Monday, June 9, 2014

Sixth Extinction

The Sixth Extinction by Elizabeth Kolbert was the subject of an essay by Luke Mitchell published last month in the London Review of Books. Because I read so little about the history of science, it is fairly easy to make a big impression on me, as Luke Mitchell definitely did, simply by reporting concepts that are common knowledge among scientists but unknown to me. I had not, for example, realized, that all educated Europeans up through the end of the 18th century and the Age of Enlightenment resolutely refused to credit the idea that any species of living creature had ever gone extinct or could go extinct. When mastodon bones were discovered in Ohio in the 1730s and sent to Paris for evaluation, "the French naturalist Georges-Louis Leclerc thought the bones might belong to a species that, uniquely in history and for reasons unknown, had disappeared from the Earth, but his conjecture was widely rejected. Thomas Jefferson put forward the consensus view in 1781, in his Notes on the State of Virginia: 'Such is the economy of nature, that no instance can be produced of her having permitted any one race of her animals to become extinct; of her having formed any link in her great work so weak as to be broken.'"

A longer passage in Mitchell's essay is the first concise summary I have encountered anywhere that credibly and comprehensibly presents the current exterminations of life on earth in the context of established historical trends 

"Palaeontologists and geologists now generally agree that the Earth has endured five major extinctions, and more than a dozen lesser ones. The first took place 450 million years ago, during the late Ordovician period, and the most lethal 200 million years ago, during the Permian-Triassic  'the great dying', when nine out of ten marine species vanished. The most terrifying of the mass extinctions, though, was surely the fifth, the Cretaceous-Paleogene incident, which began 65 million years ago when an asteroid the size of Manhattan smashed into the Yucutan Peninsula with the explosive impact of a hundred million hydrogen bombs. The palaeobiologist Peter Ward, writing last year in Nautilus, calls it 

life's worst day on Earth, when the world's global forest burned to the ground, absolute darkness from dust clouds encircled the earth for six months, acid rain burned the shells off of calcareous plankton, and a tsunami picked up all the dinosaurs on the vast Cretaceous coastal plains, drowned them, and then hurled their carcasses against whatever high elevations finally subsided the monster waves.

The lesson that mass extinction is normal is hard to accept. Scientists are beginning to recognize that we're in the middle of another event, perhaps the sixth mass extinction, but that recognition has been slow in coming. In 1963, Colin Bertram, a marine biologist and polar explorer, warned that human expansion could destroy 'most of the remaining larger mammals of the world, very many of the birds, the larger reptiles, and so many more both great and small', and in 1979 the biologist Norman Myers published a little-read book called The Sinking Ark, showing with statistics that Bertram had been correct.  But it wasn't until the 1990s that large numbers of biologists began to take such concerns seriously. In 1991, the palaeobiologist David Jablonski published a paper in Science that compared the present rate of loss to that of previous mass extinctions. Other papers followed and by 1998 a survey by the American Museum of Natural History found that seven out of ten biologists suspected another mass extinction was underway. In 2008, two such biologists, David Wake and Vance Vredenburg, asked in a widely discussed paper, 'Are We in the Midst of the Sixth Mass Extinction?' The answer arrived in 2012 from a large team of biologists and paleontologists writing in Nature: we almost certainly are. If we continue at the current rate of destruction, about three-quarters of all living species will be lost within the next few centuries.

Theories about what caused the earlier extinctions have varied  droughts, methane eruptions, volcanic ash, the ongoing problem of asteroids, the orbit of an invisible sun, our motion through the Milky Way  but there's little doubt about the culprit behind the sixth extinction. Wake and Vredenburg list the proximate causes: 'human population growth, habitat conversion, global warming and its consequences, impacts of exotic species, new pathogens, etc'. What most of these causes have in common isn't just that they are the result of human activity, but that they have been going on for a very long time. Kolbert traces the way the human wake precisely matches the millennial waves of extinction:

The first pulse, about forty thousand years ago, took out Australia's giants. A second pulse hit North America and South America some 25,000 years later. Madagascar's giant lemurs, pygmy hippos and elephant birds survived all the way into the Middle Ages. New Zealand's moas made it as far as the Renaissance. It's hard to see how such a sequence could be squared with a single climate change event. The sequence of pulses and the sequence of human settlement, meanwhile, line up almost exactly. 

We have always been very conscious of our own mortality, and now we're waking up to the existence of another, slower type of death. It's an important recognition. Palaeontologists have found Neanderthal bones everywhere from Israel to Wales, and agree that the species died out suddenly, about thirty thousand years ago, which is suspiciously close to the time that Homo sapiens began its expansion from Africa. One theory is that clever man simply murdered his stronger cousin. But there are other theories. Maybe we simply outhunted our cousins, or carried a disease that was novel to them. Or maybe our contribution to their demise was even more indirect; animals with a long reproductive cycle are vulnerable to even the slightest of disruptions. John Alroy, an American palaeobiologist, has run computer simulations that suggest it would take just a tiny bit of interference with the Neanderthal birth rate, over the course of a few thousand years, to drive it to extinction. Alroy called this a 'geologically instantaneous ecological catastrophe too gradual to be perceived by the people who unleashed it'. Such imperception is no longer possible."

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Young people confidently inform me that the near future is a place where human technology will be powerful enough to redress current ecological catastrophes and head off new ones. This, at any rate, seems to be majority opinion among young San Francisco people. And they sound so secure in this viewpoint that I have often wondered if they might not be right. At other times I'm inclined to see faith in a triumphal techno-future as a simple extension of the weird submerged belief shared by all young people (up to the day they become old people)  that they themselves certainly will never, can never die.