How will the Earth expire? Aisling Irwin looks at the usual, and some unusual, suspects.

At the opening of the sixth seal, there will be a devastating earthquake, the sun will blacken and stars will fall from the sky. Mountains will shake. At the blowing of the fifth trumpet, locusts from the bottomless pit, with human faces and scorpion tails, will savagely torment the unfaithful. Seven plagues will cause calamity upon calamity, including loathsome sores, rivers turned to blood, and a heat so severe that the world will be scorched.

The Bible's Book of Revelation contains all the vital ingredients for an end to the world that is satisfying to the human psyche. Many civilisations have believed that the end will be, in essence, a human-centred affair, a purposeful event in which evil is punished through some divine and apocalyptic means, and redemption follows in a new world for those who deserve it.

These belief-related visions of doom remain with us, but today there is also a new kind of doom-mongering based on real possibilities whose likelihood can be calculated and measured. Some of these are uncannily reminiscent of age-old themes. An asteroid crashes into earth, bringing a familiarly cataclysmic end to the human race. Wanton exploitation of nature brings about humanity's fall. Yet there is something materially different about many of the claims about the end over the past 50 years: the rise of science.

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Science has enabled us to understand the natural world better and therefore to imagine and predict many previously unthought-of apocalypses. It has also enabled us to devise means by which we could, accidentally or deliberately, finish off much of the human race, and perhaps the planet.

Paul Corcoran, associate professor of politics at the University of Adelaide, argues that this change became obvious some 50 years ago with the explosion of the atomic bomb. "This was an apocalyptic vision suddenly reduced to practical choice and the probabilities of chance error," he says.

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The 21st century is a crucial century for the survival of our species, according to Sir Martin Rees, Britain's astronomer royal. This is because we are acquiring the means by which to extinguish ourselves but we have not yet devised a way of spreading ourselves through the galaxy, a move that would enhance our chances of survival by reducing our dependence on a single planet.

The new risks we face fall into several categories, perhaps the most gruesome of which is the deliberate obliteration of humankind by humankind. Nuclear war was but the first possibility. Some argue that global annihilation through war is more likely to be the result of bioweapons - cheap to produce, easy to conceal and terrifyingly tricky to control.

We have been promised for decades that the end will come at the hand of robots, generally acting under the command of some malevolent inventor, and subsequently rebelling and taking over the earth. Scientists such as Hans Moravec, one of the founders of the robotics department at Carnegie Mellon University, predict that, despite delays, machines are getting nearer to becoming conscious every year. Once they become brighter than us, they could dominate us, wipe us out or merge with us in some sort of post-human synthesis that represents the end of humanity as we know it.

Several of the new dangers to the world resonate to a time-honoured theme - that of decline and fall brought about by moral bankruptcy. The latest incarnation of this idea is environmental degradation. In Something New under the Sun: An Environmental History of the Twentieth Century , John McNeill argues that the 20th century was unique because of the scale on which this became possible.

Human curiosity is also traditionally viewed as a precursor of doom. Modern incarnations include genetically engineered microbes. These could bring about the end of the world either through environmental disaster by breeding superweeds - a possibility discussed by US economist and environmental campaigner Jeremy Rifkin - or by their use in bioweapons.

The end could also come as the result of the blue-sky tinkerings of physicists. Pottering away at their particle accelerators with the aim of understanding abstract physical questions of the universe, scientists could trigger a chain reaction that would destroy the world. Piet Hut and Rees suggested in a 1983 article in the journal Nature that the Relativistic Heavy Ion Collider (RHIC) on Long Island, New York, could create a subatomic black hole that would slowly eat away at our planet. Alternatively, the RHIC might create exotic bits of altered matter, called strangelets, that would obliterate whatever ordinary matter they met.

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Although a panel called to address these fears has now rejected both scenarios as virtually impossible, critics say that that is not enough. "Totally impossible" is the only acceptable probability when it comes to the preservation of humanity.

Another branch of physics, nanotechnology, could also cause an exotic end. Engineers have for the past decade been building atomic-scale machines. One day they may be able to build microscopic robots that can assemble and replicate themselves. The technology, which could produce such benefits as performing surgery inside the body, could become lethal and uncontrollable. They could, says Eric Drexler, author of Engines of Creation , "reduce the biosphere to dust in a matter of days".

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Before we get around to destroying ourselves, however, the apocalypse may arrive from outer space. Once it was divine omnipotence that made these types of utterly unpredictable ends all too believable. Now it is our understanding of the vastness of galactic phenomena and Earth's cosmic insignificance. Astronomer Duncan Steel suggests in Rogue Asteroids and Doomsday Comets that we could eventually be wiped out by an asteroid. Alternatively, Earth might be obliterated by a gamma-ray burst - a sporadic explosion that produces far more energy than does our Sun. Earth's atmosphere would initially protect us from most of the burst's deadly X-rays and gamma rays. But they would slowly cook the atmosphere, a process that would destroy the ozone layer. Without the ozone layer, ultraviolet rays from the Sun would reach the Earth's surface and kill the tiny photosynthetic plankton in the ocean that underpin the world's food chain.

If the gamma-ray bursts - which are rare - fail to get us, a rogue black hole just might. Scientists have calculated that our galaxy contains about 10 million black holes and they usually orbit stars, which means that none is likely to approach us. However, if one did just scrape through the solar system, it would exert enough gravitational effect to distort planetary orbits. Earth's orbit might become elliptical, causing extreme climate swings, or Earth might shoot out of the solar system to a bitter, cold end in outer space.

There is another stellar danger whose potency we have actually witnessed in the past decade thanks to images from spacecraft. Solar flares are magnetic outbursts that bombard Earth and can disrupt power supplies, though our atmosphere and magnetic field have so far afforded us protection. There is evidence, however, that ordinary Sun-like stars can occasionally emit superflares, millions of times more powerful than ordinary ones. Bradley Schaefer of Yale University has found evidence that some perfectly normal-looking, Sun-like stars can brighten briefly and massively, and he thinks this may be because of superflares.

These could be vastly more worrying should Earth lose the protection of its magnetic field. Geologists have shown that every few hundred thousand years, this magnetic field dwindles almost to nothing for perhaps a century, then gradually reappears. The last such reversal was 780,000 years ago, so the next one may be due. One sign that it may be coming is that the strength of our magnetic field has decreased by about 5 per cent in the past century. Without protection from the magnetic field, Earth would be vulnerable to particle storms, cosmic rays from the Sun and further erosion of the ozone layer.

A particularly 20th-century terror has been of the end at the hands of aliens. Today, not only does Seti (the Search for Extra Terrestrial Intelligence) scan space for signs of intelligent life, but theologians and philosophers have prepared our moral response should aliens ever be discovered. Cutting-edge thinkers on this issue, extrapolating from the age of human exploration of the globe, argue that the greatest danger from aliens is not that they will unkindly kill us all, but that we may suffer because we get in the way of their plundering of Earth. Alternatively, they might import diseases to which we have no resistance.

Perhaps, however, divine intervention will precede all this. And if not mediated directly by God, it could be at the hands of some of his most dedicated followers. The end - decline, apocalypse and redemption - has a gripping effect on many, and these days small cults, such as the Branch Davidians and Heaven's Gate, that wish to accelerate its coming have unprecedented access to potent methods of retribution. A taste of what they might achieve was given in 1995, when members of a religious sect released nerve gas in a Tokyo underground station, killing 12 people and injuring more than 5,000. Since September 2001, we have been confronted with the concept of the global Holy War, an ancient idea made more lethal through modern technology. With improved access to bioweapons, and even to nuclear bombs, we may get the ending with which humans began.

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The THES book Big Questions in Science is published this week by Jonathan Cape, ?15.99. Call 01454 617370 to order a copy at a special THES price of pounds 12.99