Urbanization is one of the most important social forces at play around the world today. More than half of the world’s human population now lives in cities, with two billion more people expected to join urban centers over the next few decades. The concentration of people in cities provides both challenges for sustainable development, resulting from rises in living standards and increases in consumption, and opportunities afforded by economies of scale in material infrastructure and services. Does the balance between these contradictory forces result in more or less sustainable paths for human development? The answer to these questions requires a quantitative assessment over the many social, economic and environmental impacts of cities as they grow.  

Results/Conclusions

The main challenge to sustainable development – as defined by several indices of development – arises from CO₂ emissions. We analyze several new data sets on CO₂ emissions for US and a sample of worldwide cities to investigate whether urbanization leads to higher greenhouse gas emissions. We conclude that there are no indications for a positive correlation between per capita CO₂ emissions increases and urbanization. Moreover there are systematic trends indicating that denser cities achieve large savings in transportation emissions relative to sprawling metropolitan areas, which shows in turn how different development paths can lead to a reduction in the environmental impacts of human societies. Thus, although more quantitative evidence must still be collected, on the whole current data suggests that urbanization constitutes an opportunity for more sustainable economic development.