What was done
The authors synthesize the findings of a variety of paleoclimate studies based on analyses of the sediment records of several crater lakes and lakes formed by lava dams that are scattered across the Trans Mexican Volcanic Belt of central Mexico and that have an absolute chronology provided by radiocarbon dates extending back to 1500 ¹⁴C yr BP.

What was learned
Noting that the degree of coherence among the records "is remarkable," Metcalf and Davis report - in what is perhaps the key finding of their analysis - that "dry conditions, probably the driest of the Holocene [our italics], are recorded over the period 1400 to 800 ¹⁴C yr BP (ca. AD 700-1200)," the significance of which finding is augmented by their observation that "the present day climate of central Mexico is typical of most of the country." Giving the result even broader significance is the fact that it is, in the words of the two researchers, "consistent with results from the Yucatan Peninsula (Hodell et al., 1995, 2005) .. and from the Cariaco basin (Haug et al., 2003) and the Isthmus of Panama (Lachniet et al., 2004)." What is more, Mayewski et al. (2004) have identified the central portion of this period (AD 800 to 1000) as a time of truly global anomalous climate.

What it means
This study provides convincing evidence that one of the strongest manifestations of the Medieval Warm Period throughout most of Mexico, and even extending beyond its borders, was a major lack of moisture, which in this particular part of the world better delineates the temporal realm of the Medieval Warm Period than even the epoch's primary defining characteristic of elevated temperature. And this is why we sometimes rely on moisture characteristics to help identify the Medieval Warm Period in this and other parts of the world in our growing-by-the-week Medieval Warm Period Project.

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

Hodell, D.A., Curtis, J. and Brenner, M. 1995. Possible role of climate in the collapse of classic Maya civilization. Nature 375: 391-394.

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.

Lachniet, M.S., Burns, S.J., Piperno, D.R., Asmerom, Y., Polyak, V.J., Moy, C.M. and Christenson, K. 2004. A 1500-year El Niņo/Southern Oscillation and rainfall history for the Isthmus of Panama from speleothem calcite. Journal of Geophysical Research 109: 10.1029/2004JD004694.

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.