As the study put it, "70 to 80 percent of the India of 2030 is yet to be built."

Ask Sankhe how he arrived at that staggering figure, and you get the sense that it's not a precise estimate—and more a rough and impressionistic measure of untapped possibility. He applies it not just to India but also to other emerging markets like Bangladesh and Indonesia, and bases it on a variety of factors.

"Seventy percent of these cities are not yet built," he explained. "Seventy percent of almost everything: the water systems, the houses.... Sixty percent of Mumbai, for example, live in slums, so their houses are not yet built. The transportation networks: Mumbai has finished only one corridor, it requires 15 corridors of metro. So 14 corridors are not yet built.... Not the population—the population is there. Everything related to climate change, everything related to energy sufficiency, a lot of things are not yet built. So we have a rare chance of designing it right."

'Designing it right,' in Sankhe's opinion, includes integrating strategies for combating climate change into the design of cities, where energy consumption and greenhouse-gas emissions are concentrated. It's a particularly critical challenge for India, which is one of the world's largest greenhouse-gas emitters and has the second-highest number of city-dwellers living in low-elevation coastal areas that are susceptible to rising sea levels.

As his McKinsey study cautiously outlined:

[C]ities around the world already offer some excellent examples of innovative approaches for sustainability that India could incorporate into urban planning and development. One interesting example is Masdar, a clean-energy cluster city being developed on the outskirts of Abu Dhabi city. Abu Dhabi is designing Masdar to produce zero waste, be self-sufficient in terms of water, and be powered solely be renewable energy. Masdar city will support 40,000 residents and 50,000 commuters, will be car-free, and will incorporate light rail transit, personal rapid transit, and associated logistics.... Nevertheless, our analysis suggests that the scope in India to create new 'zero carbon' cities will be limited because the majority of India's urban growth will come from the expansion of existing cities—a far more complex undertaking, in fact, than building state-of-art new cities.

The additional catch is that developing all the infrastructure Sankhe is calling for—the highways, public housing, potable water—costs a lot of money. And it's money that India's urban population doesn't have. Sankhe estimates that of the 300 million people expected to migrate to Indian cities over the next two decades, 60 percent earn less than $1 per day per capita.

"Housing in this market costs some hundred times that monthly income," he said. "So it's simply unaffordable." He forecasts that the Indian government will have to invest $1.2 trillion in urban infrastructure between now and 2030, and that the city-dwellers who benefit from these expenditures will only be able to pay for a fraction of that price tag. He calls this conundrum the "mass-balance of money."