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.