Bi-directional interactions simulated by a synchronously coupled atmosphere-vegetation model: Towards building an Earth systems model

The synchronously coupled GENESIS-IBIS model is operated in the investigation of climate-vegetation interactions under a variety of boundary and initial conditions. This model improves upon iteratively coupled climate-vegetation models utilized in past work to investigate similar interactions. The simulations performed contribute to our understanding of important processes which, to some extent, determine the state of the climate system.; This model suggests that, in the high latitudes, modern climate is not sensitive enough to shift into an alternate state under the influence of an initially expanded boreal forest. Conversely, the model illustrates that the extent of the boreal forest is sensitive to CO2-induced warming, and that once vegetation cover shifts, climate exhibits potentially significant feedbacks.; For example, thriving vegetation simulated under elevated CO2 may further warm high-latitude regions during spring by masking snow and, thus, lowering surface albedo. Furthermore, denser vegetation cover may accelerate the hydrologic cycle by increasing evapotranspiration and, thus, may contribute to enhanced precipitation in many regions. However, the benefit of enhanced precipitation due to increased vegetation cover does not compensate for the sharp increase in evapotranspiration in the and mid-continental regions, so available soil moisture decreases. This is in direct contrast to the view that CO2 fertilization will enhance soil moisture and runoff across the globe: a view that neglects changes in vegetation structure and local climatic feedbacks.; Much different from the effects of rising CO2 on climate and vegetation are the effects of large ice sheets and lower CO2 concentrations found during the last glacial maximum. I confirm here previous results showing that sparser forest cover in the mid- and high latitudes may have led to conditions even cooler than simulated without vegetation feedbacks. I also verify that in the tropics, sparser forest cover may have countered the cooling, yet may have pronounced the drying. These results convey the significant role of lower atmospheric CO2 concentrations on last glacial vegetation density and the resultant climatic feedbacks.; This model will provide the opportunity for continued exploration of the climate system in future applications involving transient simulations accounting for the realistic evolution of climate and vegetation. In addition, new components of the climate system will be added, such as ocean circulation and soil CO2 dynamics. The long term goal is to simulate the Earth system in its entirety, with the hope that we will be able more confidently to provide long term guidance to policy makers.