Post-human Earth: How the planet will recover from us

WHEN Nobel prize-winning atmospheric chemist Paul Crutzen coined the word Anthropocene around 10 years ago, he gave birth to a powerful idea: that human activity is now affecting the Earth so profoundly that we are entering a new geological epoch.

The Anthropocene has yet to be accepted as a geological time period, but if it is, it may turn out to be the shortest - and the last. It is not hard to imagine the epoch ending just a few hundred years after it started, in an orgy of global warming and overconsumption.

Let's suppose that happens. Humanity's ever-expanding footprint on the natural world leads, in two or three hundred years, to ecological collapse and a mass extinction. Without fossil fuels to support agriculture, humanity would be in trouble. "A lot of things have to die, and a lot of those things are going to be people," says Tony Barnosky, a palaeontologist at the University of California, Berkeley. In this most pessimistic of scenarios, society would collapse, leaving just a few hundred thousand eking out a meagre existence in a new Stone Age.

Whether our species would survive is hard to predict, but what of the fate of the Earth itself? It is often said that when we talk about "saving the planet" we are really talking about saving ourselves: the planet will be just fine without us. But would it? Or would an end-Anthropocene cataclysm damage it so badly that it becomes a sterile wasteland?

The only way to know is to look back into our planet's past. Neither abrupt global warming nor mass extinction are unique to the present day. The Earth has been here before. So what can we expect this time?

Take greenhouse warming. Climatologists' biggest worry is the possibility that global warming could push the Earth past two tipping points that would make things dramatically worse. The first would be the thawing of carbon-rich peat locked in permafrost. As the Arctic warms, the peat could decompose and release trillions of tonnes of carbon into the atmosphere - perhaps exceeding the 3 trillion tonnes that humans could conceivably emit from fossil fuels. The second is the release of methane stored as hydrate in cold, deep ocean sediments. As the oceans warm and the methane - itself a potent greenhouse gas - enters the atmosphere, it contributes to still more warming and thus accelerates the breakdown of hydrates in a vicious circle.

"If we were to blow all the fossil fuels into the atmosphere, temperatures would go up to the point where both of these reservoirs of carbon would be released," says oceanographer David Archer of the University of Chicago. No one knows how catastrophic the resulting warming might be.

That's why climatologists are looking with increasing interest at a time 55 million years ago called the Palaeocene-Eocene thermal maximum, when temperatures rose by up to 9 °C in a few thousand years - roughly equivalent to the direst forecasts for present-day warming. "It's the most recent time when there was a really rapid warming," says Peter Wilf, a palaeobotanist at Pennsylvania State University in University Park. "And because it was fairly recent, there are a lot of rocks still around that record the event."

By measuring ocean sediments deposited during the thermal maximum, geochemist James Zachos of the University of California, Santa Cruz, has found that the warming coincided with a huge spike in atmospheric CO2. Between 5 and 9 trillion tonnes of carbon entered the atmosphere in no more than 20,000 years (Nature, vol 432, p 495). Where could such a huge amount have come from?

Volcanic activity cannot account for the carbon spike, Zachos says. Instead, he blames peat decomposition, which would have happened not from melting permafrost - it was too warm for permafrost - but through climatic drying. The fossil record of plants from this time testifies to just such a drying episode.

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