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me—gathered around to watch.
    One Tree Island is part of the Great Barrier Reef, the world’s largest reef complex, which stretches for more than 1,400 miles. The entire island is composed of bits of coral rubble, ranging from marble to basketball size, that began piling up after a peculiarly violent storm about 4,000 years ago. Even today, the island has nothing that could really be called dirt. The trees seem to rise up directly out of the rubble like flagpoles.
    When scientists first started visiting the island in the 1960s, they posed questions like, How do reefs grow? Nowadays the questions are more urgent. “Something like 25 percent of all species in the oceans spend at least part of their life in coral reef systems,” Ken Caldeira, an expert on ocean acidification at the Carnegie Institution, said one evening before heading out to collect water samples on the reef. “Corals build the architecture of the ecosystem, and it’s pretty clear if they go, the whole ecosystem goes.”
    Coral reefs are already threatened by a wide array of forces. Rising water temperatures are producing more frequent “bleaching” events, when corals turn a stark white and often die. Overfishing removes grazers that keep reefs from being overgrown with algae. Agricultural runoff fertilizes algae, further upsetting reef ecology. In the Caribbean some formerly abundant coral species have been devastated by an infection that leaves behind a white band of dead tissue. Probably owing to all these factors, coral cover in the Caribbean declined by around 80 percent between 1977 and 2001.
    Ocean acidification adds yet another threat, one that may be less immediate but ultimately more devastating to hard, reef-building corals. It undermines their basic, ancient structure—the stony skeleton that’s secreted by millions upon millions of coral polyps over thousands of years.
    Coral polyps are tiny animals that form a thin layer of living tissue on the surface of a reef. They’re shaped a bit like flowers, with six or more tentacles that capture food and feed it to a central mouth. (Many corals actually get most of their food from algae that live and photosynthesize inside them; when corals bleach, it’s because stress has prompted the polyps to expel those dark symbionts.) Each polyp surrounds itself with a protective, cup-shaped exoskeleton of calcium carbonate that contributes to the collective skeleton of the whole colony.
    To make calcium carbonate, corals need two ingredients: calcium ions and carbonate ions. Acids react with carbonate ions, in effect tying them up. So as atmospheric CO 2 levels rise, carbonate ions become scarcer in the water, and corals have to expend more energy to collect them. Under lab conditions coral skeleton growth has been shown to decline pretty much linearly as the carbonate concentration drops off.
    Slow growth may not matter much in the lab. Out in the ocean, though, reefs are constantly being picked at by other organisms, both large and small. (When I went snorkeling off One Tree Island, I could hear parrotfish chomping away at the reef.) “A reef is like a city,” said Ove Hoegh-Guldberg, who used to direct the One Tree Island Research Station and now heads the Global Change Institute at Australia’s University of Queensland. “You’ve got construction firms and you’ve got demolition firms. By restricting the building materials that go to the construction firms, you tip the balance toward destruction, which is going on all the time, even on a healthy reef. In the end you wind up with a city that destroys itself.”
    By comparing measurements made in the 1970s with those taken more recently, Caldeira’s team found that at one location on the northern tip of the reef, calcification had declined by 40 percent. (The team was at One Tree to repeat this study at the southern tip of the reef.) A different team using a different method has