Hunter-gatherers have to spend nearly all the energy they can acquire from their environment on acquiring that very energy. Most animals spend most of the energy they get from food on the task of acquiring more food. The ratio of the totality of energy acquired to the energy cost of acquiring that energy is called EROI, for energy return on investment. For living things it’s generally always between 1 and 2. It wasn’t until around 12,000 years ago when humans first started to domesticate both animals and plants that their EROI increased sufficiently to allow a burst in social complexity. Population sizes also expanded. In fact, population seems to always closely track energy return on investment.
Agricultural communities were able to store and process food in greater quantities, creating enough excess energy that some could be spent on dwellings, complex tools and increasing specialization, including roles not directly involved in food production. Farming techniques improved and more productive breeds of plants and animals were cultivated. EROI rose as high as four or so. Not only were all the present needs met. There was plenty to spare, and embodied energy in the form of preserved food and tradable goods could be accumulated.
With their excess energy empires grew socially very complex, with administrative, military, religious and economic sectors each with its own hierarchies and technicians. The number of intermediaries between the lowest and highest ranked members of society was large and the number of social roles exploded. In addition to trading and war, relations between societies could be diplomatic and involve cultural exchange.
However, every ancient Empire suffered a similar fate. Once the surrounding societies are conquered and pillaged, they have to be administered in garrison that soaks up a lot of excess energy. Each new campaign stretches lines of supply and control farther than before. Diminishing returns said and the EROI declines. To compensate for the loss of energy while maintaining homeostasis, taxes are increased, typically in the form of an ever greater share of agricultural output from vassals. This leads inexorably to resource depletion and finally the weakened empires itself conquered or simply falls to pieces.
Rome squeezed its agricultural and human resource base for every drop of energy with the result that when the Western Empire finally collapsed in the face of determined attacks from the frontier, the drop in social complexity in population and in political and cultural stability was extreme and lasted for nearly 500 years. There was no resource base left in the form of fertile soil, harvestable forests and stable human communities from which to rebuild.
Further developments in energy capture were pretty limited before the Industrial Age. There were innovations like the capture of mechanical wind power, enabling for instance the flourishing of the Dutch Empire in the 16th century and other seagoing empires, but nothing that increased EROI much above 5 or so, but with the invention of the heat engine in the 18th century and fossil fuels to produce the heat everything changed.
Coal and later oil provided an EROI of up to a hundred to one, making almost limitless energy available to drive social development of every kind. Global per capita energy use rose exponentially quadrupling by the year 2000 even as the global population increased by more than sevenfold. The result is what we have today. A globalized, industrialized, fully integrated economy with massive energy and information flows in which the level of connectivity within and between groups is so high that the complexity rivals anything previously imagined or imaginable.
To put our per capita energy use in perspective, consider that a healthy adult can sustain about 75 watts of work for eight hours. In an eight-hour day that’s about 600 watt hours of work, 0.6 of a kilowatt. If you had 10 slaves they could perform 6 kilowatt hours of work for you in a workday, rather less than you get from a single gallon of gasoline used in a typical car engine. The gallon of gasoline can propel an automobile 30 miles in 30 minutes. How long would it take you to push your car 30 miles? How many friends would you need to help you?
Whilst the peak mechanical power of a single horse can reach up to 15 horsepower, it is estimated that a typical horse can only sustain an output of 1 horsepower (746 W) for three hours and, if working for an eight-hour day, a horse might output only three quarters of one horsepower. Researchers have calculated the average power output of various animals (as a rough guide, a healthy animal can pull 10-15% of its weight for a period of four hours, so 500W for camels, oxen and mules). An average American consumer directly and indirectly uses enough energy daily to perform the work of hundreds of slaves. These energy slaves are what make global industrial civilization possible.
When oil was first exploited as a fuel source the EROI was roughly a hundred. As the oil economy grew both institutional and physical infrastructures had to be built for it, for example paved roads and these metabolic costs decreased the energy return on investment to 30 or so. As the richest and easiest to exploit deposits were exhausted, more had to be spent to exploit deposits that were smaller and harder to reach and of lower quality. By the start of the 21st century, the average EROI had dropped to 20 and the EROI of newly exploited deposits of conventional oil is now 10 or less.
Wind and solar are now competitive with oil at the point to generation, but if the electricity they produce has to be stored the EROI drops to barely more than one. Unconventional oil sources have an EROI of no more than five, possibly much less, and the pace of diminishing returns is much quicker for those than with conventional oil. Biomass has turned out to be a bust from an EROI perspective and agricultural land is becoming far too precious to waste on it. The difference between an EROI of 20 and 5 is more significant than it might appear. Established economies have a metabolic cost. Existing infrastructure has to be serviced and replaced, existing institutions have to be funded, existing populations have to be fed and clothed and housed and transported.
Embodied energy is in everything we buy. Consider a box of cornflakes from planting to growing to harvesting to processing to packaging to shipping to point-of-sale and finally to your kitchen cupboard. There are energy inputs at every step. At least half are from liquid fuels. On average, a calorie of food produced in our agricultural system requires at least 10 calories of fossil-fuel energy to produce it and deliver it to your body. In other words, the energy return on investment of food in this civilization is 10 units of energy in to get one unit of energy out.