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_THE IMPACT OF ABRUPT CLIMATE CHANGE AROUND 2650 BP IN NW-EUROPE,
EVIDENCE FOR CLIMATIC TELECONNECTIONS, AND A TENTATIVE EXPLANATION_

_Bas van Geel1 and Hans Renssen2 _

_1 The Netherlands Centre for Geo-ecological Research, University of
Amsterdam, Kruislaan 318, _
_1098 SM Amsterdam, The Netherlands. E-mail: vanGeel at bio.uva.nl_

_2 The Netherlands Centre for Geo-ecological Research, University of
Utrecht, P.O.B. 80115, 3508 TC Utrecht, The Netherlands._

A sudden and sharp rise in the 14C-content of the atmosphere, which
occurred between ca 850 and 760 calendar years BC (ca 2750-2450 BP on
the radiocarbon time scale), was contemporaneous with an abrupt
climate change. In NW-Europe (as indicated by palaeoecological and
geological evidence) climate changed from relatively warm and
continental to oceanic (cooler and wetter). Archaeological and
palaeoecological evidence for the abandonment of low-lying areas at
the Bronze Age/Iron Age transition in The Netherlands is interpreted
as the effect of a rise of the water table and the extension of fens
and bogs. Palaeoecological, geological/ geomorphological and
archaeological evidence for a synchronous climatic change elsewhere in
Europe and on other continents is presented. The discussed oscillation
of 14C at ca 2650 BP was probably caused by a reduced solar activity
which could also have been the forcing mechanism behind the recorded
cool event. An important effect of a reduced solar activity is an
increase in the cosmic-ray flux leading to a higher production of 14C
in the stratosphere and an increased cloudiness and precipitation. The
reduction in solar (ultraviolet) radiation may also have lead to a
decline in ozone production in the lower stratosphere and the latter
process may have been the trigger mechanism responsible for the
inferred climate changes, because a decrease in the stratospheric
ozone content produced cooling of the lower stratosphere by the
absorption of less sunlight. This could have resulted in a decrease of
the latitudinal extension of the Hadley Cell circulation and a
possible associated weakening of monsoons, which would be consistent
with inferred drier conditions in the tropics. Furthermore an
expansion of the Polar Cells and a repositioning of the main
depression tracks at mid-latitudes towards the equator may have
occurred. This probably has resulted in the reconstructed change to
cooler and wetter conditions at middle latitudes in both hemispheres
around 2650 BP.

_References: _

Kilian, M.R., van der Plicht, J. and van Geel, B. (1995) Dating raised
bogs: new aspects of AMS 14C wiggle matching, a reservoir effect and
climatic change, _Quaternary Science Reviews_ 14, 959-966. * Magny, M.
(1993) Un cadre climatique pour les habitats lacustres
prehistoriques?, _Compte Rendu de l'Academie des Sciences Paris_ 316,
Serrie II, 1619-1625. * Svensmark, H. and Friis-Christensen, E. (1997)
Variation of cosmic ray flux and global cloud coverage - a missing
link in solar-climate relationships. _Journal of Atmospheric and
Solar-Terrestrial Physics_ 59, 1225-1232. * van Geel, B. (1978) A
palaeoecological study of Holocene peat bog sections in Germany and
the Netherlands, _Review of Palaeobotany and Palynology_ 25, 1-120. *
van Geel, B. and Mook, W.G. (1989) High-resolution 14C dating of
organic deposits using natural atmospheric 14C variations,
_Radiocarbon_ 31, 151-156. * van Geel, B., Buurman, J. and Waterbolk,
H.T. (1996) Archaeological and palaeoecological indications of an
abrupt climate change in The Netherlands, and evidence for
climatological teleconnections around 2650 BP, _Journal of Quaternary
Science_ 11, 451-460.
___________________________________

BAS van GEEL is a paleo-ecologist in the research group Palynology and
Paleo/Actuo-ecology at the University of Amsterdam, The Netherlands.
He studies pollen and other fossil objects from peat deposits, lake
sediments and archaeological sites and reconstructs changes in
vegetation and climate in the past. Climate change during the Bronze
Age and especially the abrupt climate change at the Bronze Age/Iron
Age transition was already one of the main topics in his Ph.D.- thesis
(1976). Recently he applied the new strategy of 14C wiggle-match
dating of peat deposits, which appeared to be of crucial importance
for understanding the world-wide dramatic changes in climate around
800 BC. Changes in solar activity and related changes in cosmic rays,
14C-production, cloudiness and precipitation were very important
factors for climate change in the past, and such changes in solar
activity probably will also dominate climate change in the future. Bas
van Geel is (co)author of more than 40 papers in international
scientific journals and 6 chapters in books.
_________________________________________________________________

Presented at the SIS Conference: [1]Natural Catastrophes during Bronze
Age Civilisations.

References

1. file://localhost/www/sat/cambconf.htm