EVALUATION OF COUPLED MODEL SIMULATIONS OF NORTHERN HEMISPHERE EXTRATROPICAL CLIMATES IN THE MID-HOLOCENE

We have used the BIOME4 biogeography-biochemistry model and comparison with palaeovegetation data to evaluate the response of 9 fully-coupled ocean-atmosphere general circulation models (OAGCMs) to mid-Holocene changes in orbital forcing in the mid- to high-latitudes of the northern hemisphere. All of the models produce a:

· northward shift in the position of the northern limit of boreal forest, in response to simulated summer warming in the high-northern latitudes;

· northward expansion of temperate forest belts in North America, in response to simulated winter warming; and

· expansion of xerophytic vegetation in mid-continental North America and Eurasia, in response to reduced precipitation during the growing season.

The northward shift of the northern limit of the boreal forest and the northward expansion of temperate forests in North America are supported by the palaeovegetation data. The simulated expansion of xerophytic vegetation in Eurasia is not supported by the data. The expansion of xerophytic vegetation in mid-continental North America is consistent with palaeoenvironmental evidence, although the extent of this expansion may be over-estimated. Comparison of these results with atmosphere-only simulations, suggest that the incorporation of ocean feedback may improve the simulation of some aspects of the observed mid-Holocene climate changes, but degrades the simulation of other features of the climate. Thus ocean feedback produces a more realistic simulation of the arctic warming which controls the position of the tundra-taiga boundary, but produces less realistic conditions in mid-continental Eurasia. The results from an asynchronous coupling of one of the models to an equilibrium vegetation model suggests vegetation feedback exacerbates the mid-continental drying in Eurasia and thus produces conditions even 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 in some cases much larger than, the northward shift shown by the data) and others produce a reduction in temperate forests. Elucidation of the cause of such differences will be a major focus of the next phase of the PMIP.