Julian Cribb | City populations are large and unsustainable. There are ways to fix it

Today the world has 45 megacities - conurbations holding more than ten million citizens apiece - and some 500 smaller cities each of a million or more souls. Together they are home to well over than half of the human population, rising to 7 billion by mid-century.

For all their glorious diversity of culture and design, our cities share one thing in common: they are equally at risk of sudden collapse. Not one of them can feed itself - and few provide enough water internally for the needs of their population.

A typical megacity of 10 million or so people - like Mumbai, London, Nagoya, Hyderabad or New York for example - occupies an area of about 1000 square kilometres. But to feed and water 10 million people requires an area of land and water outside the city of 2750 square kilometres. The total urban footprint is thus nearly four times the size of the city itself.

All that food and water has to reach the city along increasingly fragile supply chains which stretch for thousands of kilometres around the globe, across other countries and oceans.

Furthermore, our cities exist within a profoundly disturbing global reality: that it currently takes about 2.75 hectares of the Earth's productive area to support the average human - yet the Earth itself only has 1.5 hectares of biologically productive land and water to sustain each one of us.

This means that today's big cities are inherently unsustainable.

Yet urban citizens, blithely shopping in supermarkets that seem to bulge with food and drink, are blind to this primal fact, or else dismiss it. There is scant awareness that the world is already, effectively, out of accessible fresh water, is losing fish, forests and farm soil at shocking rates and is increasingly exposed to climate-driven famines.

As recent history has shown, supermarkets can empty of food in 48 hours or less, if the supply chain snaps. By the end of the week, a city of 10 million may starve, unless urgently resupplied.

A disturbing scientific paper in Nature recently warned that the risk of simultaneous harvest failures in several of the world's major grain growing regions was climbing.

Global heating not only increases the strength and frequency of droughts and famines, it said, but also the probability of their occurring in more than one food bowl at once. That can seriously dislocate the food chain that feeds the megacities. And every city on Earth will feel the pinch in soaring food prices, if not outright scarcity.

Water also underlies the frailty of urban civilisation: at present some two dozen major cities face serious scarcity. These include places such as Beijing, New Delhi, Los Angeles, Cape Town, Sao Paulo, Moscow, Lahore, Istanbul, Mexico City, Jakarta, Cairo and Tokyo. Water is essential to food production as well as domestic drinking.

By 2050, water scarcity will affect at least 20 megacities, housing one-third of the human population. Such warnings place the future of our megacities in a chilling light. Not one of them is prepared for what humans, with our unassuaged hunger for resources, have already set in train. The assumption that food and water will always be plentiful is a universal delusion soon to be exploded.

Today's cities are socially constructed realities, meaning that most of their occupants view them as economic powerhouses, engines of growth and technological progress, seats of government, centres of learning or healing and fountains of culture, entertainment and the arts. Yet all these realities can vanish in a flash if food and water supplies fail.

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Nowadays, many people are aware that a collapse of global civilisation is possible - as the UN warned in its 2022 Global Risks report. They are asking: when will this collapse begin? How will we know it has started? Will there be enough forewarning to save ourselves?

There is no easy answer to such questions as collapse depends on many variables, especially on how effectively and fast nations and governments respond worldwide. At present, it is fair to say, they are completely unprepared. There is no global plan of action nor even a set of viable national plans of action to save us.

Just as the climate tipping points are signposts to an uninhabitable world, the first failure of a megacity will be a thunderous warning that global civilisation is in the collapse zone.

A failed city of 10 million will spew out refugees like a human volcano, precipitating horrors on affected regions like nothing since the famines and wars in Europe, India and Asia of last century, only on a scale four times larger. This will unleash a deadly domino-chain among neighbouring cities and countries, leading to a global economic crash and a spate of government failures.

Is there anything that can be done to prevent this from happening? The answer is yes - but it needs to happen now - and fast.

The world's cities need to:

Such measures will not prevent the collapse of global civilization. But they may cushion the shock for individual megacities when it happens and save countless lives.

Given the immense creativity, thought and effort that has gone into building our cities, it is astonishing how little care, forethought and ingenuity has gone into seeing they survive in the long run. What happens to cities individually will define what happens to the human species as a whole.

The dream of urban renewal in the current century does not mean more parks, arts centres, malls, cosy apartments and gleaming architectural monoliths. For, without secure food and water, none of these things matter.

The city is the cornerstone of human civilisation. It must have a renewable foundation of food and water - or else it is built on quicksand.