while some folks play hockey...

No, that is not a reference to Governor Palin and her children, but rather a reference to the attempt to revive the controversial "hockey stick" depiction of average global temperatures by Dr. Michael Mann, director of the Earth System Science Center at Pennsylvania State University. Mann who first published his "hockey stick" curve in the peer reviewed journal Nature in 1998, published a new study this week in the Proceedings of the National Academy of Sciences. The new analysis purports to support the findings of a decade ago with additional temperature proxy data.

As it should be in the natural sciences, other climatologists are already examining the data series on which Mann has based this newest study, and asking tough questions about Mann's criteria for including and excluding proxy series, particularly wondering why some (such as a proxy series by Yamal in 2002) that have figured prominently in other research have been excluded from this study. Many kinds of proxy studies using ice cores, tree rings, deep-sea and lake sediment cores, and coral records are used to approximate temperature readings in many geographic regions around the world.

This legitimate and vociferous scientific debate focuses not on the current warming trend, which seems accepted by all participants, but on the degree and extent of warming during the "Medieval Warm Period" which affected Europe between approximately 1100 and 1300 C.E. (Common Era). How warm did it get, and how does that compare to today?

While scientists debate the interpretation of proxies, and how warm the "Medieval Warm Period" actually was, there is another kind of less ambiguous form of evidence from the period, that is of more interest to us who are not professional climatologists. Anthropologist Brian Fagan, has assembled information from written historical sources and archaeological excavations, that chronicle the social, economic and political impacts of the "Medieval Warm Period" around the world in his 2008 book The Great Warming: Climate Change and the Rise and Fall of Civilizations (Bloomsbury Press). The "great warming" referred to in the title is not the present one, but rather the one between 1100 and 1300.

Fagan looks not only at the agricultural and technological improvements in Europe, but also at the impact of drought in central Eurasia on the western push of the Mongols, the enterprising way in which Moorish traders made drought work for them and their camel caravans, and the impact of mega-drought on the native populations of the American southwest and California coast -- including the abandonment of Chaco Canyon pueblos, and much much more. A fascinating read for those interested in the way in which climate interacts with social structure and technology.