Millennial-Scale Solar Variability in Tree Rings of Northern Fennoscandia at the End of the Holocene

Abstract

To investigate the possible Sun–climate connection during the Holocene, the Finnish super-long tree-ring chronology covering the period from 5634 B.C. to A.D. 2004 was analyzed. As an indicator of solar activity, we used a reconstruction of total solar irradiance (TSI) covering 9300 years, which is based on a composite using the cosmogenic radionuclide 10Be measured in polar ice cores, and also on neutron monitor data (Steinhilber et al. 2009). The Multiple Taper Method (MTM) wavelet decomposition and wavelet coherence analyses were applied to the time-series. The MTM spectral analysis identifies the main solar cycles at ca. 200 yr (de Vries or Suess), ca. 350 yr (unnamed) and ca. 900 years (Eddy). The strongest cross-wavelet correlation was discovered between the millennium-cycle components of TSI and tree-ring width variations. This Eddy cycle, which was recently discovered in solar activity, remains both strong and stable through almost the entire Holocene, and it reappears again at lower frequencies (ca. 1300 years) after ca. A.D. 200. Our results raise questions regarding the end of the Holocene and transition to the next glacial period and confirm the complex and nonlinear nature of the Sun–climate relationship during the Holocene Epoch.