COUPLING DYNAMIC-MODELS OF CLIMATE AND VEGETATION

Numerous studies have underscored the importance of terrestrial ecosys tems as an integral component of the Earth's climate system. This real ization has already led to efforts to link simple equilibrium vegetati on models with Atmospheric General Circulation Models through iterativ e coupling procedures. While these linked models have pointed to sever al possible climate-vegetation feedback mechanisms, they have been lim ited by two shortcomings: (i) they only consider the equilibrium respo nse of vegetation to shifting climatic conditions and therefore cannot be used to explore transient interactions between climate and vegetat ion; and (ii) the representations of vegetation processes and land-atm osphere exchange processes are still treated by two separate models an d, as a result, may contain physical or ecological inconsistencies. He re we present, as a proof concept, a more tightly integrated framework for simulating global climate and vegetation interactions. The protot ype coupled model consists of the GENESIS (version 2) Atmospheric Gene ral Circulation Model and the IBIS (version 1) Dynamic Global Vegetati on Model. The two models are directly coupled through a common treatme nt of land surface and ecophysiological processes, which is used to ca lculate the energy, water, carbon, and momentum fluxes between vegetat ion, soils, and the atmosphere. On one side of the interface, GENESIS simulates the physics and general circulation of the atmosphere. On th e other side, IBIS predicts transient changes in the vegetation struct ure through changes in the carbon balance and competition among plants within terrestrial ecosystems. As an initial test of this modelling f ramework, we perform a 30 year simulation in which the coupled model i s supplied with modern CO2 concentrations, observed ocean temperatures , and modern insolation. In this exploratory study, we run the GENESIS atmospheric model at relatively coarse horizontal resolution (4.50 la titude by 7.5 degrees longitude) and IBIS at moderate resolution (2 de grees latitude by 2 degrees longitude). We initialize the models with globally uniform climatic conditions and the modern distribution of po tential vegetation cover. While the simulation does not fully reach eq uilibrium by the end of the run, several general features of the coupl ed model behaviour emerge. We compare the results of the coupled model against the observed patterns of modern climate. The model correctly simulates the basic zonal distribution of temperature and precipitatio n, but several important regional biases remain. In particular, there is a significant warm bias in the high northern latitudes, and cooler than observed conditions over the Himalayas, central South America, an d north-central Africa. In terms of precipitation, the model simulates drier than observed conditions in much of South America, equatorial A frica and Indonesia, with wetter than observed conditions in northern Africa and China. Comparing the model results against observed pattern s of vegetation coves shows that the general placement of forests and grasslands is roughly captured by the model. In addition, the model si mulates a roughly correct separation of evergreen and deciduous forest s in the tropical, temperate and boreal zones. However, the general pa tterns of global vegetation cover are only approximately correct: ther e are still significant regional biases in the simulation. In particul ar, forest cover is not simulated correctly in large portions of centr al Canada and southern South America, and grasslands extend too far in to northern Africa. These preliminary results demonstrate the feasibil ity of coupling climate models with fully dynamic representations of t he terrestrial biosphere. Continued development of fully coupled clima te-vegetation models will facilitate the exploration of a broad range of global change issues, including the potential role of vegetation fe edbacks within the climate system, and the impact of climate variabili ty and transient climate change on the terrestrial biosphere.