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The last major change in the field took place some 780,000 years ago during a magnetic reversal, although such reversals seem to occur more often on average. A flip in the north and south poles typically involves a weakening in the magnetic field, followed by a period of rapid recovery and reorganization of opposite polarity.
Some studies in recent years have suggested the next reversal might be imminent, but the jury is out on that question.
The Earth's overall magnetic field has weakened at least 10 percent over the past 150 years, which could also point to an upcoming field reversal.