From this study, it appears that the combined energy intensity of major cities like Paris, Dhaka, Sao Paulo, London, Hong Kong, and Tokyo, amount to about one-quarter of that of the five highest scoring cities and less than half of a 50-city average.

In order to better understand these variations, data on 735 cities in six regions were analyzed. The results show that a majority of cities in Brazil, China, South Africa, India, Europe and the United States cities outperform their national average in terms of income per capita, education and employment levels.

In terms of carbon emissions, energy, electricity and water consumption, dwelling and transport patterns and motorization, however, there is a very marked difference between cities in developed and developing countries.

Whereas cities in Europe, the US and Brazil have a lower environmental impact than their respective countries, cities in India and China have a much larger impact owing to their significantly higher income levels compared with their national averages.

Patterns of urbanization in many areas also raise important social challenges. The traditional business-as-usual model of urban development - typical of many rapidly urbanizing areas - is characterized by uncontrolled horizontal expansion leading on one hand to urban sprawl of affluent populations with lower development densities and increased dependency on the private car and on the other hand to the peripheralization of the urban poor, decreasing their access to the city and its workplaces, services and infrastructure.

Typical developments further include the emergence of socially divisive neighborhoods in the form of gated communities, shopping centers and business districts and, a significant increase in the level of informal development with large swathes of slum housing with no access to basic services, infrastructure and sanitation.

At a general level, the rapid growth of many cities coupled with insufficient resources and poor management compromises fresh water and electricity supply, waste treatment, transport, and other infrastructure provision, affecting the urban poor most.

Cities can be designed, planned and managed to limit resource consumption and carbon emissions.

Or, they can be allowed to become voracious, land-hungry, all-consuming systems that ultimately damage the delicate global energy equation.

More compact urban forms, reduced travel distances and investment in green transport modes lead to greater energy efficiency.

In short, effective urban planning and governance can have significant effects on sustainable urban lifestyles, making the most of urban critical mass and reducing individual patterns of consumption.

This article is adapted from a recent UN Environment Program report on green economies.