The Sixth Extinction: an unnatural history
Henry Holt, 2014
Around 66 million years ago an asteroid somewhat larger than Manhattan ploughed into the Earth near what is now Chicxulub in the Yucután Peninsula, Mexico at 45,000 miles per hour.
Many scientists believe that about three-quarters of animals, including the non-avian dinosaurs were wiped out as a result. It took millions of years for life to recover and surpass its previous diversity, this time with a new ensemble of species that included our distant ancestors. This event, known as the Cretaceous-Paleogene extinction, is counted as one of five mass extinctions over the last 500 million years or so, where a mass extinction is defined as an event in which a significant proportion of life is eliminated in a geologically insignificant amount of time.
At first glance, the footprint of industrialised humanity on the biosphere may look small compared with that of the Chicxulub asteroid. The additional input of heat into the world's oceans resulting from greenhouse gases put there by the combustion of fossil fuels, for example, is equivalent to only about four atomic bomb detonations, or well under a tenth of a megatonne per second.
But first glances are sometimes misleading. Humans are affecting the Earth system in many ways, and have been doing so every moment for decades and indeed centuries. It may seem like a diffuse, drawn-out affair to us as individuals but compared with many natural processes, it is virtually instantaneous. Perhaps the current transformation will turn out to be more like the end-Permian 252 million years ago, the third of the "big five" extinctions, which is thought to have been kicked off by massive pulses of carbon dioxide to the atmosphere, each of which lasted only a few decades, but which together resulted in the death of up to 95% of all life.
A lot is uncertain. What is beyond reasonable doubt is that something big is under way. The best estimates are that the Earth is losing species at many times the background rate (the natural churn in which a few species go extinct every year while new ones evolve), and that 30% to 50% will be functionally extinct by 2050.
The plight of the non-human world has inspired many works of popular science over the last 20 years or so. A 1995 book by Richard Leakey and Roger Lewin, presciently titled The Sixth Extinction, David Quammen's The Song of the Dodo, in 1996, and numerous essays by EO Wilson are among those that speak with clarity and urgency.
But with extinctions and new discoveries piling up, there is a need for still more studies. And in The Sixth Extinction, Elizabeth Kolbert, a staff writer for the New Yorker, offers well-composed snapshots of history, theory and observation that will fascinate, enlighten and appal many readers.
Kolbert begins with a visit to a research station in Costa Rica, where researchers are documenting the disappearance of the golden frog, Atelopus zeteki. Amphibians, the class that includes frogs, are the most endangered group of animals in the world, with an extinction rate as much as 45,000 times the background rate. In addition to factors such as habitat loss, a kind of chytrid fungus, inadvertently spread by humans and lethal to many amphibians, is thought to be to blame.
The book then turns to the development of extinction as an idea and how it has changed our view of life. In this account the phenomenon was discovered in the early nineteenth century by the anatomist Georges Cuvier, who recognised that enormous teeth and bones recovered from sites in what is now Ohio belonged not to elephants but to hitherto unknown beasts. The mastodons, and other strange giants whose remains came across his dissecting table, had lived in "a world previous to ours", which, Cuvier suggested, had perished in a great catastrophe.
Charles Darwin accepted Cuvier's view that the deep past had been filled with extinctions, but rejected catastrophe as a principal cause in favour of a gradual, or uniformitarian view of extinction championed by the geologist Charles Lyell. Only in the 1980s was the hypothesis, proposed by Luis and Walter Alvarez, that a massive asteroid had caused mass extinction at the end of the Cretaceous period generally accepted.
Palaeontologists came to agree that life was characterised by long periods of stability occasionally interrupted by panic. Kolbert's history tour concludes with a look at the far future. Jan Zalasiewicz, a palaeobiologist at the University of Leicester, argues that although our age (for which he champions the term Anthropocene) will leave a record in the geological strata no thicker than a cigarette paper, its impact will nevertheless be great.
One of the strengths of Field Notes from a Catastrophe, Kolbert's 2006 book on global warming, was vivid reportage from exotic locations.
The Sixth Extinction shares this characteristic. There are useful, indeed exemplary, discussions of ocean acidification starting from readily observable natural effects off the Italian coast, of the fate of coral from the Great Barrier Reef, of the extent to which tropical forests in Peru can adapt to rapid change, of habitat fragmentation in the Amazon basin and beyond, and of the consequences of the mass global transference of species from one place to another. It is all pretty grim.
Towards the end, Kolbert writes: "We are deciding, without quite meaning to, which evolutionary pathways remain open and which don't." I don't know about you, but I don't recall being involved in that decision. Indeed, it seems that some of the most important decisions are being taken by those individuals who are spending hundreds of millions of dollars to keep people in the dark about climate change and who are blocking moves to a green economy. We need to decide otherwise.
The extinction crisis is so vast and complex that it is almost repels thought. We need a lot more imaginative thinking about the choices we can make and what comes next, whether it be the "Rambunctious Garden" of environment writer Emma Marris, the feral landscape of George Monbiot or a world utterly transformed by synthetic biology as envisaged by Craig Venter. We need new big stories. Is it too much to ask that we should alter Earth with compassion for the other creatures with whom we share it, and in celebration of their endless forms?
Up to half the world's species will be gone by 2050. Elizabeth Kolbert’s new book will appal readers
The Sixth Extinction: an unnatural history
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