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More than half the world’s population lives in cities. There is clearly a significant benefit to living in a large permanent settlement with many other humans–the prospect of housing, work, theatres and restaurants, potential friends and mates, to name just a few.

But is living in a large city greener than living in a small one? Today we get an answer thanks to the work of Erneson Oliveira and pals at the Federal University of Ceará in Brazil. These guys have analysed the CO2 emissions from every square 10 km in the US, as determined by study called the Vulcan project from Arizona State University. And they use this to work out the amount of carbon dioxide generated by its cities. They then ranked cities by size to see how the CO2 emissions scale.

The results make for interesting reading. It turns out that bigger cities produce more carbon dioxide per capita than small ones. That may come as a surprise, given that there are so many economies of scale at work in cities.

Indeed, Oliveira and pals result depends crucially on how you define a city. For example, if they take a standard definition known as a Metropolitan Statistical Area, then CO2 emissions scale isometrically with city size. In other words, large cities emit the same amount of CO2 per capita as small cities.

But Oliveira and pals argue that this defintion of cities doesn’t capture the truth. They use a different mechanism for defining cities based on a fine-grained measure of population density gathered by a research project claled the Rural Urban Mapping Project. This provides population data for each square kilometre of the US. An algorithm that looks for clusters in the data can then use this data to reveal the real size of cities, they say.

By this measure, large US cities have significantly smaller areas than the standard Metropolitan Statistical Area definition. And this is what leads to the conclusion that bigger cities produce more carbon dioxide per capita than small ones.

But even this shouldn’t really be a surprise, argue Oliveira and co. There are many aspects of cities that scale with population size, such as the number of jobs, houses and water consumption.

Equally, other aspects of cities do not scale with population size because of economies of scale. For example: the number of gas stations, the length of electrical cables and the amount of road materials.

But there are also quantities that scale at a rate that is greater than population size. For example, wages, income, bank deposits, rates of invention as measured by patents and so on. This suggests that when people get together, their productivity increases at a rate that is greater than the sum of the parts.

There is a flipside to this too—an increase in the amount of carbon dioxide they produce. “This result suggests that the high productivity of large cities [comes at] at the expense of a proportionally larger amount of emissions compared to small cities,” say Oliveira and pals.

And that’s why big cities produce more carbon dioxide per capita than small ones.

Of course, this work is likely to be controversial. Nobody agrees on how to define city size although we saw yesterday that “natural cities” can emerge from certain types of social media data. So it would be interesting to see how CO2 emissions scale with this type of city.

If Oliveira and co’s conclusion holds here too then that would be powerful supporting evidence that large cities really are less green.

Ref: arxiv.org/abs/1401.7720: Large Cities Are Less Green

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Tagged: Energy

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