The influence of vegetation on the global climate: an analysis of teleconnection processes using a coupled atmosphere-biosphere model

There is little doubt that the biosphere has a significant influence on the atmosphere at local and regional scales. Numerous studies have already identified the regional climate response to human modification of the landscape through changes to the biophysical exchanges of energy, water, and momentum between the land surface and the atmosphere. However, there is disagreement over whether the biosphere is capable of influencing the global climate through large-scale changes to the atmospheric dynamics by way of teleconnection mechanisms. Furthermore, given a specific surface forcing, it is not entirely clear where the response will occur, how strong it will be, and how large a surface forcing is required in order for the climate response to be felt globally.

Tropical deforestation is one example of a surface forcing that may influence the global climate. Several studies have suggested that significant changes to the Northern Hemisphere climate may occur as a result of selective tropical deforestation in the Amazon basin, yet most of these studies have tended to focus on the climate responses in the extratropics with little analyses of the mechanisms responsible for propagating the signal out of the tropics. In this study, I present results from a coupled atmosphere-biosphere model, CCM3-IBIS (Community Climate Model, version 3 – Integrated Biosphere Simulator), to illustrate the potential influence of theoretical land use and land cover change on the global climate by way of atmospheric teleconnections. Through a quantitative analysis of the response of different vegetation types to land cover change it is possible to determine those vegetation types that are most influential on the global climate as well as the biophysical processes that are most important.

As an illustration of the dynamical mechanisms of atmospheric teleconnections, I present results of a suite of numerical model simulations of tropical deforestation in the Amazon basin. The results suggest that tropical deforestation in the Amazon basin can have an influence on the climate of Asia. A weakening of deep moist convection and reductions in high-level outflow lead to an anomalous forcing of Rossby waves out of the tropics that weakens the subtropical jets and strengthens the high latitude polar front jet. This results in anomalous changes to Atlantic and European storm track activity and the horizontal eddy momentum flux. Changes in the horizontal eddy momentum flux cause enhancement and a northward shift in the thermally indirect mean meridional circulation. Since the descending branch warms adiabatically, the intensification and shift in the indirect circulation cause an anomalous warming of the lower troposphere across much of Eastern Europe and Asia. Ultimately, regional-scale positive feedbacks become important as they amplify the signal through changes in cloud cover, snow cover, and the surface albedo. The result is a large warming across parts of Asia in boreal winter. While theoretical, this approach illustrates the potentially important processes connecting regional-scale changes of the tropical land surface to regional-scale climate changes in the extratropics. Furthermore, this study shows that the biosphere has the potential to influence the global climate through biophysical changes in the region of the surface forcing and through changes to the general circulation by way of atmospheric teleconnections.