In 2011, scientists announced that the magnetic north pole was moving much faster than expected, changing position at a rate of 40 miles per year, and now recent measurements suggests that the movement may be accelerating. Magnetic pole shifts happen on average every 300,000 years. The last one took place 780,000 years ago, so the next one is long overdue. What happens is that the earth's magnetic field goes into a chaotic state, which persists for between 1,000 and 10,000 years. During the first stages, the strength of the magnetic field declines, and it is this period that will pose the greatest danger to earthly life-forms, because the planet's surface will be directly exposed to solar radiation which is normally shielded by the field. However, in the past, magnetic pole shifts have not been associated with mass extinctions, or with geologic upheavals. When the field is in flux, there will be major disruptions to such things as navigation, and migratory birds and animals that rely on Earth's magnetic field to find their routes will be compromised. Solar coronal mass ejections taking place while the field is weak would be a matter for serious concern, but the field never disappears entirely, and Earth's atmosphere also provides some shielding. A major CME striking Earth during the early phases of a reversal would be a crisis situation, however, and require careful management.
It is not known if the field slips into flux slowly, or if it reaches a 'tipping point' and suddenly becomes chaotic. The majority of scientific opinion is that it happens slowly, with magnetic north moving to a latitude about 40 degrees off true north before the field becomes so distorted that it loses its form. Even after that happens, it doesn't disappear entirely, but becomes weaker, with poles appearing at random in different places. The movement of the magnetic field is related to movement in the earth's core, but a new study suggests that its sudden and increasing acceleration is due to the melting of Greenland's ice cover. Reversals can start and then be aborted, as happened 10,000 years ago.