Dynamics of a global-scale vegetation model

Abstract

An increasing number of climate modelling studies use a general circulation model (GCM) coupled to a dynamic global vegetation model (DGVM), yet the intrinsic properties of DGVMs and the feedbacks in this coupled system have not been fully investigated. Here we present details of several intrinsic properties of an established DGVM, TRIFFID, and its coupling with the Hadley Centre GCM. A new version of TRIFFID is first developed which is able to reproduce much of the behaviour of TRIFFID, yet is simple enough to be investigated analytically. This is used to show: (1) TRIFFID has a single stable equilibrium vegetation coverage, which depends on environmental conditions, (2) the dominant timescale of changes in vegetation structure is variable and is determined by environmental conditions through photosynthesis, and (3) TRIFFID damps out variability at periods less than the response timescale. From these results we conclude that the variability of the vegetation structure, and hence vegetation feedbacks can damp or amplify atmospheric variability through a shift in the response timescale, i.e. that vegetation feedbacks to the atmosphere are sensitive to the period of the atmospheric variability.