One iconic statement from the leaked emails in the Climategate incident is this sentence from a Phil Jones email of 16 November, 1999:

“I've just completed Mike's Nature trick of adding in the real temps to each series for the last 20 years (ie from 1981 onwards) amd [sic] from 1961 for Keith's to hide the decline.”

Mike is Michael Mann, creator of the hockey-stick graph, and Keith is Keith Briffa. These folks gathered tree-ring data and tried to construct a temperature history of the earth. They plotted the data from some time in the past up to—almost—the present, and then dropped the last couple of decades because their data did not match the instrumental record. (Their data turned down—declined—instead of rising.) So they dropped that part and stitched the rest of their data to the instrumental record. What they actually hid was the fact that their proxy was no good; else why did it not work when the temperature data were actually known?

It is a fool’s errand to try to get a temperature record from tree rings, whose width depends upon amount of sunlight, nutrients, shade, insects, water, and species as well. There is a perfectly good way to use wood to learn about weather history. The wood that should be used is the pulp that has been turned into paper on which scholars have written contemporary data.

One case in particular is a recent paper [10] that uses Chinese records to show that in China “war frequency, price of rice, locust plague, drought frequency, and flood frequency were temperature-related, with cooler conditions being worse. The collapses of the Han, Tang, Song and Ming agricultural dynasties were associated with periods of cooling [4]. They say, “Our study suggests that food production during the last two millennia has been more unstable during cooler periods, resulting in more social conflicts owing to rebellions within the dynasties or/and southward aggressions from northern pastoral nomadic societies in ancient China.”

[4] Zhibin Zhang and 6 co-authors, “Periodic climate cooling enhanced natural disasters and wars in China during AD 10–1900,” Proceedings of the Royal Society, June 23, 2010, http://rspb.royalsocietypublishing.org/cont ent/early/2010/07/13/rspb.2010.0890.shor t?rss=1