HOLIVAR2006 Abstracts
Holocene climate change on decadal to centennial time scales and the potential role of solar variability and other forcing factors.
Bas van Geel
Institute for Biodiversity and Ecosystem Dynamics, Universiteit van Amsterdam, Kruislaan 318, 1098 SM Amsterdam, Netherlands
Contact: Bas van Geel (vanGeel@science.uva.nl)
Tree-ring-based temperature reconstructions show cooling effects of large explosive volcanic eruptions. Freshwater outbursts from proglacial lakes due to the final steps of deglaciation caused stagnation of the thermohaline circulation and cooling phases in the North Atlantic, but changing solar activity also had effects on the Early Holocene climatic conditions. Glaciers and lake levels are sensitive to climatic variations. Reconstructions of lake level changes and glacier records in west-central Europe show corresponding fluctuations, caused by major hydrological changes in response to Holocene climate cooling phases over mid-European latitudes. Glacier advances coincided with phases of higher lake levels. Variations in solar activity - as reflected by the record of 14C and 10Be cosmogenic isotopes - were a major driving factor behind these climate oscillations. Stalagmites and raised bog deposits are additional sources of information in the study of climate change.
The classical transition from the dry and warm Sub-Boreal to the cool, moist Sub-Atlantic around 850 calendar years BC appeared to be linked to a temporary decline of solar activity. The effects of the climate shift for (a) Late Bronze Age people living in moist conditions in the Netherlands and (b) the expansion of the Scythian culture in dry southern Siberian areas will be discussed. The hypersensitivity of the climate system to relatively small changes in solar activity points to the existence of amplification mechanisms in the atmosphere. The lack of accurate knowledge about such mechanisms still hampers balanced evaluation and modelling of present and future climate. The role of the sun - in relation to the effects of greenhouse gasses - might still be underestimated in climate models.
Bas van Geel graduated from the University of Amsterdam in 1976 with a PhD focusing on raised bog (peat) deposits as archives of vegetation history and climate change during the Holocene. He then worked for three years with archaeologists at the Amsterdam Archaeological Centre. He is presently Associate Professor (Reader) at the Department of Palaeoecology and Landscape Ecology in the Institute for Biodiversity and Ecosystem Dynamics, Universiteit van Amsterdam. Studies by Bas van Geel and colleagues have focused on the links between palaeoecology, palaeoclimate, environmental archaeology, and isotope physics. With J. van der Plicht (Groningen University) they have made significant progress in understanding the role of the sun as a factor in climate change. Based on the available evidence, he has concluded that the sun is an underestimated and understated factor in driving climate change. He is member of several committees focusing on the study of climate change (HOLIVAR; Dutch CLIVAR).