Abrupt climate change around 850 cal BC from Central Africa to northern Europe and the role of solar UV
by
Bas van Geel
Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Kruislaan 318, 1098 SM Amsterdam, The Netherlands
Coauthors: Hans Renssen (Faculty of Earth Sciences, Free University Amsterdam), Johannes van der Plicht (Center for Isotope Physics, University of Groningen)

Changing solar activity is a possible factor behind rapid climate shifts. Matching of high resolution sequences of uncalibrated ¹⁴C dates of organic deposits to the dendro-calibration curve can provide a precise core chronology. This method (¹⁴C wiggle matching) reveals relationships between atmospheric ¹⁴C variations and short-term climatic fluctuations caused by solar variations. Shifts to cool and wet climate types in the temperate zones correspond to phases of increasing and high values of the atmospheric radiocarbon content, pointing to a link between changing solar activity and climate change. In the temperate zones the transition from the Subboreal to the Subatlantic (ca 850 calendar years BC) represents a sudden and strong shift from a relatively dry and warm climate to a humid and cool episode. The climate shift is reflected in the plant macrofossil composition of NW European raised bogs, but there is also strong archaeological and dendroclimatological evidence. The climate shift occurs at the start of a sharp rise of the atmospheric ¹⁴C content, caused by a sudden decline of solar activity. Reduced solar wind permitted more cosmic rays to penetrate into the atmosphere, resulting in a higher production of the cosmogenic isotope ¹⁴C. In Cameroon and other sites in the Central African rain forest belt there was a drastic change in the vegetation cover as a consequence dryness after ca 850 cal BC. Shortly afterwards farmers migrated into the area, availing themselves of what was from the human standpoint a regional climatic improvement. A possible palaeoclimatological explanation for the dry-wet transition in the temperate zones, and the contemporaneous wet-dry transition in the tropics is the following: A reduction in solar activity (less solar UV) resulted in a decrease of stratospheric ozone, less absorption of warmth, an equatorward shift and intensification of the mid-latitude storm tracks, a constriction of the latitudinal extent of the Hadley Cell circulation and thus a weaker monsoon. The recorded paleoclimatological teleconnections point to a considerable role of solar UV in climate forcing and may indicate that changing solar wind and related changes of cosmic ray intensity (possibly causing fluctuations of cloud formation) were less important.

Date received: April 30, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cahi-53.