Millennial-scale solar variability in tree rings of northern Fennoscandia at the end of the Holocene

There is growing evidence that the Sun is one of the main drivers of the Earth’s climate in the past. To investigate the possible Sun-climate connection during the Holocene, the super-long tree-ring chronology covering the period from 5634 BC and 2004 AD is analyzed. The chronology used, consists of more than 1200 series including ones from sedimentary archives of small lakes in the region of northern Fennoscandia (68º- 70ºN, 20º - 30ºE). All series are dendrochronologically dated to calendar years. As indicator of solar activity we use a reconstruction of total solar irradiance (TSI) covering 9300 years which is based on a composite using the cosmogenic radionuclide Be^10 measured in ice cores GRIP (Greenland) and South Pole (Antarctic), and also on neutron monitor data (Steinhilber et al. 2009). To examine the relationship between tree’s growth and solar activity, the MTM, wavelet decomposition and wavelet coherence analysis were applied to the time series. MTM spectral analysis identifies in tree-ring series the main solar cycles at ~ 200 yr (de Vries or Suess) ~350 yr (unnamed) and ~ 900 yr (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 ( ~ 1300 years) frequencies since around AD 200 till now. The millennial-cycle components are modulated by a ~ 6500-year period of solar or geomagnetic origin. Our results raise questions on the subject regarding the end of the Holocene and transition to the next glacial period. In any case, the results presented confirm the complex and nonlinear nature of the Sun-climate relationship during the Holocene epoch.