Paper Reviewed
Villanueva-Díaz, J., Stahle, D.W., Therrell, M.D., Beramendi-Orosco, L., Estrada-Ávalos, J., Martínez-Sifuentes, A.R., Astudillo-Sánchez, C.C., Cervantes-Martínez, R. and Cerano-Paredes, J. 2020. The climatic response of baldcypress (Taxodium mucronatum Ten.) in San Luis Potosi, Mexico. Trees 34: 623-635.

Climate models predict rising concentrations of atmospheric CO2 will impact the hydrologic cycle. In order to evaluate or validate such model predictions, however, critical baseline data must be gathered so as to properly discern the presence or absence of an increasing CO2 fingerprint with time. The recent study of Villanueva-Díaz et al. (2020) accomplishes this important first step by reconstructing a long-term moisture record within the state of San Luis Potosi, Mexico.

Using tree-ring cores from baldcypress trees from two locations, the nine researchers applied standard dendrochronology methods to derive a long-term record of January-August Standardized Precipitation Evaporation Index (SPEI), which Index takes into account both precipitation and temperature and is therefore a useful indicator of moisture variability, including drought. The resultant reconstruction for the years 1575 to 1996 is shown below in Figure 1.

In describing the record, Villanueva-Díaz et al. report that periods of drought occurred in 1661-1674, 1680-1685, 1732-1734, 1813-1819, 1922-1929, 1947-1953 and 1993-1995, whereas wet periods occurred in 1586-1583, 1595-1625, 1640-1660, 1695-1720, 1755-1775, 1825-1845, 1885-1920 and 1965-1985. Power spectrum analysis employed on the reconstruction further revealed the existence of significant drought and pluvial frequencies at 19.5, 24.0, 66.9 and 70.6 years. The most dominant frequency, according to the authors, was at 66.9 years and "is related to the ENSO frequency where the most extreme events like droughts take place."

The observed cyclic nature of the reconstruction, coupled with what appears to be an absence of any trend in the data, suggests that rising concentrations of atmospheric CO2 have had no measurable impact on droughts or pluvials in the region of San Luis Potosi over the past 423 years. And that suggests no vindication for the models' projections on moisture. Rather, it points toward their invalidation in this regard!

Figure 1. Reconstructed January-August SPEI values for San Luis Potosi over the period 1575-1996. Adapted from Villanueva-Díaz et al. (2020).