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 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 atmosphere, regardless of what humans do: in other words, the oceans and plants will stop soaking up those three billion tonnes. The UK Meteorological Office's Hadley Centre, which specialises in climate-change research, published an alarming paper in Nature in 2000 which gave the results of a computer simulation of the future global carbon cycle. It showed that if greenhouse-gas emissions continued, the Amazon rainforest ecosystem would begin to collapse, releasing vast quantities of stored carbon into the atmosphere in addition to the man-made carbon emissions. After about 2050, even more carbon would pour into the air from warming soils around the world. The combined effect would be enough to increase CO2 in the atmosphere by another 250ppm - equivalent to a temperature rise of an extra 1.5 Celsius above previous predictions.
There is an even more chilling possibility. Deep under oceanic continental shelves right around the world, from Peru to Norway, huge quantities of methane are stored in "hydrate" form, kept solid by a combination of low temperatures and pressure from the water and sediment piled above them. It has been estimated that this methane hydrate store contains 10,000 gigatonnes - that is, ten thousand billion tonnes - of carbon, more than double the world's entire combined fossil-fuel reserves. Like carbon dioxide, methane is a greenhouse gas - in fact, it is 21 times more potent than CO2. If even a small quantity were to escape into the atmosphere, runaway global warming might become inevitable.
This nightmare, scientists say, is increasingly likely. Warming ocean temperatures will destabilise the hydrates, allowing them to bubble up to the surface. This new methane will increase temperatures further, leading to still more release from the sea floor in a potentially unstoppable spiral. In fact, geologists increasingly think this feedback to have been the mechanism that drove the end-Permian cataclysm: carbon dioxide from volcanoes first raised world temperatures enough to destabilise methane hydrates, after which prehistoric global warming gained its own deadly momentum.
A more recent geological event, 55 million years ago at the end of the Palaeocene epoch, provides even stronger evidence that a "methane burp" from the oceans has indeed happened before. Although less dramatic than the end-Permian, it was also accompanied by mass extinctions. Indeed, it was in the recovery period from this second crisis that mammals - including our primate forebears - first exploded on to the scene. The government's chief scientist, Professor Sir David King, was referring to this period when he told reporters at Tony Blair's Climate Group launch on 27 April that "Antarctica was the best place for mammals to live, and the rest of the globe would not sustain human life". He warned that these conditions, with CO2 levels as high as 1,000ppm and no ice left on earth, could again be reached by 2100.
How seriously should we take these warnings? It must be emphasised that, while scientists are now virtually unanimous about the reality of man-made global warming - new evidence published in Nature that the troposphere, the lowest level of the atmosphere, is warming at roughly the same rate as the earth's surface has removed the last doubts - they are far more cautious about suggestions that it is already moving out of control. The increase in carbon-dioxide concentrations detected by the Hawaii station, says Pieter Tans of the US National Oceanic and Atmospheric Administration, may not continue. In warmer years, he explains, the rate of bacterial decomposition in the ground speeds up, and more carbon is released from soils. Over more than a few years, he says, ecosystems adjust. However, his colleague Ralph Keeling, while agreeing that the recent change "might not be such a big deal", points out that "there is no past period where the average carbon accumulation has stayed this high". Another expert on the carbon cycle - who was prepared to speak only on condition of anonymity - said: "We simply don't have a way to tell from just one year if a positive feedback is kicking in. But if it was happening, this is what it would look like."
That is the trouble with global warming. Human beings respond to events on a daily and weekly basis, not an annual, still less a decadal one. But according to a paper from the Benfield Hazard Research Centre, to be published shortly, the impact of methane hydrate failure could be very dramatic indeed. If enough gas is released, entire continental slopes could collapse in enormous submarine landslides, triggering tsunami waves of up to 15 metres in height - enough to level entire coastal cities. Again, there is a precedent: just 7,000 years ago, an area of continental slope the size of Wales slid downhill between Norway and Iceland, triggering a tsunami that wiped out neolithic communities on the north-east coast of Scotland.
If such an event happens again, the only certainty is that there will be no warning. And yet, the danger signs are already all around: 2003 was the second-warmest year on record. Last summer's heatwave across Europe was so far off the normal statistical scale that climatologists logged it as a once-in-10,000-years event. Sea-level rise is accelerating, according to the latest satellite measurements. And last month, a truly unprecedented weather event occurred. Hurricanes were thought to be an entirely north Atlantic phenomenon. But on this occasion an Atlantic hurricane formed south of the Equator and struck Brazil with 90mph winds. Tropical meteorologists were so baffled that they had no idea what to call it, and hurricane monitoring systems may now have to be extended a thousand miles further south.
So is there any hope of persuading politicians to treat global warming with the urgency it requires? Perhaps so, now that the story has reached Hollywood, with the disaster movie The Day After Tomorrow due out this summer. Unfortunately, the events in the film are premised on an effect of global warming that remains contentious among scientists, and tends to confuse the public. This is the possibility that global warming, by increasing rainfall and ice-melt at high latitudes, shuts down the Atlantic's circulation, plunging Europe into a new ice age. A current known as the Gulf Stream transports a staggering amount of heat northwards, equivalent to the energy produced by about a million nuclear power stations. Without it, our climate would be between 5 Celsius and 10 Celsius colder - similar to that of Newfoundland. Again, the warning signs are clear: the "subpolar gyre" part of the current has already begun to slow, and through-flow of water between Iceland and the Faroes has declined by 20 per cent over the past 50 years.
However, The Day After Tomorrow's storyline - where New York is flooded and then frozen solid within a week - is not even remotely likely. Most scientists, whilst quietly approving of Hollywood's sudden conversion to an issue that many have been battling to get into the media for years, give the film itself short shrift. Writing last month in Science magazine, the oceanographer Andrew Weaver pooh-poohed the "new ice age" scenario, pointing out that such a drastic cooling of the climate would be impossible with greenhouse gases at today's elevated levels.
But whatever its flaws, there is at least a chance that the film will put global warming on the US political agenda. Indeed, the former vice-president Al Gore, who frequently delivers speeches warning of the dangers of climate change, is planning a mass rally in New York City to capitalise on the sudden media interest.
The more common mood among environmentalists, however, is one of pessimism. At the annual UN climate-change talks, scheduled for Buenos Aires in December, negotiators, instead of discussing "mitigation" (reducing greenhouse-gas emissions), are likely to focus mainly on "adaptation". In other words, having lost the battle to stop global warming, the best we can all now hope for is desperate rearguard actions to protect coastal land from flooding and to ward off large-scale starvation. Even the Pentagon is having to take note. A recent report for America's military top brass warned that mass refugee flows and competition for water and food could plunge the world into nuclear conflict. "Humans fight when they outstrip the carrying capacity of their natural environment," it warns. "The most combative societies are the ones that survive."
The report charts some of the "potential military implications of climate change", including the collapse of the EU, civil war in China and the takeover of US borders by the army to prevent refugee incursions from the Caribbean and Mexico. The report's title, Abrupt Climate Change, reflects an increasing awareness among scientists and policy-makers alike that global warming is more likely to lead to sudden climatic shifts than to slow, linear change. Once more, the earth's climate history gives a precedent: the planet swung between cold and warm periods at the end of the most recent ice age in as little as a decade.
Sudden, unanticipated events - which climatologists drily term "surprises" - could include, for example, the collapse of the west Antarctic ice sheet and the catastrophic inundation of low-lying areas all around the world. This is the scenario that causes giant waves to flood Manhattan in The Day After Tomorrow. In reality, even the most dramatic rise in sea level would still take years rather than minutes to flood coastal cities.
A better storyline might have run as follows. Rapid melting of the Greenland ice sheet causes the offshore continental shelf to rebound upwards, releasing huge quantities of methane hydrates with explosive force. The entire shelf slumps downward, triggering 15m-high tsunamis right across the north Atlantic, wiping out ReykjavIk, Lisbon and - ultimately - New York City itself. The new methane adds rapidly to global warming, causing mega-droughts and mass starvation across Asia, Africa and South America. Don't forget: it's happened before, so it could happen again.
Three months ago, scientists would have laughed if a film had portrayed a freak hurricane forming in the south Atlantic. Now nobody's smiling.
Mark Lynas is the author of High Tide: news from a warming world, published by Flamingo. www.marklynas.org