Summerfallow trends and climate in the Canadian Prairies

Global warming associated with anthropogenic greenhouse gas emissions has been of great concern due to its potential threat to ecological systems, including threats to agricultural production systems. In turn, surface changes associated with agricultural practices may have an influence on local, regional and even global climate. Such effects may have restrained climatic trends associated with global warming, and have even led to cooling, thus masking climate change signals, especially at local and regional scales. This paper presents a case study to demonstrate climatic trends in the Canadian Prairies which are likely associated with historical changes in the agricultural practice of summerfallow. Summerfallow is a practice used to conserve soil moisture and consists of leaving land fallow over a growing season, keeping it free of vegetation for 18-20 months. It has been used in the Canadian Prairies for over a century and at one time constituted 11 Mha, or a about a quarter of Canada's cultivated land. Subsequent to 1975, improvements in land management practices resulted in significant reductions to areas under summerfallow, reaching a value of 5.4 Mha by 2001. The decrease in summerfallow area, and corresponding increase in agricultural crops would likely have the greatest effect on local and regional climate between the mid-June and mid-July period, during the peak of the growing season, when agricultural crops undergo rapid foliar expansion and substantial transpiration, thus contributing to significantly higher latent heat fluxes. An analysis of 1976-2000 climate trends in maximum temperature, minimum temperature, diurnal temperature range, solar radiation, cloud cover and precipitation in several regions of the Canadian Prairies showed opposition to the trends that would be expected from climate change due to an enhanced greenhouse effect.