Evaluation of coupled ocean–atmosphere simulations of the mid-Holocene using palaeovegetation data from the northern hemisphere extratropics

We have used the BIOME4 biogeography– biochemistry model and comparison with palaeovegetation data to evaluate the response of six ocean–atmosphere general circulation models to mid-Holocene changes in orbital forcing in the midto high-latitudes of the northern hemisphere. All the models produce: (a) a northward shift of the northern limit of boreal forest, in response to simulated summer warming in high-latitudes. The northward shift is markedly asymmetric, with larger shifts in Eurasia than in North America; (b) an expansion of xerophytic vegetation in mid-continental North America and Eurasia, in response to increased temperatures during the growing season; (c) a northward expansion of temperate forests in eastern North America, in response to simulated winter warming. The northward shift of the northern limit of boreal forest and the northward expansion of temperate forests in North America are supported by palaeovegetation data. The expansion of xerophytic vegetation in mid-continental North America is consistent with palaeodata, although the extent may be over-estimated. The simulated expansion of xerophytic vegetation in Eurasia is not supported by the data. Analysis of an asynchronous coupling of one model to an equilibriumvegetation model suggests vegetation feedback exacerbates this mid-continental drying and produces conditions more unlike the observations. Not all features of the simulations are robust: some models produce winter warming over Europe while others produce winter cooling. As a result, some models show a northward shift of temperate forests (consistent with, though less marked than, the expansion shown by data) and others produce a reduction in temperate forests. Elucidation of the cause of such differences is a focus of the current phase of the Palaeoclimate Modelling Intercomparison Project. J. Wohlfahrt S. P. Harrison (&) School of Geographical Sciences, University of Bristol, University Road, Bristol BS8 1SS, UK e-mail: [email protected] J. Wohlfahrt S. P. Harrison Max Planck Institute for Biogeochemistry, PO Box 10 01 64, Jena 07701, Germany e-mail: [email protected] P. Braconnot Lab. CNRS-CEA, Lab. des Sciences du Climat et de l‘Environnement, D.S.M./Orme des Merisiers/Bat. 709, CEA/Saclay, CEA/Saclay Gif sur Yvette 91191, France e-mail: [email protected] C. D. Hewitt Hadley Centre for Climate Prediction and Research, Met Office, Fitzroy Road, Exeter, Devon EX1 3PB, UK e-mail: [email protected] A. Kitoh Climate Research Department, Meteorological Research Institute, Nagamine 1-1, Tsukuba, Ibaraki 305, Japan e-mail: [email protected] U. Mikolajewicz Max-Planck-Institute for Meteorologie, Bundesstr. 55, Hamburg 20146, Germany e-mail: [email protected] B. L. Otto-Bliesner Climate and Global Dynamics Division, National Control for Atmospheric Research, Boulder, CO 80307, USA e-mail: [email protected] S. L. Weber Royal Netherlands Meteorological Institute, P.O. Box 201, DeBilt 3730 AE, Netherlands e-mail: [email protected] 123 Clim Dyn (2008) 31:871–890 DOI 10.1007/s00382-008-0415-5