Earth's magnetic field — generated by the movement of molten iron in the outer core — protects the surface from solar wind and cosmic radiation. During a geomagnetic reversal, the field weakens significantly (potentially to 10–20% of current strength) over thousands of years before re-establishing with reversed polarity. The last reversal, the Brunhes–Matuyama event, occurred approximately 780,000 years ago.
During the transition period, reduced magnetic shielding increases radiation exposure at ground level, damages the ozone layer, increases cancer rates, and — critically for modern civilisation — makes satellite and electrical infrastructure far more vulnerable to solar events that would otherwise be deflected.
The threat is often overstated in popular media — pole reversals are slow and species have survived them before. However, the combination of weakened shielding with modern infrastructure dependency (satellites, GPS, power grids) means the civilisational impact today would be far greater than in any previous reversal.