Suggested future projects

Investigating climate changes over short time intervals: the potential of the Holocene pollen record.

Bas van Geel (vangeel@bio.uva.nl)

Several international projects using pollen data concentrate on time slices, or on sequences of ‘snapshots’ at relatively long time intervals (500 or even 1000 years). However, some of the climate changes during the Late Glacial and Holocene occurred in relatively short periods of time (decennia or maybe even shorter) and such abrupt changes generally will not be detected by these long time interval studies. According to Denton and Karlen (1973), Magny (1993), Kilian et al. (1995) and van Geel et al. (1996) changes in delta 14C, as reconstructed from radiocarbon calibration data, can be used as a climate proxy. During the Holocene there are several abrupt, temporary increases of atmospheric 14C content which seem to correspond to climatic cooling and increased precipitation. Recent studies by Svensmark, Friis-Christensen and Lassen (in press) have revealed that cosmic radiation plays a dominant role in cloud formation; this may explain the observed parallel between increases of delta 14C in the past and rising water levels in lakes, bogs and soils (14C is formed in the atmosphere by the interaction of cosmic rays with nitrogen). Changing solar activity may be the forcing mechanism behind the observed changes because during periods of reduced solar activity the earth’s magnetic field is weaker and more cosmic rays can reach the atmosphere. It is hypothesized that this radiation stimulates cloud formation and precipitation, and at the same time causes increased 14C production. Information about abrupt climate change may be extracted from pollen diagrams (at least those with sufficiently close sampling intervals and enough radiocarbon dates). This potential offers scope for new projects with the aim of exploring possible climate changes in relatively short periods of time that are characterized by sharp increases of atmospheric 14C content. Such investigations could provide new and detailed information about climate change and climatic teleconnections during the Holocene. In this way new steps perhaps can be taken in understanding mechanisms of abrupt climate change in the past and the effect of such changes both upon vegetation and upon prehistoric farming in marginal areas.

References:

Denton, G.H. and Karlen, W., 1974. Holocene climatic variations: their pattern and possible cause. Quaternary Research 3: 155-205.
Kilian, M.R., van der Plicht, J. and van Geel, B., 1995. Dating raised bogs: new aspects of AMS 14C wiggle match dating, a reservoir effect and climate change. Quaternary Science Reviews 14: 959-966.
Magny, M., 1993. Solar influence on Holocene climatic changes illustrated by correlations between past lake-level fluctuations and the atmospheric 14C record. Quaternary Research 40: 1-9.
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., 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.