Aridity and the potential physiological response of C_3 crops to doubled atmospheric CO_2: a simple demonstration of the sensitivity of the Canadian prairies

摘要

Since cultivated annual C_3 field crops cover about 50 percent of the land surface of the Canadian Prairie grassland eco-climatic zone, this vegetation influences the aridity of the climate during the growing season. The physiological response of these crops to a doubling of the atmospheric concentration of CO_2 may be a doubling of canopy resistance. If this physiological effect is not counteracted by interactive feedbacks, such as increased leaf area, evapotranspiration rates could be reduced. Todemonstrate the sensitivity of the aridity of the Prairie climate to this potential physiological effect, representative spring wheat growing-season soil moisture and Bowen ratio curves for a doubled canopy resistance (2 X CO_2) scenario were compared with a control (1 X CO_2) scenario. Lower evapotranspiration in the 2 X CO_2 scenario: (1) Increased root-zone soil moisture levels, and (2) weakened the atmospheric component of the hydrologic cycle by raising Bowen ratios, which reduces the convective available energy, and reduces the regional contribution to the atmospheric water vapour over the Prairies. A weakened hydrologic cycle implies less rainfall, and possibly, lower soil moisture levels. Thus, the net impact of a doubling of the atmospheric concentration of CO_2 on the aridity of the Canadian Prairies is uncertain. This simple sensitivity demonstration did not consider most of the potential feedback mechanisms, nor interactions of other processes. Nevertheless, the result illustrates that the physiological effect should be explicitly included in climate change models for the Canadian Prairies.