Geomorphic evidence and optical ages from seven locations indicate that widespread dune activity occurred within the last 200 years in the Great Sand Hills region of southwestern Saskatchewan. Optical ages (n = 36) define an interval of dune activity bracketed by the earliest age of back ridges in the Seward sand hills (185 ± 8 years) and the average age of stabilized dune heads (about 105 years). During this interval, parabolic dunes were active in all areas studied. These ages indicate that the most recent interval of activity was initiated about AD 1800, and continued at a level higher than present for approximately 80 years. The most likely cause of dune activation was lower-than-average precipitation (relative to 1960–1991 values) through the 1700s, culminating in drought in the late 1700s, as evidenced in dendroclimatic records from the Cypress Hills and from the Rocky Mountain foothills. Dunes affected by such climatically induced regional activity require many decades to restabilize. Historical observations show that dunes in this area have been restabilizing throughout the 20th century. For the southern Canadian Prairies, a region with serious concerns about the implications of global warming, this study highlights the sensitivity of sand dunes to drought and cumulative moisture stress.