Ecological factors often played a role in the formation of cities, but in this particular case these would appear to be only obliquely related to the intensification of agriculture. Still, there are hints of a pattern. Across many parts of Eurasia, and in a few parts of the Americas, the appearance of cities follows quite closely on a secondary, post-Ice Age shuffling of the ecological pack which started around 5000 BC. At least two environmental changes were at work here.
