What was done
The authors "developed a continuous, exactly dated, tree-ring reconstruction of maize yield variability in central Mexico from 1474 to 2001 that provides new insight into the history of climate and food availability in the heartland of the Mesoamerican cultural province." This work was made possible by latewood-width data they derived from what they describe as "the second-most southerly native stand of Douglas-fir (Pseudtosuga menziesii) trees known in the Americas." In addition, they compared their reconstruction to "historical records of crop failure and famine in order to cross-validate the tree-ring and historical records."

What was learned
Therrell et al.'s plot of reconstructed drought-induced maize-yield anomalies reveals a total of seven major decadal-scale yield shortfalls over the past 500 years, with a mean rate of occurrence of 1.5 per century over the 400-year period AD 1500-1900. Over the 20th century, however, there was only one such multi-year famine, and its magnitude pales in comparison to that of the average such event of the preceding four centuries.

Many of the more deadly earlier droughts also brought pestilence to central Mexico, one of which included the 1786 "Year of Hunger," which is described by Gibson (1964) as "the most disastrous single event in the whole history of colonial maize agriculture," wherein an associated outbreak of epidemic disease killed as many as 300,000 people (Florescano, 1986). So extraordinary was this event, Therrell et al. say Florescano hypothesized it "contributed to lasting civil unrest that may ultimately have influenced the Hidalgo Rebellion in 1810 and independence from Spain."

What it means
With respect to the threat of drought-induced crop yield reductions that are predicted by climate alarmists to accompany rising temperatures, it is clear that what they describe as the unprecedented warming of the 20th century did not produce the predicted effect in central Mexico. In fact, the threat of major drought-induced famines in this part of the world appears to have lessened with increased warming.

References
Florescano, E. 1986. Precios del Maiz y Crisis Agricolas en Mexico: 1708-1810. Ediciones Era, Mexico D.F.

Gibson, C. 1964. The Aztecs Under Spanish Rule: A History of the Indians of the Valley of Mexico. Stanford university Press, Stanford, CA, USA.