Then, having concentrated and consumed both nutrients and water, they mostly throw them away, often horribly polluted with toxins. Having half the global population, cities already concentrate more than half the world’s food nutrients. And within a generation they will also concentrate half the available freshwater, leaving insufficient water to spare for the farmers who are being asked to double food production. City planners and administrators, however, are for the most part not interested in capturing these precious resources in order to sustain all citizens in the future, seeing them only as “waste” to be disposed of. Yet, if the world is short of water and nutrients, it makes sense to harvest them where they are most readily and cheaply available. Within a de cade or so, our cities can be turned into mines for water and nutrients. All these precious substances can then be reused again and again. Humanity is thought to produce around 3 billion tonnes (3.3 billion U.S. tons) of phosphorus in its sewage, so, in theory at least, the world’s cities concentrate around 1.5 billion tonnes (1.7 billion tons)— an im mense resource that is largely wasted by flushing it into the oceans.
There is an urgency about this that has not so far received due attention. Half the world’s people now live (and, by midcentury, threequarters of the world’s people will live) in cities in circumstances where they will be totally without the means to feed themselves. Today’s mega - cities are founded on the complacent assumption by urban planners that, what ever happens, there will always be a vast river of food flowing in from outside every day of the year. Yet, as we have seen, this assumption rests on increasingly tenuous supplies of water, land, and nutrients and on other deepening uncertainties such as transportation fuels and world peace. Never has urban civilization as a whole been so at risk of catastrophe— yet so astonishingly blind to it. If food supplies to a major city of twenty to thirty million people were cut off even for a week or two, the consequences would be horrendous.
For reasons of health and amenity, city planners, investors, and health authorities have sought to banish almost all forms of food production from within the urban perimeter, driving them farther away, into less reliable regions and even into other countries. This is a pro cess that must now be reversed: cities must again become farms. They must recover a degree of the self- sufficiency they enjoyed in earlier phases of civilization. And they must cease squandering freshwater and nutrients as if there were no tomorrow. It is time to recognize that regulations about waste disposal and urban farming, created with perfectly sound public health aims in view, threaten far more lives in the event of major food scarcities than they will save.
There is a growing movement, led by a handful of architects and horticulture specialists, to “green” our cities. This refers less to aesthetics or the introduction of clean air, novel transportation systems, and “ecobuilding codes” than to the essentials of human survival: food and water. It proposes growing food crops, especially vegetables and fruits, on, in, around, and even underneath buildings.20 It proposes recapturing water and nutrients and returning them to the wider farming landscape. It proposes developing entirely novel systems of food production, such as edible plant cell cultures, microbial protein, and artificial photosynthesis for turning hydrocarbon waste into carbohydrate nutrition.
Algae farms are another important development. These can convert waste nutrients and water into food, animal feed, fertilizer, transportation fuel, and even pharmaceuticals and fine chemicals using a range of pro cesses, but driven principally by sunlight. Algae already foul many of our rivers, lakes, estuaries, and coastline waters due to the nutrient hemorrhage we have created, so the option of deliberately cultivating the beneficial ones and thus harvesting lost nutrients takes full advantage of this trend, enabling us to recapture nutrients before they vanish into the oceans.