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The radiative and physiological effects of doubled atmospheric carbon dioxide (CO[sub2] ) on climate are investigated using a coupled biosphere-atmosphere model. Five 30-yr climate simulations, designed to assess the radiative and physiological effects of doubled CO[sub2] , were compared to a 30-yr control run. When the CO[sub2] concentration was doubled for the vegetation physiological calculations only assuming no changes in vegetation biochemistry, the mean temperature increase over land was rather small (0.3 K) and was associated with a slight decrease in precipitation (-0.3%). In a second case, the vegetation was assumed to have adapted its biochemistry to a doubled CO[sub2] (2 x CO[sub2] ) atmosphere and this down regulation caused a 35% decrease in stomatal conductance and a 0.7-K increase in land surface temperature. The response of the terrestrial biosphere to radiative forcing alone--that is, a conventional greenhouse warming effect--revealed important interactions between the climate and the vegetation. Although the global mean photosynthesis exhibited no change, a slight stimulation was observed in the tropical regions, whereas 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 northern latitudes than in the Tropics (4.0 K vs 1.7 K). These interactions also resulted in an asymmetry in the diurnal temperature cycle, especially in the Tropics where the nighttime temperature increase due to radiative forcing was about twice that of the daytime, an effect not discernible in the daily mean temperatures. The radiative forcing resulted in a mean temperature increase over land of 2.6 K and 7% increase in precipitation with the least effect in the Tropics. As the physiological effects were imposed along with the radiative effects, the overall temperature increase over land was 2.7 K... [ABSTRACT FROM AUTHOR]