The reason why birds and compasses can distinguish North from South is that the earth's magnetic field is like a huge magnet, with two poles very close to the earth's axis of rotation. This field is very complex, being associated
However during the last 150 years, the magnetic field has shown signs of decay. At the current rate of decrease, it will completely disappear sometime in the next thousand years! Without the embrace of the magnetic shield, radio communication would deteriorate, navigation by magnetic compass would be difficult and migratory animals might have problems. Man-made satellites would be destroyed by high-energy cosmic and solar particles. Nothing on earth could escape being exposed to deadly radiation.
Fortunately, the disappearance of the earth's magnetic dipole is temporary. It is mainly due to the growth and southward drift of the magnetic north pole. The eventual effect of the deflexion is that compass will point to the South instead of the North; the reversal of the North and South poles.
The magnetic minerals in rocks indicate that magnetic field reversion has taken place hundreds of times on earth during the last 500 million years. However, no methods are currently available to predict precisely when it occurs. About six years ago, a complex computer program was invented to simulate the movements of the Earth's core and its magnetic effect. Now some similar programs can not only simulate the movement but also the magnetic field reversion, which may complete a cycle in 1200 years.
Even though we now appear to be in a period of declining magnetic field strength, the magnetic field will not necessarily reverse. The likelihood of this is only about one in ten thousand. And the reversion of the magnetic poles is not known to have ever directly caused any extinction.(Liu Min)