How does it look-and-feel, the big, grand city of the mid-century? If you're seven years old, everything in it feels equally wondrous. The big city is a riot of sight, sound and smells – as vivid, exciting and scary for you as any big town has ever been for anybody.

No one can overlook buildings of that colossal size – but why do they exist? A city's showplaces are always built by people anxious about their own status. In 2050, the nouveau-riche arrivistes stake their big skyline claims on the public eye. That glassy, twisting spire, as gaudy as any Christmas ornament, is owned by offshore Chinese. The gloomy tower with 85 stories of modestly greyed-out windows is an all-female enclave of Islamic business feminists. The scary heap that resembles a patchwork quilt of iron was entirely crowd-sourced.

Cars piloted by human beings were a passing thing in the ageless urban story. The urban highways are still there – far too many of them, all old – but it's network-driven robot cars, like smartphones with wheels, that deliver the payloads now. The traffic signs and signals are long gone, since machines don't need them. This city never stops – the wheeled machines flow night and day through every intersection, busy as ants, silent as eels.

There's no urban smog, but the city reeks. This dense, greenhouse stink is composed of the rot from flood damage, the decay of dead lawn and parks, and bursting, sneezy clouds of weedy pollen from invasive species. At the seashores, the great, flood-stricken port cities of the past smell like dead fish and invasive brine. This fetid greenhouse fever doesn't smell much worse than the urban smog that brought it into being. People are used to it.

Urban cats are everywhere, since people much prefer pets to children. The "human bubble" has reached its downslope. The old Population Bomb is now a rubble-clearance project. The cats are meticulously tracked by surveillance collars, and they never stray.

The same goes for the elderly. The old have become mankind's majority, for now and apparently forever, the avant-garde of the urban machine-for-living. The old pay well for their dignity, for the always-on augmentation and the ubiquitous computing. They pass their endless twilight days in padded penthouses, half spa and half life-support module, urban spaces so intensely surveilled that one will never lose a button or drop a lit match.

Modern cities are elderly, too. Brick and stone are mortal, and entropy requires no maintenance. Every major urban industry leaves its silent retinue of dead smokestacks. The early 21st Century left a rich heritage of quaint, gentlemanly rubbish: the archaic cellphone towers, the poisonous and horrifying fossil-fuel plants, the squalid paper-shuffling headquarters of extinct government bureaus. Commonly, this is where the cities stuff the climate refugees.

The poor we always have with us, because somebody is always in the business of keeping the poor that way, and the poor can always be relied upon to rob and oppress each other. The great city of the future has slums. It has red-light districts. It has pawnshops and sweatshops, and parlours for the various illicit substances that used to be called narcotics. The big city is the wicked city. No big city has ever lacked for wickedness since the time of Ur of the Chaldees. A city that failed to generate some enticing crimes would have to invent brand-new ones.

With all its timeless continuities, the mid-century metropolis does have novel and startling aspects. Ever since their invention, cities were elite barns for the sturdy peasantry of some fertile countryside. The mid-century city has created means of food production that are post-agricultural. With swordfish extinct and cattle way beyond the budget, the people eat – well, to put it bluntly, they mostly eat algae, insects and microbes. Of course this tasty goop has been effectively refined, rebranded, and skeuomorphically re-packaged as noodles, tofu, and hamburger substitute. Soylent Green is crickets.

Every urbanite loves to fuss about fine dining. The upside of a major climate crisis is the prospect it offers to entirely liberate cities from their sordid heritage in the planet's soil. A space colony is just a Dubai-style super-tall desert skyscraper – plus some zero-gravity bone depletion. A lunar colony is just a London mogul's subterranean basement, without the crusties or the labour strikes.

The urbanites in the mid-century city know that they are not the culmination of the city. No one's idea of utopia, they're not even "modern". Everybody under 30 years of age is instinctively convinced that they are the cultural radicals, the cool and daring pioneers, the youthful froth of a tsunami of some radically different way of being – and indeed, they are. Not "better" mind you – just different.

There is fear in this mid-century city. Life is frail. A vengeful super-hurricane might cross the simmering Gulf Stream and fall like an avenging angel on the coasts of Europe – but people can get used to that. Megastorms aren't that much worse than Los Angeles on a fault line, Naples on a volcano.

The scary part is what people find within themselves, when their city is gravely harmed. People can flee with relative ease, but cities are tender and sessile beings. When the survivors return to their beloved rubble, they find themselves forced to create another city – one that makes genuine technical sense under their circumstances.

Only engineers and architects will ever rub their hand at this dreadful prospect. These modernists are in secret collusion with the feral urban crows and hungry pigeons picking over the blast zone. For years, while a sentimental mankind clung to a museum economy, they have rehearsed another city, some angular, rational monster with an urban fabric that's a whole lot more nano-, robo-, and geno; buildings they can shape, and that will henceforth shape the rest of us.

To tell the truth, we never liked that city. But it just keeps happening.

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