Is the science that deals with understand past climate based on data from pollen, tree rings, macrofossils, faunal remains, and geomorphology (DeMenocal, 2001). Research for the late Holocene in the Colorado Plateau reveals two notable events during the past 2000 years.
• Medieval Warm Period from 1000 to 1350 A.D.
• Little Ice Age from about 1450 to 1850 A.D.
The Colorado Plateau is a semiarid area. It climate is characterized by cold winters. Summer days are usually hot, but nights are cool; accordingly, the diurnal variation in temperature is considerable. Annual average temperatures are 40 to 55F (4 to 13C), decreasing with rising elevation. Average annual precipitation is about 20 in (510 mm), except on the higher mountains; some parts of the province receive less than 10 in (260 mm). Summer rains are thunderstorms, with ordinary rains arriving in winter. Thus, this province differs from the Intermountain Semi desert Province, which generally lacks summer rains.
Studies showed that from 1050 to 1300 a series of significant changes occurred:
- Major droughts become common occasionally occurring as sustained intervals of substandard moisture on the order of a decade or more.
- Temperature increased, notably toward the middle of this arid period
- Unprecedented hydrologic instability occurred in both primary and secondary drainages as water table drops and erosion increased (arroyo cutting) (Agenbroad, L., 1975 and Hereford, R., 2002)
The most severe drought in the southwestern United States within the past 800 years spanned an ;22-year period between 1572 and 1593 A.D. (7) (Fig. 1C). The reconstructed spatial drought pattern at the peak of this dry period in 1583 A.D. shows extreme drought conditions extending across the American Southwest (Fig. 1C).
Although appreciably less severe than the drought of the 1580s, the 26-year “Great Drought” of the 1280s was similarly prolonged and widespread (Fig. 1D). By the time of this drought, the Anasazi, ancestors of modern Pueblo Indians, had long established elegant stone and adobe villages in the semiarid highlands and canyons of the American Southwest. Archaeological investigations of Anasazi settlements have documented that
many sites were abandoned abruptly near the end of the 13th century A.D. Cited reasons for the collapse of the Anasazi include emergent balkanization, warfare, and religious turmoil within the region, as well as the onset of severe drought conditions and regional deforestation. Whether the multidecadal drought of the 1280s was the determining factor in the collapse of the Anasazi continues to be debated, but current archaeological evidence firmly implicates drought as a contributing destabilizing factor (DeMenocal, 2001)
Figure 1: Drought history from 1200-1994 AD for the American southwest
reconstructed using a spatial network of tree ring chronologies. DeMenocal (2001)
In general terms prolonged droughts result in the following pattern (after Jones et al 1999)
Why cultural systems across the Southwest collapsed rather than splitting or implementing technological options (intensification of agriculture)?
Collapse of an economic trade system