Geomagnetic paleointensity variations can continuously be estimated using marine sediment cores having relatively uniform magnetic properties. The occurrence of large paleointensity fluctuations with periods on the order of 103 to 104 years has been recently established. The ages of paleointensity lows often correspond to those of geomagnetic excursions that have been reported so far. These results have changed our view of the geomagnetic field : it is rather unstable even within periods of constant polarity. Arguments have begun based on paleointensity records as to whether the Earth's orbital parameters and/or paleoclimate modulates the geomagnetic field. Discoveries of the Milankovitch orbital frequencies in paleomagnetic records have been reported, whereas some researchers have postulated that such frequencies are artifact caused by magnetic-property changes in sediments induced by paleoclimatic changes. It is necessary to evaluate the effect of magnetic property changes with a detailed comparison between paleointensity and magnetic properties using sediments that responded differently to paleoclimate changes : for example, between sediments with a magnetic grain-size increase in glacial periods and those with a grain-size decrease during the same period of time. The geomagnetic field produces a shielding effect in the influx of high-energy galactic cosmic rays, and a change in field intensity causes a corresponding change in the production rate of cosmogenic radionuclides. The major part of the variations in the atmospheric 14C/12C ratio during the past 50 kyrs is explained by the paleointensity changes. Studies on high-resolution paleointensity estimation are hence useful for calibration of radiocarbon timescales and a better understanding of global carbon-cycle changes. Recently, a relationship among Solar activity, influx of cosmic-rays, and climatic changes in the order of 101 to 102 years has been discussed intensively. The geomagnetic field might have modulated climatic changes over a longer timescale because its large fluctuation in the order of 103 to 104 years should have caused significant variations in the influx of cosmic rays.
