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Book Summary: How the World Really Works – The Science of Our Past, Present and Future

How the World Really Works (2022) tackles a paradox at the heart of the modern world: we’ve never had so much information at our fingertips and never known so little about how things actually work. Of course, we can’t be experts in everything. But, Vaclav Smil argues, it’s our duty as citizens to be informed about the basics – the big questions that shape our societies and their futures.

Book Summary: How the World Really Works - The Science of Our Past, Present and Future

Content Summary

Introduction: Learn the fundamentals behind energy use and food production.
The history of life on our planet is a history of energy conversions.
Energy is the ability to do work.
Fossil fuels revolutionized agriculture.
The agricultural revolution drove urbanization.
Electricity makes the modern world go round.
About the author
Table of Contents
Video and Podcast
Read an Excerpt/PDF Preview


Science, History, Economics, Politics, Technology, Environment, Business, Climate Change, Sociology, Mathematics, Probability and Statistics, History and Philosophy of Science, Social Sciences

Introduction: Learn the fundamentals behind energy use and food production.

In the year 1500, it was just about possible to meet a real Renaissance man – a savant who knew everything. Think of Leonardo da Vinci, whose knowledge extended from hydraulics to aviation and anatomy.

By the eighteenth century, though, such a figure no longer existed. When France’s greatest minds set about compiling an encyclopedia containing everything worth knowing, they needed a team.

Today, our understanding of the world has advanced so far that even an encyclopedia running to thousands of volumes couldn’t do it justice. Knowledge is so specialized that it can take a lifetime to master a single discipline. A particle physicist – let alone a professional historian – would find it very difficult to understand the first page of a new research paper in viral immunology.

But that doesn’t excuse our lack of understanding of the fundamental workings of the world. We can’t expect everyone to understand cutting-edge research in neurobiology, but we can expect them to understand, say, how the food on their plates was made.

That’s the argument Vaclav Smil makes in How the World Really Works. For Smil, such knowledge is vital to the functioning of democracies. If we don’t have that knowledge, we won’t be able to make informed decisions about the most important issues of the day.

In How the World Really Works, Smil zooms in on seven topics he thinks all of us need to know about. For this summary, we’re going to narrow that down to two: energy and food.

In this summary, you’ll learn

  • how our planet’s atmosphere came to support animal life;
  • why fossil fuels allow us to feed almost eight billion people; and
  • how electricity makes the world go round.

The history of life on our planet is a history of energy conversions.

Let’s start at the beginning – the very beginning. Some three and a half billion years ago, when much of our planet’s surface was little more than primordial soup, a new form of life emerged: simple, single-cell microbes.

These bacteria didn’t have consciousness or mobility: they merely drifted aimlessly through Earth’s seas. But they did have metabolism – the ability to convert one form of energy into another. That was how they accessed the nutrients they needed to survive and reproduce. The first form of energy was solar radiation from the sun. They used that energy to convert carbon dioxide and water into new organic compounds. While doing so, they created a by-product – oxygen.

This process, which is called photosynthesis, changed the planet’s atmosphere. Before these microbes began photosynthesizing, it was oxygenless; but after hundreds of millions of years, the atmosphere had enough oxygen to support life as we know it. Life, in other words, began with energy conversion. And it continued that way, too. The entire history of our planet is a history of energy conversions.

For example, several hundred thousand years ago, there was another epochal shift. This was the first extrasomatic use of energy – that is, the conversion of energy outside the body. Previously, all energy conversion had occurred inside the cells of living things. More metabolism, in short. Cells unlocking nutrients by converting one form of energy into another. That’s when a bunch of unusually clever apes – our ancestors – discovered the controlled combustion of plant matter.

Fire converts the chemical energy of plant matter, be it wood or peat or coal, into thermal energy and light. Homo sapiens began by using wood, of course – coal came much later. But that was enough to make indigestible foods edible, keep their shelters warm, and scare off dangerous animals. The use of fire was humanity’s first step on the long road to reshaping and controlling its environment.

The domestication of animals some 10,000 years ago is another milestone in the history of energy conversions. Before humans learned to put animals like oxen to work, they relied on their own muscles to convert chemical energy into the kinetic and mechanical energy which hauled loads, plowed fields, and drew water from wells. Domestication outsourced that role to beasts of burden. Later innovations, like sails and waterwheels, delegated that work to the wind and flowing rivers.

Then comes the next milestone – the use of fossil fuels – which brings us into the modern age. After around 1600, humans started burning coal, a fuel created over millions of years as heat and pressure fossilized plant matter. Coal gave us the steam engine – the iron workhorse which powered early industrialization. After around 1850, the history of energy conversion picks up tempo, giving us ever more new energy sources: crude oil, electricity-generating water and wind turbines, geothermal electricity, and then nuclear and solar power.

The abundance of useful energy has changed every facet of human existence. It allows us to work less, eat better, travel more, and communicate more efficiently. Put differently, if you want to understand the miracle of modern life, you have to start with how we convert energy.

Energy is the ability to do work.

Energy conversion, in short, is the basis of all life. It powered human evolution and allowed our species to dominate the planet. No one knows this better than physicists.

In 1886, Ludwig Boltzmann – one of the founders of thermodynamics – came to the conclusion that energy is the heart of everything. All life, he said, is a struggle for free energy – the energy that’s available for conversions. Erwin Schrödinger, the winner of the 1933 Nobel Prize for Physics, agreed. Every organism feeds on free energy, he wrote, and the organisms that best capture that energy hold the evolutionary advantage. We’ll come back to that, but let’s pause first to pose a fundamental question:

What is energy, anyway?

The etymology of the word, which goes back to ancient Greece, is a good place to start. It comes from the noun enérgeia, meaning “work.” And that’s pretty much the standard scientific definition: energy is the ability to do work.

In physics, work refers to the physical act of producing a change of configuration in a system in opposition to a second force which resists change. That’s a technical way of saying that work is the force that causes things to move. Energy, in other words, is about motion. If you hold a tennis ball out of a second-floor window, for example, it has potential energy – the potential for creating motion. If you drop it, that potential energy is transformed into the kinetic energy of movement.

All energy can be converted. The food you eat is a store of chemical energy; when you go for a run or tidy up, it’s transformed into kinetic energy. The chemical energy in coal, meanwhile, can be converted into mechanical energy – the force moving the pistons attached to a steam engine.

And that, really, brings us back to what energy is all about: conversions. That’s because it can’t be created or destroyed; it can only change form. This is known as the conservation of energy, which is the first law of thermodynamics.

You can see how this works if you imagine a cardboard box sliding down a loading bay ramp.

When it was sitting at the top of that ramp, the box had potential energy. But then an accidental push sent it speeding down the ramp, converting potential energy into kinetic energy. But the box slows as it comes down the ramp: friction opposes its motion. The box’s kinetic energy isn’t lost, though – it’s converted into thermal energy, which heats both the box and the ramp. What has been lost, here, is the ability of this kinetic energy to perform useful work: it can no longer move the box.

That, in a nutshell, is the physics of energy. But let’s make things a little less abstract.

We just quoted Schrödinger’s view that organisms which best capture free energy – energy available for useful conversions – hold the evolutionary advantage. That’s a pretty good description of Homo sapiens. So let’s take a look at one of the most important ways in which our species has captured energy: agriculture.

Fossil fuels revolutionized agriculture.

Between 1950 and 2019, the globe’s population rose from 2.5 billion people to 7.7 billion. More mouths to feed didn’t mean more people going hungry, though. In fact, the worldwide share of undernourished people actually fell – from 65 percent in 1950 to just 8.9 percent in 2019.

So what explains this dramatic reduction of malnutrition? One answer is that our crops yield more food than before. That in turn is down to better crop varieties, improved fertilizers, better irrigation, and the mechanization of agriculture. But all those things are inputs. What made them possible? The answer is fossil fuels.

Modern food production is hybrid – it relies on two different kinds of energy conversions. The first type is as old as life on this planet. Everything we eat, from plants to animals, is the result of photosynthesis. Solar radiation powers food production as it has since the dawn of agricultural civilizations 10,000 years ago. Without the sun, there simply wouldn’t be anything to harvest.

But solar radiation alone can’t explain today’s high-yield crops. They also rely on fossil energies like gas and oil. Labor-saving machines, like combines, require diesel to harvest crops; so do the trains, trucks, and barges which bring crops to market. Irrigation pumps, crop processing, and drying machinery run on gasoline. And the factories which manufacture the steel, rubber, plastics, glass, and electronics that go into tractors, combines, trucks, silos, and greenhouses also run on fossil fuels.

That’s only half the story, however. High-yield crops are intensely managed crops: you need fungicides, insecticides, and herbicides to minimize losses and fertilizers to boost growth. That, too, is fuel-intensive. Take nitrogen fertilizer. On average, farms use between 100 and 200 kilograms of it per hectare, making it the most important indirect energy input in farming. Plant life is impossible without nitrogen. It’s in every living cell. It’s in the chlorophyll which powers photosynthesis and in plants’ DNA and RNA. It’s also in their amino acids – the building blocks of the proteins needed to grow and repair tissue.

Nitrogen is everywhere – over 80 percent of the planet’s atmosphere is made up of this colorless, odorless compund. Thing is, though, you mostly encounter it in its nonreactive form – a form plants can’t use. To make nitrogen available to plants, you need to split the bond between the two nitrogen atoms and unlock free nitrogen. In this state, it can combine chemically with other elements and form reactive, plant-accessible nitrogen compounds like ammonia, nitrates, and nitrites.

Some natural processes, like lightning, unlock free nitrogen, but you can’t really harness lightning as an agricultural input. What you can do, by contrast, is sow nitrogen-fixing crops, such as alfalfa. The roots of these plants are home to bacteria which convert nitrogen into ammonia, thus “fixing” the nitrogen in the soil. But that takes time: you have to stop growing, say, wheat and plant alfalfa.

In the early twentieth century, German scientists worked out how to produce synthetic nitrogen fertilizers. Simply put, you upgrade natural gas by combining it with nitrogen from the air. Suddenly, there was as much fertilizer as humanity needed. The result: crop yields went through the roof, bringing us to the point where we can almost feed eight billion people. But that came at a cost of ever greater reliance on fossil fuels. Today, fertilizer production accounts for almost 1.5 percent of the entire globe’s energy supply and a huge portion of the natural gas we burn each year.

The agricultural revolution drove urbanization.

Fossil fuels powered what’s known as the green revolution – the huge increase in crop yields across the world during the twentieth century. Agricultural development didn’t just change what, or how much, we eat, though. It also changed how and where we work. To get a better sense of these big picture changes, let’s zoom in on one of the many places that revolution played out: the United States.

It’s 1801 and we’re in western New York. More precisely, we’re on the banks of the Genesee, a river which runs through a fertile valley dotted with small farms growing bread wheat. The farmers here are Americans, but they grow their wheat in the same way their ancestors did back in Europe. In fact, they grow it pretty much exactly like the ancient Egyptians did over 2,000 years ago.

They start by attaching a wooden plow with an iron-plated cutting edge to two oxen and driving that across their fields. Then they sow the seed they’ve kept back from last year. Next, they harvest the wheat with sickles before cutting and bundling it. After drying, it’s hauled into the barn and threshed. The straw is stacked and the grain is winnowed – the process of separating the wheat seeds from their tough husks known as chaff. Finally, the wheat is measured and put in sacks.

Everything is done by hand. Nothing is mechanized and everything is powered by solar radiation. All in all, it takes 120 hours of human labor plus 70 hours of ox labor per hectare. That’s about ten minutes of labor per kilogram of wheat, which is what you’d need to bake two loaves of bread.

Fast forward a century. We’re now in the Red River valley in eastern Dakota. Agriculture has already moved on a great deal. Wheat farmers use teams of four powerful horses to pull multishare steel plows. They also have mechanical seed drills and harvesters. Fossil fuels have also arrived – threshing machines, for example, are now powered by coal-fired steam engines. At 1,000 kilograms per hectare, bread wheat yields are still relatively low, but much less human labor is required to secure a wheat crop. Now, it’s just 22 hours per hectare – about one-seventh of what it was in 1801. In other words, farmers are investing between one and five minutes of their labor per kilogram of wheat.

Let’s hit the fast-forward button one last time, bringing us up to 2021. This time, we’re in Kansas – the heart of American wheat country. The world of even a hundred years ago has completely disappeared. The US Department of Agriculture stopped counting the number of draft animals on American farms in 1961, which is pretty much when diesel-guzzling tractors became ubiquitous. Every part of the process is now mechanized, including the application of inorganic fertilizers. Combines take care of harvesting and threshing before directly loading grain onto trucks. A hectare now yields 3,500 kilograms and requires just two hours of human labor. That’s just two seconds per kilogram of wheat!

The upshot of these remarkable improvements was that farming required fewer hands and produced more food. In the United States, around 83 percent of Americans worked in agriculture in 1801; by 2021, that had all fallen to just 1 percent. This same story was replicated around the world, from Denmark to China and Argentina to India. Fossil fuels, then, changed everything. They drove the mechanization of agriculture and the increase in yields. But they also severed humanity’s age-old connection with the land, driving billions of people into cities to find new work in the innovative industries which shaped the modern world.

Electricity makes the modern world go round.

Lumps of coal and canisters of gasoline are pretty tangible stores of chemical energy. When they burn, they release thermal energy – the heat which powers locomotives or motor vehicles. It’s the same with falling water. We can easily picture the water wheel which converts the river’s gravitational energy into the mechanical energy which turns a millstone.

Electricity is different. It’s less intuitive. Even physicists can’t answer the question of what exactly electricity is – they can only describe how it interacts with the world. But that’s enough to harness its life-altering power. And the effects of that can be seen everywhere.

As an energy source, electricity has many advantages. It’s always clean, for example, and mostly very efficient. A simple flip of a switch or push of a button is enough to activate thermostats, motors, heaters, and lights. There’s no need for bulky fuel storage and nothing to carry. And unlike, say, coal or gas, there’s no danger of incomplete combustion – the source of deadly carbon monoxide.

Its uses have transformed our world. Take just one application: lighting. Before electric lighting, we relied on wax candles, oil lamps, and kerosene cylinders to erase the difference between day and night. All were expensive, inefficient, and often dangerous. They were also feeble. The gas lights of early industrial cities, for example, were ten times more efficient than candles. Today’s fluorescent lights, by contrast, are 500 times as efficient. Then there are the sodium lamps we use to illuminate cities at night, which are 1,000 times more efficient!

The energy conversion which really made the difference to modern life, though, is the conversion of electricity into kinetic energy by electric motors. Electrifying machinery to lift, press, cut, and weave various goods made factories cleaner, cheaper to run, and faster. Electric trams, meanwhile, made it possible to move through vast cities, bringing workers to the gates of those factories. Between 1900 and 1930, electrification doubled American manufacturing productivity. By 1960, it had quadrupled it.

It’s hard to overstate how much we’ve come to rely on electricity. Today, the economies of developed countries are dominated by the service sector. And that sector is entirely dependent on electricity. From elevators to escalators, trash compactors, conveyor belts in warehouses, and air-conditioning units – everything runs on electricity. Regular cars now contain between 20 and 40 electric motors. And households rely on it for heating, refrigeration, lighting, and countless other smaller tasks.

Electricity also moves major resources around. Without powerful electric pumps, cities couldn’t feed water into municipal pipelines. Those pumps also move fossil fuels – above all liquid gas – from the point of extraction to the places where they are used: factories and homes. If any densely populated region were to experience severely reduced electricity supply for just a few days, it would be plunged into chaos. If an entire nation experienced such shortages, we’d be looking at an unprecedented catastrophe. Yet despite its profound importance, electricity still supplies only a relatively small share of final global energy consumption – just 18 percent. But if history is any indicator, modes of energy conversion will still continue to change well into the future.


The most important thing to take away from all this is:

The history of life on Earth is a history of energy conversions. And the history of humanity is the history of a species that has become more and more efficient in exploiting free energy. So, next time you flip a switch or take a bite of food, take a second to think about all the processes and energy conversions that went into those seemingly small luxuries.

About the author

Vaclav Smil is Distinguished Professor Emeritus at the University of Manitoba. He is the author of over forty books on topics including energy, environmental and population change, food production and nutrition, technical innovation, risk assessment, and public policy. No other living scientist has had more books (on a wide variety of topics) reviewed in Nature. A Fellow of the Royal Society of Canada, in 2010 he was named by Foreign Policy as one of the Top 100 Global Thinkers.

Vaclav Smil | Website

Table of Contents


An essential analysis of the modern science and technology that makes our twenty-first century lives possible—a scientist’s investigation into what science really does, and does not, accomplish.

We have never had so much information at our fingertips and yet most of us don’t know how the world really works. This book explains seven of the most fundamental realities governing our survival and prosperity. From energy and food production, through our material world and its globalization, to risks, our environment and its future, How the World Really Works offers a much-needed reality check—because before we can tackle problems effectively, we must understand the facts.

In this ambitious and thought-provoking book we see, for example, that globalization isn’t inevitable—the foolishness of allowing 70 per cent of the world’s rubber gloves to be made in just one factory became glaringly obvious in 2020—and that our societies have been steadily increasing their dependence on fossil fuels, such that any promises of decarbonization by 2050 are a fairy tale. For example, each greenhouse-grown supermarket-bought tomato has the equivalent of five tablespoons of diesel embedded in its production, and we have no way of producing steel, cement or plastics at required scales without huge carbon emissions.

Ultimately, Smil answers the most profound question of our age: are we irrevocably doomed or is a brighter utopia ahead? Compelling, data-rich and revisionist, this wonderfully broad, interdisciplinary guide finds faults with both extremes. Looking at the world through this quantitative lens reveals hidden truths that change the way we see our past, present and uncertain future.


“A new masterpiece from one of my favorite authors… [How The World Really Works] is a compelling and highly readable book that leaves readers with the fundamental grounding needed to help solve the world’s toughest challenges.” – Bill Gates

“Provocative but perceptive . . . You can agree or disagree with Smil—accept or doubt his ‘just the facts’ posture—but you probably shouldn’t ignore him.” – The Washington Post

“[It is] reassuring to read an author so impervious to rhetorical fashion and so eager to champion uncertainty. . . Smil’s book is at its essence a plea for agnosticism, and, believe it or not, humility — the rarest earth metal of all. His most valuable declarations concern the impossibility of acting with perfect foresight. Living with uncertainty, after all, “remains the essence of the human condition.” Even under the most optimistic scenario, the future will not resemble the past. ” – The New York Times

“How the World Really Works represents the highly readable distillation of this lifetime of scholarship… Mr. Smil looks over the horizon of the future with humility and calmness, foreseeing ‘a mixture of progress and setbacks, of seemingly insurmountable difficulties and near-miraculous advances.’” – Wall Street Journal

“The renowned energy scientist … aims to [recenter] materials rather than electronic flows of data as the bedrock of modern life — largely through examining what he calls the four pillars of modern civilization: cement, steel, plastics and ammonia.” – The New York Times Magazine

“A scientific panorama of our well-being and how it can be sustained in our current tumultuous times and beyond. [Smil] aims to combat the widespread “comprehension deficit” about basic scientific facts, and he seeks to “explain some of the most fundamental ruling realities governing our survival and our prosperity.” That aim is marvelously achieved…[this is] an exceptionally lucid, evenhanded study of the scientific basis of our current and future lives.” – Kirkus, STARRED review

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1. Understanding Energy

Fuels and Electricity

Consider a benign science fiction scenario: not travel to distant planets in search of life, but the Earth and its inhabitants as targets of remote monitoring by an exceedingly sapient civilization that sends its probes to nearby galaxies. Why do they do this? Just for the satisfaction of systematic understanding, and perhaps also to avoid dangerous surprises should the third planet orbiting around an unremarkable star in a spiral galaxy become a threat, or perhaps in case they should require a second home. Hence this planet keeps periodic tabs on Earth.

Let us imagine that a probe approaches our planet once every 100 years and that it is programmed to make a second pass (a closer inspection) only when it detects a previously unobserved kind of energy conversion-the changing of energy from one form to another-or a new physical manifestation dependent on it. In fundamental physical terms, any process-be it rain, a volcanic eruption, plant growth, animal predation, or the growth of human sapience-can be defined as a sequence of energy conversions, and for a few hundred million years after the Earth’s formation the probes would see only the same varied, but ultimately monotonous, displays of volcanic eruptions, earthquakes and atmospheric storms.

Fundamental shifts

The first microorganisms emerge nearly 4 billion years ago but passing probes do not register them, as these life forms are rare and remain hidden, associated with alkaline hydrothermal vents at the ocean’s floor. The first occasion for a closer look arises as early as 3.5 billion years ago, when a passing probe records the first simple, single-celled photosynthetic microbes in shallow seas: they absorb near-infrared radiation-that which is just beyond the visible spectrum-and do not produce oxygen. Hundreds of millions of years then elapse with no signs of change before cyanobacteria begin to use the energy of the visible incoming solar radiation to convert CO2 and water into new organic compounds and release oxygen.

This is a radical shift that will create Earth’s oxygenated atmosphere, yet a long time elapses before new, more complex aquatic organisms are seen 1.2 billion years ago, when the probes document the rise and diffusion of brilliantly colored red algae (due to the photosynthetic pigment phycoerythrin) and of much larger, brown algae. Green algae arrive nearly half a billion years later, and because of the new proliferation of marine plants the probes get better sensors to monitor the sea floor. This pays off, as more than 600 million years ago the probes make another epochal discovery: the existence of the first organisms made of differentiated cells. These flattish, soft, bottom-dwelling creatures (known as Ediacaran fauna after their Australian domicile) are the first simple animals requiring oxygen for their metabolism and, unlike algae that are merely tossed by waves and currents, they are mobile.

And then the probes begin to document what are, comparatively speaking, rapid changes: instead of passing over lifeless continents and waiting hundreds of millions of years before logging another epochal shift, they begin to record the rising, cresting, and subsiding waves of the emergence, diffusion, and extinction of a huge variety of species. This period starts with the Cambrian explosion of small marine bottom-dwellers (541 million years ago, dominated first by trilobites) through the arrival of the first fishes, amphibians, land plants, and four-legged (and hence exceptionally mobile) animals. Periodic extinctions reduce, or sometimes almost eliminate, this variety, and even just 6 million years ago the probes do not find any organism dominating the planet. Not long afterwards, the probes nearly miss the significance of a mechanical shift with enormous energetic implications: many four-legged animals briefly stand or awkwardly walk on two legs, and more than 4 million years ago this form of locomotion becomes the norm for small ape-like creatures that begin spending more time on land than in trees.

Now the intervals between reporting something noteworthy to their home base shrink from hundreds of millions to mere hundreds of thousands of years. Eventually the descendants of these early bipeds (we classify them as hominins, belonging to the genus Homo, along the long line of our ancestors) do something that puts them on an accelerated path to planetary dominance. Several hundred thousand years ago, the probes detect the first extrasomatic use of energy-external to one’s body; that is, any energy conversion besides digesting food-when some of these upright walkers master fire and begin to use it deliberately for cooking, comfort, and safety. This controlled combustion converts the chemical energy of plants into thermal energy and light, enabling the hominins to eat previously hard-to-digest foods, warming them through the cold nights, and keeping away dangerous animals.These are the first steps toward deliberately shaping and controlling the environment on an unprecedented scale.

This trend intensifies with the next notable change, the adoption of crop cultivation. About 10 millennia ago, the probes record the first patches of deliberately cultivated plants as a small share of the Earth’s total photosynthesis becomes controlled and manipulated by humans who domesticate-select, plant, tend, and harvest-crops for their (delayed) benefit. The first domestication of animals soon follows. Before that happens, human muscles are the only prime movers-that is, converters of chemical (food) energy to the kinetic (mechanical) energy of labor. Domestication of working animals, starting with cattle some 9,000 years ago, supplies the first extrasomatic energy other than that of human muscles-they are used for field work, for lifting water from wells, for pulling or carrying loads, and for providing personal transportation. And much later come the first inanimate prime movers: sails, more than five millennia ago; waterwheels, more than two millennia ago; and windmills, more than a thousand years ago.

Afterwards, the probes don’t have much to observe, following the arrival of another period of (relative) slowdown: century after century, there is just repetition, stagnation, or the slow growth and diffusion of these long-established conversions. In the Americas and in Australia (lacking any draft animals and any simple mechanical prime movers), all work before the arrival of Europeans is done by human muscles. In some of the Old World’s preindustrial regions, harnessed animals, wind and running or falling water energize significant shares of grain milling, oil pressing, grinding, and forging, and draft animals become indispensable for heavy field work (plowing above all, as harvesting is still done manually), transporting goods, and waging wars.

But at this point, even in societies with domesticated animals and mechanical prime movers, much of the work is still done by people. My estimate, using necessarily approximate past totals of working animals and people and assuming typical daily work rates based on modern measurements of physical exertion, is that-be it at the beginning of the second millennium of the Common Era or 500 years later (in 1500, at the beginning of the early modern era)-more than 90 percent of all useful mechanical energy was provided by animate power, roughly split between people and animals, while all thermal energy came from the combustion of plant fuels (mostly wood and charcoal, but also straw and dried dung).

And then in 1600 the alien probe will spring into action, and spot something unprecedented. Rather than relying solely on wood, an island society is increasingly burning coal, a fuel produced by photosynthesis tens or hundreds of millions of years ago and fossilized by heat and pressure during its long underground storage. The best reconstructions show that coal as a heat source in England surpasses the use of biomass fuels around 1620 (perhaps even earlier); by 1650 the burning of fossil carbon supplies two-thirds of all heat; and the share reaches 75 percent by 1700. England has an exceptionally early start: all the coalfields that make the UK the world’s leading 19th-century economy are already producing coal before 1640. And then, at the very beginning of the 18th century, some English mines begin to rely on steam engines, the first inanimate prime movers powered by the combustion of fossil fuel.

These early engines are so inefficient that they can be deployed only in mines where the fuel supply is readily available and does not require any transportation.But for generations the UK remains the most interesting nation to the alien probe because it is an exceptional early adopter. Even by 1800, the combined coal extraction in a few European countries and the United States is a small fraction of British production.

By 1800 a passing probe will record that, across the planet, plant fuels still supply more than 98 percent of all heat and light used by the dominant bipeds, and that human and animal muscles still provide more than 90 percent of all mechanical energy needed in farming, construction, and manufacturing. In the UK, where James Watt introduced an improved steam engine during the 1770s, the Boulton & Watt company begin to build engines whose average power is equal to that of 25 strong horses, but by 1800 they have sold less than 500 of these machines, merely denting the total power provided by harnessed horses and hard-working laborers.

Even by 1850, rising coal extraction in Europe and North America supplies no more than 7 percent of all fuel energy, nearly half of all useful kinetic energy comes from draft animals, about 40 percent from human muscles, and just 15 percent from the three inanimate prime movers: waterwheels, windmills, and the slowly spreading steam engines.The world of 1850 is much more akin to the world of 1700 or even of 1600 than that of the year 2000.

But by 1900 the global share of both fossil and renewable fuels and of prime movers shifts considerably as modern energy sources (coal and some crude oil) provide half of all primary energy, and traditional fuels (wood, charcoal, straw) the other half. Water turbines in hydro stations generate the first primary electricity during the 1880s; later comes geothermal electricity, and after the Second World War nuclear, solar, and wind electricity (the new renewables). But by 2020 more than half of the world’s electricity will still be generated by the combustion of fossil fuels, mainly coal and natural gas.

By 1900, inanimate prime movers supply about half of all mechanical energy: coal-fired steam engines make the greatest contribution, followed by better-designed waterwheels and new water turbines (first introduced during the 1830s), windmills and brand-new steam turbines (since the late 1880s), and internal combustion engines (gasoline-fueled, also first introduced in the 1880s).

By 1950, fossil fuels supply nearly three-quarters of primary energy (still dominated by coal), and inanimate prime movers-now with gasoline- and diesel-fueled internal combustion engines in the lead-provide more than 80 percent of all mechanical energy. And by the year 2000 only poor people in low-income countries depend on biomass fuels, with wood and straw providing only about 12 percent of the world’s primary energy. Animate prime movers hold only a 5 percent share of mechanical energy, as human exertions and the work of draft animals are almost completely displaced by machines fueled by liquids or by electric motors.

During the past two centuries, the alien probes will have witnessed a rapid global substitution of primary energy sources, accompanied by the expansion and diversification of fossil energy supply, and the no less rapid introduction, adoption, and growth in capacity of new inanimate prime movers-first coal-fired steam engines, then internal combustion engines (piston and turbines). The most recent visit would see a truly global society built and defined by mass-scale, stationary, and mobile conversions of fossil carbon, deployed everywhere but in some of the planet’s uninhabited regions.

Modern energy uses

What difference has this mobilization of extrasomatic energies made? Global primary energy supply usually refers to total (gross) production, but it is more revealing to look at energy that is actually available for conversion into useful forms. To do this, we need to subtract pre-consumption losses (during coal sorting and cleaning, crude oil refining, and natural gas processing), non-energy use (mainly as feedstocks for chemical industries, and also as lubricating oils for machines ranging from pumps to aircraft turbines and as paving materials), and losses during electricity transmission. With these adjustments-and rounding heavily to avoid impressions of unwarranted accuracy-my calculations show a 60-fold increase in the use of fossil fuels during the 19th century, a 16-fold gain during the 20th century, and about a 1,500-fold increase over the past 220 years.

This increasing dependence on fossil fuels is the most important factor in explaining the advances of modern civilization-and also our underlying concerns about the vulnerability of their supply and the environmental impacts of their combustion. In reality, the energy gain was substantially higher than the 1,500-fold I just mentioned, because we must take into account the concurrent increase in average conversion efficiencies. In 1800, coal combustion in stoves and boilers to produce heat and hot water was no more than 25-30 percent efficient, and only 2 percent of coal consumed by steam engines was converted into useful work, resulting in an overall conversion efficiency of no higher than 15 percent. A century later, better stoves, boilers, and engines raised the overall efficiency to nearly 20 percent, and by the year 2000 the mean conversion rate was about 50 percent. Consequently, the 20th century saw a nearly 40-fold gain in useful energy; since 1800 the gain was about 3,500-fold.

To get an even clearer picture of the magnitude of these changes, we should express these rates in per capita terms. The global population rose from 1 billion in 1800 to 1.6 billion in 1900 and 6.1 billion in the year 2000, and hence the supply of useful energy rose (all values in gigajoules per capita) from 0.05 in 1800 to 2.7 in 1900 and to about 28 in the year 2000. China’s post-2000 rise on the world stage was the main reason for a further increase in the global rate to about 34 GJ/capita by 2020. An average inhabitant of the Earth nowadays has at their disposal nearly 700 times more useful energy than their ancestors had at the beginning of the 19th century.

Moreover, within a lifetime of people born just after the Second World War the rate had more than tripled, from about 10 to 34 GJ/capita between 1950 and 2020. Translating the last rate into more readily imaginable equivalents, it is as if an average Earthling has every year at their personal disposal about 800 kilograms (0.8 tons, or nearly six barrels) of crude oil, or about 1.5 tons of good bituminous coal. And when put in terms of physical labor, it is as if 60 adults would be working non-stop, day and night, for each average person; and for the inhabitants of affluent countries this equivalent of steadily laboring adults would be, depending on the specific country, mostly between 200 and 240. On average, humans now have unprecedented amounts of energy at their disposal.