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A Precipitation History of the Southwestern United States
Rasmussen, J.B.T., Polyak, V.J. and Asmerom, Y. 2006. Evidence for Pacific-modulated precipitation variability during the late Holocene from the southwestern USA. Geophysical Research Letters 33: 10.1029/2006GL025714.

What was done
The authors have previously demonstrated that "speleothems from the Guadalupe Mountains in southeastern New Mexico are annually banded, and variations in band thickness and mineralogy can be used as a record of regional relative moisture (Asmerom and Polyak, 2004)." Here, they continue this tack, concentrating on "two columnar stalagmites collected from Carlsbad Cavern (BC2) and Hidden Cave (HC1) in the Guadalupe Mountains."

What was learned
First of all, Rasmussen et al. report that "both records, BC2 and HC1, suggest periods of dramatic precipitation variability over the last 3000 years, exhibiting large shifts unlike anything seen in the modern record [our italics]." Second, they report that the period from AD 900-1300 "includes severe drought events, consistent with tree-ring data for the western U.S. (Cook et al., 2004)." Third, whereas this time interval coincides with the well-known Medieval Warm Period and "shows dampened precipitation variability and overall drier conditions" that are "consistent with the idea of more frequent La Niņa events and/or negative PDO phases causing elevated aridity in the region during this time," they note that the preceding and following centuries (AD 100-750 and AD 1500-1800) "show increased precipitation variability ... coinciding with increased El Niņo flooding events."

What it means
First of all, climate alarmists clearly go overboard when they (1) characterize significant droughts and floods of recent times as "unprecedented" over the past millennium or more and (2) attribute them to CO2-induced global warming. Moisture extremes much greater than those observed in the modern era are neither unusual nor manmade; they are simply a normal part of earth's natural climatic variability. Second, and also contradicting climate-alarmist doctrine, Rasmussen et al.'s data clearly reveal the occurrence of the Medieval Warm Period, as well as the Dark Ages Cold Period that preceded it and the Little Ice Age that followed it, in terms of the available moisture side of the climate-change coin; for in this part of the world, global warmth is typically manifest in terms of low available moisture, while global coolness is typically manifest in terms of high available moisture. Third, the three researchers' work, like that of many others, suggests that El Niņo events are typically less significant during multi-century warm periods than they are during multi-century cold periods and the transitions between the two states, which is also something different from what climate alarmists have long proclaimed.

Asmerom, Y. and Polyak, V.J. 2004. Comment on "A test of annual resolution in stalagmites using tree rings." Quaternary Research 61: 119-121.

Cook, E.R., Woodhouse, C., Eakin, C.M., Meko, D.M. and Stahle, D.W. 2004. Long-term aridity changes in the western United States. Science 306: 1015-1018.

Reviewed 30 August 2006