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Here's How Our Fossil Fuel Addiction is Killing Coral Reefs

Coral bleaching in the Gulf of Thailand | Photo: EcoCafe Phuket, some rights reserved
Bleached staghorn coral off the coast of Queensland | Photo: Matt Kieffer, some rights reserved
Bleached staghorn coral off the coast of Queensland | Photo: Matt Kieffer, some rights reserved

In the space of geologic seconds, coral reefs that took millions of years to grow are dying – and it’s our fault. Humans have badly abused reefs for decades, if not centuries. Short-sighted fishermen pump poison chemicals into reefs to chase out valuable aquarium collectibles, like clownfish, red tangs and gobies. Trawlers drag their heavy, destructive drag nets across them, gouging wounds that can take decades or centuries to heal. Some fishermen even use dynamite to blow reefs apart and kill what lurks within. Sightseeing divers, though generally well-intentioned, may accidentally break off small pieces of ancient coral with a curious touch of the hand or the kick of a fin.

But now, the threats facing reefs are accelerating, assuming an alarming momentum that virtually nothing short of an overhaul of human civilization can stop.

Carbon dioxide emissions are probably the greatest destructive force looming in the future of coral reefs. Though less directly harmful to a reef than, say, pumping cyanide into a crevice to find Nemo or Dory for your saltwater fish tank, CO2 emissions are driving worldwide environmental changes – namely global warming and ocean acidification – that, in turn, are making ocean conditions intolerable for the organisms that build coral.

These animals – called polyps – secrete calcium carbonate, which hardens around them into protective casings. Over centuries and millennia, uncountable generations of polyps build large structures of coral – but they don’t do it alone. They share a symbiotic relationship with photosynthetic algae called zooxanthellae, which live in the tissue of the polyps and pass on energy from the sun to their hosts. The organisms depend on one another.

They also require specific environmental conditions, and they like water temperatures somewhere between roughly the mid-70s and mid-80s. Prolonged exposure to water that is just a few degrees too warm (or too cold, for that matter) causes the polyps to expel the algae en masse from the reef. Since the algae are very colorful, this makes the coral turn a ghostly white – a phenomenon called coral bleaching that scientists have been observing for decades.

As atmospheric CO2 increases (red), dissolved CO2 also increases in seawater (green), and seawater pH drops (blue). | Graph: NOAA
As atmospheric CO2 increases (red), dissolved CO2 also increases in seawater (green), and seawater becomes more acid (blue). | Graph: NOAA

Some coral bleaching occurs naturally. However, thanks to global warming, which is causing ocean temperatures to rise, bleaching has rapidly accelerated. In 1998, an El Niño-related warming event impacted coral across the globe, but it hit the reefs of the Indian Ocean especially hard. Here, 80 percent of the coral was bleached. A 2005 bleaching event affected 50 percent of the Caribbean’s coral. In January, 2010, coral reefs in the Florida Keys got bleached – though in this case, unusually cold water was the culprit. Bleaching doesn’t necessarily kill the affected coral, though often it does. A fifth of the Indian Ocean’s coral died in 1998 after the severe bleaching. In 2014, another massive bleaching event began and persisted through 2015 and 2016.  

The other crisis being driven by CO2 emissions is ocean acidification. For every two to three molecules of CO2 that enter the atmosphere, one molecule dissolves into the ocean, where it reacts with water to create carbonic acid. Carbonic acid exists naturally in water just as CO2 floats naturally in the air. But just as gas-guzzling human activity is overwhelming the atmosphere with unhealthy quantities of CO2, the acidity of the ocean is now increasing to problematic levels that directly harm many invertebrates.

Specifically, too much carbonic acid in the water limits those invertebrates’ ability to build shells and exoskeletons. For oysters, this means their free-swimming larvae have trouble building shells. For coral polyps, it hinders their capacity to build reefs. The authors of a paper published in the journal Nature in March, 2016 concluded that acidification is already slowing coral polyps’ productivity in reef building. In a groundbreaking experiment, the authors, led by Rebecca Albright of Stanford, pumped seawater with artificially boosted pH — water made more alkaline — over a parcel of the Great Barrier Reef. The idea was to replicate pre-industrial ocean conditions. They observed that coral exposed to the more alkaline water grew seven percent faster than coral in the rest of the ocean, indicating that acidification is already stunting the productivity of reef-building coral polyps.

The research also hinted at bad news to come for coral reefs.

“Acidification-induced reductions in calcification are projected to shift coral reefs from a state of net accretion to one of net dissolution this century,” the authors wrote in their paper. Instead of growing, coral reefs will start to deteriorate.

Bleaching is certainly an immediate threat to reefs. It causes dramatic and almost instantaneous effects, with coral turning white in just days, and often the bleaching is fatal. Acidification, however, seems to be emerging globally as probably the greater threat. 

“Acidification is more insidious,” says Nancy Knowlton, Smithsonian Institution's Sant Chair for Marine Science. That is, the process is slow and almost invisible but over time can turn a reef into a rubble heap.

“A coral reef is like a city, and there are construction projects going on all over the place,” says Knowlton, who has been personally studying coral reefs since the 1970s.  

There are also demolition projects underway in every direction. Fish, urchins and worms that eat coral all chisel away at reefs. So does wave erosion. Hurricanes, for instance, can have catastrophic effects on coral reefs. Healthy coral reefs can withstand, or at least rebound from, these pressures.

“There is a sort of balance between [the constructive and destructive] processes,” Knowlton says. In fact, under optimal conditions, coral growth by far outpaces the erosion of the reef – which is the very reason large coral reefs exist.

But once the water’s pH dips too far toward the acid end of the scale, that balance tilts in the other direction. The construction crews go on hiatus while the demolition goes on. The net effect is retreating reefs, the consequences of which will be felt acutely, far and wide. For instance, the fisheries that depend on dying reefs will wither. Knowlton says many species depend on reefs primarily for their structural assets. They may lay their eggs inside reefs, hide from predators in them, or use dark crevices as ambush points from which to dash out and attack prey. These animals may go on using dead reefs for years after polyp communities have died. However, like squatters in a decaying house, such creatures will watch their abode slowly fall apart. Ceilings will cave in, ledges will collapse. Eventually, a dead reef becomes uninhabitable, even for sturdy, adaptable creatures like groupers, morays and lobsters.

Herbivores like this parrotfish can control the spread of algae on a reef. | Photo: Ratha Grimes, some rights reserved
Herbivores like this parrotfish can control the spread of algae on a reef... if we leave them alone. | Photo: Ratha Grimes, some rights reserved

Years before bleaching and acidification first scored media headlines for killing coral, fishermen were already doing damage. Fishermen don’t generally target coral. However, the fish they catch may provide crucial services for a reef. For example, many species of parrotfish, surgeonfish and other reef dwellers eat seaweed. Since seaweed can overwhelm coral and essentially smother it, it follows that removing seaweed grazers from a reef bodes poorly for the coral. In the absence of the grazers, algal seaweed may thrive and all but smother the coral. Knowlton says thickets of seaweed may even promote the growth of bacteria and disease which can sicken the coral. Fishermen also damage coral with heavy gear, like trawl nets, and through the caveman-stupid practice of poisoning reef-dwelling species with cyanide.       

Funafuti, the capital of the island nation of Tuvalu, threatened by sea level rise | Photo: Inaba Tomoaki, some rights reserved
Funafuti, a coral atoll island that is 1) the capital of the nation of Tuvalu, and 2) threatened by sea level rise | Photo: Inaba Tomoaki, some rights reserved

This all creates a gloomy forecast for the many island nations surrounded by coral. Currently, coral reefs provide food and income for millions of people. They also shelter coastal communities by causing ocean waves to curl and crash on the coral structures, often hundreds of yards or even a few miles offshore, rather than directly on the land. But sea level is rising, and if coral is unable to grow upward at a pace equal to sea level rise, then the wave-breaking capacities of reefs will weaken. Pacific island nations like Nauru and Tuvalu could get pounded as large swells roll over the reefs and crash on beaches that for many ages prior had been mostly untouched by waves.

Asking whether the world’s coral reefs can be saved is like asking the same question of the rainforests: It would be a travesty to lose them. We’d like to think we won’t. Many scientists are pessimistic. They speak of tipping points in atmospheric chemistry past which processes like warming, sea level rise and acidification may continue out of control, even if emissions of greenhouse gases stopped today. For example, as the atmosphere warms, wind patterns that help circulate cool water through the ocean may shut down, leaving static warm water baking at the surface, which further heats the atmosphere. Similarly, melting ice isn’t just a result of warming; it can also be a driver of warming. That’s because ice can bounce the sun’s energy back into space, and once it melts, the reflected sunlight is absorbed by the planet instead, fueling warming.

“You can get runaway processes like this,” Knowlton says.

She guesses that roughly 50 percent of the world’s coral reefs are now dead as a result of human activities. Still, she is hopeful and believes that smart choices today can help slow emissions-related changes and reduce the loss of the planet’s biodiversity, including coral reefs.

“There are big changes happening in the energy economy,” Knowlton says. “Very few of us change the way we live because of coral reefs, but still, people are changing the way they live, especially their energy footprints.”

Under the best of scenarios, she speculates, the processes of warming-induced bleaching and acidification that seem poised to annihilate coral reefs could reverse. However, if humans of the energy-guzzling age continue with business as usual, the future will almost certainly turn out bleak for coral.

“We won’t have many reefs like we used to,” Knowlton says. “We’ll lose a lot, and others will be a pale shadow of what they once were.”

Banner: Coral bleaching in the Gulf of Thailand. Photo: EcoCafe Phuket, some rights reserved.

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