Anasazi (Pre-Columbian Native-American) Migrations During The Middle-12Th and Late-13th Centuries – Were they Drought Induced?

Abstract

Abstract. Severe droughts in the middle-12th and late-13th centuries appear to have affected Anasazi (pre-Columbian Native American) populations. During the first drought most of the great houses in the central San Juan Basin were vacated; the second drought resulted in the abandonment of the Four Corners region. During the first drought, villages may not have been completely abandoned. The multi-year drought periods probably were characterized by reductions in both winter and summer precipitation. Maize is dependent on winter precipitation for its germination and initial growth and on summer (monsoonal) precipitation for its continued growth. Reductions in precipitation are hypothesized to have resulted in low yields of maize, the dietary staple of the Anasazi. A comparison of historic climate data and tree-ring-based reconstructions of precipitation in the Four Corners region with tree-ring-based reconstructions of the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO) indicate that severe and persistent drought in the Four Corners region occurs when the PDO is negative and the AMO is positive. Historic climate data from the greater San Juan Basin indicate that a negative PDO is characterized by reductions in both water-year and summer precipitation, reinforcing the concept that at least some multi-year droughts involved weakening of the summer monsoon with attendant decreases in the yields of maize.

Possible impacts of early-11th-, middle-12th-, and late-13th-century droughts on western Native Americans and the Mississippian Cahokians

“…10a, b), tree-ring constructions for individual sites surrounding the San Juan Basin, New Mexico, indicate the presence of one to three droughts within the same approximate (AD 970-1060) time interval (Fig. 4 in Benson et al, 2006a). This indicates that drought persistence during the late-10th and early-11th centuries varied spatially.…”

“…In a recent paper, Benson et al (2006a) reiterated the concept that severe droughts in the middle-12th and late13th centuries (Fig. 10b, c) appear to have affected Anasazi populations in the Four Corners region.…”

“…However, optimum maize yields occur where growing season precipitation ranges from 40 to 60 cm (Minnis, 1981) and where the freeze-free period exceeds 120 days (Shaw, 1988). Benson et al (2006a), using freeze-free probabilities and precipitation data for 66 weather stations in the Four Corners region, determined the possible locations of areas in which dry-land farming of maize could occur, assuming that 90 freeze-free days and 30 cm of annual precipitation must be equaled or exceeded. Growing season precipitation averages $50% of the annual precipitation in the 66 sites.…”

“…However, the PDO has a very different time signature, having a pseudocyclicity ranging from 50 to 70 yr (MacDonald and Case, 2005), whereas typical ENSO events occur every 4-7 yr and persist from 6 to 18 months. Benson et al (2006a) have shown that, for the period of record, a negative PDO interval is characterized by reduced water-year (October 1-September 31) and summer (June, July, August) precipitation in the San Juan Basin area. This suggests a weakening of both winter cyclonic and summer monsoon systems in the southwest during a negative PDO.…”

Reviews of Science for Science Librarians: Drought in the Agricultural and Geosciences Literature

“…in the southwestern United States is a key reason for the abandonment of many Anasazi settlements. An evaluation of climatic and other geological data led Benson, Petersen, and Stein (2007) to support this hypothesis. As described in the section Drought and the Geosciences, geologists have methods and techniques to study historical records and paleoclimatic occurrences of drought.…”