Credit: Nelson Parker The last complete reversal of the Earth's magnetic field, the so-called Laschamps event, took place 42,000 years ago. Radiocarbon analyses of the remains of kauri trees from New Zealand now make it possible for the first time to precisely time and analyse this event and its associated effects, as well as to calibrate geological archives such as sediment and ice cores from this period. Simulations based on this show that the strong reduction of the magnetic field had considerable effects in the Earth's atmosphere. This is shown by an international team led by Chris Turney from the Australian University of New South Wales, with the participation of Norbert Nowaczyk from the German Research Centre for Geosciences Potsdam and Florian Adolphi from the Alfred Wegener Institute, in a study that now appears in the journal