The Great Barrier Reef is often vividly described, but one adjective that doesn’t make the cut is historic. And yet over thousands of years, untold billions of coral animals, like tiny construction workers, built the massive reef structure, now a 1,400-mile entity visible from space. Each anemone-like coral (most less than an inch long) makes a tiny white calcium carbonate cup as it grows, and over millennia, the cups pile up like so many bricks as new corals build on the bones of their ancestors. The resulting reef structure, riveted with nooks and crannies, then serves as a platform for a vast diversity of strange life forms that thrive in the bustling coral reef—it is these life forms that supply the brilliant color that normally dominates descriptions.
Coral researchers are now watching their subject blanch white as the Great Barrier Reef faces the largest bleaching event ever observed. Their emotional entreaties have directed the world’s focus toward the reef’s plight—and while the problems are real, this has unintentionally generated misleading media coverage proclaiming that the reef is dead and dying.
Yes, this may be the worst bleaching event we’ve witnessed. In a survey of one-third of the system’s 3,000 individual reefs, researchers report that 93 percent suffer from some degree of bleaching (along with reefs across the Indo-Pacific, some of which are just entering their bleaching season). Climate change is likely to blame. So is all hope lost?
Not quite yet.
During the most recent reefwide bleaching events, in 1998 and 2002, bleaching affected at least half of the system’s reefs (50 percent and 60 percent respectively). Yet in each case, only 5 percent of reefs suffered permanent damage. Dead coral at 150 reefs is not insubstantial, but in the end, nearly all of the bleached coral recovered.
That’s because bleached coral does not equal dead coral; it means very stressed out coral. The coral animals that build reefs rely on algae (called zooxanthellae) for energy. The algae live inside coral tissues, giving color to the otherwise transparent animals, and combine nutrients from their hosts with abundant sunlight to make food for them both. The algae can’t survive long without its coral host, and coral can’t survive long without the algae’s food—some 80 percent of its total intake.
When water gets too warm, corals’ algal partners stop functioning, churn out toxins, and begin to die. When that happens, the coral enters survival mode and kicks these malfunctioning partners out—if they didn’t, they would die, too. This is what we observe as bleaching. The coral then waits patiently for conditions to improve so that new, healthy algal partners can find them.
It’s horrible to watch fields of coral turn white, as their normally colorful bodies turn transparent and reveal the white reef below. But this is an adaptive strategy. It gives corals a chance to survive naturally occurring heat waves. If they can persist long enough for the water to cool down, corals then attract new algae partners and, given time, get back to business as usual.
The problem is that under climate change, the heat waves are more frequent and severe—even though ocean temperature has only increased modestly. “It’s not very much, but it’s a big deal for coral reefs,” says coral microbiologist Michael Lesser from the University of New Hampshire, noting that the corals are also bleaching more frequently.
This is bad, because recovering from bleaching, like recovering from illness, takes time. After the 1998 bleaching event on the Great Barrier Reef, most bleached corals that recruited new algae partners within six months survived. But it took years before they grew and reproduced normally. If, in that time, they got hit with another wave of bleaching, recovery would take even longer—if they recovered at all.
The danger is that, in the face of frequent bleaching, corals won’t be healthy enough to survive the constant onslaught. Indeed, the ongoing bleaching event has killed nearly half the coral on some northern reefs within the Great Barrier Reef. Extremely high temperatures can even kill corals on the spot. “Corals virtually melt in front of you,” says Lesser, as the coral cells and algae cells self-destruct simultaneously.
That sounds really bad—and it is—but even melted, dead reefs can recover. During the 1998 bleaching event, up to 90 percent of corals on Western Australia’s isolated Scott Reef died, and scientists expected recovery to take decades. For six years, it looked completely dead. But then, remarkably, coral larvae from faraway reefs arrived and by 2010, the reef had recovered to pre-bleaching conditions.
Still, 12 years is a long time to regroup—longer than most reefs will have as bleaching events become more frequent. If the temperature keeps going up, and it’s predicted to, things start to look pretty dire for the reefs (that is one of many reasons why reducing carbon emissions is important). But there are other ways to help corals—a study on the decline of Caribbean coral reefs provides clues.
Two years ago, the United States Geological Survey published a massive analysis of Caribbean coral reefs, featuring data from 35,000 reef surveys between 1980 and 2012. They found that overfishing and coastal pollution hurt reefs more than climate change did. So, while climate change is certainly a problem, it’s not the main killer, yet. Humans do plenty of other terrible stuff, too.
“If you could wave your magic wand and make climate change disappear tomorrow, there wouldn’t be any coral reefs in 50 or 60 years because of overfishing and coastal pollution and all the other horrible stuff that we’re doing on regional scales,” says coral biologist Jeremy Jackson, a co-author of the report. (Disclosure: Jackson is married to my former boss, coral biologist Nancy Knowlton.)
This doesn’t mean that climate change isn’t an issue for reefs—far from it. Rising temperatures and ocean acidification (the other problem carbon dioxide is causing) hurt corals more if they’re already stressed out from pollution and overfishing.
In order to give the reefs their best shot, we can focus right now on reducing all the other stressors that are currently hurting them—including pollution and overfishing, which can both be reduced more immediately than climate change can be addressed. This could buy time while politicians get it together on the other stuff.
Pollution especially is a problem on the Great Barrier Reef. Runoff from land carries sediment (which blocks light and reduces coral growth) and nutrients (which encourage seaweed to overgrow corals) into the sea. Excess nutrients also cause population explosions of a major coral predator: crown-of-thorns starfish, which crawl across the reef slurping corals from their skeletal cups. A recent study found that these starfish’s consumption was responsible for nearly half of recent coral loss on the reef—compared with 10 percent from bleaching.
Reducing pollution is no small feat, but it’s doable—it’s been done throughout the world—and importantly doesn’t require commitment from world leaders. Most pollution in the Great Barrier Reef comes from two sources: sediment carried from ranches, and fertilizers and pesticides carried from sugar cane, banana, and cotton farms. Those are addressable problems. In addition, planting trees and other vegetation inland captures fertilizer and sediment before it reaches the sea.
The good news is that the Australian government knows pollution is a problem and has a plan to address it. The bad news is that, according to a study published last month, it isn’t working. The government plan suggests that farmers use less fertilizer and that ranchers plant grasses and trees to hold soil in place. The problem is that these are mere suggestions; participation is voluntary. The authors recommend that government officials create incentives for ranchers, farmers, and other landowners to follow these recommendations—and as a result, to take better care of the Great Barrier Reef.
Once participation goes up, corals will still need time to recover from this current catastrophic event—and with climate change continuing its slow creep, they need all the time they can get. So it is important to get moving now before life becomes even more stressful for corals. By addressing the problems we know how to fix, we could buy them enough time to have a shot at survival.