Nothing to do with carbon dioxide and you confirm it with no reference to it in your lettuce leaf beating reply.
Climate change is a given.
Just ask the Mayan and their huge, abandoned cities due to same.Again, nothing to do with carbon dioxide for them as well but just natural variability.
Population estimates
[edit]
Map of the Maya region showing locations of some of the principal cities. Click to enlarge.
Until the 1960s, scholarly opinion was that the ruins of Maya centres were not true cities but were rather empty ceremonial centres where the priesthood performed religious rituals for the peasant farmers, who lived dispersed in the middle of the jungle.[11]Since the 1960s, formal archaeological mapping projects have revealed that the ceremonial centres in fact formed the centres of dispersed cities that possessed populations that at some sites could reach tens of thousands.[11]
