India's urban population grew from 290 million in 2001 to 340 million in 2008 and it is projected to reach 590 million by 2030. The country will have to build 700-900 million square meters of residential and commercial space a year to accommodate this growth, requiring an investment US$1.2 trillion to build 350-400 kilometers of subway and up to 25,000 kilometers of new roads per year.

Similarly, China's urban population is expected to increase from 636 million in 2010 to 905 million by 2030. It is predicted that by 2050 the country will need to invest 800-900 billion yuan per year to improve its urban infrastructure, about one-tenth of China's total GDP in 2001.

The nature of this investment will have significant effects on the potential of Indian and Chinese cities to be green.

As cities become more prosperous, with wider and deeper patterns of consumption and production, their environmental impacts are increasingly felt at the global level.

Urban areas in prosperous economies concentrate wealth creation as well as resource consumption and CO2 emissions.

Globally, with a population share of just above 50 per cent but occupying less than 2 per cent of the earth's surface, urban areas concentrate 80 per cent of economic output, between 60 and 80 per cent of energy consumption, and approximately 75 per cent of CO2 emissions.

This pattern is not equally distributed across the globe and reflects the concentration of particular activities within individual cities.

Buildings, transport, and industry - which are constituent components of cities and urban areas - contribute 25, 22, and 22 per cent, respectively, of global greenhouse gas emissions.

Between 1950 and 2005, the urban population grew from 29 per cent to 49 per cent of the global population while global carbon emissions from fossil-fuel burning increased by almost 500 per cent.

At the national level, urbanization goes hand in hand with increasing resource consumption.

Cities per se are neither drivers of climate change nor the source of ecosystem degradation; certain consumption and production patterns as well as certain population groups within cities are.

The relationship between carbon emissions and income levels is not straightforward, either.

Per capita incomes are generally higher in cities than in rural areas, generating higher average per capita demand in major emissions sources.

But this is the case only up to a certain income level, after which cities typically become more carbon-efficient compared with the average, as can be seen by the relatively low levels of CO2 emissions produced by high income cities like Tokyo or Paris.

A recent survey of the energy intensity (a measure of the energy efficiency of an economy calculated as units of energy per unit of GDP) of 50 cities by the World Bank confirms differential patterns of environmental performance.