What was done
The author characterized the history of Little Ice Age glacier fluctuations in eastern Tibet by studying (1) the ages of trees growing on glacier deposits, (2) chronologies of tree total ring width, which he found to be "positively correlated with temperatures of early summer (June and July) and especially with November to January preceding the growth season," and (3) chronologies of tree maximum latewood density, which he says "is highly correlated with the temperature sum of August and September," derived from trees growing on slopes above the glacier valleys.

What was learned
Just as cold is sometimes defined as the absence of heat, so too may the Little Ice Age be defined as the absence of the warmth that once sustained the Medieval Warm Period and that now sustains the Current Warm Period, as is basically done in this study. Brauning notes, for example, that (1) "tree-ring width and δ¹³C chronologies from long-living Juniperus tibetica from Qamdo in eastern Tibet have shown that the period from 1150 to 1380 was characterized by higher temperatures than today (Helle et al., 2002; Bao et al., 2003)," that (2) "the succeeding period from 1430 until the late nineteenth century witnessed a series of cold intervals indicated by growth reductions and changes in the isotopic composition in juniper trees," and that (3) the intervening period that lacked the warmth of the Medieval and Current Warm Periods "is here referred to as the 'Little Ice Age' period in Tibet."

What it means
The important message of this and numerous other studies of the Little Ice Age, in our opinion, is that there was a preceding Medieval Warm Period that helped define the Little Ice Age; and in the case of Tibet, that Medieval Warm Period was even warmer than the Current Warm Period. Consequently, since there was much less CO2 in the air between 1150 and 1380 than there is currently, there is ample reason to believe that the planet's current warmth may be due to something quite apart from its elevated atmospheric CO2 concentration, because something else was clearly the cause of the still-unsurpassed high temperatures of the Medieval Warm Period.

References
Bao, Y., Brauning, A. and Shi, Y. 2003. Late Holocene temperature fluctuations on the Tibetan Plateau. Quaternary Science Reviews 22: 2335-2344.

Helle, G., Schleser, G.H. and Brauning, A. 2002. Climate history of the Tibetan plateau for the last 1500 years as inferred from stable CARBON isotopes in tree-rings. Proceedings of the International Conference on the Study of Environmental Change Using Isotope Techniques, International Atomic Energy Agency CN-80-80, Vienna 22-27 April 2001, pp. 301-311.