Modeling global climate-vegetation interactions in a doubled CO2 world

A coupled global vegetation-climate model is used to investigate the effects of vegetation feedbacks on climate change due to doubling atmospheric CO2. The Equilibrium Vegetation Ecology model (EVE) simulates global terrestrial vegetation and is designed for interactive coupling with climate models. Terrestrial vegetation is resolved into 110 plant life forms, which represent groups of species with similar physiognomic characteristics and migrational responses to climate change, thus preserving the spatial integrity of each life-form distribution as climate changes. EVE generates a quantitative description of plant community structure defined by total vegetation cover and the fractional covers of life forms as a function of climate. The equilibrium distribution of each life form is predicted from monthly mean temperature, precipitation, and relative humidity, based on observed correlations with the present climate. The fractional covers of the life forms at each site are determined by parameterizations of dynamic ecological processes: competition for sunlight, disturbances by fire and treefall. A second model (LEAF) simulates the seasonal phenology of EVE's plant canopies, driven by the daily climate at each location, and provides the physical quantities needed for coupling vegetation and climate models. Two pairs of coupled EVE-GCM simulations are described, both with 1x and 2 x CO2: the first with prescribed fixed vegetation, and the other with fully interactive vegetation. Large effects of vegetation feedbacks in the interactive simulations are found at the northern and southern ecotones of the boreal forest. Poleward migration of boreal forests into tundra caused by warming due to elevated CO2 is enhanced by a strong snow-masking albedo feedback, consistent with earlier studies. The invasion of temperate grasslands into the southern boreal forest is also enhanced due to summer warming spreading from the north, despite the opposing sense of the grassland-forest albedo feedback. Desertification of subtropical grasslands is mostly reversed in the interactive simulations due to enhanced monsoonal precipitation. These interactions and other climate and plant community changes caused by climate-vegetation feedbacks are discussed on a regional basis.