• climate simulation;
  • greenhouse gas warming;
  • vegetation feedbacks

[1] Leaves close their stomates in response to increases of CO2. Such a rapid physiological response is included in the land component of comprehensive climate models. However, observational studies have shown that they can further close their stomates as a consequence of “down-regulation,” further reducing canopy conductance. However, they may also increase the area of their leaves, hence increasing their canopy conductance. Changes of canopy conductance change surface ET, a reduction leading to surface warming. A simulation considering these mechanisms of modifying canopy conductance is carried out for the assumption of a doubled atmospheric CO2concentration, using the Community Earth System model. It finds that down-regulation as formulated in previous studies could have as large a warming impact on land temperatures as the standard leaf physiological response. Increases in LAI, if they were to occur, appear to have but a small cooling effect. The reduction of latent cooling in the model is amplified by a reduction of low-level cloud cover, hence enhanced net absorption of solar radiation. Reduction of low level cloudiness appears to be necessary to maintain global radiation balance as reported in a previous study. Over mid to high latitudes, decreases in surface albedo associated with reduced snow cover also contribute to amplifying the warming. The physiological feedbacks of leaf stomates in the simulation increase warming by 0.6 ± 0.2°C over land and 0.3 ± 0.1°C globally, not inconsistent with previous studies. Enhanced interhemispheric temperature differences weaken the southward shift of the ITCZ associated with CO2radiative warming. Regions with relatively high LAI tend to have greater vegetation feedback; but increases in large-scale precipitation may weaken this local warming effect.