A paleoclimatic context for the drought of 1999-2001 in the northern Great Plains of North America

The least annual precipitation in the western interior of North America occurs in the northern Great Plains, including an area that encompasses parts of south-eastern Alberta, south-western Saskatchewan and eastern Montana. During 1999-2001, most climate stations in this region had record low precipitation. This paper examines this three-year drought in the context of historical climate records from Medicine Hat, Alberta and Havre, Montana and summer (June-July) and annual (August-July) precipitation reconstructed from standardized tree-ring widths (residual chronologies) from Pinus contorta (lodgepole pine) sampled in the Cypress Hills of Alberta and Saskatchewan and the Bears Paw Mountains of north-central Montana. Drought is operationally defined as precipitation in the lower 10th and 20th percentiles. Plots of reconstructed precipitation and cumulative departure from median values indicate a shift in climate variability prior to the twentieth century, when EuroCanadians settled in this region. The eighteenth and nineteenth centuries are characterized by sustained periods of progressively wetter and drier conditions, including prolonged drought. Various archival sources document the significant impacts of these prolonged droughts. While drought was frequent in the twentieth century, it tended to be of short duration and the impacts also were ameliorated by intervening periods of relatively high precipitation. Increasing aridity in response to global warming could expose a larger area of the northern Great Plains to the impacts of drought.