These cities are bigger than many industrialised nations. And they are growing at a dizzying rate, sucking in workers from rural areas.
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Economically, many of the world’s great cities are already divorced from their nation-states, with their main streams of investment come from other great cities.
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Shanghai has so much power and autonomy it has been described as effectively a city-state, within China only in geography. And on Thursday London Mayor Ken Livingstone was handed a raft of new powers over planning, housing and the environment.
He joked: "Having been to Singapore and seen how successful it was I think anything short of a fully independent city state is a lost opportunity, with its own foreign and defence policies thrown in."
There was a great series of sci-fi novels back in the ’50′s by James Blish collectively titled “Cities in Flight”. The premise was that as Earth’s infrastructure collapsed, the big cities, aided by an anti-gravity technology called “spin-dizzies”, took to space, where they mined asteroids and traded with other space cities. I don’t remember the details of any of the stories, but remember reading them with delight at their inventiveness and detail of plot and character. Beneath Blish’s sci-fi conceit was the recognition that the major cities of Earth, even at that time, had more in common with one another than they had with the population of whatever countryside they happened to have grown up in.