SENSITIVITY OF GLOBAL CLIMATE MODEL SIMULATIONS TO INCREASED STOMATAL-RESISTANCE AND CO2 INCREASES

Increasing levels of atmospheric CO2 will not only modify climate, the y will also likely increase the water-use efficiency of plants by decr easing stomatal openings. The effect of the imposition of ''doubled st omatal 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-day and doubled atmos pheric CO2 concentrations. The anticipated evapotranspiration decrease is seen most clearly in the boreal forests in the summer although, fo r the present-day climate (but not at 2 X CO2), there are also noticea ble responses in the tropical forests in South America. In the latitud e zone 44 degrees N to 58 degrees N, evapotranspiration decreases by - 15 W m(-2), temperatures increase by +2 K, and the sensible heat flux by +15 W m(-2). Soil moisture is often, but less extensively, increase d, which can cause increases in runoff. The responses at 2 X CO2 are l arger in the 44 degrees N to 58 degrees N zone than elsewhere. Globall y, the impact of imposing a doubled stomatal resistance in the present -day climate is an increase in the annually averaged surface air tempe rature of 0.13 K and a reduction in total precipitation of -0.82%. If both the atmospheric CO2 content and the stomatal resistance are doubl ed, the global response in surface air temperature and precipitation a re +2.72 K and +5.01% compared with +2.67 K and +7.73% if CO2 is doubl ed but stomatal resistance remains unchanged as in the usual ''greenho use'' experiment. Doubling stomatal resistance as well as atmospheric CO2 results in increased soil moisture in northern midlatitudes in sum mer.