Q: Evolutionists, how do you explain the decaying of the magnetic field?

Details: In the seventies creationist, Dr. Thomas Barnes, realized that the magnetic field has been measured to decay at 5% per century since 1835. archaeologists have also been able to prove that the magnetic field was 40 percent stronger in AD 1000 than today. At this rate, the magnetic field could not have been decaying for more than ten thousand years, or its strength would have been large enough to melt the earth. Now you may say that like the sun... the earths magnetic field must have reversed at some point but, if this did happen to earth it would only speed up the decaying process. However, there is no reason to believe the earth's magnetic field has reversed. It does happen to the sun every 11 years but the sun is a huge ball of hot, moving, gas unlike earth AND the energy of the sun is decreasing despite the reversal of the magnetic field. So how can you say that the earth is still many billion years old evolutionists?