MODELING THE EFFECTS OF ATMOSPHERIC CO2 ON VEGETATION ATMOSPHERE INTERACTIONS

The effect of doubling atmospheric CO2 concentration (C-a) on climate and vegetation is investigated using a combined climate-vegetation mod el. The vegetation model predicts the response of leaf area index, can opy transpiration (E(T)) and whole-plant carbon balance to changes in climate, soil moisture, and atmospheric CO2 forcing. This model has be en embedded in the UK Meteorological Office Single Column Model (SCM), which provides the climate feedback to the vegetation. The vegetation model uses an optimisation approach to predict stomatal resistance, a biochemical model to predict photosynthesis and a simple carbon balan ce model to predict leaf area. Respiration is calculated as a function of leaf area and vegetation height. Clouds are assumed to be radiativ ely passive in the SCM to avoid unrealistic feedbacks. Simulations wer e performed with the fully interactive vegetation-climate model for an Amazon location with the present-day value of C-a (1 x CO2), and twic e this value (2 x CO2). In addition, two other types of simulation wer e performed at both CO2 concentrations: one in which the vegetation co mponent was forced only with 1 x CO2, and one using a fixed surface re sistance. The latter case is equivalent to simulations using most curr ent general circulation models. In all the simulations, increased atmo spheric CO2 caused an increase in surface temperature owing to increas ed radiative forcing. With a fixed resistance, mean E(T) was increased by 5.6% and sensible heat flux was reduced by 3.8%. The fully interac tive model had significant effects on the response of both climate and productivity to C-a. Increased C-a caused stomatal closure, which res ulted in a reduction in mean E(T) Of 25%. The effect of C-a on E(T) wa s amplified by the positive feedback resulting from the effect of incr eased air humidity deficit on stomatal resistance.