Is the Earth's magnetic field changing?

Indeed it is. It currently has a strength at the Earth's surface of 0.6 Gauss. But long term observations show that it is DECREASING at a rate of about 0.07 percent PER YEAR. This means that in 1500 years from now, it will only be about 35 percent as strong as it is today, and in 4000 years it will have a strength of practically zero. Because the magnetic field is a critical buffer shielding the biosphere from cosmic rays and charged particles from the Sun, the health hazard posed by the Earth loosing its magnetic field is hard to comprehend.

The geologic record, however, shows that the Earth's polarity flips every 250,000 years or so and that presumably half way through each 'flip' cycle, the field goes to a value near zero. There are no identifiable fossil effects from previous flips, so perhaps they are not as mutagenic as we might suppose. On the other hand, perhaps other species are less susceptible to genetic damage than we humans! We know that many species of bacteria, birds and fish can sense the direction of the magnetic field and use it for migration and finding food in murky waters. These animals may perish 'suddenly' when the field changes and they can no longer rely on it to orient themselves. We have never experienced loosing our magnetic field, and the fossil record is pretty mute about the effects. No one really knows if the condition of zero-field will last 5 years, 10 years or 500 years. Stay tuned!

If you want to read more about the details of how the magnetic field has changed during the last 800,000 years, have a look at the classroom material at Magnetic Reversals and Variation at the IMAGE web site.

If you want to learn more about what happens during a magnetic reversal, read the IMAGE satelite article Magnetic Reversals: Back to the future? which discusses all of the details and likely impacts.