In his forthcoming book, Our Final Century, the British cosmologist Sir Martin Rees rates our chances as no better than 50-50. Bill Joy, the founder of Sun Microsystems and one of the leading technocrats of our time, thinks machines will soon usurp the human race. Others think nanotechnology - the construction of molecular-sized machines - will reduce the Earth's surface to a featureless goo. Genetically engineering viruses or bacteria may have the same effect. Meanwhile, we are overdue for a super-volcanic eruption - the most recent was 74,000 years ago - or an asteroid or comet impact like the one that wiped out the dinosaurs 65m years ago. Then there's global warming. And don't get me started on the impending vacuum metastability disaster.The good news is you can make money out of this. William Hill offered me absurd odds of 1m to 1 against the end of the world by 2200, based on the assumption, as one William Hill spokesman said, that 'bookmakers will go to heaven - albeit temporarily - to pay those punters who have bet on the end of the world'. I told them Rees's odds, but they were unmoved on the basis that he and his publishers would say that, wouldn't they?

But the bookies are wrong. The threats to our survival are numerous and imminent. There are two categories of catastrophe to be considered: the end of the world (killing all humans and perhaps destroying the planet), and the end of the world as we know it (a disaster large enough to transform human civilisation, killing millions if not billions, but leaving survivors). Both scenarios can be caused by nature or by humanity.

And why is this suddenly an issue? The answer is knowledge. We now understand that the human race has prospered in a period of unusual cosmic and geological calm. We know that the issue is not if some awful natural disaster will happen, but when. And our knowledge has brought us to a technological brink: we have the means to destroy the world. 'If hamadryas baboons had nuclear weapons, they would destroy the world in a week,' said the great biologist E O Wilson. But just a few fragments of DNA separate us from baboons. Furthermore, when the Americans tested the first atom bomb in 1945, some scientists believed the test would set the atmosphere on fire or start a catastrophic chain reaction in the hydrogen locked up in the oceans. But they went ahead and did it on that well-known principle of modern physics: 'What the hell, let's give it a whirl.'Here, then, are the top-10 terrible things that might happen. The only question is whether they will finish us off completely, or just knock us back. We are 6 billion ill-tempered neurotics clinging to a rock in an utterly indifferent, violent universe. Never has the fragility of our existence been more apparent. That thought alone might be of some hope. The first images of Earth from space fired the environmental movement with the vision of a species that trod more lightly on our delicate planet. New awareness of how easily we might be extinguished by our own or nature's actions might lead us to draw back from more obvious dangers. But it might not. Life on Earth will then turn out to have been a temporary anomaly, a pale flicker, a brief cry in the darkness and silence, seen and heard only by God.

1: The experiment that backfires

It is the 'What the hell, let's give it a whirl' principle that lies behind the first, and seemingly most exotic, risk to the planet in the near future: a 'phase transition in the cosmic vacuum energy' brought about by high-energy physics experiments. This, perhaps, is the ultimate end-of-everything scenario. And it would be all our own work.

Particle colliders have so far been vast, circular underground tunnels that accelerate particles almost to the speed of light and then smash them into other particles. The objects that are being studied are so minuscule that you may not imagine they represent much of a threat. But they do. Recently, serious reports suggested that the Relativistic Heavy Ion Collider in Long Island might create a tiny black hole that would slowly suck all the Earth's matter into itself. There was also the possibility that they might generate another Big Bang, thus destroying our present universe.

New particle colliders need only be about 12ft long and may be capable of producing energy levels almost unprecedented in the universe; if you imagine the energy released by two aircraft colliding, compressed into the space of a subatomic particle, you may get the picture. The early universe was just a mass of energy fields known as a virtual vacuum. This was highly unstable, and at some point it flipped over into the more stable vacuum of the universe we know. But even our universe may not be that stable. Energy levels created by new colliders may flip it once again. A bubble would spread out from that 12ft collider that would engulf us, the galaxy and then the entire universe.

The philosopher John Leslie has suggested, with a completely straight face, that any aliens who were passing and saw us about to conduct such experiments would have to destroy the Earth in their own best interests.

2: Machines taking over

It is routinely claimed that computers will reach 'human equivalence' by the year 2030. This means that they will have the storage capacity and the processing speed of our brains. It does not necessarily mean that they will be conscious or self-aware beings. Most science-fiction disaster scenarios - for example, the one played out in the film Terminator - focus on consciousness as the key to machines' power. But unconscious machines may prove in time to be just as dangerous.

The idea of the machines taking over was floated as an optimistic prospect by the American scientist Ray Kurzweil. In his book The Age of Spiritual Machines, Kurzweil foresaw a future utopia in which humans became immortal by, in effect, becoming robots. Similarly, the scientist Hans Moravec argues that life is about to transcend mere humanity because robots will take over.

Others, however, are not so sanguine. Theodore Kaczynski, the 'Unabomber' whose 17-year terror campaign in the US killed three people, was driven by his conviction that 'engineered human beings' would be 'reduced to the status of domestic animals'.