Climate in northern Eurasia 6000 years ago reconstructed from pollen data
Introduction
In order to accurately predict future changes in the climate, robust models are required. As the past has experienced major climatic changes, one of the best ways to test and improve the forecasting power of climate models is to compare model-simulated conditions for a particular past time period, with the conditions estimated from geological data. The last 20 ka cover a wide variety of climates against which atmospheric general circulation models (AGCMs) can be tested [1]. Within this time period, the mid-Holocene (6 ka) represents a key time-slice. It has been used by modellers to test whether solar forcing has a direct effect on the Earth's climate system [2]. At that time, ice sheets were reduced to their modern extent, the mid- and high-latitude summer insolation were notably higher than today and the winter insolation lower than today.
Here we present a quantitative climatic reconstruction for northern Eurasia at 6 ka based on pollen data. The aim of this paper is three fold: (1) to test the new `PFT method' [3]on the latest pollen dataset from northern Eurasia; (2) to compare the results with earlier continental-scale climate reconstructions based on different statistical and non-statistical methods 4, 5; and (3) to discuss the reconstructed climate patterns in comparison with some AGCM simulations.
The method of climate reconstruction [3]used in the present study uses the concept of `biomization' [6], which attributes the appropriate biome to any given pollen spectra, by grouping the pollen taxa into plant functional types (PFTs — the broad classes of plants selected by their ecology, leaf morphology and bioclimatic tolerance [7]). The method [3]uses PFT scores instead of the pollen taxa abundances used in the traditional best modern analogue method [8]. The `PFT method' has already been successfully applied to the scarce pollen data of the Last Glacial Maximum from southern Europe [3]and northern Eurasia [9]. These studies demonstrate robust and homogeneous results of climate reconstruction, proving the hypothesis [7]that groups of taxa have a better-defined response to climate than individual taxa, whose responses to climatic change may be obscured by competitive and migrational processes.
Section snippets
Pollen and climate data
A set of 1245 modern surface pollen spectra covering northern Eurasia and western North America has already been used by Tarasov et al. [9]to establish the relationships between the numerical scores of the PFTs and the modern climate at the sampling sites. The present-day climate was calculated at each site by weighted distance interpolation [10]of values taken from the present-day climate database [11].
In the present study we have reconstructed four bioclimatic variables considered as
Results
The biome distribution in northern Eurasia at 6 ka was previously reconstructed by Tarasov et al. [12], using the standard method of biome reconstruction [6]. The improvement here was the ability to define steppe biome as cool steppe and desert as cool desert (Fig. 1a), using a modification of the biomization method [14]. Cool and warm steppes (and deserts) require different climatic conditions and the ability to determine them from pollen data is a valuable contribution to the validation of
Discussion and conclusions
Reconstructed values of 6 ka climate show spatially coherent and generally significant patterns. The large error bars obtained with the applied `PFT method' make the reconstructions partly non-significant (Fig. 3), suggesting that the method is less precise than the best modern analogues method. However, this is not true, as the small error ranges of the analogues method are often underestimated [5].
The results of Tc, Tw, GDD5 and α reconstruction are consistent with the qualitative
Acknowledgements
PT thanks the Délégation aux Relations Internationales et à la Coopération (Ministère de l'Education Nationale, de l'Enseignement Supérieur, et de la Recherche) for the financial support. The present work is a contribution to the international PMIP project and has been funded by EEC contract ENV4-CT95-0075 and by the French PNEDC (projet `Climat glaciaire: extrême et variabilité; approche conjointe modèles-données'). PT compiled the 6 ka pollen data used in this study within the framework of
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2015, Quaternary Research (United States)Citation Excerpt :However, in general, the regional differences in the importance of climate variables are an underappreciated issue in continental-scale climate reconstructions. In many studies the large-scale modern pollen datasets have been applied to a high number of fossil records for reconstructing the same climate variables over continents or globally (Tarasov et al., 1999; Davis et al., 2003; Viau et al., 2006; Bartlein et al., 2011). It is likely that in such studies the results may be less reliable in regions where the variables of interest are not among the most significant regional variables.