The first warning signs would have come from the sunsets. Weird splashes of red, yellow and purple-painted evening horizons all around the globe. Only those living near the eruption site would have seen the cause--vast volcanic outgassings of carbon dioxide, ash and sulphur. The end-Permian apocalypse had begun. By its conclusion, up to 95 per cent of species had been wiped out, the oceans transformed into black, oxygen-starved graveyards as millions of animal carcasses and uprooted plants rotted in the inky depths. It was the worst mass extinction ever to hit the planet, and it happened 251 million years ago because of global warming. (For a full description, see Michael Benton's When Life Nearly Died, published by Thames & Hudson last year.)
Today, the world stands on the brink of a similar cataclysm, with one crucial difference. The agent of death at the end of the Permian period was volcanism. Now the agent of death is man.
But how close are we to this catastrophe? Is it still avoidable? In the pre-industrial era, levels of carbon dioxide per cubic metre of air stood at roughly 278 parts per million (ppm). Today, they have soared to 376ppm, the highest in at least 420,000 years, and probably much longer. This means that every breath of air we take is chemically different from the air breathed throughout the evolutionary history of the human species. And if the current rate of carbon accumulation continues, the rise in temperature could be as much as 6[degrees] Celsius by the end of the century, according to the UN Intergovernmental Panel on Climate Change. That is roughly the same as the temperature increase that delivered the coup de grace to the prehistoric world of the Permian.
All the efforts of the climate-change panel, all the international conferences and protocols, all the green campaigning, are based on the assumption that, if we act now, the worst can be avoided. Although some global warming is already inescapable--temperatures will continue to rise for many years, and there is no power on earth that can stop them--we assume, none the less, that it is not too late; if we do the right things within the next couple of decades, temperatures will eventually stabilise.
But what if this is wrong? What if global warming is already unstoppable and is now accelerating uncontrollably? What if we have reached the point of no return and there is nothing we can do except wait for the end? Scientists are naturally cautious people, but a growing number fear that this may be the case. One ominous indicator comes from a US atmospheric sampling station 3,000 metres up on the northern flank of the Mauna Loa volcano in Hawaii. Since the 1950s, this station--and dozens of others dotted around the globe from Alaska to the South Pole--have recorded a steady increase in carbon-dioxide concentrations. The average year-on-year rise is 1.5ppm. Over the past two years, the rate of accumulation has doubled--to nearly 3ppm. This could mean that the rate of fossil-fuel burning has doubled--but it hasn't. The alternative explanation is that the biosphere "sinks", which used to absorb carbon, have suddenly shut down.
To understand the implications of this second possibility, we need to look at how global warming works. Every year, humans burn enough coal, oil and gas to add roughly six billion tonnes of carbon to the global atmosphere. This carbon was formerly trapped underground, laid down between rock deposits from much earlier (and warmer) phases in the earth's history. About half of this extra annual dose of carbon--three billion tonnes--is soaked up by oceans and plants. It is the other half that steadily accumulates in the atmosphere and causes all the trouble.
The fear is that, as temperatures rise, global warming, in a process that scientists call "positive feedback", will itself increase the amount of carbon released into the …