From the press release:
"Our results show rather modest rainfall reductions between times when the Classic Maya Civilization flourished and its collapse – between AD 800-950. These reductions amount to only 25 to 40 per cent in annual rainfall. But they were large enough for evaporation to become dominant over rainfall, and open water availability was rapidly reduced. The data suggest that the main cause was a decrease in summer storm activity."
Science 24 February 2012:Vol. 335 no. 6071 pp. 956-959 DOI: 10.1126/science.1216629
Collapse of Classic Maya Civilization Related to Modest Reduction in Precipitation
Martín Medina-Elizalde, Eelco J. Rohling
The disintegration of the Classic Maya civilization in the Yucatán Peninsula and Central America was a complex process that occurred over an approximately 200-year interval and involved a catastrophic depopulation of the region. Although it is well established that the civilization collapse coincided with widespread episodes of drought, their nature and severity remain enigmatic. We present a quantitative analysis that offers a coherent interpretation of four of the most detailed paleoclimate records of the event. We conclude that the droughts occurring during the disintegration of the Maya civilization represented up to a 40% reduction in annual precipitation, probably due to a reduction in summer season tropical storm frequency and intensity.