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Native Americans and the Medieval Warm Period
Benson, L.V., Berry, M.S., Jolie, E.A., Spangler, J.D., Stahle, D.W. and Hattori, E.M. 2007. Possible impacts of early-llth-, middle-12th-, and late-13th-century droughts on western Native Americans and the Mississippian Cahokians. Quaternary Science Reviews 26: 336-350.

What was done
The authors review and discuss possible impacts of early-11th-, middle-12th-, and late-13th-century droughts on three Native American cultures that occupied parts of the western United States (Anasazi, Fremont, Lovelock) plus another culture that occupied parts of southwestern Illinois (Cahokia).

What was learned
Benson et al. report that "population declines among the various Native American cultures were documented to have occurred either in the early-11th, middle-12th, or late-13th centuries" - AD 990-1060, 1135-1170, and 1276-1297, respectively - and that "really extensive droughts impacted the regions occupied by these prehistoric Native Americans during one or more of these three time periods." In particular, they say the middle-12th-century drought "had the strongest impact on the Anasazi and Mississippian Cahokia cultures," noting that "by AD 1150, the Anasazi had abandoned 85% of their great houses in the Four Corners region and most of their village sites, and the Cahokians had abandoned one or more of their agricultural support centers, including the large Richland farming complex." In addition, they write that "the sedentary Fremont appear to have abandoned many of their southern area habitation sites in the greater Unita Basin area by AD 1150 as well as the eastern Great Basin and the Southern Colorado Plateau," so that "in some sense, the 13th century drought may simply have 'finished off' some cultures that were already in decline." Lastly, they state that these "major reductions in prehistoric Native American habitation sites/population" occurred during a period of "anomalously warm" climatic conditions, which characterized the Medieval Warm Period throughout much of the world at that particular time. As may be seen from the Interactive Map and Time Domain Plot of our Medieval Warm Period (MWP) Project, for example, the deadly AD 1150 date falls right at the midpoint of the global MWP time domain plot as defined by all pertinent data we have analyzed there to this point in time.

What it means
Among other things, the fact that the deadly North American droughts of the MWP have never been equaled throughout all the ensuing years argues strongly that what Benson et al. call the anomalous warmth of that period has also "never been equaled throughout all the ensuing years," which further suggests (since the air's CO2 content was so much less during the MWP than it is now) that the lesser warmth of today need not in any way be related to the much higher CO2 concentration of earth's current atmosphere.

Reviewed 8 August 2007