Analyses of temperatures from boreholes in previously forested areas in western Canada disclose sudden increases of one to two degrees in ground surface temperature at the times of deforestation at each site. This is the first clear evidence for deforestation induced warming. These findings suggest that any land development changing climatic parameters such as the amounts of water evaporated from the earth's surface contributes to regional climatic change. A warming of the ground surface over a large area of central Canada, synchronous with the deforestation of southern Ontario and neighbouring regions in the nineteenth century, may be an example of climate change linked to the widespread creation of agricultural lands. Such a warming also affects the surrounding regions. ¿ 1998 American Geophysical Union