Modeling global climate-vegetation interactions in a doubled CO2 world

A coupled global vegetation-climate model is used to investigate the effect s of vegetation feedbacks on climate change due to doubling atmospheric CO2 . The Equilibrium Vegetation Ecology model (EVE) simulates global terrestri al vegetation and is designed for interactive coupling with climate models. Terrestrial vegetation is resolved into 110 plant life forms, which repres ent groups of species with similar physiognomic characteristics and migrati onal responses to climate change, thus preserving the spatial integrity of each life-form distribution as climate changes. EVE generates a quantitativ e description of plant community structure defined by total vegetation cove r and the fractional covers of life forms as a function of climate. The equ ilibrium distribution of each life form is predicted from monthly mean temp erature, precipitation, and relative humidity, based on observed correlatio ns with the present climate. The fractional covers of the life forms at eac h site are determined by parameterizations of dynamic ecological processes: competition for sunlight, disturbances by fire and treefall. A second mode l (LEAF) simulates the seasonal phenology of EVE's plant canopies, driven b y 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 inter active simulations are found at the northern and southern ecotones of the b oreal forest. Poleward migration of boreal forests into tundra caused by wa rming due to elevated CO2 is enhanced by a strong snow-masking albedo feedb ack, consistent with earlier studies. The invasion of temperate grasslands into the southern boreal forest is also enhanced due to summer warming spre ading from the north, despite the opposing sense of the grassland-forest al bedo feedback. Desertification of subtropical grasslands is mostly reversed in the interactive simulations due to enhanced monsoonal precipitation. Th ese interactions and other climate and plant community changes caused by cl imate-vegetation feedbacks are discussed on a regional basis.