Quantifying the impact of global climate change on potential natural vegetation

Impacts of climate change on vegetation are often summarized in biome maps, representing the potential natural vegetation class for each cell of a gri d under current and changed climate. The amount of change between two biome maps is usually measured by the fraction of cells that change class, or by the kappa statistic. Neither measure takes account of varying structural a nd Aoristic dissimilarity among biomes. An attribute-based measure of dissi milarity (Delta V) between vegetation classes is therefore introduced. Delt a V is based on (a) the relative importance of different plant life forms ( e.g, tree, grass) in each class, and (b) a series of attributes (e.g. everg reen-deciduous, tropical-nontropical) of each life form with a weight for e ach attribute. Delta V is implemented here for the most used biome model, B IOME 1 (Prentice, I. C. et al., 1992). Multidimensional scaling of pairwise Delta V values verifies that the suggested importance values and attribute weights lead to a reasonable pattern of dissimilarities among biomes. Diss imilarity between two maps (Delta V) is obtained by area-weighted averaging of Delta V over the model grid. Using Delta V, present global biome distri bution from climatology is compared with anomaly-based scenarios for a doub ling of atmospheric CO2 concentration (2 x CO2), and for extreme glacial an d interglacial conditions. All scenarios are obtained from equilibrium simu lations with an atmospheric general circulation model coupled to a mixed-la yer ocean model. The 2 x CO2 simulations are the widely used OSU and GFDL r uns from the 1980's, representing models with low and high climate sensitiv ity, respectively. The palaeoclimate simulations were made with CCM1, with sensitivity similar to GFDL. Delta V values for the comparisons of 2 x CO2 with present climate are similar to values for the comparisons of the last interglacial and mid-Holocene with present climate. However, the two simula ted 2 x CO2 cases are much more like each other than they are to the simula ted interglacial cases. The largest Delta V values were between the last gl acial maximum and all other cases, including the present. These examples il lustrate the potential of Delta V in comparing the impacts of different cli mate change scenarios, and the possibility of calibrating climate change im pacts against a palaeoclimatic benchmark.