If I was a young person interested in demography there is one job that I would pursue relentlessly. I would do whatever it took to manoeuvre my way into the United Nations Department of Economic and Social Affairs Population Division. Seriously, this group does the coolest stuff. Plus, it is based in New York. How good is that!
In an extraordinary feat of technical proficiency the UN population division has produced five-yearly estimates of the population of every city on the planet between 1950 and 2030 where the base population in 2015 is greater than 300,000. Eight Australian cities make the grade: Sydney, Melbourne, Brisbane, Perth, Adelaide, Gold Coast, Newcastle and Canberra.
There are 1692 cities listed in this database. Collectively they accommodate 2.2 billion people or 31 per cent of humanity. The biggest city on earth is Tokyo with 38 million but which is expected to subside to just 37 million by 2030 and which is in line with Japan's expected overall population decline. Tokyo will have a surfeit of housing in 15 years; not a good omen for residential property prices. 
Tokyo wrested the title of world's biggest city from New York in the mid-1950s and it will hold it for 75 years. At some point in the early 2030s the largest city will be Delhi which is now second with 26 million; however by 2030 Delhi is expected to accommodate 36 million residents.
By the middle of this century Delhi could evolve into an urban mass of 45 million. Who knows, by the end of the century this city could approach 60 million. After all, India will replace China as the most populous nation on earth at some point in the 2020s. As long as India remains socially and politically cohesive Delhi could be -humanity's first true urban coagulation, the quintessential fright city of the 21st century.
Closer to the current time the UN city database identifies cities that have been catapulted from nothing to something within a few decades, and which might be termed the shock cities of the early 21st century.
And in terms of shock cities Beijing stands apart. Beijing has doubled its population by adding 10 million residents over the past 15 years. No other city has matched this scale and pace of growth. An average of 680,000 residents per year have been added to Beijing in the early years of this century, which was a time of low fertility in China.
By my reckoning Beijing attracts and has attracted for the better part of a decade close to 600,000 rural-to-urban migrants every year. Every day 1600 people move to Beijing. This must be physically and visually evident on the outskirts of the city and in the bus and train stations within Beijing. Rural migrants arriving and alighting into what they hope will be a better life. If Delhi is to be the fright city of the mid-century then Beijing is the shock city of the early century. In 1950 Beijing contained 1.6 million residents; today it contains 20 million.
But there are other urban agglomerations that have attracted similar numbers over the period from 2000 but off a higher base. Shanghai added 10 million to reach 24 million, Dhaka added seven million to reach 18 million, Karachi added seven million to reach 17 million and Lagos added six million to reach 13 million.
I would go so far as to suggest that no city in human history has added as many people as have Beijing, Shanghai and Delhi since the year 2000. Other smaller cities may have doubled, tripled, quadrupled over the same time frame but none have attracted such a quantum of humanity over this time. Something powerful is driving this urban shift; that something is an irrepressible aspiration to achieve a better life if not for adult migrants then for the migrant's children.
The shock cities of the early years of the 21st century are being driven by a mix of economic attraction (Beijing, Shanghai, Delhi and perhaps Sao Paulo) and rural poverty (Dhaka, Karachi, Lagos and perhaps Cairo). Given the right circumstances cities can blossom and burgeon within a single generation. Just to put the scale of shock-city growth into perspective: Melbourne is now adding 95,000 new residents a year, about one-sixth of that being added to Beijing, Delhi and Shanghai.
Consider some of the most remarkable urban transformations that have taken place over the last generation. Shenzen, the sixth biggest city in China, now has 10 million residents. Located on the mainland near Hong Kong, it contained 174,000 residents in 1985. In 1950 Shenzhen was about the same size as Australia's township of Gundagai. What would be the circumstances that could take Gundagai to double Sydney's population by 2090?
Chongqing, China's fourth biggest city with 13 million residents, has added five million people over the last 15 years. About 40 million people have gravitated to six cities in China over the last 15 years and a further 25 million have been added to six cities in India.
The rise of Asia's middle class, be it in China or in India or indeed elsewhere, is more likely to have resulted from rural-urban migration than from any in situ generational transition with which Westerners are more familiar.
The UN city database offers more than the mere documentation of where and when urban growth has occurred, it provides a glimpse into what the world might look like mid-century.
With a steadily rising proportion of the world's population living in cities there will be increased demand for Australian energy (coal, uranium, gas), steel (iron ore), resources (copper, nickel, bauxite) and commodities (wheat, meat, dairy).
But there is another insight that derives from the database that is even more challenging. The primacy of American power is eroded by the rise of Chinese cities and by economic and military power. China will seek to shore up its supply lines through trade with -nations like Australia.
Australia will become increasingly enmeshed with Chinese culture and interests through immigration, through business ties including property investment, through education and tourism, and through aviation connectivity.
Eventually there will be, there must be, direct air links between Sydney and Melbourne and China's rapidly expanding second-tier cities such as Chongqing (population 14 million), Guangzhou (12 million), Tianjin (11 million) and Shenzhen.
The rural to urban migration that has delivered a middle-class "western" quality lifestyle will not abate in China or indeed in India. The ensuing social and lifestyle division would otherwise lead to internal dissent within these nations. It must be concluded that this century's shock cities in China and in India have further to grow.
The real challenge globally is not so much in managing and making functional China's middle-class megacities but in managing the mass of humanity living in subsistence circumstances in -places like Lagos, Dhaka, Karachi and Cairo.
For Australia there is economic opportunity in supplying the wherewithal to deliver bigger middle-class cities. But there is also risk in the coalescing of large numbers of disadvantaged and disaffected into large poorly managed and frightening cities.Bernard Salt is a KPMG partner and an adjunct professor at Curtin University Business School; bsalt@kpmg.com.au