Potential biosphere response to climate change and its impact on hydrology and regional climate prediction over western Africa

Date of Completion

January 2009

Keywords

Engineering, Environmental

Degree

Ph.D.

Abstract

This research includes two major types of studies, done in a two-stage fashion. In the first type, global simulations are performed with a land surface/vegetation model driven with climate changes projected by eight General Circulation Models (GCMs) to examine biospheric changes and feedback in response to future changes in CO2 and climate. Simulated vegetation response ranges from mild changes in fractional plant functional type (PFT) coverage to the rather dramatic changes of dominant PFTs. Overall, a quite consistent spatial pattern emerges, characterized by a considerable poleward spread or shift of temperate and boreal forests in the northern hemisphere high latitudes, and a substantial degradation of vegetation type in the tropics (e.g., increase of drought deciduous trees coverage at the expense of evergreen trees) especially in portions of West and southern Africa and South America. Despite the widespread degradation of vegetation type in the tropics, net primary productivity (NPP) and growing-season leaf area index (LAI) are predicted to increase under most GCM scenarios over most of the globe due to the synergistic effects of CO2 concentration changes, climate changes, and vegetation changes. Generally, feedback from the changes in vegetation structure enhances winter warming and reduces summer warming in the northern mid- and high-latitudes. Structural vegetation feedback also increases evapotranspiration under future climate, thereby accelerating the hydrological cycle. Measured by evapotranspiration flux, this hydrological acceleration by structural vegetation feedback is at a magnitude comparable to that due to CO2 and climate changes. ^ In the second type, a dynamic vegetation model is asynchronously coupled to a regional climate model applied to western Africa, and the coupled climate-vegetation model is used to examine the role of vegetation dynamics in predictions of future climate change in western Africa. The focus here is on the hydrological cycle. Without vegetation feedback, elevated CO2 and associated global warming lead to drier conditions over a large region between the equator and about 12°N. Including the effects of vegetation changes substantially modifies this, resulting in wetter conditions in a future climate. These studies demonstrate that CO2- and climate-induced changes in vegetation structure substantially influence hydrological processes and climate, thus evincing the importance of including vegetation feedback in future climate change predictions. ^

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