Richard C. Brill, an associate professor of geology and geophysics at Honolulu Community College in Hawaii, provides the following answer:
Image: National Geophysical Data Center

RESTLESS FIELDS. Computer models indicate that the strength of Earth's magnetic field declined about 10 percent over the past century.

The Earth's magnetic field is constantly changing, and the way which it changes also changes. When describing the magnetic field of the Earth we must specify both the direction and the intensity of the field. Since both of these change, and change differently in different places, it is not easy to say how the field of the Earth as a whole is changing. It must be measured at many places to get a good picture of its distribution.

Because of the complexity of the Earth's magnetic field and the constant changes it is impossible to predict what the field will be like anytime in the distant future. Using data from many magnetic observatories, geophysicists can make mathematical representations of the field and how it is changing. These models must be constantly revised as new data is added from many observations.

Although magnetic reversals have occurred many times in the geologic past, it is not yet possible to predict when the next reversal will occur. The most recent reversal was about 700,000 years ago. In the past 200 million years reversals have taken place every half million years or so, but with no discernible regularity or pattern.

Image: National Geophysical Data Center
WORLDWIDE NETWORK of geomagnetic observatories monitors changes in Earth's magnetic field.

At most places there has been a general decrease in the strength over the past century, typically ten percent or so. No one can say with any certainty whether this represents a fluctuation or whether it is a decrease which will eventually lead to a reversal. Past reversals have taken place over a short period of time geologically speaking, 10,000 years or so. In order for a reversal to take place there must be a brief time during which the field is non existent.

It is not easy to predict the effects of the decrease. The Earth's magnetic field extends outward from the Earth forming a shield which focuses cosmic rays towards the magnetic poles and away from other latitudes. In the absence of this magnetic field, or magnetosphere, these cosmic rays would bombard the entire Earth with a higher intensity that today. The effects of cosmic rays on life in general are not known for sure, but it is expected that they would cause tissue damage similar to the effects of x-rays.

Some scientists theorize that some, if not all of the mass extinctions that have wiped out thousands of species of life on Earth might be correlated with magnetic reversals. There have been so many reversals that it is not likely that all have caused the demise of a large number of species.

No one really knows how important magnetism is to life in general. Many migrating animals such as birds, whales, and butterflies use magnetism for navigation. Other organisms have small areas in which the magnetic mineral known as magnetite is concentrated. There is some evidence that our own brains may contain grains of magnetite.

When the next reversal occurs, the several generations of scientists who study it will know much more about the effects, and our research may eventually learn more about the causes of changes and reversals of Earth's magnetic field.