The world's cities are already responsible for the majority of energy consumption and rapid urbanisation is increasing their dominance. Inconsistent definition and limited data has made it difficult to compare emissions trends among the world's big cities. Here we use a series of remotely sensed proxies to estimate both the size of cities each year and their emissions. We decompose the emissions trends into changes in area, population density and per capita emissions. We see three dominant clusters. Most developed countries show significant growth in urban area counterbalanced by decreasing per capita emissions. Developing countries show increases in both these terms with per capita emissions rising faster in cities than in the country as a whole.

Almost alone among developing countries, China shows a "developed" pattern with rapid increases in area but reductions in per capita emissions; most likely a result of explicit policies around urban air quality. We also see a negative correlation between population density and per capita emissions, supporting the role of densification in emissions reduction. The results suggest that urbanisation is likely to remain a driver of accelerating emissions for some time but that this may reverse with continued economic development.