What was done
The authors derived a 16,200-year-long palaeoclimatic record with nearly annual time-resolution from a sediment core extracted from Lake Huguang Maar (21°9'N, 110°17'E) in southeast China, based on continuous measurements of sediment titanium content and magnetic susceptibility and the acquisition of accelerator mass spectrometry ¹⁴C dates of five leaves and four bulk sediment samples.

What was learned
Yancheva et al. report that comparison of the titanium record they obtained from Lake Huguang Maar with the titanium record obtained by Haug et al. (2001) from the Cariaco Basin on the Northern Shelf of Venezuela "reveals similarities, including both a general shift towards drier climate at about AD 750 and a series of three multi-year rainfall minima within that generally dry period, the last of which coincides with the final stage of Maya collapse [AD 910] as well as the end of the Tang dynasty [AD 907]."

What it means
The ten researchers say the two sets of observations from opposite sides of the Pacific Ocean suggest the occurrence of "global [our italics] climatic changes" - in this case a worsening period of reduced precipitation interspersed with socially-devastating multi-year droughts - driven by "migrations of the intertropical convergence zone," which a number of studies, in addition to theirs, place at the approximate time-of-transition from the Dark Ages Cold Period to the Medieval Warm Period (Haug et al., 2003; Mayewski et al., 2004; Almeida-Lenero et al., 2005; Hodell et al., 2005). Consequently, the study of Yancheva et al. adds further credence to the ever-more-hard-to-deny fact that a non-anthropogenic-induced millennial-scale oscillation of climate is what produced the Roman Warm Period, Dark Ages Cold Period, Medieval Warm Period, Little Ice Age, and the Current Warm Period (see also, in this regard, the many items archived under the general heading of Climate Oscillations (Millennial-Variability) in our Subject Index).

References
Almeida-Lenero, L., Hooghiemstra, H., Cleef, A.M. and Van Geel, B. 2005. Holocene climatic and environmental change from pollen records of Lakes Zempoala and Quila, central Mexican highlands. Review of Palaeobotany and Palynology 136: 63-92.

Haug, G.H., Gunther, D., Peterson, L.C., Sigman, D.M., Hughen, K.A. and Aeschlimann, B. 2003. Climate and the collapse of Maya civilization. Science 299: 1731-1735.

Haug, G.H., Hughen, K.A., Sigman, D.M., Peterson, L.C. and Rohl, U. 2001. Southward migration of the intertropical convergence zone through the Holocene. Science 293: 1304-1308.

Hodell, D.A., Brenner, M. and Curtis, J.H. 2005. Terminal Classic drought in the northern Maya lowlands inferred from multiple sediment cores in Lake Chichancanab (Mexico). Quaternary Science Reviews 24: 1413-1427.

Mayewski, P.A., Rohling, E.E., Stager, J.C., Karlen, W., Maasch, K.A., Meeker, L.D., Meyerson, E.A., Gasse, F., van Kreveld, S., Holmgren, K., Lee-Thorp, J., Rosqvist, G. Rack, F., Staubwasser, M., Schneider, R.R. and Steig, E.J. 2004. Holocene climate variability. Quaternary Research 62: 243-255.