Agriculture and Tornadoes on the Canadian Prairies: potential impact of increasing atmospheric CO2 on Summer Severe Weather

On the Canadian Prairies, a significantportion of the transpiration is derived from a ratherhomogenous agro-ecosystem comprised of springwheat and other annual C₃ field crops with similar water use patterns. The seasonal pattern oftranspiration is determined, to alarge extent, by crop phenology. By increasing thespecific humidity of the atmospheric boundarylayer, regional transpiration has a large positive effect on themagnitude of the potential energy available fordeep convection and thus, on the likelihood of occurrence andintensity of severe thunderstorms. Acomparison of the average wheat phenologycurve, for a representative aridgrassland and for a representativetransitional grassland site, with theaverage number of tornado days per week inthe entire eco-climatic zone demonstrated that the twoare linked. With increasing atmospheric concentrations ofCO₂, the physiological response of C₃ cropsmight lower transpiration rates and climate warming may advance cropseeding dates. The former would reduce the specifichumidity of the convective boundary layer, and thereby, reducethe potential energy available for deep convection.Thus, future summer severe weather seasons might, on average,be more benign and occur earlier in the seasonthan at present. This conclusion is far from certain as there are amultitude of complex feedback mechanisms. What ismore certain is that global circulation models (GCMs) mustadequately handle the inter-action between theatmosphere and the agro-ecosystem of the Canadian Prairies beforethey can correctly simulate the thermodynamic propertiesof the convective boundary layer and determine the impactof increasing atmospheric concentrations of CO₂ onfuture summer severe weather.