Thursday, 8 April 2004
A reversal of the Earth's magnetic field, a rare but feared event due to the catastrophic effect it could have on human life, takes about 7000 years to complete, a U.S. scientist predicts.
Associate Professor Bradford Clement, from Florida International University in Miami, publishes his work in today's issue of the journal Nature.
The so-called flip between the Earth's North and South poles occurs at long but unpredictable intervals, the most recent one occurring about 790,000 years ago.
A compass needle, if one existed then, would have pointed to the south and not to the north.
The 180º switch occurs when there is a change in the circulation patterns in the molten iron that flows around the Earth's outer core and, like a dynamo, creates the magnetic field.
The intensity of the field drops for a while before the circulation rhythm is established and the new polarity occurs.
But scientists have only estimated how long the switching process takes before the new poles become established. Estimates have ranged from a couple of thousand years to 28,000.
Clement cast light on this area of uncertainty by analysing records taken from sedimentary rock samples drilled from various sites around the world.
These samples, deposited at four different ages in Earth's history, had a residual magnetic echo from the magnetic field at the time.
"These records yield an average estimate of about 7000 years for the time it takes for the directional change to occur," Clement said.
But the big switchover does not take place in one swoop. It happens faster at the equator and takes longer at higher latitudes, the closer you get to the poles.
He calculated it took 2000 years at the equator and about 10,000 closer to the poles.
The reason for this, said Clement, is that in the absence of the main north-south magnetic field, the Earth's core develops a weaker secondary field that has many mini-poles at the surface.
Eventually the two main poles are established again, but on opposite sides of the planet, and restore their primacy.
No-one knows what would happen to life on Earth if the flip occurred today but the speculation borders on the doomsday.
Many aspects of life today would be literally turned upside down, both for humans, given our dependence on magnets for navigation, and for migrating animals that use an inner compass.
We would also be more exposed to deadly busts of solar radiation, from which we are normally protected by Earth's magnetic field. And the loss of that shield would cause solar particles to smash into the upper atmosphere, warming it and potentially causing wrenching climate change.
There was a scare in 2002 after French geophysicist Professor Gauthier Hulot discovered a weakening of Earth's magnetic field near the poles, which could be interpreted as an early sign that a flip is near.
Polarity reversals "seem to occur randomly in time", wrote geophysicist Professor Ronald Merrill from the University of Washington in Seattle in a commentary in the same issue of the journal.
The shortest interval between flips is between 20,000 and 30,000 years, and the longest is 50 million years.
Merrill said Clement's model for pole reversal was "probably not physically realistic" but was an "important contribution" to our understanding of these magnetic flips.
Among Merrill's criticisms were Clement's assumptions about the symmetry of Earth's magnetic fields and that he didn't account for the different behaviour of different types of magnetic fields in his model.