British teenagers hoping to earn their diver's certification. The Bonsai's itinerary calls for a stop at Eastern Dry Rocks, but it will be a brief one. There just isn't much to look at. Below the surface, Wassylenko guides his student divers past the skeletons of antlerlike elkhorn corals still clinging to the side of a mass of coral rock as lifeless as a pile of poured concrete. The water here is a murky blue, and there are few fish. A lone barracuda emerges from a crevice, sniffs around and moves on.

A few miles away, Wassylenko has better luck at a shallow reef known as Rock Key. Here the reef still pulses with life: golden brain corals, rust-colored elkhorns and orange fire corals, along with dense, shimmering schools of brightly colored tropical fish. But there are also signs of trouble: Some of the corals have white spots that are indicators of disease or bleaching.

Scientists believe that most Caribbean reefs are heading toward a fate similar to Eastern Dry Rocks'. For a 2003 report published in Nature, researchers analyzed data from 263 sites across the region and found an 80 percent drop in the amount of live coral over 25 years, an estimated average annual loss of 5.5 percent. Especially in the Caribbean, with its large population and dominant tourist industry, a confluence of negative forces appears to be at work. 

Craig and DeeVon Quirolo, a Key West couple who cofounded a charter-boat service in the 1970s, first noticed changes in the mid-1980s, as amateur divers and sewage outflows took a toll on local reefs. As the Quirolos watched, the problems progressed from minor to serious to catastrophic. Excessive nutrients in the water from sewage spurred an explosion of nuisance algae that turned the water green and murky. A succession of plagues hit the reefs. "Whatever the problem, the Keys saw it first," says DeeVon Quirolo, who with her husband created an environmental group, Reef Relief, to push for greater protections for corals. "We first saw major bleaching in 1997. The next year it was worldwide."

If reefs were contending with a single threat, they could probably adapt. But when corals already are weakened by pollution, overfishing and other stresses, changes in water temperature and chemistry can push them over the edge. "It is like being nibbled to death by ducks," says James Porter, a University of Georgia marine biologist who in the 1990s identified a previously unknown disease, called whitepox, that has since contributed to the death of 90 percent of the Florida Keys' elkhorn corals, once the region's dominant coral species.

Disease may have doomed the elkhorns, but Porter's research shows that it was directly linked to warmer temperatures in the Caribbean. The bacteria responsible for whitepox had long existed in the region's waters, he says, but global warming weakened the corals and created a perfect environment for the disease to grow and spread. As a result, the Keys' dominant reef-building species has disappeared entirely from reefs like Eastern Dry Rocks. And it may soon vanish altogether. "If that happens, you've lost more than a species," Porter says. "You've lost an ecosystem."

Joe Dupree is a freelance writer who lives in Northern Virginia. He visited coral reefs around Key West while researching this article.

Some Like It Cold

Once thought to exclusively inhabit warm, shallow oceans of the Tropics, coral reefs are increasingly being discovered in cold, dark waters worldwide at depths as great as 1,000 feet. Unlike tropical species, cold-water corals do not house symbiotic algae inside their polyps, which is why they do not need sunlight. Instead, the corals feed by capturing food particles from surrounding nutrient-rich waters.

Found primarily in areas with fast currents--such as the edges of continental shelves and beside deep-ocean ridges and seamounts--cold-water corals are slow-growing and long-lived. The oldest reefs found so far, which have been growing for about 8,000 years, are in Norway (where a cuckoo wrasse swims beside dead man's fingers, left). Such traits make cold-water corals, along with the long-lived, slow-reproducing fish that live among them, particularly vulnerable. The greatest threat to these reefs today, scientists say, is bottom trawling for deep-sea fish.

NWF Priority: Global Warming and Coral Reefs

Combating the threat global warming poses to coral reefs and other wildlife habitats is a top priority for NWF, which is backing congressional legislation to reduce greenhouse gases, publishing reports on warming's impact on wildlife and collaborating with state affiliates on grassroots efforts. In Florida, following last year's publication of the report An Unfavorable Tide (which spotlights the risks global warming poses to coastal habitats, including coral reefs), NWF has been working with community leaders, government officials, dive shops and sportfishing enthusiasts to build support for curbing global warming pollution at the state and national levels. For more information, go to

Web Exclusive

Corals: More Complex Than People?

Among the simplest creatures on Earth, corals, it turns out, possess as many genes as humans--or possibly even more.

"Four years ago, researchers were predicting that coral would be found to have about 10,000 genes, but we've found almost that many already and clearly have a long way to go," says David Miller of Australia's ARC Centre of Excellence in Coral Reef Studies, a biologist who is studying the coral genome. "Based on the rate of gene discovery, we estimate that corals have as many as 20,000 to 25,000 genes, compared with the human complement of 20,000 to 23,000."

Miller also has discovered that corals, though they are only distantly related to humans, have many of the same immune system genes that protect people from disease. Such similarities may provide insight into combating an upsurge in coral diseases, including white pox, white plague and black band, that are decimating coral reefs worldwide.--Laura Tangley

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