Large-scale vegetation feedbacks on a doubled CO2 climate

Changes in vegetation cover are known to influence the climate system by mo difying the radiative, momentum, and hydrologic balance of the land surface . To explore the interactions between terrestrial vegetation and the atmosp here for doubled atmospheric CO2 concentrations, the newly developed fully coupled GENESIS-IBIS climate-vegetation model is used. The simulated climat ic response to the radiative and physiological effects of elevated CO2 conc entrations, as well as to ensuing simulated shifts in global vegetation pat terns is investigated. The radiative effects of elevated CO2 concentrations raise temperatures and intensify the hydrologic cycle on the global scale. In response, soil mois ture increases in the mid- and high latitudes by 4% and 5%, respectively. T ropical soil moisture, however, decreases by 5% due to a decrease in precip itation minus evapotranspiration. The direct, physiological response of plants to elevated CO2 generally acts to weaken the earth's hydrologic cycle by lowering transpiration rates acr oss the globe. Lowering transpiration alone would tend to enhance soil mois ture. However, reduced recirculation of water in the atmosphere, which lowe rs precipitation, leads to more arid conditions overall (simulated global s oil moisture decreases by 1%), particularly in the Tropics and midlatitudes . Allowing structural changes in the vegetation cover (in response to changes in climate and CO2 concentrations) overrides the direct physiological effe cts of CO2 on vegetation in many regions. For example, increased simulated forest cover in the Tropics enhances canopy evapotranspiration overall, off setting the decreased transpiration due to lower leaf conductance. As a res ult of increased circulation of moisture through the hydrologic cycle, prec ipitation increases and soil moisture returns to the value simulated with j ust the radiative effects of elevated CO2. However, in the highly continent al midlatitudes, changes in vegetation cover cause soil moisture to decline by an additional 2%. Here, precipitation does not respond sufficiently to increased plant-water uptake, due to a limited source of external moisture into the region. These results illustrate that vegetation feedbacks may operate differently according to regional characteristics of the climate and vegetation cover. In particular, it is found that CO2 fertilization can cause either an incre ase or a decrease in available soil moisture, depending on the associated c hanges in vegetation cover and the ability of the regional climate to recir culate water vapor. This is indirect contrast to the View that CO2 fertiliz ation will enhance soil moisture and runoff across the globe: a view that n eglects changes in vegetation structure and local climatic feedbacks.