^ yes, earth's last magnetic field change was around 42,0000 years ago. The magnetic field diverts a lot of solar radiation, which otherwise would have a "bit of an impact" on life.
Other than accelerating the earth's mean temperature by raising the CO2 level in the atmosphere, in this case mankind has nothing to do with it/cannot do anything against it (other than is this film "The Core"..)
This planet acts like a giant dynamo, the iron core moving with another speed and direction than the crust, thus generating a magnetic field.