  We have assembled CO2 emission figures from collections of urban GHG emission
estimates published in peer reviewed journals or reports from research
institutes and non-governmental organizations. Analyzing the scaling with
population size we find that the exponent is development dependent with a
transition from super- to sub-linear scaling. From the climate change
mitigation point of view, the results suggest that urbanization is desirable in
developed countries and should be avoided in developing ones. Further, we
compare this analysis with a second scaling relation, namely the fundamental
allometry between city population and area, and propose that density might be
the decisive quantity. Last, we derive the theoretical country-wide urban
emissions by integration and obtain a dependence on the size of the largest
city.
