What was done
The authors collected and analyzed various proxy climate data derived from ice cores, tree rings, river and lake sediments, lake terraces and paleosols, as well as historical documents, that enabled them to assess the climatic state of northwest China during the Western and Eastern Han Dynasties (206 BC-220 AD) relative to that of the past two millennia.

What was learned
Bao et al.'s analysis revealed, in their words, "strong evidence for a relatively warm and humid period in northwest China between 2.2 and 1.8 kyr B.P.," during the same time interval as what in regions surrounding the North Atlantic Ocean is referred to as the Roman Warm Period.  In fact, they determined that this period experienced "higher temperatures and higher precipitation than today." What is more, they report that "the warm-wet climate period during 2.2-1.8 kyr B.P. also occurred in central and east China, after [which] temperatures decreased rapidly (Zhu, 1973; Hameed and Gong, 1993; Yan et al., 1991, 1993; Shi and Zhang, 1996; Ge et al., 2002)," noting additionally that historical records report "an abrupt climate change from warmer and wetter to cooler and drier conditions occurred around A.D. 280 (Zhang et al., 1994)."  Last of all, they state that "three alternate China-wide temperature composites covering the last 2000 years display an obvious warm stage in 0-240 A.D. (Yang et al., 2002)," and that "according to a 2650-year warm-season temperature reconstruction from a stalagmite from Shihua Cave of Beijing (Tan et al., 2003), the temperatures during 2.1-1.8 ka B.P. were basically above the average of the entire temperature series."

What it means
In addition to helping to document the global nature of the Roman Warm Period and its superiority to the current world climatic state in terms of temperature and precipitation (all without the help of any of the increase in atmospheric CO2 concentration we have experienced over the course of the Industrial Revolution), Bao et al. note that "the warm and moist conditions during the Western and Eastern Han Dynasties might have been responsible for the large-scale agricultural production and the local socioeconomic boom that is documented by the occurrence of the famous ruin groups of Loulan, Niya, and Keriya."  Citing the existence of plant remains such as walnuts, rice, barley, millet and wheat grains, they also indicate that "water and temperature conditions were suitable for rice cultivation and much better than today."

References
Ge, Q., Zheng, J.Y., Man, Z.M., Fang, X.Q. and Zhang, P.Y.  2002.  Reconstruction and analysis on the series of winter-half-year temperature changes over the past 2000 years in eastern China.  Earth Science Frontiers 9: 169-181.

Hameed, S. and Gong, G.F.  1993.  Temperature variation in China during historical times.  In: Climate Change and Its Impact.  (Y. Zhang et al., Eds.) China Meteorology, Beijing, China, pp. 57-69.

Shi, Y. and Zhang, P.Y.  (Eds.) Climatic Variation in Historical Time in China.  Shandong Science and Technology, Jinan, China.

Tan, M., Liu, T.S., Hou, J. Qin, X., Zhang, H. and Li, T.  2003.  Cyclic rapid warming on centennial-scale revealed by a 2650-year stalagmite record of warm season temperature.  Geophysical Research Letters 30: 10.1029/2003GL017352.

Yan, Z.W., Ye, D.Z. and Wang, C.  1991.  Climatic jumps in the flood/drought historical chronology of central China.  Climate Dynamics 6: 153-160.

Yan, Z.W., Li, Z.Y. and Wang, C.  1993.  Analysis on climatic jump in historical times on decade-century timescales.  Sci. Atmos. Sin. 17: 663-672.

Yang, B., Braeuning, A., Johnson, K.R. and Shi, Y.F.  2002.  General characteristics of temperature variation in China during the last two millennia.  Geophysical Research Letters 29: 10.1029/2001GL104485.

Zhang, P.Y., Wang, Z., Liu, X.L. and Zhang, S.H.  1994.  Climatic evolution in China during the recent 2000 years.  Sci. China Ser. B. 24: 998-1008.

Zhu, K.Z.  1973.  A preliminary study on the climatic fluctuations during the last 5,000 years in China.  Sci. Sin. 16: 226-256.