Sensitivity of Global Climate Model Simulations to Increased Stomatal Resistance and 
C02 Increases

A. HENDERSON-SELLERS 
Climatic Impads Centre. Macquarie University, Sydney, New South Wales. Australia 
K. McGUFFIE 
Departmem of@Applied Physics, University of Technology, Sydney, New South Wales, 
Australia 
C. GROSS 
Climatic Impacts Centre, Macquarie University. and Department of@Applied Physics. 
University of Technology, Sydney, New South Wales. Australia 

ABSTRACT 
Increasing levels of atmospheric C02 will not only modify climate, they will also 
likely increase the water-use efficiency of plants by decreasing stomatal openings. 
The effect of the imposition of "doubled stomatal resistance" on climate is 
investigated in off-line simulations with the Biosphere-Atmosphere Transfer Scheme 
( BATS ) and in two sets of global climate model simulations: for present<lay and 
doubled atmospheric C02 concentrations. The anticipated evapotranspiration decrease 
is seen most clearly in the boreal forests in the summer although, for the present<lay 
climate ( but not at 2 X C02 ) . there are also noticeable responses in the tropical 
forests in South America. In the latitude zone 44'N to 58'N, evapotranspiration 
decreases by -15 W m^-2 , temperatures increase by +2 K, and the sensible heat flux 
by + I 5 W m^-2 . Soil moisture is often, but less extensively. increased. which can 
cause increases in runoff. The responses at 2 X C02 are larger in the 44'N to 58'N 
zone than elsewhere. Globally, the impact of imposing a doubled stomatal resistance 
in the present-day climate is an increase in the annually averaged surface air 
temperature of@O.13 K and a reduction in total precipitation of -0.82%. If both the 
atmospheric C02 content and the stomatal resistance are doubled, the global response 
in surface air temperature and precipitation are +2.72 K and +5.01 % compared with 
+2.67 K and +7.73% if C02 is doubled but stomatal resistance remains unchanged as 
in the usual "greenhouse" experiment. Doubling stomatal resistances well as 
atmospheric C02 results in increased soil moisture in northern midlatitudes m summer.