This story appears in the May 2011 issue of National Geographic magazine.
Not far beneath the surface of the Coral Sea, where the Great Barrier Reef lives, parrotfish teeth grind against rock, crab claws snap as they battle over hiding spots, and a 600-pound grouper pulses its swim bladder to announce its presence with a muscular whump. Sharks and silver jacks flash by. Anemone arms flutter and tiny fish and shrimp seem to dance a jig as they guard their nooks. Anything that can't glom on to something rigid is tugged and tossed by each ocean swell.
The reef's sheer diversity is part of what makes it great. It hosts 5,000 types of mollusks, 1,800 species of fish, 125 kinds of sharks, and innumerable miniature organisms. But the most riveting sight of all—and the main reason for World Heritage status—is the vast expanse of coral, from staghorn stalks and wave-smoothed plates to mitt-shaped boulders draped with nubby brown corals as leathery as saddles. Soft corals top hard ones, algae and sponges paint the rocks, and every crevice is a creature's home. The biology, like the reef, transforms from the north—where the reef began—to the south. The shifting menagerie is unmatched in the world.
Time and tides and a planet in eternal flux brought the Great Barrier Reef into being millions of years ago, wore it down, and grew it back—over and over again. Now all the factors that let the reef grow are changing at a rate the Earth has never before experienced. This time the reef may degrade below a crucial threshold from which it cannot bounce back.
West Meets Reef
Europeans were introduced to the Great Barrier Reef by British explorer Capt. James Cook, who came upon it quite by accident. On a June evening in 1770, Cook heard the screech of wood against stone; he couldn't have imagined that his ship had run into the most massive living structure on Earth: more than 10,000 square miles of coral ribbons and isles waxing and waning for some 1,400 winding miles.
Cook's team had been exploring the waters offshore of what is now Queensland when the H.M.S. Endeavour became trapped in the labyrinth. Not far beneath the surface, jagged towers of coral tore into the ship's hull and held the vessel fast. As timbers splintered and the sea poured in, the crew arrived on deck "with countenances which sufficiently expressed the horrors of our situation," Cook later wrote in his diary. Captain and crew were able to limp to a river mouth to patch their vessel.
Aborigines had lived in the region for thousands of years before Europeans hit the rocks. Culturally, the reef has been a rich part of the landscape for Aboriginal and Torres Strait Islander peoples, who have canoed it and fished it and shared myths about its creatures for generations. But historians aren't sure how deep their knowledge went of the reef's geology and animal life. A few decades after Cook's run-in with the behemoth beneath the sea, English cartographer Matthew Flinders—who also had a mishap or two while "threading the needle" among the reefs—gave the entity its name, inspired by its size. All told, if the reef's main chunks were plucked from the sea and laid out to dry, the rock could cover all of New Jersey, with coral to spare.
Expansion and Erosion
This mammoth reef owes its existence to organisms typically no bigger than a grain of rice. Coral polyps, the reef's building blocks, are tiny colonial animals that house symbiotic algae in their cells. As those algae photosynthesize—using light to create energy—each polyp is fueled to secrete a "house" of calcium carbonate, or limestone. As one house tops another, the colony expands like a city; other marine life quickly grabs on and spreads, helping cement all the pieces together.
Off Australia's eastern edge, conditions are ripe for this building of stone walls. Corals grow best in shallow, clear, turbulent water with lots of light to support photosynthesis. Millions of polyp generations later, the reef stands not as a singular thing but as a jumble whose shapes, sizes, and life-forms are determined by where in the ocean they lie—how close to shore, for example—and what forces work on them, such as heavy waves. Go far enough from the coast, where the light is low and the waters are deeper, and there's no reef at all.
"In the Great Barrier Reef, corals set the patterns of life from end to end," says Charlie Veron, coral expert and a longtime chief scientist for the Australian Institute of Marine Science. With over 400 species in the region, "they structure the entire environment; they're the habitat for everything else here." The perfect temperature, clarity, and currents enable plate corals, for example, to increase in diameter up to a foot a year. The reef continuously erodes as well, worn down by waves, ocean chemistry, and organisms that eat limestone. This vanishing act is far slower than the constant building up; still, as much as 90 percent of the rock eventually dissipates into the waters, forming sand. So the living veneer of this reef, the part a diver sees, is ever changing.
And the layers beneath are relatively young, geologically speaking, at less than 10,000 years. The reef's true beginnings go back much further. Closer to 25 million years ago, Veron says, as Queensland edged into tropical waters with the movement of the Indo-Australian tectonic plate, coral larvae began riding south-flowing currents from the Indo-Pacific, grabbing footholds wherever they could. Slowly, rocky colonies grew and spread along the seafloor flush with diverse marine life.
A Rocky Course
Since the reef first found footing, ice ages have come and gone, tectonic plates have crept forward, and ocean and atmospheric conditions have fluctuated wildly. The reef has seen many iterations—expanding and eroding, being defaced and reinhabited at nature's whim.
"A history of the Great Barrier Reef," Veron says, "is a catalog of disasters" caused by planetary chaos. But they are disasters from which the reef has always recovered.
Today new disasters endanger the reef, and the prospect for recovery is uncertain. The relatively quick shift in the world's climate, scientists say, appears to be devastating for reefs. In corals, warming temperatures and increased exposure to the sun's ultraviolet rays lead to a stress response called bleaching—when the colorful algae in coral cells become toxic and are expelled, turning the host animals skeletal white. Fleshy seaweeds may then choke out the remains.
Major bleaching in the Great Barrier Reef and elsewhere in 1997-98 was linked to a severe El Niño year and record-high sea-surface temperatures—in some spots 3°F higher than normal. Another round began in 2001 and again in 2005. By 2030, some reef experts say, these destructive episodes will occur every year.
Heat is also implicated in a 60-year decline in ocean phytoplankton—the microscopic organisms that not only gobble greenhouse gases but also feed, directly or indirectly, almost every other living thing in the sea. Reef fish, too, respond to warmer waters—sometimes with bolder, more aggressive behavior toward both predators and prey. Changes in sea level, either up or down, have a dire impact as well, exposing shallow corals to too much sun or drowning them in deeper water, where they're hidden from the light.
A more immediate concern is massive flooding in Australia that earlier this year sent huge plumes of sediment and toxin-laden waters onto the reef off Queensland. The full harm to marine life won't be clear for years, but long stretches of the Great Barrier Reef could experience disastrous die-offs.
And then there's the acid test.
Reef ecosystems worldwide took a pounding during each of Earth's five mass extinctions, the first about 440 million years ago. Greenhouse gases spiked naturally over the millennia, and Aussie biologist Veron says massive spewing of carbon dioxide during periods of heavy volcanic activity was likely a big player in coral decimation, notably the most recent mass extinction some 65 million years ago. At that time, oceans absorbed more and more of those greenhouse gases from the atmosphere, causing ocean acidity to rise. The lower pH—a sign of high levels of acidity—ultimately thwarted the ability of marine creatures to build their limestone shells and skeletons.
In some oceans this acidification is once again happening. The most vulnerable to acid's corrosive bite are the fast-growing branching corals and vital calcium-excreting algae that help bind the reef. The more brittle the reef's bones, the more wave action, storms, diseases, pollutants, and other stresses can break them.
In ancient times many corals adapted to changing ocean acidity, says Veron, who paints a particularly bleak picture of the Barrier Reef's future. "The difference is there were long stretches in between; corals had millions of years to work it out." He fears that with unprecedented CO2, sulfur, and nitrogen emissions by human industry, added to the increasing escape of methane as a result of Earth's melting ice, much of the reef will be nearly bereft of life within 50 years. What will be left? "Coral skeletons bathed in algal slime," he says.
Of course, to the two million tourists who visit the reef each year, the promise of an underwater paradise teeming with life is still fulfilled. But the blemishes are there if you know where to look. The reef bears a two-mile-long scar from a collision with a Chinese coal carrier in April of last year. Other ship groundings and occasional oil spills have marred the habitat. Sediment plumes from flooding and nutrients from agriculture and development also do very real damage to the ecosystem. But Aussies aren't inclined to let the reef fall apart without a national outcry. The captain of the boat who took me diving put it this way: "Without the reef, there's nothing out here but a whole lot of salty water." To many locals, he adds, "the reef is a loved one whose loss is too sad to contemplate." And it is also crucial economically: The visitors he motors to the reef's edges provide more than one billion dollars annually for Australia's books.
The challenge scientists face is to keep the reef healthy despite rapid change. "To fix a car engine, you need to know how it works," says marine biologist Terry Hughes of James Cook University. "The same is true for reefs." He and others have been investigating how these ecosystems function so that efforts to prevent damage can be doubly effective.
High on the to-do list: Determine the full impact of overfishing. Traditionally, commercial fishermen could work along the reef, even after 133,000 square miles of ocean habitat was designated a marine park in 1975. But with rising concern about the big take, the Australian government in 2004 made a third of that area, in strategically placed zones, off-limits to all fishing—including for sport. The biological recovery has been bigger and faster than expected; within two years after the ban, for example, numbers of coral trout doubled on once heavily fished reef. Some scientists speculate that protective zones may also lead to declines in outbreaks of a devastating coral-eating sea star.
Scientists also want to know what makes specific corals extra tenacious during times of change. "We know some reefs experience much more stressful conditions than others," says reef ecologist Peter Mumby of the University of Queensland. "Looking at decades of sea temperature data, we can now map where corals are most acclimated to warmth and target conservation actions there." He says understanding how corals recover from bleaching—and figuring out where new polyps are likely to grow—can help in designing reserves. Even the outspoken Veron acknowledges that coral survival is possible long-term if the onslaughts against reefs are halted—soon.
Nature has some safeguards of her own, including a genetic script for corals that may have helped them ride out past environmental disruptions. Many reef builders evolve through hybridization—when different species mix genes. As Veron puts it, "everything is always on its way to becoming something else." On the reef, about a third of the corals reproduce in annual mass spawning. During such events, as many as 35 species on a single patch of reef release their egg and sperm bundles simultaneously, which means millions of gametes from genetically different parents mingle in a slick at the ocean surface. "This provides outstanding opportunities to produce hybrids," explains marine biologist Bette Willis of James Cook University. Especially with climate and ocean chemistry in such flux, she says, hybridization can offer a speedy path to adaptation and hardiness against disease.
Indeed, one lesson is that despite today's weighty threats, the Great Barrier Reef won't easily crumble. It has, after all, toughed it out through catastrophic change before. And all kinds of marine life are around to help keep the reef whole. In studies conducted in 2007, scientists found that where grazing fish thrive, so do corals, especially in waters polluted with excess nutrients. "If you take away herbivores, say through overfishing, seaweed replaces corals," says Hughes. If voracious vegetarians are protected, corals can prevail.
A human visitor to the reef can see the fish doing their vital job. In dappled afternoon light toward the reef's northern tip, palatial walls of coral tower over a rare species of batfish, long finned and masked in black, that nibbles back strands of sargassum. And a school of parrotfish—fused teeth like wire cutters—chip away noisily at the rocks, where algae in mats of green and red have quietly taken hold.