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31 Centuries of Drought in North-Central Minnesota, USA
Reference
Tian, J., Nelson, D.M. and Hu, F.S. 2006. Possible linkages of late-Holocene drought in the North American mid-continent to Pacific Decadal Oscillation and solar activity. Geophysical Research Letters 33: 10.1029/2006GL028169.

What was done
The authors derived a high-resolution δ18O record of endogenic calcite, which they obtained from sediments extracted from Steel Lake (4658'N, 9441'W) in north-central Minnesota, USA.

What was learned
Tian et al. first demonstrated that at Steel Lake, evaporative 18O enrichment related to aridity was the "dominant control" of calcite δ18O, which they found to be "strongly correlated with the index of the Pacific Decadal Oscillation during the past 100 years." In addition, they note that "time series of δ18O and atmospheric Δ14C [a proxy for solar activity] are generally coherent after 700 AD."

Other findings of interest were that "the region was relatively dry during the Medieval Climate Anomaly (~1400-1100 AD) and relatively wet during the Little Ice Age (~1850-1500 AD), but that the moisture regime varied greatly within each of these two periods." Their most striking finding of all, however, was the fact that "drought variability was anomalously low during the 20th century." In fact, it was so depressed, as they describe it, that "~90% of the variability values during the last 3100 years were greater than the 20th-century average."

What it means
Climate alarmists contend that global warming brings with it greater variability in all types of extreme weather conditions, drought being one of the primary phenomena they cite in this regard. Throughout the entire course of the 20th century, however, over which period of time they claim the earth warmed at a rate and to a temperature unprecedented over the past thousand to one million years (Hansen et al., 2006), drought variability in north-central Minnesota was nowhere near "unprecedented." In fact, it was anomalously low.

Reference
Hansen, J., Sato, M., Ruedy, R., Lo, K., Lea, D.W. and Medina-Elizade, M. 2006. Global temperature change. Proceedings of the National Academy of Sciences USA 103: 14,288-14,293.

Reviewed 5 December 2007