In the past 20 million years the magnetic poles of the Earth have reversed as many as 100 times. These reversals can take up to 1,000 years to settle, and leave traces in the rocks that allow scientists to see when they occured. Sediment cores are taken from up to 300 meters deep in the ocean floor which contain varying amounts of cosmogenic isotopes, which indicate increased cosmic ray interaction with atmospheric atoms at certain points.
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When a polarity reversal happens, the magnetic field weakens by  90% until a new configuration is established. During this time, the Earth is bombarded with high-energy cosmic due to the the lack of a protective magnetic shield, where they collide with nitrogen and oxygen in the atmosphere.

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Our last pole shift happened around 770,000 years ago, but there have been 15 excursion events since then, where  field strength drops significantly but does not reach the required threshold for a reversal and rises back to normal levels. It has been 40,000 years since the last excursion event, but scientist warn that the next one could be soon. These excursions pose a significant risk while the magnetic field weakens over centuries. 

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Geoscientists at CEREG in Aix-en-Provence, France explained in a recent phys.org article that Earth's magnetic field has been in decline for almost 3,000 years and could potentially spiral to critical levels within a century. Of course, this is more unlikely than not to happen so soon, but a little alarmism is good for raising research money, and this a particular cause that I am not opposed to more examination.