Evidence from the past: Solar forcing of climate change by way of cosmic rays and/or by solar UV?

Major Holocene shifts to cool and wet climate types in the temperate zones correspond to suddenly increasing values of the atmospheric 14 content, suggesting a link between changing solar activity and climate change. In the temperate zones the transition from the Subboreal to the Subatlantic (ca 850 cal BC) represents a sudden, strong shift from a relatively dry and warm climate to a humid and cool episode. The moment of change occurred at, or maybe even just before the start of a sharp rise of the atmospheric C-14 content. In previous studies, we postulated two amplification mechanisms: a) increased cosmic ray flux causes an increase in atmospheric C-14 content, and also a climate shift, b) a decline of solar UV causes a reduced stratospheric ozone concentration, leading to climate change at the earth surface. Two phenomena indicate that mechanism a) is much less likely than mechanism b): 1) The enhancement of cosmic ray intensity to relatively high levels took place several decades after the climate shift. 2) In Central Africa and in Western India there was a shift to dryness. Chronological differentiation in solar output may play a role, but this is purely hypothetical.