Reference
Hodell, D.A., Brenner, M., Curtis, J.H. and Guilderson, T. 2001. Solar forcing of drought frequency in the Maya lowlands. Science 292: 1367-1370.
What was done
The authors analyzed lake sediment cores taken from Lake Chichancanab on the Yucatan Peninsula of Mexico, reconstructing the climatic history of this region over the past 2600 years.
What was learned
Long periods of drought were noted throughout the record, and spectral analysis revealed a significant recurrent drought periodicity of 208 years that matched well with a cosmic ray-produced 14C record preserved in tree rings that is believed to reflect variations in solar activity. Because of the good correspondence between the two data sets, the authors concluded that "a significant component of century-scale variability in Yucatan droughts is explained by solar forcing."
What it means
The results of this study demonstrate that drought is a recurring feature of climate in the Yucatan Peninsula (and therefore not CO2-induced, as the air's CO2 content has not varied in an oscillatory manner over the period of study). Quite to the contrary, the data suggest that variable solar activity has been the major player in orchestrating the cyclical history of drought in this region.