Large variations in Holocene solar activity: Constraints from 10Be in the Greenland Ice Core Project ice core

Cosmogenic radionuclides extracted from ice cores hold a unique potential for reconstructing past solar activity changes beyond the direct instrumental period. Taking the geomagnetic modulation into account, the solar activity in terms of the heliospheric modulation function can quantitatively be reconstructed in high resolution throughout the Holocene. For this period our results reveal changes in heliospheric modulation of galactic cosmic rays significantly larger than the variations reconstructed on the basis of neutron monitor measurements of galactic cosmic rays for the last 50 years. Moreover, the 10 Be data from the Greenland Ice Core Project ice core as well as 14 C support a high current solar activity. However, although the reconstruction of solar activity on long timescales is difficult, our result suggests that the modern activity state of the Sun is not that exceptional regarding the entire Holocene. This extended solar activity record provides the basis for further detailed investigations on solar and cosmic ray physics, as well as on solar forcing of the Earth's climate whose importance is suggested by increasing paleoclimatic evidences.