Study of the geomagnetic variation and its relation to climate change

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We studied the variation of the intensity of the Earth's magnetic field for a wide range of time scale, and examined its relation to climate change. For the paleointensities of the Archean to Proterozoic period, we collected samples from the stable cratons in Canada, Australia, and Africa and the paleointensity measurements were made by the Thellier's method. The results indicate that the average value of paleointensity during the last 2 billion years are rather similar to the geomagnetic intensity of the present field, whereas the variation of the intensity around 2.5 billion years ago was found to be very large. During the last 200,000 years, several transient increases or decreases of the intensity were observed. The variations are attributed to the change of the dipole moment. The decreases of the intensity to about 1/10 of the present field corresponds to the excursion time of the magnetic field, and are known to relate to climate change. On the other hand, the relation between the transient increases of the intensity and the climate change is not evident. For the last 2000 years, we obtained a standard curve of the field intensity variation in Japan. The variation of the field intensity for this time scale is mainly controlled by the change of the non- dipole component of the geomagnetic field, although an abrupt increase of the field intensity at about 500 AC is probably due to the change of the dipole moment. As for the recent variation of the field intensity, we developed a new method for decomposing the observed magnetic field into the external, induced, and internal component of core origin by using spherical harmonic analysis, and short period variations of core field with a time scale of several years are identified.