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Medieval Droughts of the Western United States
Kleppe, J.A., Brothers, D.S., Kent, G.M., Biondi, F., Jensen, S. and Driscoll, N.W. 2011. Duration and severity of Medieval drought in the Lake Tahoe Basin. Quaternary Science Reviews 30: 3269-3279.

The authors write that "evidence for medieval droughts of duration and magnitude much larger than those in the instrumental record has been reported throughout much of the world, but a particularly robust signal is expressed in the western United States (e.g., Cook et al., 2010; Woodhouse et al., 2010)," and they say that "this period of pronounced aridity, the Medieval Climate Anomaly of Stine (1994), is best expressed between ~ 800 and 1250 AD," when "the centennial average precipitation decreased to 75% and decadal averages decreased to 60% of twentieth-century values (Graham and Hughes, 2007)."

What was done
In a study designed to expand the MCA drought database in the western United States, Kleppe et al. (2011) reconstructed the duration and magnitude of extreme droughts in the northern Sierra Nevada region based on dendrochronology, geomorphic analysis, and hydrologic modeling of the Fallen Leaf Lake (California) watershed in order to estimate paleo-precipitation near the headwaters of the Truckee River-Pyramid Lake watershed of eastern California and northwestern Nevada.

What was learned
In the words of the six scientists, "submerged Medieval trees and geomorphic evidence for lower shoreline corroborate a prolonged Medieval drought near the headwaters of the Truckee River-Pyramid Lake watershed," when and where water-balance calculations independently indicate precipitation to have been "less than 60% normal." And they say these findings "demonstrate how prolonged changes of Fallen Leaf's shoreline allowed the growth and preservation of Medieval trees far below the modern shoreline." In addition, they note that age groupings of such trees suggest that similar mega-droughts "occurred every 600-1050 years during the late Holocene.".

What it means
Considered in their entirety, the several findings of Kleppe et al. (and many others who they cite) suggest that the Medieval Climate Anomaly - or Medieval Warm Period - experienced far less precipitation and far longer and more severe drought than what has been experienced to date in the Current Warm Period. In addition, their data suggest that such dry conditions have occurred regularly, in cyclical fashion, "every 650-1150 years during the mid- and late-Holocene." And all of these observations suggest that there is nothing unusual, unnatural or unprecedented about the nature of drought during the Current Warm Period, in contradiction of the unfounded claim of the world's climate alarmists to the contrary.

Cook, E.R., Seager, R., Heim, R.R., Vose, R.S., Herweijer, C. and Woodhouse, C. 2010. Mega-droughts in North America: placing IPCC projections of hydroclimatic change in a long-term palaeoclimate context. Journal of Quaternary Science 25: 48-61.

Graham, N.E. and Hughes, M.K. 2007. Reconstructing the Mediaeval low stands of Mono Lake, Sierra Nevada, California, USA. The Holocene 17: 1197-1210.

Stine, S. 1994. Extreme and persistent drought in California and Patagonia during medieval time. Nature 369: 546-549.

Woodhouse, C.A., Meko, D.M., MacDonald, G.M., Stahle, D.W. and Cook, E.R. 2010. A 1,200-year perspective of 21st century drought in southwestern North America. Proceedings of the National Academy of Sciences USA 107: 21,283-21,288.

Reviewed 8 February 2012