Moderate drought was downfall of the Maya

BBC : From about AD 250 to 800, Mayan civilization, which thrived in what is now southern Mexico and Guatemala, developed a culturally sophisticated empire made up of several independent city-states. Between AD 800 and 900, however, large-scale architectural construction ceased and the civilization went into lengthy decline, although Mayan civilization overall continued until the Spanish conquered the last city-state, Tayasal, in 1697. Many reasons for the decline have been proposed, ranging from severe drought to trade route collapse. Eelco Rohling of Southampton University in the UK and Martin Medina-Elizalde of the Yucatan Center for Scientific Research in Mexico have published a study in Science in which they suggest that a sustained but relatively mild drought was the cause. Using quantitative analysis to examine the paleoclimate records of the event, they were able to estimate rainfall and evaporation rates between AD 800 and 950. They found that a reduction of 25–40% in rainfall was enough to deplete the Mayan freshwater storage systems.