Long-term changes to incoming solar energy on the Canadian Prairie

Many regions of the earth experienced a steady decline in solar radiation (global dimming) from the late 1950s to the late 1980s–early 1990s and a steady brightening thereafter. To determine trends in solar energy across the agricultural region of the Canadian Prairie, we analyzed incoming solar radiation, sunshine, and climate data gathered between 1951 and 2005 from 7 locations on the prairies. From the 1950s, there was a decreasing trend in annual average daily incoming solar radiation, no trend in annual average daily sunshine hours, and increasing trends in annual average daily temperature and in the annual number of precipitation/rain events. As well, we found the annual average daily incoming solar radiation decreased as the annual number of precipitation/rain events increased, but increased as the annual average precipitation intensity increased. We found the annual average daily sunshine hours decreased as the annual precipitation amount increased and as the annual average precipitation intensity increased. Thus, incoming solar radiation and sunshine responded differently to precipitation amount/events. Nevertheless, climate trends coupled with the relationship of solar radiation/sunshine to precipitation amount/events suggested that increased cloudiness played a key role in the extinction of solar radiation by the atmosphere over the past 50–60 years on the Canadian Prairie. Increasing greenhouse gas concentrations act to reduce cloudiness, whereas increasing aerosol concentrations often increase cloudiness. Specific to the Canadian Prairie, further research is needed to determine the influence of changing greenhouse gas and aerosol concentrations on cloudiness and cloud characteristics and the impact these changes would have on solar energy measured at the earthʹs surface.