On a recent trip to Heron Island, a speck of sand and foliage on the southern end of Australia’s Great Barrier Reef, I found myself on a walking tour of the local birdlife. My group’s guide was young, determinedly friendly, and seemed to feel not a little trapped. She looked as though she might, at any moment, reconsider everything and run as far as she could—which is to say, not very far: Heron is just half a mile long. We began with the egrets, which inspired the island’s not altogether accurate name, and which are born either black or white; our guide pointed out monochrome pairs roosting together in the trees. Then it was on to the buff-banded rails, which reminded me of thin, shifty, omnivorous quails. Beneath a Pisonia tree, also known as a grand devil’s-claws, we encountered a rowdy group of white-capped noddies. (The caps, our guide told us cheerfully, were to stop the birds’ brains from overheating in the sun.) Noddies, which feed on fish and make their nests by glueing together fallen Pisonia leaves, lead perilous lives. The trees’ seeds are sticky, often adhering to the birds’ charcoal-feathered bodies. Sometimes, a noddie will get covered in so many seeds that it can no longer fly, and so it falls to the ground and starves to death, its carcass fertilizing the nutrient-poor sand in which the Pisonia grows. “The noddies have a really special relationship with the devil’s-claws,” our guide said.

Heron Island is also home to the Barrier Reef’s oldest research station, where Sophie Dove, a biology professor at the University of Queensland, has lately been studying the effects of climate change on corals. Though she was off on the mainland when I visited, we caught up a few days later. The problem, Dove explained, is twofold. As humanity pumps carbon dioxide into the atmosphere, the planet’s over-all temperature rises; at the same time, some of the CO 2 from the air is absorbed by the oceans, acidifying the water. For corals, particularly those that live in the shallows, the resulting environmental changes can be catastrophic. Many species around Heron rely for their survival on a group of tiny photosynthesizing organisms called zooxanthellae, which, in exchange for a safe home in the corals’ body tissues, furnish their hosts with food and oxygen and give them their vibrant colors. When the water gets too acidic, the corals’ rock-like skeletons break down. When it gets too warm, the zooxanthellae go into overdrive, producing dangerous amounts of oxygen. To protect themselves, the corals will expel the organisms, turning bone-white in the process—a phenomenon known as bleaching. Two springs ago, the Great Barrier Reef experienced its worst bleaching event in history. Another one followed in 2017, though Heron Island was spared the worst of it by an unseasonable influx of cold weather.

For the past several decades, marine biologists have devoted themselves to investigating how and why bleaching occurs, from the cellular level on up to the global level. Dove is interested in the area in between. Her lab setup consists of twelve tanks, each containing a miniature reef: one or two sea cucumbers, a few small fish, and the same seven species of coral. Dove refers to the tanks as mesocosms (“medium worlds”). Over the past eight years, by varying the temperature and carbon-dioxide levels of the water, she has been able to simulate how these mini-reefs respond to five climate-change scenarios. The first mimics the cooler, less acid ocean environment before the industrial revolution. The second reproduces the ocean of today, based on readings from buoys in the Coral Sea. The third conjures a world where we reduce emissions somewhat; Dove refers to this as the “pulling our belts in” model. The fourth mesocosm, which she calls “really pulling our belts in,” envisions what the future might look like if the signatories to the 2015 Paris climate agreement honor their pledges. The fifth, “business as usual,” imagines what will happen by the end of the century if humans continue to burn fossil fuels at their current rate.

So far, Dove has found, the pre-industrial and present-day mini-reefs appear healthy. Left to their own devices, she said, they are “chock-a-block” with coral. Dove still needs to crunch the numbers on the Paris tanks, but she hasn’t noticed a visual difference between this scenario and the present day. In the other two mesocosms, though, her results have been “really worrying.” The corals in those tanks thrive during the day, but, at night, when the zooxanthellae are no longer photosynthesizing (and thus no longer converting carbon dioxide to oxygen), the water acidity climbs and the corals begin to dissolve. In the pulling-in-our-belts tanks, there had been a little more growth than dissolution; in the business-as-usual tanks, there had been no growth at all. This is bad news not only for corals but for all the other species, on land and at sea, that depend on them.

It didn’t take much time on the reef for me to grasp the abundance of what the corals had built. Snorkeling in the pale-green waters of Shark Bay, on Heron’s eastern side, I saw whitetip sharks and blacktip sharks; giant shovel-nosed rays and their pale young; and person-size pink whiptail rays, which shimmied away suddenly from the seafloor, sand falling from their backs. A pair of turtles attempted mating, the knocking of their shells audible under the water. On the other side of the island, in what a Kiwi dive instructor had described as the “Powerade blue” of the harbor, I explored the rusted hull of a sunken ship. Schools of bright fish hugged the wreck’s sides; a turtle had wedged itself into a corner so that it could sleep without drifting away. Later, when the tide had gone out, I went for a reef walk, stepping carefully between semi-submerged coral colonies. A small epaulette shark, gold with brown spots, swam between my legs, on the hunt for fish that had been trapped in rock pools. I exchanged glances with an eel as a group of Australian children passed by. “Without the reefs we wouldn’t even have sand,” one said. “Without reefs we wouldn’t have anything!” another said.

I had travelled to Heron Island on the advice of Charlie Veron, an Australian reef expert who has named a fifth of the world’s coral species. Like Dove, Veron believes that the only way to save the Great Barrier Reef is to put a stop to the burning of fossil fuels; unlike Dove, he seems to hold little hope that this will happen. When we first spoke, he mentioned a proposal by the Indian resources giant Adani to build a new coal mine about four hundred miles northwest of Heron, in the state of Queensland. If approved, it would be the largest such project in Australia, a country that already has the highest per-capita carbon emissions in the world.

“I’ve been despondent for a very long time,” Veron said. A decade ago, ahead of the U.N. climate-change conference in Copenhagen, he was called upon to address the Royal Society’s Coral Reef Crisis Working Group, in London. The last photograph he took before he boarded his flight to the U.K., he told me, was of an eel on Lizard Island, in the northern Great Barrier Reef—an area that, in 2016, suffered what one of Dove’s university colleagues characterized as a “complete ecosystem collapse.” “You can’t talk underwater, but I said, ‘O.K., fella, I’m going to help look after you and your family,’ ” Veron recalled. “I failed him. Yep. Or her. I failed their family.” He appeared to have tears in his eyes, and then I definitely did. A healthy reef bustles with the color and the activity and, especially, the noise of thousands of species “talking to each other all the time,” Veron said. When that goes away, the effect is profound. “There’s nothing deader than a dead coral reef,” he said. “It really is like a graveyard.”

One of the difficult things about climate change is that we struggle to imagine it. A living edifice such as the Great Barrier Reef can, to the human mind, seem too permanent, too complicated to fail. But here, on Heron Island, the world was just small enough—a mesocosm—for its precariousness to feel real. Remove one link from the breathing chain––the noddies from the trees, the zooxanthellae from the corals––and the others would be lost quickly, easily. Veron had said that a reef represents “the greatest concentration of life on this planet.” Now I could imagine its opposite. If Heron’s corals died, the fish would starve, leave, or be eaten. The noddies would follow, and after a while the trees that feed on them might die, too. The island would be abandoned, sandy and silent, until it slipped beneath the waves.