Regional climatic warming and associated twentieth century land-cover changes in north-western North America

Twentieth century (1900 to 1990) changes in annual surface air temperature (SAT) are compared with contemporaneous changes in annual ground surface temperature (GST) over an area extending from east of the Cordillera in north-western Canada, to Texas in the south-central United States. One of the largest SAT increases over the past century has occurred in the north-western portion of this study area. It also coincides (spatial regression coefficient r = 0.70) with the largest positive GST anomaly in northern North America. However, there are large areas of Alberta, Saskatchewan, Manitoba and southern Northwest Territories that exhibit spatially coherent patterns of differences between SAT and GST warming. These differences appear to be related to twentieth century land-use and land-cover changes. The highest GST warming has been observed in large areas where extensive land-cover changes, such as the clearing of forests, increased forest fire activity, and conversion of prairie grassland to agricultural land, have occurred. It is hypothesized that land-cover change dramatically alters surface characteristics affecting the radiation budget and energy balance. The partitioning between sensible and latent fluxes is altered and the potential for land drying is increased. Calculated flux changes associated with land-cover change are comparable in magnitude with greenhouse gas radiative forcing. It therefore appears that through a step change in GST, land-cover changes have contributed to a portion of the observed SAT warming in this region.