The Sixth Extinction: An Unnatural History

by Elizabeth Kolbert

A useful mnemonic for remembering the geologic periods of the last half-billion years is: Camels Often Sit Down Carefully, Perhaps Their Joints Creak (Cambrian-Ordovician-Silurian-Devonian-Carboniferous-Permian-Triassic-Jurassic-Cretaceous). The mnemonic unfortunately runs out before the most recent periods: the Paleogene, the Neogene, and the current Quaternary.

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.

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.

“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 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.

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.)

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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.

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(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.)

Ocean acidification increases the cost of calcification by reducing the number of carbonate ions available to begin with.

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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.

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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.

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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.

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Seawater is naturally basic, so as the pH falls the process usually referred to as ocean acidification could, less catchily, be called a decline in ocean alkalinity.

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.

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.

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.

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.

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.

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.”

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.

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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.

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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 eggsperm bundles start to bulge out from the polyps, and the whole colony develops what looks like goose bumps.

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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.

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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.

(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 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.”

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 tropic...

(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.)

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.

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 11.6 degrees Celsius. In Plot 3, which is about two hundred and fifty metres higher, it’s 10.5 degrees Celsius, and in Plot 5, which is about two hundred and fifty metres lower, it’s 13.3 degrees Celsius. 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.

(One member of the group, Schefflera arboricola, from Taiwan, commonly known as the dwarf umbrella tree, is often grown as a houseplant.)

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.

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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.

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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.

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.

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.* A typical example of the species-area relationship, showing the shape of the curve.

SAR - species-area relationship

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.

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.”

Islands—we are talking about real islands now, rather than “islands” of habitat—tend to be species-poor, or, to use the term of art, depauperate. This is true of volcanic islands situated in the middle of the ocean, and it is also, more intriguingly, true of so-called land-bridge islands that are located close to shore. Researchers who have studied land-bridge islands, which are created by fluctuating sea levels, have consistently found that they are less diverse than the continents they once were part of.

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Why is this so? Why should diversity drop off with isolation? For some species, the answer seems pretty straightforward: the slice of the habitat they’ve been marooned on is inadequate. A big cat that requires a range of a hundred square kilometres isn’t likely to make it for long in an area of only fifty square kilometres. A tiny frog that lays its eggs in a pond and feeds on a hillside needs both a pond and a hillside to survive.

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Ecologists account for relaxation by observing that life is random. Smaller areas harbor smaller populations, and smaller populations are more vulnerable to chance.

He offered his own theory for why life in the tropics is so various, which is that diversity tends to be self-reinforcing. “A natural corollary to high species diversity is low population density, and that’s a recipe for speciation—isolation by distance,” he explained. It’s also, he added, a vulnerability, since small, isolated populations are that much more susceptible to extinction.

Army ants—there are dozens of species in the tropics—differ from most other ants in that they have no fixed home. They spend their time either on the move, hunting for insects, spiders, and the occasional small lizard, or camped out in temporary “bivouacs.”

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