Why did they leave? It is impossible to find a single cause that can explain it, but there appear to be several that contributed. First, the climate during the Pueblo III period was somewhat unstable with erratic rainfall patterns and periods of drought. This weather problem climaxed with a thirty-year drought starting about 1270 that coincided with a cooling trend that significantly shortened the growing season. Perhaps the expanding population had pressed the limits of the land's capacity to support the people so that they were unable to survive the climatic upheavals of the thirteenth century.