INTERCOMPARISON OF SIMULATED GLOBAL VEGETATION DISTRIBUTIONS IN RESPONSE TO 6 KYR BP ORBITAL FORCING

The response of ten atmospheric general circulation models to orbital forcing at 6 kyr BP has been investigated using the BIOME model, which predicts equilibrium vegetation distribution, as a diagnostic. Severa l common features emerge: (a) reduced tropical rain forest as a conseq uence of increased aridity in the equatorial zone, (b) expansion of mo isture-demanding vegetation in the Old World subtropics as a consequen ce of the expansion of the Afro-Asian monsoon, (c) an increase in warm grass/shrub in the Northern Hemisphere continental interiors in respo nse to warming and enhanced aridity, and (d) a northward shift in the tundra-forest boundary in response to a warmer growing season at high northern latitudes. These broadscale features are consistent from mode l to model, but there are differences in their expression at a regiona l scale. Vegetation changes associated with monsoon enhancement and hi gh-latitude summer warming are consistent with palaeoenvironmental obs ervations, but the simulated shifts in vegetation belts are too small in both cases. Vegetation changes due to warmer and more arid conditio ns in the midcontinents of the Northern Hemisphere are consistent with palaeoenvironmental data from North America, but data from Eurasia su ggests conditions were wetter at 6 kyr BP than today. The models show quantitatively similar vegetation changes in the intertropical zone, a nd in the northern and southern extratropics. The small differences am ong models in the magnitude of the global vegetation response are not related to differences in global or zonal climate averages, but reflec t differences in simulated regional features. Regional-scale analyses will therefore be necessary to identify the underlying causes of such differences among models.