Interactions between vegetation and climate: Radiative and physiological effects of doubled atmospheric CO2

The radiative and physiological effects of doubled atmospheric carbon dioxi de (CO2) on climate are investigated using a coupled biosphere-atmosphere m odel. Five 30-yr climate simulations, designed to assess the radiative and physiological effects of doubled CO2, were compared to a 30-yr control run. When the CO2 concentration was doubled for the vegetation physiological cal culations only assuming no changes in vegetation biochemistry, the mean tem perature increase over land was rather small (0.3 K) and was associated wit h a slight decrease in precipitation (-0.3%). In a second case, the vegetat ion was assumed to have adapted its biochemistry to a doubled CO2 (2 x CO2) atmosphere and this down regulation caused a 35% decrease in stomatal cond uctance and a 0.7-K increase in land surface temperature. The response of t he terrestrial biosphere to radiative forcing alone-that is, a conventional greenhouse warming effect-revealed important interactions between the clim ate and the vegetation. Although the global mean photosynthesis exhibited n o change, a slight stimulation was observed in the tropical regions, wherea s in the northern latitudes photosynthesis and canopy conductance decreased as a result of high temperature stress during the growing season. This was associated with a temperature increase of more than 2 K greater in the nor thern latitudes than in the Tropics (4.0 K vs 1.7 K). These interactions al so resulted in an asymmetry in the diurnal temperature cycle, especially in the Tropics where the nighttime temperature increase due to radiative forc ing was about twice that of the daytime, an effect not discernible in the d aily mean temperatures. The radiative forcing resulted in a mean temperatur e increase over land of 2.6 K and 7% increase in precipitation with the lea st effect in the Tropics. As the physiological effects were imposed along w ith the radiative effects, the overall temperature increase over land was 2 .7 K but with a smaller difference (0.7 K) between the northern latitudes a nd the Tropics. The radiative forcing resulted in an increase in available energy at the earth's surface and, in the absence of physiological effects, the evapotranspiration increased. However, changes in the physiological co ntrol of evapotranspiration due to increased CO2 largely compensated for th e radiative effects and reduced the evapotranspiration approximately to its control value.