The Sixth Extinction: An Unnatural History

by Elizabeth Kolbert

Everything (and everyone) alive today is descended from an organism that somehow survived the impact. But it does not follow from this that they (or we) are any better adapted. In times of extreme stress, the whole concept of fitness, at least in a Darwinian sense, loses its meaning: how could a creature be adapted, either well or ill, for conditions it has never before encountered in its entire evolutionary history?

At the end of the Ordovician, some 444 million years ago, the oceans emptied out. Something like eighty-five percent of marine species died off. For a long time, the event was regarded as one of those pseudo-catastrophes that just went to show how little the fossil record could be trusted. Today, it’s seen as the first of the Big Five extinctions, and it’s thought to have taken place in two brief, intensely deadly pulses.

The study revealed that in addition to the five major mass extinctions, there had been many lesser extinction events. When all of these were considered together, a pattern emerged: mass extinctions seemed to take place at regular intervals of roughly twenty-six million years.

The current theory is that the end-Ordovician extinction was caused by glaciation.

One theory has it that the glaciation was produced by the early mosses that colonized the land and, in so doing, helped draw carbon dioxide out of the air. If this is the case, the first mass extinction of animals was caused by plants.

The end-Permian extinction also seems to have been triggered by a change in the climate. But in this case, the change went in the opposite direction. Right at the time of extinction, 252 million years ago, there was a massive release of carbon into the air—so massive that geologists have a hard time even imagining where all the carbon could have come from. Temperatures soared—the seas warmed by as much as eighteen degrees—and the chemistry of the oceans went haywire, as if in an out-of-control aquarium. The water became acidified, and the amount of dissolved oxygen dropped so low that many organisms probably, in effect, suffocated. Reefs collapsed.

One of the ways we’ve accomplished this is through our restlessness. Often purposefully and just as often not, humans have rearranged the earth’s biota, transporting the flora and fauna of Asia to the Americas and of the Americas to Europe and of Europe to Australia. Rats have consistently been on the vanguard of these movements, and they have left their bones scattered everywhere,

whatever the future holds for rats, the extinction event that they are helping to bring about will leave its own distinctive mark. Not yet anywhere near as drastic as the one recorded in the mudstone at Dob’s Linn or in the clay layer in Gubbio, it will nevertheless appear in the rocks as a turning point. Climate change—itself a driver of extinction—will also leave behind geologic traces, as will nuclear fallout and river diversion and monoculture farming and ocean acidification. For all of these reasons, Zalasiewicz believes that we have entered a new epoch, which has no analog in earth’s history. “Geologically,” he has observed, “this is a remarkable episode.”

the Anthropocene will be marked by a unique “biostratigraphical signal,” a product of the current extinction event on the one hand and of the human propensity for redistributing life on the other.

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 s...

“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.”

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).

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.

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.”

“It is the rate of CO2 release that makes the current great experiment so geologically unusual, and quite probably unprecedented in earth history,”

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.

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.

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.

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.

How did the plants and animals of the Pleistocene cope with these temperature swings? According to Darwin, they did so by moving.

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.

animals are critical to the survival of the forest. They are the pollinators and seed dispersers, and the birds prevent the insects from taking over. At the very least, Silman’s work suggests, global warming will restructure ecological communities. Different groups of trees will respond differently to warming, and so contemporary associations will break down. New ones will form. In this planet-wide restructuring, some species will thrive. Many plants may in fact benefit from high carbon dioxide levels, since it will be easier for them to obtain the CO2 they need for photosynthesis. Others will fall behind and eventually drop out.

CURRENTLY, about fifty million square miles of land on the planet are ice-free, and this is the baseline that’s generally used for calculating human impacts. According to a recent study published by the Geological Society of America, people have “directly transformed” more than half of this land—roughly twenty-seven million square miles—mostly by converting it to cropland and pasture, but also by building cities and shopping malls and reservoirs, and by logging and mining and quarrying. Of the remaining twenty-three million square miles, about three-fifths is covered by forest—as the authors put it, “natural but not necessarily virgin”—and the rest is either high mountains or tundra or desert.