Earth's magnetic field is weakening, may flip within the next 1000 years. This happens occasionally, and it's a bit of a problem in that we'll be barraged by intense radioactive bursts.
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'These solar particles can have profound effects,' said Dr Paul Murdin, of the Institute of Astronomy, Cambridge. 'On Mars, when its magnetic field failed permanently billions of years ago, it led to its atmosphere being boiled off. On Earth, it will heat up the upper atmosphere and send ripples round the world with enormous, unpredictable effects on the climate.'