CHANGING DRIVERS OF HOLOCENE DROUGHTS IN THE NORTHERN GREAT PLAINS

The Northern Great Plains (NGP) region of North America has a Holocene record of high frequency drought occurrence, as demonstrated by numerous paleolimnologic, pollen and tree ring records. Many of these droughts were of greater magnitude and duration than those of the 20th century “Dust Bowl” years, which had such a profound economic and societal impact. However, not all these droughts of the Holocene NGP can be analogs for future droughts because the underlying processes or drivers that produced them, and the conditions under which those drivers operated, have changed through time. Conditions driving drought on the NGP in the early Holocene appear to have been intense westerly winds and a strong polar front, whereas droughts of the late Holocene appear to have been triggered by variability along the tropical front. Geologically speaking, late Holocene droughts are not as great in magnitude or duration as those of the early and mid-Holocene, although their impact on society and the economy have proven to be extensive. This difference may be because the wind-driven droughts of the early Holocene were a product of global air mass configurations with temporal persistence, whereas the late Holocene droughts appear to have been driven by changes in the tropical front, reflecting shorter term moisture variability. The appropriate choice of analogs for future drought conditions on the NGP will depend on 1) developing a clear understanding of the sets of processes that can produce sustained drought, 2) more intensive work on high resolution records that capture decadal scale events, and 3) more integrated studies of climate-driven hydrologic processes. NGP droughts of the near future may have much in common with late Holocene droughts because the processes that drive them are similar, yet a fourth process, one of unprecedented anthropogenic activities, may be multiplying the intensity of naturally occurring drought. Rising temperatures, increased usage of surface water and groundwater, and continued expansion of human needs all escalate drought conditions and have serious implications for NGP ecology.