Mutualistic Cities, Chapter One:
The Lightbulb at the End of Time, Part I

“The relationship that cities have developed with the rest of the Earth System is essentially parasitic, and predicated on using massive energy resources. This does not seem sustainable over long (geological) timeframes. To survive, cities of the near future must evolve a relationship that is mutualistic – that is, a relationship that benefits the city and the rest of the Earth System at the same time. Such relationships are widespread in nature.”

Image: 'The Spiders Create Tightropes from Bulb to Bulb' by Nicki Varkevisser. Sourced via Flickr Commons

This four-part blog series comes from collaborative work, spanning the humanities and the geosciences, between SEI and colleagues at the University of Leicester, in the UK. We want to understand and articulate the ecological dynamics of urban areas, now and in the future. We also want to imagine paths toward urban mutualism, or reciprocally beneficial relationships between cities and the planet. Our first instalment, from Mark Williams and Jan Zalasiewicz, puts human and urban energy use in striking perspective.

An Urban Species

Humans are a young species from a geological perspective, evolving only about 300,000 years ago in Africa1,2. For most of that time, they have existed in small groups as hunter-gatherers, with limited impact on their immediate environments. One could characterize this relationship as essentially commensal: humans gained from the landscape around them without causing significant damage to its environments or ecologies. The evolution of urban humans over the past 10,000 years has – gradually at first, then much more rapidly – changed this relationship, as cities became the foci of agricultural and industrial consumption. Over the past fifty years, the energy consumption of urban humans has grown to exceed that of rural humans. The International Energy Agency estimated that about 67% of world primary energy use in 2006 was in urban areas3. Just five years later, in 2011, urban humans exceeded the rural population for the first time in history4. By 2050, when there may be 10 billion people on Earth, 60% of them will live in towns and cities. Humans have evolved to become an urban species, and a ‘colonial’ species, too. Their concrete-, glass-, and steel-framed cities are the structures in which they reside, analogous to the calcium carbonate skeletons of colonial bryozoans or corals, or the nests of termites and ants. As with corals and coral reefs, the skeletal infrastructure of cities may become the fossil record of humans long into the future.

The Three Hundred-Billion-Year Lightbulb

In 2018 there are 7.6 billion humans on Earth. Based on current projections, the planet’s population is predicted to exceed 8 billion, and possibly 10 billion, by midcentury.5,6 Collectively, humans consumed the primary energy of 13,699 million tonnes of oil (equivalent) in 20147, or – to put the thing differently – at a rate of 572 exajoules (EJ) of energy per year. The exajoule is something of an unintelligible figure, one really only useful to physicists. But we can examine how much energy this really is by considering an old-fashioned (non-fluorescent or -LED) 60-Watt light bulb. Such a bulb will burn 60 joules of energy per second. One EJ = 1,000,000,000,000,000,000 of those joules (yes, there are eighteen 0s after the 1), and so to burn through one EJ, a 60-watt light bulb must stay ablaze for some 528 million years. That’s equivalent to all of the time that has elapsed since the first trilobites swam in the seas of the Cambrian Period. To this we must add another 571 of those 60-watt light bulbs, each burning for 528 million years. This adds up to a little over 300 billion years, which takes us back far, far beyond the beginning of the universe at a paltry 13.8 billion years ago. Put another way, all of the lights in a large human house would, using that year’s worth of energy, appear to burn until the end of time.

Human demands on energy don’t – or won’t – stop there. We also consume energy from plants in the biosphere, harvesting or destroying a further 373 EJ each year8 (or another 373 of those 60-watt light bulbs each burning for half a billion years). These are staggering figures, made more alarming by estimates of human energy consumption in the coming years. By the mid-21st century, when 60% of us live in urban areas, those of us in towns and cities may burn through 730 EJ of energy each year9. Human energy consumption will then be approaching that of the above-ground harvestable energy of all the world’s vegetation, which is 1241 EJ/year. This would be a staggering achievement, so to speak, for a single species, and almost certainly unprecedented in four billion years of biological evolution on Earth.

Where might our energy consumption go in the near future, if it continues to double every 40 years or so? Such levels of consumption, if based on energy resources such as coal, oil and gas, would yield vast amounts of carbon to the atmosphere, leading to a dangerous accumulation of greenhouse gases that would likely push Earth’s climate into a new ‘hothouse’ state. This climate trajectory might well damage civilization itself, and, in tandem with pressures on the biosphere, may result in massive biodiversity loss10. Or maybe humans can quickly develop new energy sources, utilizing the sun’s energy directly to produce a futuristic ‘Kardashev Type 1’ civilization – one utilizing all of the energy from our sun that arrives on Earth.

In the second chapter of this series, Killian Quigley considers how humans might imagine their way toward new, mutually beneficial relations between their selves, their cities, and their planet. He asks whether “biodiversity” is sufficiently useful for conceiving those relations, and highlights some of the writers and filmmakers who conjure possible urban futures.


  1. Richter, D., et al. (2017). ‘The age of the hominin fossils from Jbel Irhoud, Morocco, and the origins of the Middle Stone Age.’ Nature 546, pp.293-296.
  2. Hublin, J.J., et al. (2017). ‘New fossils from Jebel Irhoud, Morocco and the pan-African origin of Homo sapiens.’ Nature 546, pp.289-292.
  3. Grubler, A., Schulz, N.B., (2013). Urban energy use (pp. 57-70). In A.Grubler and D. Fisk (eds.) Energizing Sustainable Cities. Oxon, UK: Routledge, Abingdon.
  4. United Nations (2014). Department of Economic and Social Affairs, Population Division 2014. World Urbanization Prospects: The 2014 Revision, Highlights (ST/ESA/SER.A/352).
  5. Hoornweg, D., and Pope, K. (2014). ‘Socioeconomic pathways and regional distribution of the world’s 101 largest cities.’ Global Cities Institute Working Paper No. 4.
  6. United Nations, Department of Economic and Social Affairs, Population Division 2015. World Population Prospects: The 2015 Revision, Key Findings and Advance Tables. Working Paper No. ESA/P/WP.241.
  7. Key World Energy Statistics (2016). International Energy Agency.
  8. Rogner, H.H., et al. (2012). Energy Resources and Potentials (chapter 7, pp. 423-512.). In Global Energy Assessment – Toward a Sustainable Future. Cambridge, UK and New York, USA:  Cambridge University Press and the International Institute for Applied Systems Analysis, Laxenburg, Austria.
  9. Creutzig, F., et al. (2014). Global typology of urban energy use and potentials for an urbanization mitigation wedge. PNAS 112, pp.6283-6288.
  10. Ceballos, G., Ehrlich, P., Dirzo, R. (2017). Biological annihilation via the ongoing sixth mass extinction signaled by vertebrate population losses and declines. PNAS, pp.6089-6096.

Mark Williams is a Professor of Palaeobiology at the University of Leicester, UK. He is interested in the evolution of the biosphere over geological timescales, with an emphasis on understanding the rate and degree of current biological change. He is a founding member of The Anthropocene Working Group, and with Jan Zalasiewicz, is the author of the popular science books ‘The Goldilocks Planet’ (2012), ‘Ocean Worlds’ (2014) and ‘Skeletons: the frame of life’ (2018, Oxford University Press).

Jan Zalasiewicz is a Professor of Palaeobiology at the University of Leicester, UK. In early career he was a field geologist and palaeontologist at the British Geological Survey. Now, he teaches geology and Earth history to undergraduate and postgraduate students, and studies fossil ecosystems and environments across over half a billion years of geological time. Over the last few years he has helped develop ideas on the Anthropocene, the concept that humans now drive much geology on the surface of Earth, and chairs the Anthropocene Working Group of the International Commission on Stratigraphy. His writing includes the books The Earth After Us (2008), The Planet in a Pebble (2010) and Rocks: A Very Short Introduction (2016).