Urban environments are often assumed to save energy, and I have no reason to dispute that assumption. But I wondered... do countries with high percentages of their population living in urban environments emit less carbon dioxide as a whole? I found the carbon dioxide emissions (per capita) here, the population distribution data here, and here's the chart:
As you can see, there is no negative relationship between the urban population of the country and its carbon dioxide emissions. In fact, there is a small positive relationship, although the correlation (r square = 0.068) is very small.
There are two conclusions we can draw here:
1) A third variable, such as levels of gas tax or alternative energy subsidies, is the primary force impacting carbon dioxide emissions per country.
2) Large urban populations may not be as effective at curbing pollution as assumed, perhaps because waste disposal is more difficult or because individuals have less space to grow their own food and must import resources from far away, which has a high transport cost.