Read The Sixth Extinction, pages 161-240.



Patella caerulea

Castello Aragonese is a tiny island that rises straight out of the Tyrrhenian Sea, like a turret. Eighteen miles west of Naples, it can be reached from the larger island of Ischia via a long, narrow stone bridge. At the end of the bridge there’s a booth where ten euros buys a ticket that allows you to climb—or, better yet, take the elevator—up to the massive castle that gives the island its name. The castle houses a display of medieval torture instruments as well as a fancy hotel and an outdoor café. On a summer evening, the café is supposed to be a pleasant place to sip Campari and contemplate the terrors of the past.

Like many small places, Castello Aragonese is a product of ” “very large forces, in this case the northward drift of Africa, which every year brings Tripoli an inch or so closer to Rome. Along a complicated set of folds, the African plate is pressing into Eurasia, the way a sheet of metal might be forced into a furnace. Occasionally, this process results in violent volcanic eruptions. (One such eruption, in 1302, led the entire population of Ischia to take refuge on Castello Aragonese.) On a more regular basis, it sends streams of gas bubbling out of vents in the sea floor. This gas, as it happens, is almost a hundred percent carbon dioxide.

Carbon dioxide has many interesting properties, one of which is that it dissolves in water to form an acid. I have come to Ischia in late January, deep into the off-season, specifically to swim in its bubbly, acidified bay. Two marine biologists, Jason Hall-Spencer and Maria Cristina Buia, have promised to show me the vents, provided the predicted rainstorm holds off. It is a raw, gray day, and we are thumping along in a fishing boat that’s been converted into a research vessel. We round Castello Aragonese and anchor about twenty yards from its “its rocky cliffs. From the boat, I can’t see the vents, but I can see signs of them. A whitish band of barnacles runs all the way around the base of the island, except above the vents, where the barnacles are missing.”

“Barnacles are pretty tough,” Hall-Spencer observes. He is British, with dirty blond hair that sticks up in unpredictable directions. He’s wearing a dry suit, which is a sort of wet suit designed to keep its owner from ever getting wet, and it makes him look as if he’s preparing for a space journey. Buia is Italian, with reddish brown hair that reaches her shoulders. She strips down to her bathing suit and pulls on her wet suit with one expert motion. I try to emulate her with a suit I have borrowed for the occasion. It is, I learn as I tug at the zipper, about half a size too small. We all put on masks and flippers and flop in.”

“The water is frigid. Hall-Spencer is carrying a knife. He pries some sea urchins from a rock and holds them out to me. Their spines are an inky black. We swim on, along the southern shore of the island, toward the vents. Hall-Spencer and Buia keep pausing to gather samples—corals, snails, seaweeds, mussels—which they place in mesh sacs that drag behind them in the water. When we get close enough, I start to see bubbles rising from the sea floor, like beads of quicksilver. Beds of seagrass wave beneath us. The blades are a peculiarly vivid green. This, I later learn, is because the tiny organisms that usually coat them, dulling their color, are missing. The closer we get to the vents, the less there is to collect. The sea urchins drop away, and so, too, do the mussels and the barnacles. Buia finds some hapless limpets attached to the cliff. Their shells have wasted away almost to the point of transparency. Swarms of jellyfish waft by, just a shade paler than the sea.

“Watch out,” Hall-Spencer warns. “They sting.”

“SINCE the start of the industrial revolution, humans have burned through enough fossil fuels—coal, oil, and natural gas—to add some 365 billion metric tons of carbon to the atmosphere. Deforestation has contributed another 180 billion tons. Each year, we throw up another nine billion tons or so, an amount that’s been increasing by as much as six percent annually. As a result of all this, the concentration of carbon dioxide in the air today—a little over four hundred parts per million—is higher than at any other point in the last eight hundred thousand years. Quite probably it is higher than at any point in the last several million years. If current trends continue, CO2 concentrations will top five hundred parts per million, roughly double the levels they were in preindustrial days, by 2050.” “It is expected that such an increase will produce an eventual average global temperature rise of between three and a half and seven degrees Fahrenheit, and this will, in turn, trigger a variety of world-altering events, including the disappearance of most remaining glaciers, the inundation of low-lying islands and coastal cities, and the melting of the Arctic ice cap. But this is only half the story.”

“Ocean covers seventy percent of the earth’s surface, and everywhere that water and air come into contact there’s an exchange. Gases from the atmosphere get absorbed by the ocean and gases dissolved in the ocean are released into the atmosphere. When the two are in equilibrium, roughly the same quantities are being dissolved as are being released. Change the atmosphere’s composition, as we have done, and the exchange becomes lopsided: more carbon dioxide enters the water than comes back out. In this way, humans are constantly adding CO2 to the seas, much as the vents do, but from above rather than below and on a global scale. This year alone the oceans will absorb two and a half billion tons of carbon, and next year it is expected they will absorb another two and a half billion tons. Every day, every American in effect pumps seven pounds of carbon into the sea.”

“Thanks to all this extra CO2, the pH of the oceans’ surface waters has already dropped, from an average of around 8.2 to an average of around 8.1. Like the Richter scale, the pH scale is logarithmic, so even such a small numerical difference represents a very large real-world change. A decline of .1 means that the oceans are now thirty percent more acidic than they were in 1800. Assuming that humans continue to burn fossil fuels, the oceans will continue to absorb carbon dioxide and will become increasingly acidified. Under what’s known as a “business as usual” emissions scenario, surface ocean pH will fall to 8.0 by the middle of this century, and it will drop to 7.8 “by the century’s end. At that point, the oceans will be 150 percent more acidic than they were at the start of the industrial revolution.*

Owing to the CO2 pouring out of the vents, the waters around Castello Aragonese provide a near-perfect preview of what lies ahead for the oceans more generally. Which is why I am paddling around the island in January, gradually growing numb from the cold. Here it is possible to swim—even, I think in a moment of panic, to drown—in the seas of tomorrow today.”

“BY the time we get back to the harbor in Ischia, the wind has come up. The deck is a clutter of spent air tanks, dripping wet suits, and chests full of samples. Once unloaded, everything has to be lugged through the narrow streets and up to the local marine biological station, which occupies a steep promontory overlooking the sea. The station was founded by a nineteenth-century German naturalist named Anton Dohrn. Hanging on the wall in the entrance hall, I notice, is a copy of a letter Charles Darwin sent to Dohrn in 1874. In it, Darwin expresses dismay at having heard, through a mutual friend, that Dohrn is overworked.”

“Installed in tanks in a basement laboratory, the animals Buia and Hall-Spencer gathered from around Castello Aragonese at first appear inert—to my untrained eye, possibly even dead. But after a while, they set about waggling their tentacles and scavenging for food. There is a starfish missing a leg, and a lump of rather rangy-looking coral, and some sea urchins, which move around their tanks on dozens of threadlike “tube feet.” (Each tube foot is controlled hydraulically, extending and retracting in response to water pressure.) There is also a six-inch-long sea cucumber, which bears an unfortunate resemblance to a blood sausage or, worse yet, a turd. In the chilly lab, the destructive effect of the vents is plain. Osilinus turbinatus is a common Mediterranean snail with a shell of alternating black and white splotches arranged in a snakeskin-like pattern. The Osilinus turbinatus in the tank has no pattern; the ridged outer layer of its shell has been eaten away, exposing the smooth, all-white layer underneath. The limpet Patella caerulea is shaped like a Chinese straw hat. Several Patella caerulea shells have deep lesions through which their owners’ putty-colored bodies can be seen. They look as if they have been dunked in acid, which in a manner of speaking they have.

“Because it’s so important, we humans put a lot of energy into making sure that the pH of our blood is constant,” Hall-Spencer says, raising his voice to be heard over the noise of the running water. “But some of these lower organisms, they don’t have the physiology to do that. They’ve just got to tolerate what’s happening outside, and so they get pushed beyond their limits.”

“Later, over pizza, Hall-Spencer tells me about his first trip to the vents. That was in the summer of 2002, when he was working on an Italian research vessel called the Urania. One hot day, the Urania was passing by Ischia when the crew decided to anchor and go for a swim. Some of the Italian scientists who knew about the vents took Hall-Spencer to see them, just for the fun of it. He enjoyed the novelty of the experience—swimming through the bubbles is a bit like bathing in champagne—but beyond that, it set him thinking.

At the time, marine biologists were just beginning to recognize the hazards posed by acidification. Some disturbing calculations had been done and some preliminary experiments performed on animals raised in labs. It occurred to Hall-Spencer that the vents could be used for a new and more ambitious sort of study. This one would involve not just a few species reared in tanks, but dozens of species living and breeding in their natural (or, if you prefer, naturally unnatural) environment.”

“At Castello Aragonese, the vents produce a pH gradient. On the eastern edge of the island, the waters are more or less unaffected. This zone might be thought of as the Mediterranean of the present. As you move closer to the vents, the acidity of the water increases and the pH declines. A map of life along this pH gradient, Hall-Spencer reasoned, would represent a map of what lies ahead for the world’s oceans. It would be like having access to an underwater time machine.

It took Hall-Spencer two years to get back to Ischia. He did not yet have funding for his project, and so he had trouble getting anyone to take him seriously. Unable to afford a hotel room, he camped out on a ledge in the cliffs. To collect samples, he used discarded plastic water bottles. “It was a bit Robinson Crusoe-ish,” he tells me.”

“Eventually, he convinced enough people, including Buia, that he was onto something. Their first task was producing a detailed survey of pH levels around the island. Then they organized a census of what was living in each of the different pH zones. This involved placing metal frames along the shore and registering every mussel, barnacle, “and limpet clinging to the rocks. It also involved spending hours at a stretch sitting underwater, counting passing fish.”

“In the waters far from the vents Hall-Spencer and his colleagues found a fairly typical assemblage of Mediterranean species. These included: Agelas oroides, a sponge that looks a bit like foam insulation; Sarpa salpa, a commonly consumed fish that, on occasion, causes hallucinations; and Arbacia lixula, a sea urchin with a lilac tinge. Also living in the area was Amphiroa rigida, a spiky, pinkish seaweed, and Halimeda tuna, a green seaweed that grows in the shape of a series of connecting disks. (The census was limited to creatures large enough to be seen with the naked eye.) In this vent-free zone, sixty-nine species of animals and fifty-one species of plants were counted.”

“When Hall-Spencer and his team set up their quadrants closer to the vents, the tally they came up with was very different. Balanus perforatus is a grayish barnacle that resembles a tiny volcano. It is common and abundant from west Africa to Wales. In the pH 7.8 zone, which corresponds to the seas of the not-too-distant future, Balanus perforatus was gone. Mytilus galloprovincialis, a blue-black mussel native to the Mediterranean, is so adaptable that it’s established itself in many parts of the world as an invasive. It, too, was missing. Also absent were: Corallina elongata and Corallina officinalis, both forms of stiff, reddish seaweed; Pomatoceros triqueter, a kind of keel worm; three “species of coral; several species of snails; and Arca noae, a mollusk commonly known as Noah’s Ark. All told, one-third of the species found in the vent-free zone were no-shows in the pH 7.8 zone.

“Unfortunately, the biggest tipping point, the one at which the ecosystem starts to crash, is mean pH 7.8, which is what we’re expecting to happen by 2100,” Hall-Spencer tells me, in his understated British manner. “So that is rather alarming.”

“SINCE Hall-Spencer’s first paper on the vent system appeared, in 2008, there has been an explosion of interest in acidification and its effects. International research projects with names like BIOACID (Biological Impacts of Ocean Acidification) and EPOCA (the European Project on Ocean Acidification) have been funded, and hundreds, perhaps thousands, of experiments have been undertaken. These experiments have been conducted on board ships, in laboratories, and in enclosures known as mesocosms, which allow conditions to be manipulated on a patch of actual ocean.

Again and again, these experiments have confirmed the hazards posed by rising CO2. While many species will apparently do fine, even thrive in an acidified ocean, lots of others will not. Some of the organisms that have been shown to be vulnerable, like clownfish and Pacific oysters, are familiar from aquariums ”

“and the dinner table; others are less charismatic (or tasty) but probably more essential to marine ecosystems. Emiliania huxleyi, for example, is a single-celled phytoplankton—a coccolithophore—that surrounds itself with tiny calcite plates. Under magnification, it looks like some kind of crazy crafts project: a soccer ball covered in buttons. It is so common at certain times of year that it turns vast sections of the seas a milky white, and it forms the base of many marine food chains. Limacina helicina is a species of pteropod, or “sea butterfly,” that resembles a winged snail. It lives in the Arctic and is an important food source for many much larger animals, including herring, salmon, and whales. Both of these species appear to be highly sensitive to acidification: in one mesocosm experiment Emiliania huxleyi disappeared altogether from enclosures with elevated CO2 levels.”

“Ulf Riebesell is a biological oceanographer at the GEOMAR-Helmholtz Centre for Ocean Research in Kiel, Germany, who has directed several major ocean acidification studies, off the coasts of Norway, Finland, and Svalbard. Riebesell has found that the groups that tend to fare best in acidified water are plankton that are so tiny—less than two microns across—that they form their own microscopic food web. As their numbers “increase, these picoplankton, as they are called, use up more nutrients, and larger organisms suffer.

“If you ask me what’s going to happen in the future, I think the strongest evidence we have is there is going to be a reduction in biodiversity,” Riebesell told me. “Some highly tolerant organisms will become more abundant, but overall diversity will be lost. This is what has happened in all these times of major mass extinction.”

“Ocean acidification is sometimes referred to as global warming’s “equally evil twin.” The irony is intentional and fair enough as far as it goes, which may not be far enough. No single mechanism explains all the mass extinctions in the record, and yet changes in ocean chemistry seem to be a pretty good predictor. Ocean acidification played a role in at least two of the Big Five extinctions (the end-Permian and the end-Triassic) and quite possibly it was a major factor in a third (the end-Cretaceous). There’s strong evidence for ocean acidification during an extinction event known as the Toarcian Turnover, which occurred 183 million years ago, in the early Jurassic, and similar evidence at the end of the Paleocene, 55 million years ago, when several forms of marine life suffered a major crisis.”

“Oh, ocean acidification,” Zalasiewicz had told me at Dob’s Linn. “That’s the big nasty one that’s coming down.”

*   *   *

WHY is ocean acidification so dangerous? The question is tough to answer only because the list of reasons is so long. Depending on how tightly organisms are able to regulate their internal chemistry, acidification may affect such basic processes as metabolism, enzyme activity, and protein function. Because it will change the makeup of microbial communities, it will alter the availability of key nutrients, like iron and nitrogen. For similar reasons, it will change the amount of light that passes through the water, and for somewhat different reasons, it will alter the way sound propagates. (In general, acidification is expected to make the seas noisier.) It seems likely to promote the growth of toxic algae. It will impact photosynthesis—many plant species are apt to benefit from elevated CO2 levels—and it will alter the compounds formed by dissolved metals, in some cases in ways that could be poisonous.”

“Of the myriad possible impacts, probably the most significant involves the group of creatures known as calcifiers. (The term calcifier applies to any organism that builds a shell or external skeleton or, in the case of plants, a kind of internal scaffolding out of the mineral calcium carbonate.) Marine calcifiers are a fantastically varied lot. Echinoderms like starfish and sea urchins are calcifiers, as are mollusks like clams and oysters. So, too, are barnacles, which are crustaceans. Many species of coral are calcifiers; this is how they construct the towering structures that become reefs. Lots of kinds of seaweed are calcifiers; these often feel rigid or brittle to the touch. Coralline algae—minute organisms that grow in colonies that look like a smear of pink paint—are also calcifiers. Brachiopods are calcifiers, and so are coccolithophores, foraminifera, and many types of pteropods—the list goes on and on. It’s been estimated that calcification evolved at least two dozen separate times over the course of life’s history, and it’s quite possible that the number is higher than that.”

“From a human perspective, calcification looks a bit like construction work and also a bit like alchemy. To build their shells or exoskeletons or calcitic plates, calcifiers must join calcium ions (Ca2+) and carbonate ions (CO32−) to form calcium carbonate (CaCO3). But at the concentrations that they’re found in ordinary seawater, calcium and carbonate ions won’t combine. At the site of calcification, organisms must therefore alter the chemistry of the water to, in effect, impose a chemistry of their own.”

“Ocean acidification increases the cost of calcification by reducing the number of carbonate ions available to begin with. To extend the construction metaphor, imagine trying to build a house while someone keeps stealing your bricks. The more acidified the water, the greater the energy that’s required to complete the necessary steps. At a certain point, the water becomes positively corrosive and solid calcium carbonate begins to dissolve. This is why the limpets that wander too close to the vents at Castello Aragonese end up with holes in their shells.”

“Lab experiments have indicated that calcifiers will be particularly hard-hit by falling ocean pH, and the list of the disappeared at Castello Aragonese confirms this. In the pH 7.8 zone, three-quarters of the missing species are calcifiers. These include the nearly ubiquitous barnacle Balanus perforatus, the hardy mussel Mytilus galloprovincialis, and the keel worm Pomatoceros triqueter. Other absent calcifiers are Lima lima, a common bivalve; Jujubinus striatus, a chocolate-colored sea snail; and Serpulorbis arenarius, a mollusk known as a worm snail. Calcifying seaweed, meanwhile, is completely absent.”

“According to geologists who work in the area, the vents at Castello Aragonese have been spewing carbon dioxide for at least several hundred years, maybe longer. Any mussel or barnacle or keel worm that can adapt to lower pH in a time frame of centuries presumably already would have done so. “You give them generations on generations to survive in these conditions, and yet they’re not there,” Hall-Spencer observed.”

“And the lower the pH drops, the worse it goes for calcifiers. Right up near the vents, where the bubbles of CO2 stream up in thick ribbons, Hall-Spencer found that they are entirely absent. In fact, all that remains in this area—the underwater equivalent of a vacant lot—are a few hardy species of native algae, some “species of invasive algae, one kind of shrimp, a sponge, and two kinds of sea slugs.

“You won’t see any calcifying organisms, full stop, in the area where the bubbles are coming up,” he told me. “You know how normally in a polluted harbor you’ve got just a few species that are weedlike and able to cope with massively fluctuating conditions? Well, it’s like that when you ramp up CO2.”

“ROUGHLY one-third of the CO2 that humans have so far pumped into the air has been absorbed by the oceans. This comes to a stunning 150 billion metric tons. As with most aspects of the Anthropocene, though, it’s not only the scale of the transfer but also the speed that’s significant. A useful (though admittedly imperfect) comparison can be made to alcohol. Just as it makes a big difference to your blood chemistry whether you take a month to go through a six-pack or an hour, it makes a big difference to marine chemistry whether carbon dioxide is added over the course of a million years or a hundred. To the oceans, as to the human liver, rate matters.”

“If we were adding CO2 to the air more slowly, geophysical processes, like the weathering of rock, would come into play to counteract acidification. As it is, things are moving too fast for such slow-acting forces to keep up. As Rachel Carson once observed, referring to a very different but at the same time profoundly similar problem: “Time is the essential ingredient, but in the modern world there is no time.”

“A group of scientists led by Bärbel Hönisch, of Columbia’s Lamont-Doherty Earth Observatory, recently reviewed the evidence for changing CO2 levels in the geologic past and concluded that, although there are several severe episodes of ocean acidification in the record, “no past event perfectly parallels” what is happening right now, owing to “the unprecedented rapidity of CO2 release currently taking place.” It turns out there just aren’t many ways to inject billions of tons of carbon into the air very quickly. The best explanation anyone has come up with for the end-Permian extinction is a massive burst of vulcanism in what’s now Siberia. But even this spectacular event, which created the formation known as the Siberian Traps, probably released, on an annual basis, less carbon than our cars and factories and power plants.”

“By burning through coal and oil deposits, humans are putting carbon back into the air that has been sequestered for tens—in most cases hundreds—of millions of years. In the process, we are running geologic history not only in reverse but at warp speed.

“It is the rate of CO2 release that makes the current great experiment so geologically unusual, and quite probably unprecedented in earth history,” Lee Kump, a geologist at Penn State, and Andy Ridgwell, a climate modeler from the University of “Bristol, observed in a special issue of the journal Oceanography devoted to acidification. Continuing along this path for much longer, the pair continued, “is likely to leave a legacy of the Anthropocene as one of the most notable, if not cataclysmic events in the history of our planet.”



Acropora millepora”

Excerpt From: Elizabeth Kolbert. “The Sixth Extinction.” iBooks. https://itunes.apple.com/us/book/the-sixth-extinction/id687060053?mt=11

“Half a world away from Castello Aragonese, One Tree Island sits at the southernmost tip of the Great Barrier Reef, about fifty miles off the coast of Australia. It has more than one tree, which surprised me when I got there, expecting—cartoonishly, I suppose—a single palm sticking up out of white sand. As it turned out, there wasn’t any sand, either. The whole island consists of pieces of coral rubble, ranging in size from small marbles to huge boulders. Like the living corals they once were part of, the rubble chunks come in dozens of forms. Some are stubby and finger-shaped, others branching, like a candelabra. Still others resemble antlers or dinner plates or bits of brain. It is believed that One Tree Island was created during a particularly vicious storm that occurred some four thousand years ago. (As one geologist who has studied the place put it to me, “You wouldn’t have wanted to be there when that happened.”) The island is still in the process of changing shape; a storm that passed through in March 2009—Cyclone Hamish—added a ridge that runs along the island’s eastern shore.”

“One Tree would qualify as deserted except for a tiny research station operated by the University of Sydney. I traveled to the island, as just about everyone does, from another, slightly larger island about twelve miles away. (That island is known as Heron Island, also a misnomer, since at Heron there are no herons.) When we docked—or really moored, since One Tree has no dock—a loggerhead turtle was heaving herself out of the water onto the shore. She was nearly four feet long, with a large welt on her shell, which was encrusted with ancient-looking barnacles. News travels fast on a nearly deserted island, and soon the entire human population of One Tree—twelve people, including me—had come out to watch. “Sea turtles usually lay their eggs at night, on sandy beaches; this was in the middle of the day, on jagged coral rubble. The turtle tried to dig a hole with her back flippers. After much exertion, she produced a shallow trough. By this point, one of her flippers was bleeding. She heaved herself farther up the shore and tried again, with similar results. She was still at it an hour and a half later, when I had to go get a safety lecture from the manager of the research station, Russell Graham. “He warned me not to go swimming when the tide was going out, as I might find myself “swept off to Fiji.” (This was a line I would hear repeated many times during my stay, though there was some disagreement about whether the current was heading toward Fiji or really away from it.) Once I’d taken in this and other advisories—the bite of a blue-ringed octopus is usually fatal; the sting of a stonefish is not, but it is so painful it will make you wish it were—I went back to see how the turtle was doing. Apparently, she had given up and crawled back into the sea.”

“The One Tree Island Research Station is a bare-bones affair. It consists of two makeshift labs, a pair of cabins, and an outhouse with a composting toilet. The cabins rest directly on the rubble, for the most part with no floor, so that even when you’re indoors you feel as if you’re out. Teams of scientists from all around the world book themselves into the station for stays of a few weeks or a few months. At one point, someone must have decided that every team should leave a record of its visit on the cabin walls. GETTING TO THE CORE IN 2004, reads one inscription, drawn in magic marker. Others include:




“The American-Israeli team that was in residence at the time of my arrival had already made two trips to the island. The epigram from its first visit, DROPPING ACID ON CORALS, was accompanied by a sketch of a syringe dripping what looked like blood onto a globe. The group’s latest message referred to its study site, a patch of coral known as DK-13. DK-13 lies out on the reef, far enough away from the station that, for the purposes of communication, it might as well be on the moon.”

“The writing on the wall said, DK-13: NO ONE CAN HEAR YOU SCREAM.

*   *   *

THE first European to encounter the Great Barrier Reef was Captain James Cook. In the spring of 1770, Cook was sailing along the east coast of Australia when his ship, the Endeavour, rammed into a section of the reef about thirty miles southeast of what is now, not coincidentally, Cooktown. Everything dispensable, including the ship’s cannon, was tossed overboard, and the leaky Endeavour managed to creak ashore, where the crew spent the next two months repairing its hull. “Cook was flummoxed by what he described as “a wall of Coral Rock rising all most perpendicular out of the unfathomable Ocean.” He understood that the reef was biological in origin, that it had been “formed in the Sea by animals.” But how, then, he would later ask, had it come to be “thrown up to such a height?”

The question of how coral reefs arose was still an open one sixty years later, when Lyell sat down to write the Principles. Although he had never seen a reef, Lyell was fascinated by them, and he devoted part of volume two to speculating about their origins. Lyell’s theory—that reefs grew from the rims of extinct underwater volcanoes—he borrowed more or less wholesale from a Russian naturalist named Johann Friedrich von Eschscholtz. (Before Bikini Atoll became Bikini Atoll, it was called, rather less enticingly, Eschsholtz Atoll.)”

“When his turn came to theorize about reefs, Darwin had the advantage of actually having visited some. In November 1835, the Beagle moored off Tahiti. Darwin climbed to one of the highest points on the island, and from there he could survey the neighboring island of Moorea. Moorea, he observed, was encircled by a reef the way a framed etching is surrounded by a mat.”

“I am glad that we have visited these islands,” Darwin wrote in his diary, for coral reefs “rank high amongst the wonderful objects in the world.” Looking over at Moorea and its surrounding reef, he pictured time running forward; if the island were to sink away, Moorea’s reef would become an atoll. When Darwin returned to London and shared his subsidence theory with Lyell, Lyell, though impressed, foresaw resistance. “Do not flatter yourself that you will be believed until you are growing bald like me,” he warned.”

“In fact, debate about Darwin’s theory—the subject of his 1842 book The Structure and Distribution of Coral Reefs—continued until the nineteen-fifties, when the U.S. Navy arrived in the Marshall Islands with plans to vaporize some of them. In preparation for the H-bomb tests, the Navy drilled a series of cores on an atoll called Enewetak. As one of Darwin’s biographers put it, these cores proved his theory to be, in its large lines at least, “astoundingly correct.”

“Darwin’s description of coral reefs as “amongst the wonderful objects of the world” also still stands. Indeed, the more that has been learned about reefs, the more marvelous they seem. Reefs are organic paradoxes—obdurate, ship-destroying ramparts constructed by tiny gelatinous creatures. They are part animal, part vegetable, and part mineral, at once teeming with life and, at the same time, mostly dead.”

“Like sea urchins and starfish and clams and oysters and barnacles, reef-building corals have mastered the alchemy of calcification. What sets them apart from other calcifiers is that instead of working solo, to produce a shell, say, or some calcitic plates, corals engage in vast communal building projects that stretch over generations. Each individual, known unflatteringly as a polyp, adds to its colony’s collective exoskeleton. On a reef, billions of polyps belonging to as many as a hundred different species “are all devoting themselves to this same basic task. Given enough time (and the right conditions), the result is another paradox: a living structure. The Great Barrier Reef extends, discontinuously, for more than fifteen hundred miles, and in some places it is five hundred feet thick. By the scale of reefs, the pyramids at Giza are kiddie blocks.”

“The way corals change the world—with huge construction projects spanning multiple generations—might be likened to the way that humans do, with this crucial difference. Instead of displacing other creatures, corals support them. Thousands—perhaps millions—of species have evolved to rely on coral reefs, either directly for protection or food, or indirectly, to prey on those species that come seeking protection or food. “This coevolutionary venture has been under way for many geologic epochs. Researchers now believe it won’t last out the Anthropocene. “It is likely that reefs will be the first major ecosystem in the modern era to become ecologically extinct” is how a trio of British scientists recently put it. Some give reefs until the end of the century, others less time even than that. A paper published in Nature by the former head of the One Tree Island Research Station, Ove Hoegh-Guldberg, predicted that if current trends continue, then by around 2050 visitors to the Great Barrier Reef will arrive to find “rapidly eroding rubble banks.”

“ CAME to One Tree more or less by accident. My original plan had been to stay on Heron Island, where there’s a much larger research station and also a ritzy resort. On Heron, I was going to watch the annual coral spawning and observe what had been described to me in various Skype conversations as a seminal experiment on ocean acidification. Researchers from the University of Queensland were building an elaborate Plexiglas mesocosm that was going to allow them to manipulate CO2 levels on a patch of reef, even as it allowed the various creatures that depend on the reef to swim in and out. By changing the pH inside the mesocosm and measuring what happened to the corals, they were going to be able to generate predictions about the reef as a whole. I arrived at Heron in time to see the spawning—more on this later—but the experiment was way behind schedule and the mesocosm still in pieces. Instead of the reef of the future, all there was to see was a bunch of anxious graduate students hunched over soldering irons in the lab.”

“As I was trying to figure out what to do next, I heard about another experiment on corals and ocean acidification that was under way at One Tree, which, by the scale of the Great Barrier Reef, lies just around the corner. Three days later—there is no regular transportation to One Tree—I managed to get a boat over.

The head of the team at One Tree was an atmospheric scientist named Ken Caldeira. Caldeira, who’s based at Stanford, is “often credited with having coined the term “ocean acidification.” He became interested in the subject in the late nineteen-nineties when he was hired to do a project for the Department of Energy. The department wanted to know what the consequences would be of capturing carbon dioxide from smokestacks and injecting it into the deep sea. At that point, almost no modeling work had been done on the effects of carbon emissions on the oceans. Caldeira set about calculating how the ocean’s pH would change as a result of deep-sea injection, and then compared that result with the current practice of pumping CO2 into the atmosphere and allowing it to be absorbed by surface waters. In 2003, he submitted his results to Nature. The journal’s editors advised him to drop the discussion of deep-ocean injection because the calculations concerning the effects of ordinary atmospheric release were so startling. Caldeira published the first part of his paper under the subheading “The Coming Centuries May See More Ocean Acidification Than the Past 300 Million Years.”

“Under business as usual, by mid-century things are looking rather grim,” he told me a few hours after I had arrived at One Tree. We were sitting at a beat-up picnic table, looking out over the heartbreaking blue of the Coral Sea. The island’s large and boisterous population of terns was screaming in the background. Caldeira paused: “I mean, they’re looking grim already.”

“CALDEIRA, who is in his mid-fifties, has curly brown hair, a boyish smile, and a voice that tends to rise toward the end of sentences, so that it often seems he is posing a question even when he’s not. Before getting into research, he worked as a software developer on Wall Street. One of his clients was the New York Stock Exchange, for whom he designed a computer program to detect insider trading. The program functioned as it was supposed to, but after a while Caldeira came to believe that the NYSE wasn’t really interested in catching insider traders, and he decided to switch professions.”

“Unlike most atmospheric scientists, who focus on one particular aspect of the system, Caldeira is, at any given moment, working on four or five disparate projects. He particularly likes computations of a provocative or surprising nature; for example, he once calculated that cutting down all the world’s forests and replacing them with grasslands would have a slight cooling effect. (Grasslands, which are lighter in color than forests, absorb less sunlight.) Other calculations of his show that to keep pace with the present rate of temperature change, plants and animals would have to migrate poleward by thirty feet a day, and that a molecule of CO2 generated by burning fossil fuels will, in the course of its lifetime in the atmosphere, trap a hundred thousand times more heat than was released in producing it.”

“At One Tree, life for Caldeira and his team revolved around the tides. An hour before the first low tide of the day and then an hour afterward, someone had to collect water samples out at DK-13, so named because the Australian researcher who had set up the site, Donald Kinsey, had labeled it with his initials. A little more than twelve hours later, the process would be repeated, and so on, from one low tide to the next. The experiment was slow tech rather than high tech; the idea was to measure various properties of the water that Kinsey had measured back in the nineteen-seventies, then compare the two sets of data and try to tease out how calcification rates on the reef had changed in the intervening decades. In daylight, the trip to DK-13 could be made by one person. In the dark, in deference to the fact that “no one can hear you scream,” the rule was that two had to go.

My first evening on One Tree, low tide fell at 8:53 PM. Caldeira was making the post–low-tide trip, and I volunteered to go with him. At around nine “o’clock, we gathered up half a dozen sampling bottles, a pair of flashlights, and a handheld GPS unit and started out.”

“From the research station, it was about a mile walk to DK-13. The route, which someone had plugged into the GPS unit, led around the southern tip of the island and over a slick expanse of rubble that had been nicknamed the “algal highway.” From there it veered out onto the reef itself.”

“Since corals like light but can’t survive long exposure to the air, they tend to grow as high as the water level at low tide and then spread out laterally. This produces an expanse of reef that’s more or less flat, like a series of tables, which can be crossed the way a kid, after school, might jump from desk to desk. The surface of One Tree’s reef flat was brittle and brownish and was known around the research station as the “pie crust.” It crackled ominously underfoot. Caldeira warned me that if I fell through, it would be bad for the reef and even worse for my shins. I recalled another message I had seen penned on the wall of the research station: DON’T TRUST THE PIE CRUST.”

“The night was balmy and, beyond the beams of our flashlights, pitch-black. Even in the dark, the extraordinary vitality of the reef was evident. We passed several loggerhead turtles waiting out low tide with what looked like bored expressions. We encountered bright blue starfish, and leopard sharks stranded in shallow pools, and ruddy octopuses doing their best to blend into the reef. Every few feet, we had to step over a giant clam, which appeared to be leering with garishly painted lips. (The mantles of giant clams are packed with colorful symbiotic algae.) The sandy strips between the blocks of coral were littered with sea cucumbers, which, despite the name, are animals whose closest relations are sea urchins. On the Great Barrier Reef, the sea cucumbers are the size not of cucumbers but of bolster cushions. Out of curiosity, I decided to pick one up. It “was about two feet long and inky black. It felt like slime-covered velvet.”

“After a few wrong turns and several delays while Caldeira tried to photograph the octopuses with a waterproof camera, we reached DK-13. The site consisted of nothing more than a yellow buoy and some sensing equipment anchored to the reef with a rope. I glanced back in what I thought was the direction of the island, but there was no island, or land of any sort, to be seen. We rinsed out the sampling bottles, filled them, and started back. The darkness was, if anything, even more complete. The stars were so bright they appeared to be straining out of the sky. For a brief moment I felt I understood what it must have been like for an explorer like Cook to arrive at such a place, at the edge of the known world.”

“CORAL reefs grow in a great swath that stretches like a belt around the belly of the earth, from thirty degrees north to thirty degrees south latitude. After the Great Barrier Reef, the world’s second-largest reef is off the coast of Belize. There are extensive coral reefs in the tropical Pacific, in the Indian Ocean, and in the Red Sea, and many smaller ones in the Caribbean. Yet curiously enough, the first evidence that CO2 could kill a reef came from Arizona, from the self-enclosed, supposedly self-sufficient world known as Biosphere 2.”

“A three-acre, glassed-in structure shaped like a ziggurat, Biosphere 2 was built in the late nineteen-eighties by a private group largely funded by the billionaire Edward Bass. It was intended to demonstrate how life on earth—Biosphere 1—could be re-created on, say, Mars. The building contained a “rainforest,” a “desert,” an “agricultural zone,” and an artificial “ocean.” The first group of Biospherians, four men and four women, remained sealed inside the place for two years. They grew all of their own food and, for a stretch, breathed only recycled air. Still, the project was widely considered a failure. The Biospherians spent much of their time hungry, and, even more ominously, they lost control of their artificial atmosphere. “In the various “ecosystems,” decomposition, which takes up oxygen and gives off carbon dioxide, was supposed to be balanced by photosynthesis, which does the reverse. For reasons mainly having to do with the richness of the soil that had been imported into the “agricultural zone,” decomposition won out. Oxygen levels inside the building fell sharply, and the Biospherians developed what amounted to altitude sickness. Carbon dioxide levels, meanwhile, soared. Eventually, they reached three thousand parts per million, roughly eight times the levels outside.”

“Biosphere 2 officially collapsed in 1995, and Columbia University took over the management of the building. The “ocean,” a tank the size of an Olympic swimming pool, was by this point a wreck: most of the fish it had been stocked with were dead, “and the corals were just barely hanging on. A marine biologist named Chris Langdon was assigned the task of figuring out something educational to do with the tank. His first step was to adjust the water chemistry. Not surprisingly, given the high CO2 content of the air, the pH of the “ocean” was low. Langdon tried to fix this, but strange things kept happening. Figuring out why became something of an obsession. After a while, Langdon sold his house in New York and moved to Arizona, so that he could experiment on the “ocean” full-time.

“Although the effects of acidification are generally expressed in terms of pH, there’s another way to look at what’s going on that’s just as important—to many organisms probably more important—and this is in terms of a property of seawater known, rather cumbersomely, as the “saturation state with respect to calcium carbonate,” or, alternatively, the “saturation state with respect to aragonite.” (Calcium carbonate comes in two different forms, depending on its crystal structure; aragonite, which is the form corals manufacture, is the more soluble variety.) The saturation state is determined by a complicated chemical formula; essentially, it’s a measure of the concentration of calcium and carbonate ions floating around. When CO2 dissolves in water, it forms carbonic acid—H2CO3—which effectively “eats” carbonate ions, thus lowering the saturation state.”

“When Langdon showed up at Biosphere 2, the prevailing view among marine biologists was that corals did not much care about the saturation state as long as it remained above one. (Below one, water is “undersaturated,” and calcium carbonate dissolves.) Based on what he was seeing, Langdon became convinced that corals did care about the saturation state; indeed, they cared about it deeply. “To test his hypothesis, Langdon employed a straightforward, if time-consuming, procedure. Conditions in the “ocean” would be varied, and small colonies of corals, which were attached to little tiles, would be periodically lifted out of the water and weighed. If the colony was putting on weight, it would show that it was growing—adding more mass through calcification. The experiment took more than three years to complete and yielded more than a thousand measurements. It revealed a more or less linear relationship between the growth rate of the corals and the saturation state of the water. Corals grew fastest at an aragonite saturation state of five, slower at four, and still slower at three. At a level of two, they basically quit building, like frustrated contractors throwing up their hands. In the artificial world of Biosphere 2, the implications of this discovery were interesting. In the real world—Biosphere 1—they were rather more worrisome.”

“Prior to the industrial revolution, all of the world’s major reefs could be found in water with an aragonite saturation state between four and five. Today, there’s almost no place left on the planet where the saturation state is above four, and if current emissions trends continue, by 2060 there will be no regions left “above 3.5. By 2100, none will remain above three. As saturation levels fall, the energy required for calcification will increase, and calcification rates will decline. Eventually, saturation levels may drop so low that corals quit calcifying altogether, but long before that point, they will be in trouble. This is because out in the real world, reefs are constantly being eaten away at by fish and sea urchins and burrowing worms. They are also being battered by waves and storms, like the one that created One Tree. Thus, just to hold their own, reefs must always be growing.

“It’s like a tree with bugs,” Langdon once told me. “It needs to grow pretty quickly just to stay even.”

“Langdon published his results in 2000. At that point many marine biologists were skeptical, in no small part, it seems, because of his association with the discredited Biosphere project. Langdon spent another two years redoing his experiments, this time with even tighter controls. The findings were the same. In the meantime, other researchers launched their own studies. These, too, confirmed Langdon’s discovery: reef-building corals are sensitive to the saturation state. This has now been shown in dozens more lab studies and also on an actual reef. A few years ago, Langdon and some colleagues conducted an experiment on a stretch of reef near a volcanic vent system off Papua New Guinea. The experiment, modeled on Hall-Spencer’s work at Castello Aragonese, again used the volcanic vents as a natural source of acidification. “As the saturation state of the water dropped, coral diversity plunged. Coralline algae declined even more drastically, an ominous sign since coralline algae act like a kind of reef glue, cementing the structure together. Seagrass, meanwhile, thrived.

“A few decades ago I, myself, would have thought it ridiculous to imagine that reefs might have a limited lifespan,” J. E. N. Veron, former chief scientist of the Australian Institute of Marine Science, has written. “Yet here I am today, humbled to have spent the most productive scientific years of my life around the rich wonders of the underwater world, and utterly convinced that they will not be there for our children’s children to enjoy.” A recent study by a team of Australian researchers found that coral cover in the Great Barrier Reef has declined by fifty percent just in the last thirty years.

Not long before their trip to One Tree, Caldeira and some of the other members of his team published a paper assessing the future of corals, using both computer models and data gathered in the field. The paper concluded that if current emissions trends continue, within the next fifty years or so “all coral reefs will cease to grow and start to dissolve.”

“IN between trips out over the reef to collect samples, the scientists at One Tree did a lot of snorkeling. The group’s preferred spot was about a half a mile offshore, on the opposite side of the island from DK-13, and getting there meant cajoling Graham, the station manager, into taking out the boat, something that he did only with reluctance and a fair amount of grumbling.”

“Some of the scientists, who had dived all over—in the Philippines, in Indonesia, in the Caribbean, and in the South Pacific—told me that the snorkeling at One Tree was about as good as it gets. I found this easy to believe. The first time I jumped off the boat and looked down at the swirl of life beneath me, it felt unreal, as if I’d swum into the undersea world of Jacques Cousteau. Schools of small fish were followed by schools of larger fish, which were followed by sharks. Huge rays glided by, trailed by turtles the size of bathtubs. I tried to keep a mental list of what I’d seen, but it was like trying to catalog a dream. After each outing, I spent hours looking through a huge volume called The Fishes of the Great Barrier Reef and the Coral Sea. “Among the fish that I think I may have spotted were: tiger sharks, lemon sharks, gray reef sharks, blue-spine unicorn fish, yellow boxfish, spotted boxfish, conspicuous angelfish, Barrier Reef anemonefish, Barrier Reef chromis, minifin parrotfish, Pacific longnose parrotfish, somber sweetlips, fourspot herring, yellowfin tuna, common dolphinfish, deceiver fangblenny, yellow spotted sawtail, barred rabbitfish, blunt-headed wrasse, and striped cleaner wrasse.”

“Reefs are often compared to rainforests, and in terms of the sheer variety of life, the comparison is apt. Choose just about any group you like, and the numbers are staggering. An Australian researcher once broke apart a volleyball-sized chunk of coral and found, living inside of it, more than fourteen hundred polychaete worms belonging to 103 different species. More recently, American researchers cracked open chunks of corals to look for crustaceans; in a square meter’s worth collected near Heron Island, they found representatives of more than a hundred species, and in a similar-sized sample, collected at the northern tip of the Great Barrier Reef, they found representatives of more than a hundred and twenty. It is estimated that at least half a million and possibly as many as nine million species spend at least part of their lives on coral reefs.”

“This diversity is all the more astonishing in light of the underlying conditions. Tropical waters tend to be low in nutrients, like nitrogen and phosphorus, which are crucial to most forms of life. (This has to do with what’s called the thermal structure of the water column, and it’s why tropical waters are often so beautifully clear.) As a consequence, the seas in the tropics should be barren—the aqueous equivalent of deserts. Reefs are thus not just underwater rainforests; they are rainforests in a marine Sahara. The first person to be perplexed by this incongruity was Darwin, and it has since become known as “Darwin’s paradox.” Darwin’s paradox has never been entirely resolved, but one key to the puzzle seems to be recycling. Reefs—or, really, reef creatures—have developed a fantastically efficient system by which nutrients are passed from one class of organisms to another, as at a giant bazaar. Corals are the main players in this complex system of exchange, and, at the same time, they provide the platform that makes the trading possible. Without them, there’s just more watery desert.

“Corals build the architecture of the ecosystem,” Caldeira told me. “So it’s pretty clear if they go, the whole ecosystem goes.”

“One of the Israeli scientists, Jack Silverman, put it to me this way: “If you don’t have a building, where are the tenants going to go?”

“REEFS have come and gone several times in the past, and their remains crop up in all sorts of unlikely places. The ruins of reefs from the Triassic, for example, can now be found towering thousands of feet above sea level in the Austrian Alps. The Guadalupe Mountains in west Texas are what’s left of reefs from the Permian period that were elevated in an episode of “tectonic compression” about eighty million years ago. Reefs from the Silurian period can be seen in northern Greenland.”

“All these ancient reefs consist of limestone, but the creatures that created them were quite different. Among the organisms that built reefs in the Cretaceous were enormous bivalves known as rudists. In the Silurian, reef builders included spongelike creatures called stromatoporoids, or “stroms” for short. In the Devonian, reefs were constructed by rugose corals, which grew in the shape of horns, and tabulate corals, which grew in the shape of honeycombs. Both rugose corals and tabulate corals were only distantly related to today’s scleractinian corals, and both orders died out in the great extinction at the end of the Permian. This extinction shows up in the geologic record as (among other things) a “reef gap”—a period of about ten million years when reefs went missing altogether.”

“Reef gaps also occurred after the late Devonian and the late Triassic extinctions, and in each of these cases it also took millions of years for reef construction to resume. This correlation has prompted some scientists to argue that reef building as an enterprise must be particularly vulnerable to environmental change—yet another paradox, since reef building is also one of the oldest enterprises on earth.”

“Ocean acidification is, of course, not the only threat reefs are under. Indeed, in some parts of the world, reefs probably will not last long enough for ocean acidification to finish them off. The roster of perils includes, but is not limited to: overfishing, which promotes the growth of algae that compete with corals; agricultural runoff, which also encourages algae growth; deforestation, which leads to siltation and reduces water clarity; and dynamite fishing, whose destructive potential would seem to be self-explanatory. All of these stresses make corals susceptible to ”

“pathogens. White-band disease is a bacterial infection that, as the name suggests, produces a band of white necrotic tissue. It afflicts two species of Caribbean coral, Acropora palmata (commonly known as elkhorn coral) and Acropora cervicornis (staghorn coral), which until recently were the dominant reef builders in the region. The disease has so ravaged the two species that both are now listed as “critically endangered” by the International Union for Conservation of Nature. Meanwhile coral cover in the Caribbean has in recent decades declined by close to eighty percent.”

“Finally and perhaps most significant on the list of hazards is climate change—ocean acidification’s equally evil twin.

Tropical reefs need warmth, but when water temperatures rise too high, trouble ensues. The reasons for this have to do with the fact that reef-building corals lead double lives. Each individual polyp is an animal and, at the same time, a host for microscopic plants known as zooxanthellae. The zooxanthellae produce carbohydrates, via photosynthesis, and the polyps harvest these carbohydrates, much as farmers harvest corn. Once water temperatures rise past a certain point—that temperature varies by location and also by species—the symbiotic relation between the corals and their tenants breaks down. The zooxanthellae begin to produce dangerous concentrations of oxygen radicals, and the polyps respond, desperately and often self-defeatingly, by expelling them. “Without the zooxanthellae, which are the source of their fantastic colors, the corals appear to turn white—this is the phenomenon that’s become known as “coral bleaching.” Bleached colonies stop growing and, if the damage is severe enough, die. Bleached colonies stop growing and, if the damage is severe enough, die. There were major bleaching events in 1998, 2005, and 2010, and the frequency and intensity of such events are expected to increase as global temperatures climb. A study of more than eight hundred reef-building coral species, published in Science in 2008, found a third of them to be in danger of extinction, largely as a result of rising ocean temperatures. This has made stony corals one of the most endangered groups on the planet: the proportion of coral species ranked as “threatened,” the study noted, exceeds “that of most terrestrial animal groups apart from amphibians.

“ISLANDS are worlds in miniature or, as the writer David Quammen observed, “almost a caricature of nature’s full complexity.” By this account, One Tree is a caricature of a caricature. The whole place is less than 750 feet long and 500 feet wide, yet hundreds of scientists have worked there, drawn to it, in many cases, by its very diminutiveness. In the nineteen-seventies, a trio of Australian scientists set about producing a complete biological census of the island. They spent the better part of three years living in tents and cataloging every single plant and animal species they could find, including: trees (3 species), grasses (4 species), birds (29 species), flies (90 species) and mites (102 species). The island, they discovered, has no resident mammals, unless you count the scientists themselves or a pig that was once brought over and kept in a cage until it was barbecued. The monograph that resulted from this research ran to four hundred pages. It opened with a poem attesting to the charms of the tiny cay:”

“An island slumbering—

Clasped in a shimmering circlet

Of waters turquoise and blue.

Guarding her jewel from the pounding surf

On her coral rim.”

“On my last day at One Tree, no snorkeling trips were planned, so I decided to try to walk across the island, an exercise that should have taken about fifteen minutes. Not very far into my journey, I ran into Graham, the station manager. A rangy man with bright blue eyes, ginger-colored hair, and a walrus mustache, Graham looked to me like he would have made an excellent pirate. We fell into walking and talking together, and as we wandered along, Graham kept picking up bits of plastic that the waves had carried to One Tree: the cap of a bottle; a scrap of insulation, probably from a ship’s door; a stretch of PVC pipe. He had a whole collection of these bits of flotsam, which he displayed in a wire cage; the point of the exhibit, he told me, was to demonstrate to visitors “what our race is doing.”

“Graham offered to show me how the research station actually functioned, and so we threaded our way behind the cabins and the labs, toward the island’s midsection. It was breeding season, and everywhere we walked, there were birds strutting around, screaming: bridled terns, which are black on top and white on their chests; lesser crested terns, which are gray with black and white faces; and black noddies, which have a patch of white on their heads. I could see why humans had had such an easy time killing off nesting seabirds; the terns seemed completely unafraid and were so much underfoot it took an effort not to step on them.”

“Graham brought me to see the photovoltaic panels that provide the research station with power, and the tanks for collecting rain to supply it with water. The tanks were mounted on a platform, and from it we could look over the tops of the island’s trees. According to my very rough calculations, these numbered around five hundred. They seemed to be growing directly out of the rubble, like flagpoles. Just beyond the edge of the platform, Graham pointed out a bridled tern that was pecking at a black noddy chick. Soon, the chick was dead. “She won’t eat him,” he predicted, and he was right. The bridled tern walked away from the chick, who shortly thereafter was consumed by a gull. “Graham was philosophical about the episode, versions of which he had obviously seen many times; it would keep the island’s bird population from outstripping its resources.”

“That night was the first night of Hanukkah. For the holiday, someone had crafted a menorah out of a tree branch and strapped two candles onto it with duct tape. Lighted out on the beach, the makeshift menorah sent shadows skittering across the rubble. Dinner that evening was kangaroo meat, which I found surprisingly tasty, but which, the Israelis noted, was distinctly not kosher.”

“Later, I set out for DK-13 with a postdoc named Kenny Schneider. By this point, the tides had crept forward by more than two hours, so Schneider and I were scheduled to arrive at the site a few minutes before midnight. Schneider had made the journey before but still hadn’t quite mastered the workings of the GPS unit. About halfway there, we found that we had wandered off the prescribed route. The water was soon up to our chests. This made walking that much slower and more difficult, and the tide was now coming in. A variety of anxious thoughts ran through my mind. Would we be able to swim back to the station? Would we even be able to figure out the right direction to swim in? Would we finally settle the Fiji question?”

“Long after we were supposed to, Schneider and I spotted the yellow buoy of DK-13. We filled the sampling bottles and headed back. I was struck again by the extraordinary stars and the lightless horizon. I also felt, as I had several times at One Tree, the incongruity of my position. The reason I’d come to the Great Barrier Reef was to write about the scale of human influence. And yet Schneider and I seemed very, very small in the unbroken dark.”

“LIKE the Jews, the corals of the Great Barrier Reef observe a lunar calendar. Once a year, after a full moon at the start of the austral summer, they engage in what’s known as mass spawning—a kind of synchronized group sex. I was told that the mass spawning was a spectacle not to be missed, and so I planned my trip to Australia accordingly.”

“For the most part, corals are extremely chaste; they reproduce asexually, by “budding.” The annual spawning is thus a rare opportunity to, genetically speaking, mix things up. Most spawners are hermaphrodites, meaning that a single polyp produces both eggs and sperm, all wrapped together in a convenient little bundle. No one knows exactly how corals synchronize their spawning, but they are believed to respond to both light and temperature.”

“In the buildup to the big night—the mass spawning always occurs after sundown—the corals begin to “set,” which might be thought of as the scleractinian version of going into labor. The egg-sperm bundles start to bulge out from the polyps, and “the whole colony develops what looks like goose bumps. Back on Heron Island, some Australian researchers had set up an elaborate nursery so they could study the event. They had gathered up colonies of some of the most common species on the reef, including Acropora millepora, which, as one of the scientists put it to me, functions as the “lab rat” of the coral world, and were raising them in tanks. Acropora millepora produces a colony that looks like a cluster of tiny Christmas trees. No one was allowed to go near the tanks with a flashlight, for fear that it would upset the corals’ internal clocks. Instead everyone was wearing special red headlamps. With a borrowed headlamp, I could see the egg-sperm bundles straining against the polyps’ transparent tissue. The bundles were pink and resembled glass beads.”

“The head of the team, a researcher named Selina Ward, from the University of Queensland, bustled around the tanks of gravid corals like an obstetrician preparing for a delivery. She told me that each bundle held somewhere between twenty and forty eggs and probably thousands of sperm. Not long after they were released, the bundles would break open and spill their gametes, which, if they managed to find partners, would result in tiny pink larvae. As soon as the corals in her tanks spawned, Ward was planning to scoop up the bundles and subject them to different levels of acidification. She had been studying the effects of acidification on spawning for the past several years, and her results suggested that lower saturation levels led to significant “declines in fertilization. Saturation levels also affected larval development and settlement—the process by which coral larvae drop out of the water column, attach themselves to something solid, and start producing new colonies.”

“Broadly speaking, all our results have been negative so far,” Ward told me. “If we continue the way we are, without making dramatic changes to our carbon emissions immediately, I think we’re looking at a situation where, in the future, what we’ve got at best is remnant patches of corals.”

“Later that night, some of the other researchers at Heron Island, including the graduate students who were trying to weld together the overdue mesocosm, heard that Ward’s corals were getting ready to spawn and organized a nocturnal snorkel. This was a much more elaborate affair than the snorkeling trips at One Tree, complete with wet suits and underwater lights. There wasn’t enough equipment for everyone to go at once, so we went in two shifts. I was in the first, and initially I was disappointed, because nothing seemed to be happening. Then, after a while, I noticed a few corals releasing their bundles. Almost immediately, countless others followed. “The scene resembled a blizzard in the Alps, only in reverse. The water filled with streams of pink beads floating toward the surface, like snow falling upward. Iridescent worms appeared to eat the bundles, producing an eerie glow, and a slick of mauve began to form on the surface. When my shift was over, I reluctantly climbed out of the water and handed over my light.



Alzatea verticillata”

“Trees are stunning,” Miles Silman was saying. “They are very beautiful. It’s true they take a little more appreciation. You walk into a forest, and the first thing you notice is, ‘That’s a big tree,’ or ‘That’s a tall tree,’ but when you start to think about their life history, about everything that goes into getting a tree to that spot, it’s really neat. It’s kind of like wine; once you start to understand it, it becomes more intriguing.” We were standing in eastern Peru, at the edge of the Andes, on top of a twelve-thousand-foot-high mountain, where, in fact, there were no trees—just scrub and, somewhat incongruously, a dozen or so cows, eyeing us suspiciously. The sun was sinking, and with it the temperature, but the view, in the orange glow of evening, was extraordinary. To the east was the ribbon of the Alto Madre de Dios River, which flows into the Beni River, which flows into the Madeira River, which eventually meets the Amazon. Spread out before us was Manú National Park, one of the world’s great biodiversity “hot spots.”

“In your field of vision is one out of every nine bird species on the planet,” Silman told me. “Just in our plots alone, we have over a thousand species of trees.”

Silman and I and several of Silman’s Peruvian graduate students had just arrived on the mountaintop, having set out that morning from the city of Cuzco. As the crow flies, the distance we’d traveled was only about fifty miles, but the trip had taken us an entire day of driving along serpentine dirt roads. The roads wound past villages made of mud brick and fields perched at improbable angles and women in colorful skirts and brown felt hats carrying babies in slings on their backs. At the largest of the towns, we’d stopped to have lunch and purchase provisions for a four-day hike. These included bread and cheese and a shopping bag’s worth of coca leaves that Silman had bought for the equivalent of about two dollars.”

“Standing on the mountaintop, Silman told me that the trail we were going to take down the following morning was often used by coca peddlers walking up. The cocaleros carried the “eaves from the valleys where they are grown to high Andean villages of the sort we’d just passed, and the trail had been used for this purpose since the days of the conquistadors.

Silman, who teaches at Wake Forest University, calls himself a forest ecologist, though he also answers to the title tropical ecologist, community ecologist, or conservation biologist. He began his career thinking about how forest communities are put together, and whether they tend to remain stable over time. This led him to look at the ways the climate in the tropics had changed in the past, which led him, naturally enough, to look into how it is projected to change in the future. What he learned inspired him to establish the series of tree plots that we are about to visit. Each of Silman’s plots—there are seventeen in all—sits at a different elevation and hence has a different average annual temperature. In the mega-diverse world of Manú, this means that each plot represents a slice of a fundamentally different forest community.

“In the popular imagination, global warming is mostly seen as a threat to cold-loving species, and there are good reasons for this. As the world warms, the poles will be transformed. In the Arctic, perennial sea ice covers just half the area it did thirty years ago, and thirty years from now, it may well be gone entirely. Obviously, any animal that depends on the ice—ringed seals, say, or polar bears—is going to be hard-pressed as it melts away.”

“But global warming is going to have just as great an impact—indeed, according to Silman, an even greater impact—in the tropics. The reasons for this are somewhat more complicated, but they start with the fact that the tropics are where most species actually live.”

“CONSIDER for a moment the following (purely hypothetical) journey. You are standing on the North Pole one fine spring day. (There is, for the moment, still plenty of ice at the pole, so there’s no danger of falling through.) You start to walk, or better yet ski. Because there is only one direction to move in, you have to go south, but you have 360 meridians to choose from. Perhaps, like me, you live in the Berkshires and are headed to the Andes, so you decide that you will follow the seventy-third meridian west. You ski and ski, and finally, about five hundred miles from the pole, you reach Ellesmere Island. All this time, of course, you will not have seen a tree or a land plant of any kind, since you are traveling across the Arctic Ocean. On Ellesmere, you will still not see any trees, at least not any that are recognizable as such. The only woody plant that grows on the island is the Arctic willow, which reaches no higher than your ankle. (The writer Barry Lopez has noted that if you spend much time wandering around the Arctic, you eventually realize “that you are “standing on top of a forest.

“As you continue south, you cross the Nares Strait—getting around is now becoming more complicated, but we’ll leave that aside—then traverse the westernmost tip of Greenland, cross Baffin Bay, and reach Baffin Island. On Baffin, there is also nothing that would really qualify as a tree, though several species of willow can be found, growing in knots close to the ground. Finally—and you are now roughly two thousand miles into your journey—you reach the Ungava Peninsula, in northern Quebec. Still you are north of the treeline, but if you keep walking for another 250 miles or so, you will reach the edge of the boreal forest. Canada’s boreal forest is huge; it stretches across almost a billion acres and represents roughly a quarter of all the intact forest that remains on earth. But diversity in the boreal forest is low. Across Canada’s billion acres of it, you will find only about twenty species of tree, including black spruce, white birch, and balsam fir.”

“Once you enter the United States, tree diversity will begin, slowly, to tick up. In Vermont, you’ll hit the Eastern Deciduous Forest, which once covered almost half the country, but today remains only in patches, most of them second-growth. Vermont has something like fifty species of native trees, Massachusetts around fifty-five. North Carolina (which lies slightly to the west of your path) has more than two hundred species. Although the seventy-third meridian misses Central America altogether, it’s “worth noting that tiny Belize, which is about the size of New Jersey, has some seven hundred native tree species.”

“The seventy-third meridian crosses the equator in Colombia, then slices through bits of Venezuela, Peru, and Brazil before entering Peru again. At around thirteen degrees south latitude, it passes to the west of Silman’s tree plots. In his plots, which collectively have an area roughly the size of Manhattan’s Fort Tryon Park, the diversity is staggering. One thousand and thirty-five tree species have been counted there, roughly fifty times as many as in all of Canada’s boreal forest.”

“And what holds for the trees also holds for birds and butterflies and frogs and fungi and just about any other group you can think of (though not, interestingly enough, for aphids). As a general rule, the variety of life is most impoverished at the poles and richest at low latitudes. This pattern is referred to in the scientific literature as the “latitudinal diversity gradient,” or LDG, and it was noted already by the German naturalist Alexander von Humboldt, who was amazed by the biological splendors of the tropics, which offer “a spectacle as varied as the azure vault of the heavens.”

“The verdant carpet which a luxuriant Flora spreads over the surface of the earth is not woven equally in all parts,” Humboldt wrote after returning from South America in 1804. “Organic development and abundance of vitality gradually increase from the poles towards the equator.” More than two centuries later, why this should be the case is still not known, though more than thirty theories have been advanced to explain the phenomenon.”

“One theory holds that more species live in the tropics because the evolutionary clock there ticks faster. Just as farmers can produce more harvests per year at lower latitudes, organisms can produce more generations. The greater the number of generations, the higher the chances of genetic mutations. The higher the chances of mutations, the greater the likelihood that new species will emerge. (A slightly different but related theory has it that higher temperatures in and of themselves lead to higher mutation rates.)”

“A second theory posits that the tropics hold more species because tropical species are finicky. According to this line of reasoning, what’s important about the tropics is that temperatures there are relatively stable. Thus tropical organisms tend to possess relatively narrow thermal tolerances, and even slight climatic differences, caused, say, by hills or valleys, can constitute insuperable barriers. (A famous paper on this subject is titled “Why Mountain Passes Are Higher in the Tropics.”) Populations are thus more easily isolated, and speciation ensues.”

“Yet another theory centers on history. According to this account, the most salient fact about the tropics is that they are old. A version of the Amazon rainforest has existed for many millions of years, since before there even was an Amazon. Thus, “in the tropics, there’s been lots of time for diversity to, as it were, accumulate. By contrast, as recently as twenty thousand years ago, nearly all of Canada was covered by ice a mile thick. So was much of New England, meaning that every species of tree now found in Nova Scotia or Ontario or Vermont or New Hampshire is a migrant that’s arrived (or returned) just in the last several thousand years. The diversity as a function of time theory was first advanced by Darwin’s rival, or, if you prefer, codiscoverer, Alfred Russel Wallace, who observed that in the tropics “evolution has had a fair chance,” while in glaciated regions “it has had countless difficulties thrown in its way.

“THE following morning, we all crawled out of our sleeping bags early to see the sunrise. Overnight, clouds had rolled in from the Amazon basin, and we watched them from above as they turned first pink and then flaming orange. In the chilly dawn, we packed up our gear and headed down the trail. “Pick out a leaf with an interesting shape,” Silman instructed me once we’d descended into the cloud forest. “You’ll see it for a few hundred meters, and then it will be gone. That’s it. That’s the tree’s entire range.”

“Silman was carrying a two-foot-long machete, which he used to hack away at the undergrowth. Occasionally, he waved it in the air to point out something interesting: a spray of tiny white orchids with flowers no bigger than a grain of rice; a plant in the blueberry family with vivid red berries; a parasitic shrub with bright orange flowers. One of Silman’s graduate students, William Farfan Rios, handed me a leaf the size of a dinner plate.”

“This is a new species,” he said. Along the trail, Silman and his students have found thirty species of trees new to science. (Just this grove of discoveries represents half again as many species as in Canada’s boreal forest.) And there are another three hundred species that they suspect may be new, but that have yet to be formally classified. What’s more, they’ve discovered an entirely new genus.”

“That’s not like finding another kind of oak or another kind of hickory,” Silman observed. “It’s like finding ‘oak’ or ‘hickory.’” Leaves from trees in the genus had been sent to a specialist at the University of California-Davis, but, unfortunately, he had died before figuring out where on the taxonomic tree to stick the new branch.

Although it was winter in the Andes and the height of the dry season, the trail was muddy and slick. It had worn a deep channel into the mountainside, so that as we walked along, the ground was at eye level. At various points, trees had grown across the top and the channel became a tunnel. The first tunnel we hit was dark and dank and dripping with fine rootlets. Later tunnels were longer and darker and even in the middle of the “day required a headlamp to navigate. Often I felt as if I’d entered into a very grim fairy tale.

We passed Plot 1, elevation 11,320 feet, but did not stop there. Plot 2, elevation 10,500 feet, had been recently scoured by a landslide; this pleased Silman because he was interested to see what sorts of trees would recolonize it.

“The farther we descended, the denser the forest became. The trees were not just trees; they were more like botanical gardens, covered with ferns and orchids and bromeliads and strung with lianas. In some spots, the vegetation was so thick that soil mats had formed above the ground, and these had sprouted plants of their own—forests in the air. With nearly every available patch of light and bit of space occupied, the competition for resources was evidently fierce, and it almost seemed possible to watch natural selection in action, “daily and hourly” scrutinizing “every variation, even the slightest.” (Another theory of why the tropics are so diverse is that greater competition has pushed species to become more specialized, and more specialists can coexist in the same amount of space.) I could hear birds calling, but only rarely could I spot them; it was difficult to see the animals for the trees.”

“Somewhere around Plot 3, elevation 9,680 feet, Silman pulled out the shopping bag full of coca leaves. He and his students were carrying what seemed to me to be a ridiculous amount of heavy stuff: a bag of apples, a bag of oranges, a seven-hundred-page bird book, a nine-hundred-page plant book, an iPad, bottles of benzene, a can of spray paint, a wheel of cheese, a bottle of rum. Coca, Silman told me, made a heavy pack feel lighter. It also staved off hunger, alleviated aches and pains, and helped counter altitude sickness. I had been given little to carry besides “my own gear; still, anything that would lighten my pack seemed worth trying. I took a handful of leaves and a pinch of baking soda. (Baking soda, or some other alkaline substance, is necessary for coca to have its pharmaceutical effect.) The leaves were leathery and tasted like old books. Soon my lips grew numb, and my aches and pains began to fade. An hour or two later, I was back for more. (Many times since have I wished for that shopping bag.)

“In the early afternoon, we reached a small, soggy clearing where, I was informed, we were going to spend the night. This was the edge of Plot 4, elevation 8,860 feet. Silman and his students had often camped there before, sometimes for weeks at a stretch. The clearing was strewn with bromeliads that had been pulled down and gnawed upon. Silman identified these as the leavings of a spectacled bear. The spectacled bear, also known as the Andean bear, is South America’s last surviving bear. It is black or dark brown with beige around its eyes, and it lives mainly off plants. I hadn’t realized that there were bears in the Andes, and I couldn’t help thinking of Paddington, arriving in London from “deepest, darkest Peru.”

“EACH of Silman’s seventeen tree plots is two and a half acres, and the plots are arranged along a ridge a bit like buttons on a cloak. They run from the top of the ridge all the way down to the Amazon basin, which is pretty much at sea level. In the plots, someone—Silman or one of his graduate students—has tagged every single tree over four inches in diameter. Those trees have been measured, identified by species, and given a number. Plot 4 has 777 trees over four inches, and these belong to sixty different species. Silman and his students were preparing to recensus the plots, a project that was expected to take several months. All the trees that had already been tagged would have to be remeasured, and any tree that had shown up or died since the last count would have to be added or subtracted. “There were long, Talmudic discussions, conducted partly in English and partly in Spanish, about how, exactly, the recensus should be conducted. One of the few that I could follow centered on asymmetry. A tree trunk is not perfectly circular, so depending on how you orient the calipers when you’re measuring, you’ll get a different diameter. Eventually, it was decided that the calipers should be oriented with their fixed jaw on a dot spray-painted on every tree in red.

“Owing to the differences in elevation, each of Silman’s plots has a different average annual temperature. For example, in Plot 4 the average is fifty-three degrees. In Plot 3, which is about eight hundred feet higher, it’s fifty-one degrees, and in Plot 5, which is about eight hundred feet lower, it’s fifty-six degrees. Because tropical species tend to have narrow thermal ranges, these temperature differences translate into a high rate of turnover; trees that are abundant in one plot may be missing entirely from the next one down or up.”

“Some of the dominants have the narrowest altitudinal range,” Silman told me. “This suggests that what makes them such good competitors in this range makes them not so good outside of it.” In Plot 4, for example, ninety percent of the tree species are different from those species found in Plot 1, which is only about twenty-five hundred feet higher.”

“Silman first laid out the plots in 2003. His idea was to keep coming back, year after year, decade after decade, to see what happened. How would the trees respond to climate change? One possibility—what might be called the Birnam Wood scenario—was that the trees in each zone would start moving upslope. Of course, trees can’t actually move, but they can do the next best thing, which is to disperse seeds that grow into new trees. Under this scenario, species now found in Plot 4 would, as the climate warmed, start appearing higher upslope, in Plot 3, while Plot 3’s would appear in Plot 2, and so on. Silman and his students completed the first recensus in 2007. Silman thought of the effort as part of his long-term project and couldn’t imagine that much of interest would be found after just four years. But one of his postdocs, Kenneth Feeley, insisted on sifting through all the data, anyway. Feeley’s work revealed that the forest was already, measurably, in motion.”

“There are various ways to calculate migration rates: for instance, by the number of trees or, alternatively, by their mass. Feeley grouped the trees by genus. Very roughly speaking, he found that global warming was driving the average genus up the mountain at a rate of eight feet per year. But he also found the average masked a surprising range of response. Like cliques of kids at recess, different trees were behaving in wildly different ways.

“Take, for example, trees in the genus Schefflera. Schefflera, which is part of the ginseng family, has palmately compound leaves; these are arrayed around a central point the way your fingers are arranged around your palm. (One member of the group, Schefflera arboricola, from Taiwan, commonly known as the dwarf umbrella tree, is often grown as a houseplant.) Trees in Schefflera, Feeley found, were practically hyperactive; they were racing up the ridge at the astonishing rate of nearly a hundred feet a year.

On the opposite extreme were trees in the genus Ilex. These have alternate leaves that are usually glossy, with spiky or serrated edges. (The genus includes Ilex aquifolium, which is native to Europe and known to Americans as Christmas holly.) The trees in Ilex were like kids who spend recess sprawled out on a bench. While Schefflera was sprinting upslope, Ilex was just sitting there, more or less inert.”

“ANY species (or group of species) that can’t cope with some variation in temperatures is not a species (or group) whose fate we need be concerned about right now, because it no longer exists. Everywhere on the surface of the earth temperatures fluctuate. They fluctuate from day to night and from season to season. Even in the tropics, where the difference between winter and summer is minimal, temperatures can vary significantly between the rainy and the dry seasons. Organisms have developed all sorts of ways of dealing with these variations. They hibernate or estivate or migrate. They dissipate heat through panting or conserve it by growing thicker coats of fur. Honeybees warm themselves by contracting the muscles in their thorax. Wood storks cool off by defecating on their own legs. (In very hot weather, wood storks may excrete on their legs as often as once a minute.)

“Over the lifetime of a species, on the order of a million years, longer-term temperature changes—changes in climate—come into play. For the last forty million years or so, the earth has been in a general cooling phase. It’s not entirely clear why this is so, but one theory has it that the uplift of the Himalayas exposed vast expanses of rock to chemical weathering, and this in turn led to a drawdown of carbon dioxide from the atmosphere. At the start of this long cooling phase, in the late Eocene, the world was so warm there was almost no ice on the planet. By around thirty-five million years ago, global temperatures had declined enough that glaciers began to form on Antarctica. By three million years ago, temperatures had dropped to the point that the Arctic, too, froze over, and a permanent ice cap formed. Then, about two and a half million years “ago, at the start of the Pleistocene epoch, the world entered a period of recurring glaciations. Huge ice sheets advanced across the Northern Hemisphere, only to melt away again some hundred thousand years later.”

“Even after the idea of ice ages was generally accepted—it was first proposed in the eighteen-thirties by Louis Agassiz, a protégé of Cuvier—no one could explain how such an astonishing process could take place. In 1898, Wallace observed that “some of the most acute and powerful intellects of our day have exerted their ingenuity” on the problem, but so far “altogether in vain.” It would take another three-quarters of a century for the question to be resolved. It is now generally believed that ice ages are initiated by small changes in the earth’s orbit, caused by, among other things, the gravitational tug of Jupiter and Saturn. These changes alter the distribution of sunlight across different latitudes at different times of year. When the amount of light hitting the far northern latitudes in summer approaches a minimum, snow begins to build up there. “This initiates a feedback cycle that causes atmospheric carbon dioxide levels to drop. Temperatures fall, which leads more ice to build up, and so on. After a while, the orbital cycle enters a new phase, and the feedback loop begins to run in reverse. The ice starts to melt, global CO2 levels rise, and the ice melts back farther.

During the Pleistocene, this freeze-thaw pattern was repeated some twenty times, with world-altering effects. “So great was the amount of water tied up in ice during each glacial episode that sea levels dropped by some three hundred feet, and the sheer weight of the sheets was enough to depress the crust of the earth, pushing it down into the mantle. (In places like northern Britain and Sweden, the process of rebound from the last glaciation is still going on.)”

“How did the plants and animals of the Pleistocene cope with these temperature swings? According to Darwin, they did so by moving. In On the Origin of Species, he describes vast, continental-scale migrations.

As the cold came on, and as each more southern zone became fitted for arctic beings and ill-fitted for their former more temperate inhabitants, the latter would be supplanted and arctic productions would take their places.… As the warmth returned, the arctic forms would retreat northward, closely followed up in their retreat by the productions of the more temperate regions.

“Darwin’s account has since been confirmed by all sorts of physical traces. Researchers studying ancient beetle casings, for example, have found that during the ice ages, even tiny insects migrated thousands of miles to track the climate. (To name just one of these, Tachinus caelatus is a small, dullish brown beetle “that today lives in the mountains west of Ulan Bator, in Mongolia. During the last glacial period, it was common in England.)”

“In its magnitude, the temperature change projected for the coming century is roughly the same as the temperature swings of the ice ages. (If current emissions trends continue, the Andes are expected to warm by as much as nine degrees.) But if the magnitude of the change is similar, the rate is not, and, once again, rate is key. Warming today is taking place at least ten times faster than it did at the end of the last glaciation, and at the end of all those glaciations that preceded it. To keep up, organisms will have to migrate, or otherwise adapt, at least ten times more quickly. In Silman’s plots, only the most fleet-footed (or rooted) trees, like the hyperactive genus Schefflera, are keeping pace with rising temperatures. How many species overall will be capable of moving fast enough remains an open question, though, as Silman pointed out to me, in the coming decades we are probably going to learn the answer, whether we want to or not.”

“MANÚ National Park, where Silman’s plots are laid out, sits in the southeastern corner of Peru, near the country’s borders with Bolivia and Brazil, and it stretches over nearly six thousand square miles. According to the United Nations Environment Programme, Manú is “possibly the most biologically diverse protected area in the world.” Many species can be found only in the park and its immediate environs; these include the tree fern Cyathea multisegmenta, a bird known as the white-cheeked tody flycatcher, a rodent called Barbara Brown’s brush-tailed rat, and a small, black toad known only by its Latin name, Rhinella manu.”

“The first night on the trail, one of Silman’s students, Rudi Cruz, insisted that we all go out looking for Rhinella manu. He had seen several of the toads during a previous visit to the spot, and he felt sure we could find them again if we tried. I’d recently read a paper on the spread of the chytrid fungus to Peru—according to the authors, it had already arrived in Manú—but I decided not to mention this. Perhaps Rhinella manu was still out there, in which case I certainly wanted to see it.”

“We strapped on headlamps and set out down the trail, like a line of coal miners filing down a shaft. The forest at night had become an impenetrable tangle of black. Cruz led the way, shining his lamp along the tree trunks and peering into the bromeliads. The rest of us followed suit. This went on for maybe an hour and turned up only a few brownish frogs from the genus Pristimantis. After a while, people started getting bored and drifting back to camp. Cruz refused to give up. Perhaps thinking that the problem was the rest of us, he headed up the trail in the opposite direction. “Did you find anything?” someone would periodically call out to him through the darkness.

“Nada,” came the repeated response.”

“The next day, after more arcane discussions about tree measurements, we packed up to continue down the ridge. On a trip to fetch water, Silman had found a spray of white berries interspersed with what looked like bright purple streamers. He’d identified the arrangement as the inflorescence of a tree in the Brassicaceae, or mustard, family, but he had never seen anything quite like it before, which made him think, he told me, that it might represent yet another new species. It was pressed in newspaper for transport down the mountain. The idea that I might have been present at the discovery of a species, even though I’d had absolutely nothing to do with it, filled me with an odd sort of pride.”

“BACK on the trail, Silman hacked away with his machete, pausing every now and then to point out a new botanical oddity, like a shrub that steals water from its neighbors by sticking out needlelike roots. Silman talks about plants the way other people speak about movie stars. One tree he described to me as “charismatic.” Others were “hilarious,” “crazy,” “neat,” “clever,” and “amazing.”

“Sometime in the mid-afternoon, we emerged onto a rise with a view across a valley to the next ridge. On the ridge, the trees were shaking. This was a sign of woolly monkeys making their way through the forest. Everyone stopped to try to get a glimpse of them. As they sailed from branch to branch, the monkeys made a chirruping noise, a bit like the whine of crickets. Silman pulled out the shopping bag and passed it around.”

“A little while later, we reached Plot 6, elevation 7,308 feet, where the tree from the new genus had been found. Silman waved his machete at it. The tree looked pretty ordinary, but I tried to see it through his eyes. It was taller than most of its neighbors—perhaps it could be described as “stately” or “statuesque”—with smooth, ruddy bark and simple, alternate leaves. It belonged to the Euphorbiaceae, or spurge, family, whose members include poinsettia. Silman was eager to learn as much as possible about the tree, so that when a new taxonomist could be found to replace the one who had died, he’d be able to send him all the necessary material. He and Farfan went to see what they could come up with. They returned with some seed capsules, which were as thick and tough as hazelnut shells, but delicately shaped, like flowering lilies. The capsules were dark brown on the outside and inside the color of sand.”

“That evening, the sun set before we reached Plot 8, where we were going to camp. We hiked on through the dark, then set up our tents and made dinner, also in the dark. I crawled into my sleeping bag around 9 PM, but a few hours later, I was woken by a light. I assumed someone had gotten up to pee, and rolled over. In the morning, Silman told me that he was surprised I’d been able to sleep through all the commotion. “Six groups of cocaleros had tromped through the campsite overnight. (In Peru, though the sale of coca is legal, all purchases are supposed to go through a government agency known as ENACO, a restriction growers do their best to avoid.) Every single group had tripped over his tent. Eventually he’d gotten so annoyed, he’d yelled at the cocaleros, which, he had to admit, probably hadn’t been the wisest idea.”

“IN ecology, rules are hard to come by. One of the few that’s universally accepted is the “species-area relationship,” or SAR, which has been called the closest thing the discipline has to a periodic table. In its broadest formulation, the species-area relationship seems so simple as to be almost self-evident. The larger the area you sample, the greater the number of species you will encounter. This pattern was noted all the way back in the seventeen-seventies by Johann Reinhold Forster, a naturalist who sailed with Captain Cook on his second voyage, the one after his unfortunate collision with the Great Barrier Reef. In the nineteen-twenties, it was codified mathematically by a Swedish botanist, Olof Arrhenius. (As it happens, Olof was the son of the chemist Svante Arrhenius, who, in the eighteen-nineties, showed that burning fossil fuels would lead to a warmer planet.) And it was further refined and elaborated by E. O. Wilson and his colleague Robert MacArthur in the nineteen-sixties.”

“The correlation between the number of species and the size of the area is not linear. Rather, it’s a curve that slopes in a predictable way. Usually, the relationship is expressed by the formula S = cAz, where S is the number of species, A is the size of the area, and c and z are constants that vary according to the region and taxonomic group under consideration (and hence are not really constants in the usual sense of the term). The relationship counts as a rule because the ratio holds no matter what the terrain. You could be studying a chain of islands or a rainforest or a nearby state park, and you’d find that the number of species varies according to the same insistent equation: S = cAz.*”

“For the purposes of thinking about extinction, the species-area relationship is key. One (admittedly simplified) way of conceiving of what humans are doing to the world is that we are everywhere changing the value of A. Consider, for example, a grassland that once covered a thousand square miles. Let’s say the grassland was home to a hundred species of birds (or beetles or snakes). If half of the grassland were eliminated—converted into farmland or shopping malls—it should be possible to calculate, using the species-area relationship, the proportion of bird species (or beetles or snakes) that would be lost. Very roughly speaking, the answer is ten percent. (Here again, it’s important to remember that the relationship is not linear.) Since it takes a long time for the system to reach a new equilibrium, you wouldn’t expect the species to disappear right away, but you would expect them to be headed in that direction.”

“In 2004, a group of scientists decided to use the species-area relationship to generate a “first-pass” estimate of the extinction risk posed by global warming. First, the members of the team gathered data on the current ranges of more than a thousand plant and animal species. Then they correlated these ranges with present-day climate conditions. Finally, they imagined two extreme scenarios. In one, all of the species were assumed to be inert, much like the Ilex trees in Silman’s plots. “As temperatures rose, they stayed put, and so, in most cases, the amount of climatically suitable area available to them shrank, in many instances down to zero. The projections based on this “no dispersal” scenario were bleak. If warming were held to a minimum, the team estimated that between 22 and 31 percent of the species would be “committed to extinction” by 2050. If warming were to reach what was at that point considered a likely maximum—a figure that now looks too low—by the middle of this century, between 38 and 52 percent of the species would be fated to disappear.”

“Here’s another way to express the same thing,” Anthony Barnosky, a paleontologist at the University of California-Berkeley, wrote of the study results. “Look around you. Kill half of what you see. Or if you’re feeling generous, just kill about a quarter of what you see. That’s what we could be talking about.”

“In the second, more optimistic scenario, species were imagined to be highly mobile. Under this scenario, as temperatures climbed, creatures were able to colonize any new areas that met the climate conditions they were adapted to. Still, many species ended up with nowhere to go. As the earth warmed, the conditions they were accustomed to simply disappeared. (The “disappearing climates” turned out to be largely in the tropics.) Other species saw their habitat shrink because to track the climate they had to move upslope, and the area at the top of a mountain is smaller than at the base.”

“Using the “universal dispersal” scenario, the team, led by Chris Thomas, a biologist at the University of York, found that, with the minimum warming projected, 9 to 13 percent of all species would be “committed to extinction” by 2050. With maximum warming, the numbers would be 21 to 32 percent. Taking the average of the two scenarios, and looking at a mid-range warming projection, the group concluded that 24 percent of all species would be headed toward extinction.”

“The study ran as the cover article in Nature. In the popular press, the welter of numbers the researchers came up with was condensed down to just one. “Climate Change Could Drive a Million of the World’s Species to Extinction,” the BBC declared. “By 2050 Warming to Doom a Million Species” is how the headline in National Geographic put it.”

“The study has since been challenged on a number of grounds. It ignores interactions between organisms. It doesn’t account for the possibility that plants and animals can tolerate a broader range of climates than their current range suggests. It looks only as far as 2050 when, under any remotely plausible scenario, warming will continue far beyond that. It applies the species-area relationship to a new, and therefore untested, set of conditions.”

“More recent studies have come down on both sides of the Nature paper. Some have concluded that the paper overestimated the number of extinctions likely to be caused by climate change, others that it understated it. For his part, Thomas has acknowledged that many of the objections to the 2004 paper may be valid. But he has pointed out that every estimate that’s been proposed since then has been the same order of magnitude. Thus, he’s observed, “around 10 or more percent of species, and not 1 percent, or .01 percent,” are likely to be done in by climate change.”

“In a recent article, Thomas suggested that it would be useful to place these numbers “in a geological context.” Climate change alone “is unlikely to generate a mass extinction as large as one of the Big Five,” he wrote. However, there’s a “high likelihood that climate change on its own could generate a level of extinction on par with, or exceeding, the slightly ‘lesser’ extinction events” of the past.

“The potential impacts,” he concluded, “support the notion that we have recently entered the Anthropocene.”

Elizabeth Kolbert. “The Sixth Extinction.

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