Paper Reviewed
Liu, N., Liu, Y., Bao, G., Bao, M., Wang, Y., Ge, Y., Zhang, L., Bao, W. and Tian, H. 2015. A tree-ring based reconstruction of summer relative humidity variability in eastern Mongolian Plateau and its associations with the Pacific and Indian Oceans. Palaeogeography, Palaeoclimatology, Palaeoecology 438: 113-123.

In the past two decades, much scientific research has been conducted to examine the uniqueness (or non-uniqueness) of Earth's current climate in an effort to discern whether or not rising atmospheric CO2 concentrations are having any measurable impact on various climatic indices. The study of Liu et al. (2015) adds to the growing list of such studies.

As their contribution to this Herculean effort, the team of nine Chinese researchers collected tree-ring samples from the eastern Mongolian Plateau to reconstruct a long-term moisture record that would extend beyond the limited set of observations preserved in meteorological records for this area since the 1950s. The resultant chronology, as shown in the figure below, presents a 178-yr record of summer (Jun-Aug) relative humidity, tripling the temporal domain for which this hydroclimatic parameter can be examined. And what does this record reveal?

Figure 1. Reconstructed summer (Jun-Aug) mean relative humidity for the eastern Mongolian Plateau over the period 1835-2013. The bold line represents a 10-year low pass filter of the data and the horizontal line indicates the mean value of the entire span. Adapted from Liu et al. (2015).

According to Liu et al., there are multiple extremely wet (31) and extremely dry (25) years spaced across the record, defined as one standard deviation above and below the long-term mean, respectively. In addition, they note there are "several prolonged dry (>mean) and (<mean) wet periods" of 5 or more years duration. And making particular mention of the most recent dry period, Liu et al. state that the severe drought and low humidity experienced in this region during the late 1990s to 2000s "were not unusual from the long-term perspective" of their record, citing the similar multi-year low humidity/drought that occurred in this region during the1850s/60s. Moreover, in comparing their reconstruction with four other moisture-related proxies from the Mongolian Plateau, they similarly conclude that the most recent drought is "within the range of the hydroclimatic fluctuations ... over the past several centuries."

It would thus appear that there is nothing unusual, unnatural or unprecedented about the current hydroclimate of the Mongolian plateau. If CO2-induced climate change is having any impact at all, it is not patently obvious, as recent moisture deviations fall well within the bounds of natural variability.