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Energy is the capacity for ‘doing work’, for ‘making something happen’ – for simply ‘getting things done’.

Energy is what drives change, and all activity is an expression of energy flow. Human activity is also a function of energy flows but this is not usually regarded as a topic to include in history courses. Our total dependence on plant energy, like our dependence on the oxygen we breathe (also supplied by plants), is not addressed directly in history books because plants are taken for granted: they are a simple necessity of existence, a ‘given’. But when lives are threatened, the consequences gather historical significance to become part of environmental, as well as economic and cultural history. We see this today in relation to food security, climate change, and our human sustainability on planet earth.

The significance of plant energy for human history is at last gaining some recognition (see history in 10,000 words) which explains how human dependence on plant energy has increased exponentially through three major phases of human history (Natura, Agraria, Industria) whose mode of existence and degree of social organization was constrained by the available energy, its method of capture, and use, and how we are now entering a fourth phase, Informatia

This article discusses the physics and biology of energy before considering the ways that the energy of plants, derived from the Sun, has become woven into the fabric of society in ever more complex ways.

Forms of energy

We generally think of energy in terms of sources that are suitable for human use – the non-renewable fuels like coal, gas, oil and nuclear power, and renewable fuels like solar, wind, bioenergy and hydro. With the exception of nuclear energy, all these forms of energy can be traced back to the Sun which is the power generator for life on Earth. It is the energy from the Sun that drives the Earth’s climate by influencing winds, the patterns of rainfall and evaporation, and the heating of the oceans to produce climate-affecting water currents.

Radiation from the Sun is also absorbed by plants during photosynthesis and stored in carbon compounds that are formed from the combination of water taken from the soil, and carbon dioxide extracted from the atmosphere. This fundamental life process captures, in plant cells, energy that was formed by nuclear fusion in the Sun. Leaving the surface of the Sun, this radiant energy travelled through space at the speed of light for about 8.5 minutes before being absorbed by the plants on Earth. It is this energy that passes through the food chain from plants to animals and that powers our own bodies when we eat. Energy does not cycle like water and other elements and compounds, but passes through the biosphere to eventually leave in the form of heat.

Some of the ancient energy of the Sun has remained locked up in plants that became fossilised many millions of years ago. This highly concentrated energy, stored in coal, oil and natural gas, drives human industry. It is released, together with carbon dioxide, when fossil fuels are used, returning the long-stored carbon to a very different atmosphere and world from the one in which it was collected. Fossil fuels are intimately bound up in the history of the biosphere as well as our own history and way of life. The transition to a post-industrial society and a modern standard of living in the West can be attributed to the use of vast quantities of cheap fossil fuels. We now depend on these in almost every aspect of our daily lives, from our alarm clocks in the morning, to our travel to work, the lighting, heating, and cooling of buildings, and the production of the food we eat.

Energy & CO2

But it is this carbon dioxide that also contributes to the enhanced greenhouse effect that is driving climate change. Finding ways to beat our fossil carbon addiction is one of the greatest challenges for sustainability science. Any management of atmospheric carbon dioxide must start with the knowledge of how carbon cycles between land, plants, the atmosphere, and the oceans. The movement of carbon between the major sinks is known as the carbon cycle and it is closely linked to the flow of energy through living organisms

Stabilising the world’s climate will require high income countries to reduce their emissions by 60-90% over 2006 levels by 2050. This should stabilise atmospheric carbon dioxide levels at 450-650 parts per million (ppm) from current levels of about 380 ppm. Above this level and temperatures would probably rise by more than 2o C to produce ‘catastrophic’ climate change.

For us, as global citizens and consumers, it is important to know, and recognize through our behaviour, that energy flows are both ‘direct’ (as when we ‘burn’ the petrol in our cars) but is also ‘indirect’ or ‘embodied’ in the goods and services we use. A lot of energy would have been consumed in the manufacture of our car. When we buy a product we are, in effect, also buying (and, ideally, accepting responsibility for) all the resources and all the environmental and social impacts that were ‘embodied’ in that product … except that we rarely have any idea what these are.


Australian annual solar radiation (2006):
indicating regions most suited to harvest the Sun’s energy

Biological energy

Biological energy is the energy that is derived from the Sun and stored in plant tissue during photosynthesis. Plant tissue then becomes the food energy that sustains the entire community of life. In humans most of this energy is used to power metabolic activity in the heart, lungs, stomach, brain, and muscles. However, some biological energy is also directed outside the body to achieve social objectives. This portion of biological energy can be called social energy.

Social energy is energy that is directed to social ends. It may be biological energy, made more efficient when leveraged by mental and physical tools, or it may come from totally different energy sources.

Human labour

The Industrial Revolution, it was hoped, would relieve humans once-and-for-all of the burden of physical toil.

The energy we use up in physical activity is derived from the food we eat. Like all other animals, we obtain body energy either directly from plants (as cereals, fruit, nuts and vegetables) or indirectly through the consumption of animal meat (which ultimately derives its energy from plant matter either directly or through an animal food chain). Since the Industrial Revolution we now know that the energy we expend as individuals must also include the embodied energy we consume in the goods and services we use.

A short diversion into the food-energy requirements of our own bodies when we are resting and going about our daily lives will help us get a feel for the energy flows that occur in other organisms, within ecosystems, and throughout the human food chain. Like a running engine our bodies are constantly using fuel. At rest they tick over at basal metabolic rate (BMR). The approximate human daily energy requirement is about 12,500 kJ, equivalent to an energy generating capacity (at BMR) of 80 watts, about the same as an incandescent light (about 20 watts of this energy is being used by the brain). A 100 watt light bulb therefore works 1.25 times as hard as our body (100/80 watts). We could create a human energy unit called a human-equivalent (H-e), and say that the 100 watt light bulb is running at 1.25 H-e.

Human body energy

The human body obtains metabolic energy from the energy contained in plant tissue and eaten as food. This includes the meat protein that is also derived from plants. Plants obtain their energy from the Sun, converting light into chemical energy in a process (photosynthesis) that humans have been unable to replicate as a means of energy capture and storage. Our bodies make use of chemical energy by breaking down biomolecules like glucose, amino acids and fatty acids. To build tissues (anabolism) others must be broken down (catabolism)., a process that goes on simultaneously all the time in the body, though not always at the same rate. The heart, stomach, lungs, muscles, and brain are especially energy hungry.

The body stores long-term energy in lipids (fats and oils) which store a lot of chemical energy. The food we eat is digested in the stomach liquids that include acids and enzymes. Carbohydrates (sugars and starches) are broken down into another type of sugar called glucose which is a short-term source of energy. Glucose is, however, a large molecule and not an efficient source of fast energy. The immediate chemical we use to obtain body energy is called adenosine triphosphate (ATP) whose chemical bonds, when broken, release the direct energy that powers all our body functions and reactions.

ATP is thus the currency of biological energy: it can be used directly from the food source or stored in the body as fat. Energy generation occurs in tiny cell organelles called mitochondria which are the body’s energy factory. Active cells, like those in muscle, have many thousands of mitochondria whose ATP is ‘spent’ when the bonds between the phosphate groups and the rest of the molecule are broken. This is more a process of energy conversion from the chemical energy of food and fats into usable cell energy taken to the place where it is needed – as with the breakdown of adenosine triphosphate to adenosine diphosphate releasing the energy needed for muscle contraction.


See here for a discussion of human population.
Biological studies on the growth of populations of organisms show that their numbers will increase until either a) they come into equilibrium with their environment (determined mainly by resource availability but also predation and other factors) or b) there is a repeated pattern of flourishing and collapse or c) if numbers greatly exceed sustainability then there may be a sudden and total collapse leading to extinction.

Human populations have tended to grow until reined in by wars, famine, disease, and natural disasters. This was the thesis of the English curate Thomas Malthus in his Essay on the Principles of population (1798). Today we live in societies where medicine has greatly reduced the risk of disease and the energy that underpins growth is cheap and abundant. We know that societies thrive when they are growing and that is reflected in our economic theory and practice. Growth cannot be limitless and there is always the danger that it will be halted by one of Malthus’s limiting factors if it is not carefully managed. There is now the possibility of artificial birth control, although the use of this as a means of regulating global population in a non-voluntary way is highly controversial. Based on current figures the global human population should plateau by 2050 but by this time will have increased from 7.5 billion to between 9 and 10 billion. Needless to say, thi is a critical period in global history.

Social energy

For an account of the role of energy in history see history in 10,000 words.

If history is about ‘getting things done’ then we must ask how this is achieved in terms of energy flows. Up to the time of the Industrial Revolution, with its machines powered by fossil fuels, the work needed to physically maintain complex societies was provided by human (and to a lesser extent animal) muscles. The awe-inspiring monuments of the ancient world – the Great Pyramids, Stone Henge, the magnificent buildings of Ancient Greece and Rome like the Parthenon and Colosseum, the temples of Angkor Wat – all were achieved using human toil, and not always by choice but as slave labour. Though craftsmen and artisans might willingly provide their labour to such causes the European empires of the 17th and 18th centuries were built on products like sugar, cotton, tobacco, tea, coffee, and rubber that were provided from plantations maintained by slave labour. Much of human social and political history has been bound up in the subtle linguistic distinctions and working conditions associated with ‘rulers and administrators’, ‘employers’, ‘managers’, ‘employees’, ‘servants’, and ‘slaves’. The owner of a large country estate simply cannot do the cooking, cleaning, and maintenance needed to keep that estate going, for efficiency there must be a division of labour and that, historically, has led to hierarchies and inequities. (For a discussion of hierarchies see Great Chain of Being, and reductionism).

World Emissions Flow Chart
The flow of energy through complex social organization

Global greenhouse gas emissions by sector, end use, and gas


Energy is an elemental force in the universe that takes many forms. The social transition to agriculture in the Neolithic Revolution marked a stage of human energy capture when increasing numbers of people could be supported by nearby energy farms (food plants of one kind concentrated together into fields that can be managed efficiently isolated from other plants). The large settled communities made possible by this way of harnessing energy created a division of labour as a hierarchy of rulers, administrators, and officials as well as workers and religiosi. This was loosely the form of society that we know today. Urban societies also developed spatial distinctions familiar to us today – domestic housing, administrative buildings, markets and meeting places, entertainment areas, parks and gardens. There arose a host of mental distinctions that we are also familiar with today: wild and cultivated land, urban and rural regions, house and garden and so on. This marked a transition, based on the ready supply of plant food energy) from life in direct contact with the forces of nature (that had forged the human body in evolutionary time) to artificial human-constructed environments. It was the origins of the familiar distinction between nature and culture. From this time on biological evolution would be supercharged by cultural evolution based on the written word and more efficient energy sources.

With the advent of the Industrial Revolution the numbers of agricultural workers rapidly decreased as people moved into the cities, mines, and manufacturing industries. Europe was developing its global empires as many of the mills and factories of Europe processed products supplied by labourers in colonial plantations.

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... Forms of energy

... Energy and CO2


... Human labour

... Human body energy









Date of origin

Base state   -       human muscle

Hand tools        -     3.5 M BP
Mental tools     -     3.5 M BP



Fire                     -     1.7-2 M BP
Animal muscle -     12000 BP
Wind & water   -  ...  5000 BP ...
Coal                    -      1600 ...
Gas                     -      1820 ...
Oil                       -      1860 ...
Electricity           -      1880 ...
Nuclear              -      1950 ...


             Natura       -   0.25m
             Agraria      -   0.48
             Industria   -   0.26 - 0.31


The energy derived from the Sun, stored in plant tissue during photosynthesis, then used (as food) to power the bodies of living organisms. Most biological energy drives internal metabolic processes within organisms but some is transformed directly into social energy via muscles and brains.

The food energy needed to sustain an individual human body has remained about the same throughout history (though physically active people require more calories) at about 12,500 kJ. while the human body has an energy generating capacity (at basal metabolic rate) of around 80 watts (about 20 watts of this being used by the brain), about the same as an incandescent light bulb). To derive a physical 'feel' for what this means, a 100 watt light bulb works 1.25 times harder than our body, that is, 1.25 H-e or 1.25 human equivalents.


The energy that powers the social activity that may be directed towards the maintenance or enhancement of social organization.

The energy of human social activity is derived partly from the biological energy that powers human bodies, and partly from external sources like water, wind, animal muscle, fire and more recently, fossil fuels, nuclear fuels etc.

Historically, the proportion of social energy derived from human bodies has decreased over time to become negligeable today. Fossil fuels provided a concentrated, abundant, and cheap source of social energy that facilitated growth in populations and economies. The use of this energy has been leveraged by the increasing efficiency of technology as both material and mental tools.

Media Gallery

Top Countries by CO₂ Emissions per Capita 1950 to 2018

Animated Stats – 2019 – 5:49


First published on the internet – 1 March 2019
. . . revised 1 October 2020

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