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On the Canadian Prairies, a significant portion of the transpiration is derived from a rather homogenous agro-ecosystem comprised of spring wheat and other annual C-3 field crops with similar water use patterns. The seasonal pattern of transpiration is determined, to a large extent, by crop phenology. By increasing the specific humidity of the atmospheric boundary layer, regional transpiration has a large positive effect on the magnitude of the potential energy available for deep convection and thus, on the likelihood of occurrence and intensity of severe thunderstorms. A comparison of the average wheat phenology curve, for a representative arid grassland and for a representative transitional grassland site, with the average number of tornado days per week in the entire eco-climatic zone demonstrated that the two are linked. With increasing atmospheric concentrations of CO2, the physiological response of C-3 crops might lower transpiration rates and climate warming may advance crop seeding dates. The former would reduce the specific humidity of the convective boundary layer, and thereby, reduce the potential energy available for deep convection. Thus, future summer severe weather seasons might, on average, be more benign and occur earlier in the season than at present. This conclusion is far from certain as there are a multitude of complex feedback mechanisms. What is more certain is that global circulation models (GCMs) must adequately handle the inter-action between the atmosphere and the agro-ecosystem of the Canadian Prairies before they can correctly simulate the thermodynamic properties of the convective boundary layer and determine the impact of increasing atmospheric concentrations of CO2 on future summer severe weather.