What was done
Based on Qilian juniper (Sabina przewalskii) tree-ring width chronologies obtained from both living trees and archaeological wood located in the mid-eastern Tibetan Plateau (which data were calibrated against measured air temperatures for the period AD 1958-2000), Liu et al. obtained a mean annual temperature history of the region spanning the 2,485-year period 484 BC - AD 2000, which they demonstrated to be well correlated with several temperature histories of the Northern Hemisphere.

What was learned
In contrast to climate-alarmist claims that temperatures near the end of the 20th century (which have not since been eclipsed) were unprecedented over the past couple of millennia, the eight researchers report that there were four periods to have average temperatures similar to "or even higher than" the mean of AD 1970-2000, beginning with the warm period AD 401-413, which they say "was the warmest period within the last 2.5 thousand years." And in further support of their "even higher than" statement, they note that an archaeological documentary record from Loulan in Xinjiang province shows that pomegranate (a vitamin-C rich fruit) was employed as currency during the Eastern Jin Dynasty (AD 317-589), because the appearance of pomegranate during that period "suggests that the temperature at that time was higher than nowadays," citing Zhang and Zhang (2006). In addition, they indicate that the rate of warming that led to the early ultra-warm period of their record was "unprecedented in the last 2500 years." And we note that the last of the four ultra-warm periods was also slightly warmer than it was at the end of the 20th century.

What it means
Liu et al. report that the high-temperature intervals of the AD 400-1000 period in the mid-eastern Tibetan Plateau were what could be described as relatively good times, as they indicate that the downfalls of most major dynasties in China coincided with intervals of low temperature, or at least the beginnings of their downfalls did, citing in this regard the demise of the Qin, Three Kingdoms, Tang, Song (North and South), Yuan, Ming and Qing Dynasties. And so we have yet another set of instances where "warm" is nearly always better than "cold," and where pre-20th century warmth was equal to, or even greater than, earth's current level of warmth.

Reference
Zhang, X.W. and Zhang, J.B. 2006. Handbook of Xinjiang Weather. Meteorological Press, Beijing, China.