Background
In the words of Cai et al. (2014), drought is "one of the most devastating natural disasters throughout the world." As an example of the type of devastation it can cause, Cai et al. cite the consequences of an "extraordinary drought" that occurred in China in the late 1920s, which caused widespread famine and disease in five northern provinces and resulted in the death of 4 million people, citing the work of Liang et al. (2006) and Wang (2006). Against this backdrop, Cai et al. state that "investigations of the natural climate background of the Chinese Loess Plateau (CLP) are crucial for understanding the processes and characteristics of climate change in this region as well as the current status of the climate." Thus, they set out to conduct a historical analysis of drought for this region, attempting to answer the following two key questions: (1) "Did the drought severity or frequency increase in response to the global warming?" and (2) Is the present drought in the region "unprecedented during the last three centuries?"

What was done
Cai et al. developed a new regional tree-ring chronology from Chinese pine trees located in three different sites of the Lingkong Mountain area (112°10'-112°15' E, 36°31'-36°43' N) of the southeast CLP. The resulting chronology was positively correlated with monthly Palmer Drought Severity Index (PDSI) values obtained from meteorological data for the region over the period 1954-2005. Based on that correlation, the researchers were able to reconstruct a record of historic PDSI since 1703 AD.

What was learned
Seven dry periods and six wet periods were noted in the 306-year reconstruction. The driest interval occurred between 1867 and 1932, while the wettest interval followed between 1934 and 1957. With respect to the most recent drought (1993-2008), there was nothing unique or unprecedented about it. Rather, Cai et al. report it is "still within the frame of natural climate variability." In addition, multi-taper spectral analysis identified periodicities of 37.9 and 102 years in the reconstructed PDSI record, which Cai et al. say "resemble the 35-year Bruckner (Raspopov et al., 2004) and Gleissberg cycles of solar activity (Sonett et al., 1990; Braun et al., 2005), respectively." Further analysis of the PDSI reconstruction and the NOAA sunspot time series revealed a significant correlation between the two variables that Cai et al. say "convincingly [supports] the influence of solar activity on moisture variations in the Lingkong Mountain area."

What it means
The results of the analysis speak for themselves in answering the two questions posed by the authors at the beginning of their study. Recent drought in the CLP is not unprecedented and is more likely the product of natural, as opposed to anthropogenic, forcings.

References
Braun, H., Christl, M., Rahmstorf, S., Ganopolski, A., Mangini, A., Kubatzki, C., Roth, K. and Kromer, B. 2005. Possible solar origin of the 1,470-year glacial climate cycle demonstrated in a coupled model. Nature 438: 208-211.

Liang, E.Y., Liu, X.H., Yuan, Y.J., Qin, N.S., Fang, X.Q., Huang, L., Zhu, H.F., Wang, L.L. and Shao, X.M. 2006. The 1920s drought recorded by tree rings and historical documents in the semi-arid and arid areas of northern China. Climatic Change 79: 403-432.

Raspopov, O.M., Dergachevb, V.A. and Kolström, T. 2004. Periodicity of climate conditions and solar variability derived from dendrochronological and other palaeo-climatic data in high latitudes. Palaeogeography, Palaeoclimatology, Palaeoecology 209: 127-139.

Sonett, C.P., Finney, S.A. and Berger, A. 1990. The spectrum of radiocarbon. Philosophical Transactions of the Royal Society of London A 330: 413-426.

Wang, Y. 2006. The preliminary study on the natural disaster in north Shaanxi during 1923-1931. Meteorology and Disaster Reduction Research 29: 34-38.