- The Earth could be about to turn upside down. The planet's
magnetic field is showing signs of wanting to make a gigantic somersault,
so that magnetic north heads towards Antarctica, and magnetic south goes
north. Compasses will point the wrong way, and migrating birds, fish and
turtles are going to be very confused.
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- Just when this will happen, how long it will take and
what the consequences will be, is difficult to fathom. What is not in doubt,
though, is that it will happen. About every half a million years or so,
the Earth's magnetic field flips upside down.
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- The story begins in 1600, when Sir William Gilbert, physician
to Queen Elizabeth I, suggested that the Earth was a giant magnet. At the
magnetic poles, a compass needle would stand up and point straight down
into the Earth. And he was right, up to a point. The magnetic poles are
where all the lines of force of Earth's magnetic field are drawn together.
It does not coincide with the geographic poles, the axis on which the Earth
spins, but it is close.
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- Yet the Earth is not a solid magnet. For one thing, its
magnetic poles are constantly drifting around. At present, magnetic north
is heading out of Canadian territory into the Arctic Ocean at about 10
miles per year. Also, a bar magnet quickly loses its power, yet the Earth's
magnetic field has been around for billions of years, so something is regenerating
it. This is why Einstein remarked that the origin of the Earth's magnetic
field was one of the greatest mysteries of physics.
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- Today, we think that magnetic power comes from the Earth's
hot outer shell of molten iron sloshing around a solid inner core. As this
subterranean ocean of liquid metal slowly whirls around, it behaves like
a dynamo generating electrical currents and magnetic fields. Just like
the flickering light on a bicycle powered by a dynamo, the Earth's currents
are a little erratic, and so the magnetic field at the surface of the Earth
fluctuates. We know the magnetic polarity goes topsy-turvy from rocks on
the bed of the Atlantic Ocean.
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- Along the middle of the Atlantic runs a gigantic crack
from which lava oozes. As the lava solidifies into rock, it records the
Earth's magnetic polarity at the time. These records show that we are due
for another flip about now. But the Earth does not keep a regular rhythm,
so no one could make a prediction based on past performance alone. There
is, however, more convincing proof that we are heading for a tumble. Each
time the magnetic field heads for a reversal, it grows weaker over several
thousand years until it almost disappears. Then it switches and starts
up again with renewed vigour.
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- Magnetism trapped in ancient pottery shows that over
the past 4,000 years, the magnetic field has weakened by more than 50%.
This past century, the strength has dropped by 5%. At this rate, the field
might disappear in the next few hundred or thousand years. Another warning
sign of an imminent flip has come recently from satellite measurements
of the Earth's magnetic field.
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- A team led by Gauthier Hulot, of the Institut de Physique
du Globe de Paris, has spotted patches of reversed magnetism concentrated
in two places just underneath the Earth's outer mantle. In the largest
patch, beneath the southern tip of Africa, the magnetic field is pointing
towards the centre of the Earth, instead of outwards. The other patch is
near the north pole.
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- Some experts have stuck their necks out to predict that
we can expect the next reversal some time in the next 2,000 years. The
process would probably then take anything between 100 and a few thousand
years - not even a blink in the history of Earth. We can only guess what
life would be like during that reversal. Anyone trying to navigate with
a magnetic compass is going to have a tough time, but what is going to
happen to all those birds, fish and other animals that migrate vast distances
using their own internal magnetic compass? Will they have time to re-draw
their magnetic maps and get new bearings?
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- Even more creatures such as bees and some bacteria use
a sense of magnetism for finding their way around their local territories,
for a north/south or up/down axis. The Earth's magnetic field also stretches
several hundred miles into space and protects us from the sun's charged
particles and cosmic rays by focusing them towards the poles. This is where
they appear as the northern and southern lights as they excite gases in
the atmosphere. As the magnetic poles migrate across the world, those night
lights are going to light up some very strange places where they have never
been seen before.
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- During a field reversal, this protective magnetic shield
is going to be weak and might even disappear for a century or more. That
might drastically affect the weather. There is a growing body of evidence
that the sun's highly charged particles batter the upper atmosphere so
hard that some of the assault filters down into the atmosphere around us,
influencing the wind, atmospheric pressure and temperature.
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- Without our magnetic shield, those solar particles might
create havoc with the weather. That cosmic radiation blasting the Earth's
surface could cause genetic mutations and cancers. Yet when palaeontologists
scoured the fossil records looking for signs of mass extinctions or bursts
of evolution during previous magnetic field flips, they found nothing.
Living organisms seem to have survived intact. But what will happen next
time?
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- http://www.guardian.co.uk/science/story/0,3605,748510,00.html
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