Purple dye tracks the movement of deliberately acidified water at One Tree Island. Aaron Takeo Ninokawa

Sixty miles away from the Australian mainland, a small part of the immense Great Barrier Reef pokes out of the ocean and is known as One Tree Island. It’s a tiny, secluded paradise. Its waters teem with sharks. Its skies and shores swell with seabirds. Occasionally, sea eagles dive into the water to pluck out sea snakes. At low tide, you could walk around the island in half an hour—provided you could even get there. The waters around the island are heavily protected, and people can’t even sail there unless they have a scientific permit. “It is one of my favorite places on earth,” says Rebecca Albright from the California Academy of Sciences.

It’s also where she built what she describes as the world’s biggest soda stream.

Albright and her colleagues created a huge inflatable container that would rise from the ocean floor and trap 15,000 liters of seawater, “like submerging a bucket underwater and pulling it up,” she says. They then used a ring of air diffusers, much like those that send bubbles into aquarium tanks, to saturate the trapped water with carbon dioxide. This gas acidified the water, which Albright then pumped over a nearby patch of corals, simulating what these creatures will experience 100 years from now.

Corals are nature’s greatest builders, but humanity is slowly depriving them of mortar. A quarter of the carbon dioxide that we pump into the atmosphere dissolves in the oceans, gradually making them more acidic. This depletes the carbonate ions that corals need to build their mighty reefs. Eventually, they won’t be able to lay down new material fast enough to offset the effects of hungry fish, battering storms, and other eroding forces. Their reefs will start to recede and dissolve. But the One Tree Island experiment suggests that the tipping point may come even sooner than anyone expected.

There’s a value called Ω, or omega, which represents the amount of carbonate ions in the water. From a high of 4.5 during pre-industrial times, it plummeted to around 3.8 in the mid-1990s. For corals to thrive, Ω should ideally stay higher than three. It’s likely to hit that level in a few decades, and sink even further to 2.3 by century’s end. If it falls below one, their reefs will start to dissolve.

But even before then, they’ll be in trouble. Albright’s team found that if Ω falls by one from current levels, the growth of reefs will slow down by 40 percent. “We were surprised,” says Albright. “It was a much stronger response than has been reported in most lab studies,” which have found growth reduction of just 15 percent for the same amount of environmental change.

Many researchers have done similar experiments in laboratories, wafting acidified water over captive corals. These studies have revealed a lot about how individual species will cope, “but they’re so simplified when compared to one of the most biodiverse ecosystems in the world,” says Albright.

By contrast, her new study “shows for the first time, experimentally, in a real field setting, and at a scale that is environmentally relevant, how increasing carbon dioxide reduces the capacity of coral reefs to grow,” says Katharina Fabricius from the Australian Institute for Marine Science, who was not involved in the work. “We sort of knew that ocean acidification will affect reef growth. This experiment not only confirms that prediction, but informs us about the severity of the effect. It’s outstanding.”

It’s actually the sequel to an earlier experiment, in which Albright’s team reduced the acidity over the same patch of corals at One Tree Island. They used the same apparatus, but this time, they dumped sodium hydroxide into the trapped water to make it more alkaline, as it would have been 100 years ago. They then released this water over the corals, once per day for almost a month. By starting at low tide, they ensured that the water wafted over the corals in one particular direction. By adding a harmless dye, they could track the water’s flow. And by collecting samples upstream and downstream of the corals, they could work out how much carbonate they were extracting—and thus, how fast they would grow.

They found that the reduced acidity increased the corals’ growth rate by about 15 percent, which was itself a striking result. “Ocean acidification is often talked about as this thing of the future,” says Albright. “But it’s already happened. Reefs are already calcifying more slowly today than 100 years ago.”

If that experiment was the ghost of corals past, its sequel with the soda stream is the ghost of corals future. It suggests that even if the pace of acidification continues linearly, corals will fare even worse than they have. The previous century of change reduced their growth by 15 percent; the next century will do so by around 40 percent.

“This is just one study and it needs to be repeated,” says Albright. But if it’s right, it shows that reefs will “get to a tipping point where the system just starts to collapse.” This supports the conclusions from another recent study, which estimated that corals will reach this tipping point before the end of the century. And that’s bad news for the millions of people who depend on those reefs to provide them with food, tourism dollars, and protection from storms.

But Albright’s team subjected the One Tree Island corals to a century’s worth of changing climate in a matter of days. If those changes play out over actual time, could the corals acclimate or adapt? “It’s still an open question,” Albright says. “Overall, the scientific community thinks these changes are happening too quickly. If corals were able to acclimate to the rate we’re changing things, why have we lost 50 percent of our reefs in the past 30 years?”

The Great Barrier Reef, for example, has recently lost more than a quarter of its corals to unprecedentedly bad mass-bleaching events, where high temperatures forced them to evict the algae that they depend on for sustenance. The northern portion was slammed, but the southernmost section, where One Tree Island resides, was largely spared. Still, the reef’s misfortunes gave Albright pause while conducting her experiment. “We caused no long-term impact to this reef,” she says. “At the end, the calcification rates were comparable to what they were at the start. If there had been long-term declines in health, it would have been difficult for me to sleep at night.”

“This is a very important study,” says Abel Valdivia, a conservation biologist at the Center for Biological Diversity. “Globally, coral reefs are being drastically affected by mass-bleaching events. This study suggests that in the near future ocean acidification will considerably reduce the capacity of coral reefs to recover from such events.”

Some coral-reef researchers are now looking at ways of mitigating the disastrous effect of climate change by, say, identifying the sturdiest coral reefs and protecting them, or breeding hardy super-corals. Others feel that such projects are distractions from the only solution that ultimately matters: halting climate change. “I don’t think these things are mutually exclusive,” says Albright. “Ultimately, the solution is reducing global carbon emissions, but we’re not gaining traction in the ways we need to gain traction. Helping reefs in the interim, while we’re garnering political will, is important.”

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