Like a Kryptonite-challenged Superman, its strength has steadily and mysteriously waned, leaving parts of the planet vulnerable to increased radiation from space.
Some satellites already feel the effects.
What is uncertain is whether the weakened field is on the way to a complete collapse and a reversal that would flip the North and South Poles.
Compasses pointing North would then point South.
It is not a matter of whether it will happen, but when, said scientists who presented the latest research on the subject at a recent meeting of the American Geophysical Union in San Francisco.
But when is hard to pinpoint. The dipole reversal pattern is erratic.
"We can have periods without reversals for many millions of years, and we can have four or five reversals within one million years," said Yves Gallet, from Institut de Physique du Globe de Paris, France, who studies the palaeomagnetic record and estimates that the current decay started 2,000 years ago.
Some satellites already feel the effects.
What is uncertain is whether the weakened field is on the way to a complete collapse and a reversal that would flip the North and South Poles.
Compasses pointing North would then point South.
It is not a matter of whether it will happen, but when, said scientists who presented the latest research on the subject at a recent meeting of the American Geophysical Union in San Francisco.
But when is hard to pinpoint. The dipole reversal pattern is erratic.
"We can have periods without reversals for many millions of years, and we can have four or five reversals within one million years," said Yves Gallet, from Institut de Physique du Globe de Paris, France, who studies the palaeomagnetic record and estimates that the current decay started 2,000 years ago.
Flip or flop
Over the last century and a half, since monitoring began, scientists have measured a 10% decline in the dipole.
At the current rate of decline it would take 1,500 to 2,000 years to disappear.
SEAFLOOR RECORDER
As molten rock rises, spreads out and cools, magnetised minerals record field direction
Over millions of years, the seafloor rocks retain a 'barcode' of pole reversals
These pole reversal events may take perhaps 10,000 years to complete
The last major pole flip appears to have been about 780,000 years ago A particular weakness in the field has been observed off the coast of Brazil in the so-called Southern Atlantic Anomaly. Here, eccentricities in the Earth's core have caused a "dip" in the field, leaving it 30% weaker than elsewhere.
The extra dose of radiation creates electronic glitches in satellites and spacecraft that fly through it. Even the Hubble telescope has been affected.
Magnetic reversals were always preceded by weakened magnetic fields, said Dr Gallet, but not all weakened fields bring on a flip-flop.
The Earth's invisible shield could also grow back in strength. "Then sometime, maybe 10,000 years from now, the dipole will decay again and that will lead to a reversal," said Harvard physicist Jeremy Bloxham.
The theme was recently taken up by Hollywood in the movie The Core, in which the Earth's core mysteriously stops spinning, effectively turning off the electromagnetic field.
The movie is nonsense, scientists told BBC News Online, except that the Earth's magnetic field is generated by activity deep inside it.
At the current rate of decline it would take 1,500 to 2,000 years to disappear.
SEAFLOOR RECORDER
As molten rock rises, spreads out and cools, magnetised minerals record field direction
Over millions of years, the seafloor rocks retain a 'barcode' of pole reversals
These pole reversal events may take perhaps 10,000 years to complete
The last major pole flip appears to have been about 780,000 years ago A particular weakness in the field has been observed off the coast of Brazil in the so-called Southern Atlantic Anomaly. Here, eccentricities in the Earth's core have caused a "dip" in the field, leaving it 30% weaker than elsewhere.
The extra dose of radiation creates electronic glitches in satellites and spacecraft that fly through it. Even the Hubble telescope has been affected.
Magnetic reversals were always preceded by weakened magnetic fields, said Dr Gallet, but not all weakened fields bring on a flip-flop.
The Earth's invisible shield could also grow back in strength. "Then sometime, maybe 10,000 years from now, the dipole will decay again and that will lead to a reversal," said Harvard physicist Jeremy Bloxham.
The theme was recently taken up by Hollywood in the movie The Core, in which the Earth's core mysteriously stops spinning, effectively turning off the electromagnetic field.
The movie is nonsense, scientists told BBC News Online, except that the Earth's magnetic field is generated by activity deep inside it.
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