At present the north-seeking end of a compass points approximately in the direction of the North Pole but paleomagnetic studies have shown that the Earth's magnetic field has been reversed in the past so that the same compass needle would have pointed south. Since the geomagnetic field normally helps to shield the Earth's surface from low-energy radiation, reversals permit a higher incidence of radiation to reach the surface, possibly causing extinctions and mutations. The more highly specialized species would be more vulnerable to such changes. Some bird migration patterns would be severely affected.
It is known that the earth's magnetic field has gone down to zero and reversed itself nine times during the last four million years, with at least 171 reversals in the last 80 million years. Some 45 million years ago the reversal frequency doubled to a rate of 5 reversals per million years although remaining irregular. There is evidence for 5 short-lived reversals in the last 450,000 years. It therefore seems to be impossible to predict when the next reversal will take place although the field intensity is known to have been decreasing over the past century. The polarity inversion apparently occurs over a period of 2,000 years although the field intensity change takes from 15,000-20,000 years.
Around 440 million years ago the land that is now the Sahara was at the South Pole. When continental land masses are located over the poles, huge ice sheets readily form. Massive continental glaciers significantly chill both oceanic currents and the atmospheric circulation, cooling down the whole planet.