All living beings draw energy from the environment for their subsistence. Humans also consume energy for many other needs related to their activities and well-being, such as industrial processes, temperature control in buildings (heating and cooling), transport (cars, trains, ships and planes), and household appliances. Indeed, energy consumption increases proportionally to the stage of development of a population. Cultural progress and well-being are linked to energy consumption: the more the former grows, the more the latter increases. Therefore, with regard to the development of the human species and its impact on nature, it is not only the increase in population that must be considered, but also the increase in energy consumption.
Total energy consumption (E) increases with population growth (p) and the incremental energy consumption per individual (e), so that we have to look at the sum, E = p + e. This value can be high even if we reach zero population growth (p), as is happening in developed countries, if the value of the increase in energy consumption (e) remains high. Currently the total index (p + e) at global level remains, in fact, very high, because it indicates the doubling of the total energy consumption every 20 years.
In the last 100 years the world population has grown at a rapid pace. It is estimated that in the first centuries of our era it numbered about 200 million people. It grew slowly, so that in the 18th century there were 600 million human beings on Earth and in the 19th century we reached one billion. But since then, growth has been rapid: in 1950 there were 2.5 billion of us and in 2022 we reached 7.9 billion. In recent times this rate of growth is slowing down, following the reduction in the birth rate, linked to progress, and it is believed that by 2050 we will reach a maximum of 11 billion individuals.
Energy is expressed in various units: first of all the unit of work, that is the joule (J), with its multiples gigajoule (1 GJ = 109 J) and exajoule (1 EJ = 1018 J). One joule is equal to the work done by a force of one newton acting through one meter (J = N x m). A newton is in turn the force required to impart to the mass of a kilogram the acceleration of one meter per second squared (N = kg x m/s2). Another widely used and better known criterion measures energy in terms of units of power (work per unit time): the watt, which is equivalent to one joule per second (W = J/s). The power of one watt produced for the time of one hour, the watt-hour (Wh), represents the energy used in the process. Its best known multiples are the kilowatt-hour (kWh), the energy of 1,000 watts maintained for one hour, and the terawatt-hour (1 TWh = 109 kWh). One kilowatt-hour is equivalent to 3.6 million joules (3.6 x 106 J). As for multiples, 1 exajoule is equivalent to 280 TWh.