Hydrological Impact of the Potential Future Vegetation Response to Climate Changes Projected by 8 GCMs

This study uses offline simulations with a land surface model to explore how the future response of potential vegetation to elevated CO2 and attendant climate changes feeds back to influence surface hydrological processes. Climate changes are those projected by eight General Circulation Models (GCMs) under the SRESAlB, and the potential natural vegetation structure corresponding to the Preindustrial control and SRESAlB 2100 climate of the 8 GCMs are simulated by a dynamic global vegetation model integrated to equilibrium. For climate change forcing from each GCM, comparisons are made among three surface hydrology simulations using CLM3.0 driven with different combinations of climate forcing, atmospheric CO2 concentration, and potential natural vegetation. These simulations are designed to separate the effect of structural vegetation feedback from the combined influence of climate and CO2 changes. With the exception of the HadCM scenario, all other GCM scenarios broadly agree on the spatial patterns of structural vegetation feedbacks on surface temperature and surface water budget, although the response of soil moisture varies considerably among the GCM scenarios especially in the tropics. With the HadCM excluded, averages over the seven GCM scenarios indicate that the CO2-induced warming in winter is stronger than in summer in the northern mid and high latitudes, and structural vegetation feedback enhances the winter warming and reduces the summer warming over a large portion of these regions; the global hydrological cycle is expected to accelerate in a warmer future climate, while the structural vegetation feedback further increases evapotranspiration in a major portion of the globe, including parts of South America, tropical Africa, Southeast Asia, and regions north of approximately 45°N, suggesting that vegetation feedback could further accelerate the hydrological cycle. Averaged over the globe, this increase in evapotranspiration due to structural vegetation feedback is equivalent to 78% of that due to climate and CO2 changes. When changes in vegetation structure are not considered, the 7-model average response of soil moisture to climate and CO2 changes is characterized by wetter soil conditions in the northern high latitudes and parts of the midlatitudes. Structural vegetation feedback, however, causes strong midlatitude dryness both in the winter and in the summer. The impact of vegetation changes corresponding to the HadCM-projected climate changes is markedly different, being either more extreme or in a different direction than that corresponding to the other GCMs examined. Our results are constrained by the lack of consideration for human land use changes and vegetation feedback to climate, as well as the uncertainty related to the highly disputed physiological response of ecosystems to elevated CO 2. Nevertheless, this study demonstrates that climate- and CO 2-induced changes in potential vegetation structure substantially influence the surface hydrological processes, thus emphasizing the importance of including vegetation feedback in future climate change prediction