In contemplating how heavy a city is, we tend to think of the mass of solid, constructed stuff—buildings, roads, vehicles, bridges, etc.—contained within its limits. We might ponder including the people of the city or its trees, grass, and soil, despite the fact that none are manufactured. Surely the city is nothing without its people—and how could they function without green space to enjoy? This surely begs further questions: What about the food and water the citizens eat and drink (or bathe in)—should that be on the scales? What about the citizens’ waste products? Now we find we have started asking questions not just about a city’s mass, but also about its metabolism.
We have also put liquid on the scales, and a lot of it. London, for example, consumes around 460 million tons of water each year. That is a renewable resource, but what about other liquids like the fuel going into the vehicles of the city? And what about the gas (mostly carbon dioxide) that comes out of their tailpipes? That also weighs something, as did the oxygen that went into the engine. So too the oxygen the citizens breathe in, and the carbon dioxide they exhale. Just because these gases are invisible does not make them weightless. Official figures state that each year London emits 28.4 million tons of CO2 equivalents (Mt CO2e per year), which means it consumed a comparable mass of hydrocarbon fuels, food, and oxygen. But if we consider the emissions associated with everything consumed within the city, the figure is more like 120 Mt CO2e per year.
This is not a stock of material, but a flow. Nonetheless, it adds to a corresponding stock of CO2 in the atmosphere. Notably, to make every ton of cement in a city, 0.6–0.8 ton of CO2 was emitted. For each ton of steel, around 1.85 tons of CO2 were emitted. There is an invisible city of CO2 that weighs as much as the visible, manufactured city. That ghost city of CO2 is now spread throughout the atmosphere. Shouldn’t it also be on the scales?
Gaia
This metabolic approach to a city mirrors how James Lovelock started taking a new look at life on Earth in the 1960s. Rather than consider life as objects (organisms) which can be put on the scales, Lovelock considered life as a process and started tracing its exchanges of energy and materials beyond the organisms themselves, as far as he could. In so doing, he discovered that living beings produce what we call their “environment.” Life has completely transformed the atmosphere of our planet. It has also transformed the land surface, the cycling of many elements, the climate, and the composition of the oceans, in ways that tend (overall) to support the persistence and flourishing of life.
Lovelock’s Gaia hypothesis recognizes that if living beings produce their environment, then their material boundaries no longer stop at their membranes; they expand as far as their influence can reach. Things that were thought of before as “abiotic,” like the atmosphere, can now be seen as a living part: “like the fur of a mink or the shell of a snail.”1 This process is as old as life itself, a staggering four billion years old.
There is nothing new in a city’s metabolism altering the atmosphere. To stay alive, organisms must take in materials and free energy from their surroundings, use them to build their bodies, and excrete less useful energy and altered material waste products. Cities do the same. Like an organism, a city does not have firm boundaries or an “environment” in the way we usually think of it. But if we look closer, there is something different about the metabolism of a city.
Industrial Metabolism
Cities—and humanity more generally—are heterotrophic (like other animals, fungi, and many single-celled organisms). They do not synthesize their own food. Instead, they obtain energy and materials by consuming other sources of organic carbon. But unlike other heterotrophs, industrialized humans and our cities don’t just consume food. We consume fossil fuels, which are concentrated forms of ancient organic carbon. This massively boosts our energy supply, and we use it to source all sorts of materials. The issue is we consume far more fossil organic carbon than food—about thirty times more for a UK citizen—which generates a massive increase in waste products.
In the biosphere, autotrophs like plants, algae, and cyanobacteria capture energy, often from sunlight, and use it to synthesize their own food, from carbon dioxide and other elements. They recycle the carbon and other elements excreted by heterotrophs, powered by solar energy. But this natural recycling system cannot cope with the massive new flux of waste products coming from humans and their cities. Hence the waste products pile up: in the atmosphere, oceans, sediments, and landfill sites.
This current “industrial metabolism” has a further, alarming quality: the waste products are piling up ever faster. Cities play a key role in the “Great Acceleration” of our impact on Gaia, because they have an unusual quality: as they grow, their economic output grows faster. Compare this to an animal: as they get larger, from a mouse to an elephant, their metabolism tends to slow down. Cities are the hives of exponential economic growth, but this—like a cancer—is potentially pathological. So long as that growth is powered by fossil fuels it will translate into increasing waste, the consequences of which are coming back to bite us.
Gaia invites us to trace the metabolic consequences of a city beyond its boundaries, as far as we can trace them. This leads to some striking discoveries. Cities have a vast ecological footprint: for London it is 49 million hectares, roughly twice the entire area of the UK.2 From a Gaian point of view, their industrial metabolism is a disaster. So, how do we need to change it?
A Choice
Some say that to achieve long-term sustainability, we should retreat into a world of lower energy, lower material consumption, and lower population. But this is at odds with current global trends. Energy demand, material consumption, and population are all increasing. Population is projected to peak at around ten billion later this century and then start to decline, but growing per capita consumption is already dominating our impact on Gaia.3 The rich certainly have plenty of scope to reduce their consumption without any detriment to their wellbeing, and in the UK that is starting to happen (decoupling growth from energy and meat consumption, for example). However, at a deeper level, retreat seems at odds with the self-propelling growth that manifests as cities, and the continual force of innovation that lies behind it.
There is another path we can take: a revolution into a high energy, high recycling world that can support billions of people as part of a thriving biosphere. The key to this path is to learn some lessons from Gaia: our societies must be powered by sustainable energy and use some of that energy to recycle all the materials we need to flourish. Luckily, there is an awful lot of sustainable energy available for human use, and there is nothing fundamentally wrong about high energy use per se. It is our increasing flow of material waste products that is the real problem—they are what cause the damages, and ultimately Earth’s surface has only a finite supply of materials. What innovations then will we need to enjoy sustainable flourishing?
Gaia Devices
Imagine a contraption that converts whatever material waste products you put into it into new products that you desire. To do this without breaking the laws of thermodynamics, the contraption needs a source of energy, and (assuming it is not a nuclear reactor) the stuff that comes out must have the same masses and proportions of elements as what goes in. Perfect remanufacture is difficult to achieve because it involves separating all the materials that come in before combining them in a new way. This means creating order—working locally against the second law of thermodynamics (which tells us the universe is inexorably tending toward disorder). This also always costs useful energy. But if there is an abundant enough energy supply, the efficiency of recycling can approach 100 percent.
This thought experiment implies that achieving a sustainable economy is limited by energy supply, not by material supply, provided that recycling is highly efficient. In principle, if humans have an abundant source of energy, we can minimize the problems of accumulating waste products by increasing the efficiency of recycling. This is commonly referred to as the “circular economy.” We can save a lot of the energy needed to power a circular economy from what is currently used to extract materials from the Earth’s crust or the atmosphere but then goes to waste, often after a single use. For example, it takes four times as much energy to make new steel from iron ore as it does to recycle existing steel, and twenty times as much energy to make aluminum from bauxite as it does to recycle aluminum.
You might have pictured this recycling contraption as a metallic and industrial form, but the closest example we have of this is not a human construction at all: it is Gaia itself. Gaia recycles every atom of carbon entering from the Earth’s crust around 400 times before losing it back to the crust, and every atom of phosphorus over 1000 times. Every atom of nitrogen that life fixes from the atmosphere is cycled around 100 times before being returned to the atmosphere.
If we want to flourish long-term, we need to emulate Gaia. We need to switch to sustainable energy and use some of it to power “Gaia devices” that we design to recycle our waste products far more efficiently than we are currently doing. We need to do this fast, because we are heading into a storm of climate change, which we will not be able to ride out if we stick with our current trajectory.
In this vision, cities will become “the mines of the future,” as Jane Jacobs foresaw in 1961.4 But they will be remaking themselves continuously as part of a much more Gaia-like human enterprise. This is akin to making the industrial metabolism more autotrophic.
Power
The most massive unrecycled waste product of the current industrial metabolism is the invisible city of CO2 that we continue to add to the atmosphere. Hence, the single biggest reduction in human waste production would come from stopping burning fossil fuels and switching to sustainable energy sources.
Burning stuff to get energy is a prehistoric way of living. It is dirty and wildly inefficient: nearly two-thirds of the “primary” energy in fossil fuels is wasted before it does any useful work or produces any benefit to us. But we have kept doing it because fossil fuel energy has been relatively cheap: its cost has stayed roughly the same for over a century. Fossil fuels also have a high energy density—a small volume of fuel can produce a lot of energy—making them convenient to propel things a long way or create intense heat or thrust.
Alternatives have existed for a long time. In 1884 Charles Fritts installed the first solar cell on a New York city rooftop. In the 1890s, Poul la Cour produced a wind turbine generating electricity, and in 1903 helped establish the Danish Wind Electricity Company. By the 1920s, hydropower was being widely used to split water in electrolysis and create “green hydrogen,” which could then be combined with nitrogen to make “green ammonia” for fertilizer (in the Haber-Bosch process). It is just that fossil fuels outcompeted these options on price.
But now renewable energy is the cheapest source of power almost everywhere in the world, and the more it gets deployed, the cheaper and better it gets. This is thanks to well-known reinforcing feedback loops of learning-by-doing and economies of scale. Solar energy is also abundant. The power in sunlight reaching the Earth’s land surface exceeds current total human power consumption by a factor of over a thousand, and solar photovoltaic (PV) cells can capture up to 30 percent of it. This means that considerably less than 1 percent of the Earth’s land surface needs to be covered in solar panels to meet current energy demand.
Solar PV is set to dominate future power supply, complemented by some wind power. Some cities’ future power supply will be generated on solar rooftops, roads, and walkways, but much of it will have to come from outside (just as it does now). That hinterland of solar panels and wind turbines will cover a larger area of land or sea than the current fossil energy supply, but it will eliminate a massive footprint on the atmosphere.
The future city will be thoroughly electrified with the likes of LED lighting, electric motors, and heat pumps. Electrification is highly efficient. Where energy needs to be stored—for example to power an electric vehicle—batteries are increasing in energy density and declining in price the more they get deployed. For those end uses that cannot be electrified, renewable electricity can be used (once again) to make green hydrogen, which in turn can be used to synthesize a range of green fuels and chemicals, including green ammonia.
The greater overall efficiency of an electrified economy means that switching to renewable power will tend to reduce overall energy demand, but demand to power Gaia devices will grow.
Concrete and Metals
The next largest waste product of industrial metabolism is concrete. It takes a lot of energy to make and is also a significant source of CO2, which is released in the process of calcining cement (to make clinker). Around half of all mass currently going into landfills is concrete. Concrete recycling can radically reduce this material footprint by nearly 100 percent.5 It uses less energy than making fresh cement and can eliminate most of the emissions—although it faces a host of practical challenges.6
Iron (often as steel) is the next most massive waste stream, and by far the largest metal flux due to humans. Extracting iron ore then making steel is energy intensive and responsible for around 7 percent of all CO2 emissions. Together iron and aluminum represent more than 95 percent by mass of all metals mined. Globally, there is already significant recycling of iron and aluminum, and there is considerable scope to increase recycling and an energy incentive to do so. Recycling steel with an electric arc furnace uses over 70 percent less energy than making it from iron ore with a fossil-fueled blast oxygen furnace. Even in a steady state economy, some new steel will be needed, because it cannot all be recycled. But green hydrogen and renewable energy can be used to turn iron ore into steel. For aluminum, recycling saves 95 percent of energy, and could approach 100 percent efficiency.
Copper and zinc are the next most massive metal waste streams, followed by chromium, lead, and nickel. Around 95 percent of used copper is potentially recyclable. Currently an increasing range of critical metals are being put into technology, but the urban mining of e-waste is already more energy efficient and cost effective than virgin mining. Complex alloys and complex products that mix different materials together can make recycling very difficult, but there is considerable scope for better design to improve recyclability.
The Gaia devices doing all this material recycling will involve a lot of human employment. They would logically be placed within cities to minimize the energy costs of transporting heavy materials. Thus, the future city will become more like the biosphere, with solar energy flowing in from outside (in the form of electricity) and material cycling within.
Food and Land
Unless we evolve a symbiotic relationship with photosynthetic organisms under our skin, the citizens of the city (and their pets) will remain heterotrophic. Therefore, food will continue to have to flow in (largely) from outside. The most effective way to reduce the land footprint of the city is for citizens to eat less animal products, starting with beef and lamb. Just eliminating beef from diets worldwide would halve the global land area needed for agriculture. Eliminating dairy products would halve it again. Eating less meat also improves citizens’ health. If citizens really cannot face this, then alternative proteins can mimic meat products with a fraction of the footprint, or meat could be cultured, powered by solar energy.
Urban agriculture cannot feed a whole city, even with vertical farming, and it is not an efficient way to try. Currently its carbon footprint is six times higher than conventional agriculture on average, although some specific crops like tomatoes and some home gardens can be efficient. The most efficient food system is a globalized not a localized one. This is because it uses the most productive land for each crop, wherever that is, and also because the emissions associated with transporting food are relatively small compared to those associated with producing it. Furthermore, those transport emissions can be eliminated by electrification or powering long-distance shipping with green ammonia.
The globalized future food system needs to shift to increased efficiency and recycling. Currently, nitrogen and phosphorus are poured onto farmland as fertilizers that are inefficiently utilized by plants, inefficiently transferred to livestock, rarely make it into our food, and ultimately lost in our waste products. There is a finite supply of phosphorus from rock phosphate reserves, from which it takes a lot of energy to extract. Nitrogen can be fixed from the atmosphere indefinitely to make ammonia for fertilizer, but this is energetically costly. Instead of using fossil fuels as a source of hydrogen, this can be replaced with green hydrogen to make green ammonia fertilizer. But still, fertilizer loading needs to be reduced to lower impacts on freshwaters and the generation of the potent greenhouse gas nitrous oxide. Precision agriculture can greatly improve the efficiency of nutrient use by crop plants. Nutrients removed with the crop need to be recycled locally to the land as compost, animal dung, and urea. Crucially, to close the phosphorus cycle, phosphate needs to be extracted from cities’ wastewater and recycled globally for agricultural use.
The invisible city of CO2 that accompanies the visible city can also be recycled, but the best way to do this depends on reducing the city’s land footprint. Already some of the invisible city of CO2 is being taken up by natural ecosystems and the ocean (albeit the latter acidifies in the process). The best way to remove the rest would be to regenerate forests and soil carbon stores that have been removed by the city’s past agricultural expansion, driven largely by the growing appetite for meat. The way to create the space to regrow forests is to shrink agricultural land by eating less meat and maximizing the efficiency of the globalized food system. Forests will be regrown wherever less productive pastureland goes out of use. Ultimately, however, the visible city may come to store some of its invisible legacy of CO2, like in high-tech wooden buildings made from sustainable harvesting of regrowing forests.
Incentives
Clearly the how of achieving all this is another matter. To transform, our industrial metabolism needs a change in incentive structure. One would think survival and flourishing would be enough of an incentive, but sadly our current economic system doesn’t make Earth-system sense. It must change, and it is in the hands of governments to change it, for the public good. A price should be put on the ecological damages caused by our material waste products, and recycling and remanufacture from waste should be rewarded beyond the energy savings it often achieves. This means valuing nature and the life support it provides us beyond valuing carbon. That way we can get to a point where it will make economic sense to put an increasing fraction of renewable energy into enhancing recycling. Then as renewable energy gets ever cheaper, this will further incentivize using it to power the Gaia devices of the future. This vision is unlikely to materialize without a social movement and a host of innovations behind it. The city often thinks of itself as the cradle of social and technological innovation. Are the citizens ready to push for the changes needed?
James Lovelock, Gaia: A New Look at Life on Earth (Oxford: Oxford University Press, 1979).
“London’s Ecological Footprint: A Review,” Greater London Authority Economics, 2003.
“World Population Prospects 2024,” United Nations, Department of Economic and Social Affairs, Population Division, 2024.
Jane Jacobs, The Death and Life of Great American Cities, (New York: Random House, 1961).
Chunbo Zhang et al., “Life cycle assessment of material footprint in recycling: A case of concrete recycling,” Waste Management 155, no. 1 (2023): 311–19.
Abdulmalek K. Badraddin et al., “Main Challenges to Concrete Recycling in Practice," Sustainability 13, no. 19 (2021): 11077.
