People use energy to accomplish the tasks of production and everyday life. For eons, our only motor was our muscle-power, our energy inputs food and the warming rays of the sun. To these bodily energy inputs could perhaps be added our use of the sun and wind for drying things, and the heating effect of the sun on our crude shelters. An active person consumes 3000 kilocalories, or 13 megajoules (MJ) of food energy a day.[1] If we assume that a person also uses two square meters' worth of solar heating power, at a temperate latitude, and a typical number of the days are cloudy, her consumption of solar heat would average 20 to 40 megajoules a day.[2] The amount of wind energy involved in drying would be much less, perhaps 105 joules a day or less. Regarding the efficiency of utilisation of these energy inputs, most of the food energy we consume operates our bodies' metabolic processes, keeps us happy, and goes off as heat. Besides this, a strong person, working hard with her muscles, can do about 2.9 MJ of mechanical work in a day. (This would probably cause her to consume somewhat more than 13MJ of food energy in a day – perhaps more like 17 MJ.) The efficiency of the body in turning food energy into mechanical work (force x distance) is roughly 15 percent – after basic metabolic needs have been met.[3] As for our prehistoric friend's technical relations with the sun, we can speculate that a lean-to made of tree boughs captures solar energy less efficiently than, say, a well-oriented thermopane window over a concrete or stone heatsink (as one might find in a present-day solar house). The smallness of the lean-to, however, is an efficiency advantage: she is not heating vast uninhabited atriums, dinig rooms, and parlours in addition to the lair she is actually curled up in.

As people progressed technologically they began harnessing more of the energy flows around them. Wood for burning and the muscle-power of draught animals were early captures. Then waterwheels, windmills, and the sails of boats began siphoning off little trickles of the natural flux. An old-fashioned overshot waterwheel 8 meters in diameter, 0.8 meters wide and making one revolution every minute and a half delivers a few hundred Watts, or a few times 107 joules a day. It spills a lot and is not geometrically optimal, and thus captures only perhaps a tenth of the available falling-water energy.

With the industrial revolution which began around 1800, per-capita energy use has increased dramatically. The main source of this energy has been fossil fuels, first coal, then also oil and natural gas. In 2012 we burned 460 exajoules (EJ, 1018 joules) worth of fossil fuels.[4] Our other major sources of energy today are traditional biomass fuels such as wood and peat, flowing water which we use to generate hydroelectricity, nuclear power, and `new renewables' such as sun and wind captures by solar panels and wind turbines. All together, from fossil fuels plus these other energy sources, we diverted for our own use about 560 EJ of energy in 2012.[5] This works out to 80,000 MJ per person per day for each of the 7 billion people on the Earth. It is evident that a great divide separates the modern from the pre-industrial world in terms of energy use. The average person now uses about a thousand times as much energy as a person did before the industrial revolution. But in some ways, even this understates the difference, because 80,000 MJ is the modern world average; people in poor countries use less than this, and people in rich countries considerably more. The per-capita figure for the U.S., given by the International Energy Agency, is 285,000 MJ, and even this may underestimate the consumption attributable to the average American, because she imports products, and the energy used in making those products shows up in the energy budgets of other countries. The per-capita energy consumption in many poorer countries, as given by the IEA, is in the 13,000 to 17,000 MJ range.[6]

Our high levels of energy use bring many pleasures and advantages but also have bad side effects such as danger, noise, instability, uncertainty, and environmental degradation. One of the more serious environmental effects is global warming. This is caused mainly by carbon dioxide (CO2) gas, which, along with steam, is the major exhaust component from the burning of our largest energy source, fossil fuels. Carbon dioxide gas in the atmosphere forms a sort of selective blanket around the Earth, which lets sunlight in, but is relatively resistant to letting infrared radiation (radiant heat from the warm surface of the Earth) escape into outer space. The concentration of CO2 in the atmoshere was about 270 parts per million (ppm) before the industrial revolution; it is now 360 ppm and rising about 2 ppm a year[7] The increase is partly due to deforestation and other changes in land-use patterns, and partly due to cement production, but mostly due to the burning of fossil fuels. We naked apes' yearly injection of carbon into the atmoshere, as of 2012, is 9 gigatonnes (GT). Our cumulative injection, 1850-2009, is about 350 GT.[8] Global average surface temperature rose 0.6ºC ± 0.2ºC over the 20th century.[9] It is estimated that our cumulative emission by the year 2100 will have reached to between 700 and 2500 GT. Keeping it to the low end of that range will require determined measures.

About half of the carbon we inject into the atmosphere remains there for the medium term – a century or more. The other half is removed fairly rapidly by dissolution into the ocean, and by increased uptake by plants, soil organisms, and ocean life.[10] In the pre-industrial era, the mass of CO2 in the atmosphere was about 600 GT.[11] It is now (based on a 360 ppm concentration figure), about 760 GT. According to recent climate models, a doubling of atmospheric CO2 results in a temperature rise of about 3ºC.


  1. Australia, National Health and Medical Research Council, Nutrient Reference Values, Dietary Energy
  2. See insolation maps at Wikimedia Commons: Insolation.png Europe solar map
  3. Erin Baumgartner, `Human Horsepower',  accessed 2014.
  4. International Energy Agency, Key World Energy Statistics 2014.
  5. Primary energy production, as given by International Energy Agency, Key World Energy Statistics 2014.
  6. Intenational Energy Agency, Key World Energy Statistics, op. cit. The national figure are total primary energy supply, divided by population.
  7. In 2000 it was about 355 ppm: Michael McElroy, The Atmosheric Environment (2002), around page 141.
  8. From fuel and cement emissions. Josep Canadell, Corinne Le Quere, et al., `Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks.' Proceedings Nat'l Academy Sciences U.S.A. Nov 20, 2007; 104(47).
  9. IPCC, cited in Barrie Pittock, Climate Change (2005), p 4.
  10. McElroy, p 142. After analysing a simple atmospheric model, he says: `It follows that only 56% of the carbon emitted due to combustion of fossil fuels has remained in the atmosphere.'
  11. McElroy, p 144.
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