Scientists have discovered that its strength has dropped precipitously over the past two centuries and could disappear over the next 1,000 years.

The effects could be catastrophic. Powerful radiation bursts, which normally never touch the atmosphere, would heat up its upper layers, triggering climatic disruption. Navigation and communication satellites, Earth's eyes and ears, would be destroyed and migrating animals left unable to navigate.

'Earth's magnetic field has disappeared many times before - as a prelude to our magnetic poles flipping over, when north becomes south and vice versa,' said Dr Alan Thomson of the British Geological Survey in Edinburgh.

'Reversals happen every 250,000 years or so, and as there has not been one for almost a million years, we are due one soon.'

For more than 100 years, scientists have noted the strength of Earth's magnetic field has been declining, but have disagreed about interpretations. Some said its drop was a precursor to reversal, others argued it merely indicated some temporary variation in field strength has been occurring.

But now Gauthier Hulot of the Paris Geophysical Institute has discovered Earth's magnetic field seems to be disappearing most alarmingly near the poles, a clear sign that a flip may soon take place.

Using satellite measurements of field variations over the past 20 years, Hulot plotted the currents of molten iron that generate Earth's magnetism deep underground and spotted huge whorls near the poles.

Hulot believes these vortices rotate in a direction that reinforces a reverse magnetic field, and as they grow and proliferate these eddies will weaken the dominant field: the first steps toward a new polarity, he says.

And as Scientific American reports this week, this interpretation has now been backed up by computer simulation studies.

How long a reversal might last is a matter of scientific controversy, however. Records of past events, embedded in iron minerals in ancient lava beds, show some can last for thousands of years - during which time the planet will have been exposed to batterings from solar radiation. On the other hand, other researchers say some flips may have lasted only a few weeks.

Exactly what will happen when Earth's magnetic field disappears prior to its re-emergence in a reversed orientation is also difficult to assess. Compasses would point to the wrong pole - a minor inconvenience. More importantly, low-orbiting satellites would be exposed to electromagnetic batterings, wrecking them.

In addition, many species of migrating animals and birds - from swallows to wildebeests - rely on innate abilities to track Earth's magnetic field. Their fates are impossible to gauge.

As to humans, our greatest risk would come from intense solar radiation bursts. Normally these are contained by the planet's magnetic field in space. However, if it disappears, particle storms will start to batter the atmosphere.

'These solar particles can have profound effects,' said Dr Paul Murdin, of the Institute of Astronomy, Cambridge. 'On Mars, when its magnetic field failed permanently billions of years ago, it led to its atmosphere being boiled off. On Earth, it will heat up the upper atmosphere and send ripples round the world with enormous, unpredictable effects on the climate.'

It is unlikely that humans could do much. Burrowing thousands of miles into solid rock to set things right would stretch the technological prowess of our descendants to bursting point, though such limitations do not worry film scriptwriters. Paramount's latest sci-fi thriller, The Core - directed by Englishman Jon Amiel, and starring Hilary Swank and Aaron Eckhart - depicts a world beset by just such a polar reversal, with radiation sweeping the planet.

The solution, according to the film, to be released next year, involves scientists drilling into Earth's mantle to set off a nuclear blast that will halt the reversal.

Given that temperatures at such depths rival those of the Sun's surface, such a task would seem impossible - except, of course, in Hollywood.