On coupling global biome models with climate models

Abstract

The BIOME model of Prentice et al. (1992; J. Biogeogr. 19: 117-134), which predicts global vegetation patterns in equilibrium with climate, was coupled with the ECHAM climate model of the Max-Planck-Institut fiir Meteorologie, Hamburg, Germany. It was found that incorporation of the BIOME model into ECHAM, regardless at which frequency, does not enhance the simulated climate variability, expressed in terms of differences between global vegetation patterns. Strongest changes are seen only between the initial biome distribution and the biome distribution computed after the first simulation period, provided that the climate-biome model is started from a biome distribution that resembles the present-day distribution. After the first simulation period, there is no significant shrinking, expanding, or shifting of biomes. Likewise, no trend is seen in global averages of land-surface parameters and climate variables. Significant differences in the results of the climate-biome model are found when single-year and multl-year climatologies are compared regardless whether climate and biome model are used in an off-line mode or are interactively integrated. It is concluded that a biome model should be coupled with a climate model in the following way: firstly, the climate model should be integrated over several years; secondly, a biome distribution should be computed from the corresponding multi-year simulated climatology; finally, land-surface parameters are to be deduced from the biome distribution as boundary condition of the climate model for a subsequent integration, and so on until an equilibrium is established. Starting the climate-biome model from a biome map which drastically differs from today's global distribution of biomes, but keeping present-day ocean temperatures fixed, it takes several iterations until the model finds a new equilibrium differing from the present-day vegetation distribution in certain parts of the globe. This study indicates that the results of the climatebiome model are dependent on the initial land-surface conditions.