On the impact of urbanisation on CO₂ emissions

We use a globally consistent, time-resolved data set of CO₂ emission proxies to quantify urban CO₂ emissions in 91 cities. We decompose emission trends into contributions from changes in urban extent, population density and per capita emission. We find that urban CO₂ emissions are increasing everywhere but that the dominant contributors differ according to development level. A cluster analysis of factors shows that developing countries were dominated by cities with the rapid area and per capita CO₂ emissions increases. Cities in the developed world, by contrast, show slow area and per capita CO₂ emissions growth. China is an important intermediate case with rapid urban area growth combined with slower per capita CO₂ emissions growth. Urban per capita emissions are often lower than their national average for many developed countries, suggesting that urbanisation may reduce overall emissions. However, trends in per capita urban emissions are higher than their national equivalent almost everywhere, suggesting that urbanisation will become a more serious problem in the future. An important exception is China, whose per capita urban emissions are growing more slowly than the national value. We also see a negative correlation between trends in population density and per capita CO₂ emissions, highlighting a strong role for densification as a tool to reduce CO₂ emissions.