Ecological economists distinguish (following A. Lotka’s ideas) between ‘endosomatic’ and ‘exosomatic’ use of energy by humans. Inside the body, as food energy, adult humans spend per day between 1,500 and 2,500 kcal on average. A convenient number easy to remember is 2,400 kcal, equivalent to 10 MJ (megajoules). Per year, the endosomatic energy use would then be 3.65 GJ (gigajoules, thousands of millions joules).
Exosomatically, the use of energy varies according to income and style of living. However, if one person has five times the income of another person, he or she is not going to eat five times more in terms of kcal or joules. All humans spend some energy for cooking food (typically more energy is spent in cooking than that in the food itself), and they use energy also for their houses and to produce clothes. In agricultural societies, much energy is used also for domestic animals. In industrial societies, the amount of energy for exosomatic use is drastically higher, whether it is for factories, for transport or for domestic use. In the case of South Africa, while poor people lack minimal access to electricity, mining companies use enormous amounts at subsidized rates.
A typology of societies (from hunter-gatherers to agriculturalists using animals to industrial society) reveals a pattern of exosomatic use of energy that increases from 20 GJ per person per year, to 60 GJ per person per year, to 200–300 GJ per person per year (Fischer-Kowalski and Haberl, 2007). The question arises whether the 7,000 million people at present on earth, or the 8,500 million at the estimated ‘peak population’ towards 2050, will have enough available energy to supply the current level of 200–300 GJ per person per year in the very rich societies. This is unlikely.
Biomass for food and for fuel (firewood) continues to be an important source of energy for humans. There are programmes to produce ‘biofuels’ for transport needs, but as an energy source biofuels show a low EROI, and moreover they increase the HANPP to the detriment of other species, and they require much ‘virtual’ water in order to grow. The other main sources of energy at world level are oil (about 34 percent), coal (about 25 percent, increasing slowly), and gas (24 percent, increasing quickly). Oil extraction, at 87 million barrels per day in 2012, is reaching a maximum level (peak oil). Coal is plentiful but noxious locally, and also globally, because of carbon dioxide emissions. About 7 percent of the energy supply comes from nuclear energy, 7 percent is hydroelectricity, and the rest comes from wind energy and solar energy (thermal or photovoltaic) which are increasing rapidly. However, it would be extremely difficult to completely substitute renewable energy for fossil fuels (coal, oil and gas) at the present level of energy use, let alone to increase energy use within a system based on renewable sources of energy.
Fischer-Kolwalski, M. and Haberl, H. eds. (2007) Socio-ecological transitions, Edward Elgar, Cheltenham.
For further reading:
Lotka, A. J. (1956). Elements of Mathematical Biology. New York: Dover Publications.