Summary

In order to understand the global relevance of basic ecological and physiological processes, their role in biogeochemistry and climate, and the impacts of human-induced changes in the Earth's biosphere, terrestrial net primary productivity (NPP) must be studied globally. Global terrestrial biosphere models (TBMs) aim to understand and eventually predict the cycling of the major elements (C, N) within vegetation and soils, as well as their fluxes with other reservoirs. TBMs can calculate the global fluxes of CO₂ and H₂O between ecosystems and the atmosphere, two key components of the climate system. Because the spatial aggregation of existing and planned in situ flux measurements does not allow full global coverage, and given the (large) heterogeneity of most land ecosystems, TBMs are an important technique for extrapolating our understanding of terrestrial processes to the regional and global level. Alternatively, one can use atmospheric measurements together with global atmospheric transport models (TMs) in order to attempt to infer indirectly the distribution of the terrestrial fluxes. This chapter is subdivided into three parts. First, we make an overall presentation of the different types of TBMs and of their range of application. Second, we review how the major processes that control the functioning of the biosphere are incorporated in TBMs. Third, we discuss how global data sets, namely, remote-sensed data and atmospheric CO₂ and other tracer measurements, can be used to constrain indirectly the NPP distribution.