The American Scientist and the Scientific American both have an article in their July edition on collapse of ancient societies in America. The Scientific American article is called Simulating Ancient Societies and is written by Tim Kohler, George Gumerman and Bob Reynolds (pp. 76-84). They report on a number of studies using agent-based models to explain the disappearance of a number of Anasazi cites in the South West of the USA. They conclude that it can not just have been a drought that led to the collapse, also cultural factors need to be included to explain the observations. The article in the American Scientist is by Larry Peterson and Gerald Haug (Climate and the Collapse of Maya Civilization, pp. 322-329). They report on detailed climate recordings from analyzing titanium concentrations in a lake in Venezuela, which is a strong indicator for the weather patterns in the Maya sites, more north in Mexico, Guatamala and Belize. They find a strong correlation between periods of regional collapse and droughts. The spatial pattern of collapse can be affected by the local conditions since people in northern Yucatan had a natural storage of water in caves, which led to a delay of the collapse.
Both cases show that both climate and social factors play an important role to understand collapse of the diverse ancient societies in America.