Paper Reviewed
Hodgson, J.Y.S., Ward, A.K. and Dahm, C.N. 2013. An independently corroborated, diatom-inferred record of long-term drought cycles occurring over the last two millennia in New Mexico, USA. Inland Waters 3: 459-472.

Major droughts and floods of our day and age are typically said by climate alarmists to be the results of CO2-induced climatic aberrations. One way by which this claim can be evaluated is to reconstruct long-term histories of these phenomena that stretch from the present far back into pre-industrial times, when the air's CO2 content was much reduced compared to that of today.

This tack was taken by Hodgson et al. (2013), who, as they describe it, "investigated the late Holocene (2000 YBP [years before present] to present) drought in northern New Mexico, USA, using diatom values sampled from a lake sediment core," which they say they "analyzed with a combination of multivariate ordination and time series analysis to identify significant changes in community dynamics and corresponding significant bifurcations between periods of increased and decreased precipitation." They also report that their "diatom-inferred precipitation regime was statistically corroborated against an independently derived tree ring record of precipitation in northern New Mexico [Grissino-Mayer, 1996]." And they add that both of these records were further tested "for concordance with indirect radiocarbon solar intensity data," which had been obtained by Reimer et al. (2004). So what did they learn from their reconstruction?

The three U.S. researchers report that "periods of drought aligned with periods of decreased solar intensity during ~1400-1000 and ~600-200 YBP," while "periods of increased solar activity aligned with periods of increased precipitation during ~1000-600 YBP and ~200 YBP to present day." In fact, the "prevailing trend," as they described it, was that "as solar intensity decreased, tree rings indicated decreased precipitation and diatoms suggested lower lake levels."

Faced with these multiple findings, about the only thing Hodgson et al. could conclude was that "while correlation cannot prove causation, this significant 3-way correlation between solar intensity, diatoms and tree rings implied that these independent proxies could be linked and suggested that decreased solar intensity may have been a driving force in causing prolonged droughts in this climatic region." And given such correlation, one should be very cautious about inferring droughts in this corner of the world are primarily caused by anything else, including CO2-induced global warming.

References
Grissino-Mayer, H.D. 1996. A 2129-year reconstruction of precipitation for northwestern New Mexico, USA. In: Dean, J.S., Meko, D.M. and Swetnam, T.M. (Eds.). Tree Rings, Environment, and Humanity. Department of Geosciences, University of Arizona, Tucson, Arizona, USA, p. 191-204.

Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Bertrand, C., Blackwell, P.G., Buck, C.E., Burr, G., Cutler, K.B. et al. 2004. INTCAL04 terrestrial radiocarbon age calibration, 0-26 cal KYRBP. Radiocarbon 46: 1029-1058.