What was done
The authors analyzed pollen profiles derived from sediment cores retrieved from Lake Zempoala (19°03'N, 99°18'W) and nearby Lake Quila (19°04'N, 99°19'W) in the central Mexican highlands about 65 km southwest of Mexico City.

What was learned
Almeida-Lenero et al. determined that it was generally more humid than at present in the central Mexican highlands during the mid-Holocene.  Thereafter, however, there was a gradual drying of the climate; and their data from Lake Zempoala indicate that "the interval from 1300 to 1100 cal yr BP was driest and represents an extreme since the mid-Holocene," noting further that this interval of 200 years "coincides with the collapse of the Maya civilization."  Likewise, they report that their data from Lake Quila are also "indicative of the most arid period reported during the middle to late Holocene from c. 1300 to 1100 cal yr BP."  In addition, they note that "climatic aridity during this time was also noted by Metcalfe et al. (1991) for the Lerma Basin [central Mexico]," that "dry climatic conditions were also reported from Lake Patzcuaro, central Mexico by Watts and Bradbury (1982)," and that "dry conditions were also reported for [Mexico's] Zacapu Basin (Metcalfe, 1995) and for [Mexico's] Yucatan Peninsula (Curtis et al., 1996, 1998; Hodell et al., 1995, 2001)."

What it means
Based on proxy temperature data from North America, it would appear that some of the driest conditions of the Late Holocene throughout much of Mexico may have occurred during the climatic transition between the Dark Ages Cold Period and the Medieval Warm Period.

References
Curtis, J., Hodell, D. and Brenner, M.  1996.  Climate variability on the Yucatan Peninsula (Mexico) during the past 3500 years, and implications for Maya cultural evolution.  Quaternary Research 46: 37-47.

Curtis, J., Brenner, M., Hodell, D. Balser, R., Islebe, G.A. and Hooghiemstra, H.  1998.  A multi-proxy study of Holocene environmental change in the Maya Lowlands of Peten Guatemala.  Journal of Paleolimnology 19: 139-159.

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

Hodell, D., Brenner, M., Curtis, J. and Guilderson, T.  2001.  Solar forcing of drought frequency in the Maya Lowlands.  Science 292: 1367-1370.

Metcalfe, S.E.  1995.  Holocene environmental change in the Zacapu Basin, Mexico: a diatom based record.  The Holocene 5: 196-208.

Metcalfe, S.E., Street-Perrott, F.A., Perrott, R.A. and Harkness, D.D.  1991.  Palaeolimnology of the Upper Lerma Basin, central Mexico: a record of climatic change and anthropogenic disturbance since 11,600 yr B.P.  Journal of Paleolimnology 5: 197-218.

Watts, W.A. and Bradbury, J.P.  1982.  Paleoecological studies at Lake Patzcuaro on the West Central Mexican plateau and at Chalco in the Basin of Mexico.  Quaternary Research 17: 56-70.