This article is adapted from the author’s new book, The Sixth Extinction: An Unnatural History, on sale today.
Castello Aragonese is a tiny island that rises straight out of the Tyrrhenian Sea, like a turret, eighteen miles west of Naples, Italy. The marine biologist Jason Hall-Spencer first visited in the summer of 2002, while working on an Italian research vessel. One hot day, the crew decided to anchor and go for a swim. Some of the Italian scientists knew about a series of undersea vents in the seafloor near Castello Aragonese, where streams of gas bubble to the surface. This gas, as it happens, is almost a hundred percent carbon dioxide, and swimming through the bubbles is a bit like bathing in champagne. Hall-Spencer enjoyed the novelty of the experience, but beyond that, it set him thinking.
At the time, marine biologists were just beginning to recognize the hazards posed by a process known as ocean acidification, which is sometimes referred to as global warming’s “equally evil twin.” 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. As a result of all this, the concentration of carbon dioxide in the air today—about 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.
Ocean covers seventy percent of the earth’s surface, and everywhere that water and air come into contact there’s an exchange. 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 near Castello Aragonese 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 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, by the end of this century, 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, Hall-Spencer realized, the waters around Castello Aragonese could provide a near-perfect preview of what lies ahead for the oceans. 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.
Which is why I have come to Castello Aragonese in January, deep into the off-season, to take a swim in the acidic seas of tomorrow. Hall-Spencer and one of his research partners, marine biologist Maria Cristina Buia, have promised to show me the vents in its bubbly, acidified bay, 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 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. 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.”
By the time we get back to the harbor, 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.
Installed in tanks in a basement laboratory, the animals gathered from around Castello Aragonese plainly show the destructive effect of the vents. 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.”
In the waters far from the vents, Hall-Spencer and his colleagues have found a fairly typical assemblage of Mediterranean species, counting sixty-nine animals and fifty-one plants. (The census was limited to creatures large enough to be seen with the naked eye.) 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 zone that 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 more acidic 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. Hundreds, perhaps thousands, of 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.”
Over the last half a billion years, there have been five mass extinctions, when the diversity of life on earth suddenly and dramatically contracted. Scientists around the world are currently monitoring what’s being called “the sixth extinction,” predicted to be the most devastating since the asteroid impact that wiped out the dinosaurs. No single mechanism explains all the mass extinctions in the prehistoric 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. And now, in the sixth extinction, ocean acidification—this time triggered by humanity’s use of fossil fuels—again appears to be a driving force.
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.
Ocean acidification increases the cost of calcification by reducing the number of carbonate ions available to organisms that build shells or exoskeletons. 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.
According to geologists who work in the area, the vents 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.
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. 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.”
Adapted from The Sixth Extinction: An Unnatural History by Elizabeth Kolbert published February 2014 by Henry Holt and Company, LLC. Copyright 2014 by Elizabeth Kolbert. All rights reserved. Users are warned that this work is protected under copyright laws and reproduction of the text in any form for distribution is strictly prohibited. The right to reproduce or transfer the work via any medium must be secured with the copyright owner.
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