Energy in History - The Basic Environmental History - page 4

sense, energy is the main input; that is to say, the main input is that part of matter
(resources R) transformed by the converters, that is by workers (L), who metabolize
food, and capital (K), which transforms some materials such as

rewood, coal, oil,

gas and electricity into mechanical work, heat and light.
The increase in productivity of energy, as a consequence both of discoveries
of new sources and technologies (macro-inventions) or improvements in the
exploitation of those already existing (micro-inventions) 3 can be represented by

the following ratio:

Y
E

p ¼

where Y is output (in value) and E is the total input of energy in physical terms
(in Calories or joules or any other energy measure). The formula represents the
productivity of energy, that is the product generated by the unit of energy. It is
the reciprocal of the better known energy intensity (i), or the energy we need to
produce an unit of GDP:

E
Y

i ¼

In the previous diagram, energy productivity is the result of the ratio between the

nal product (in money) and the input of matter (food, coal, oil … ) transformed into

energy by the converters (in kcal, joules ciency of the

energy converters from a technical viewpoint. The result is also conditioned
by changes in the structure of the product. The increasing importance of less energy
intensive sectors can result in an increase in energy productivity (or decline in
energy intensity) even without any technical change.

… ). It is a measure of the ef

1.2.3 Energy and History

At the end of the 20th century, per capita energy consumption, on a world scale,
was about 50,000 kcal per day; that is 76 GJ per year, including traditional sources.
About 80 % of this consumption was represented by organic fossil sources; coal,
oil and natural gas. Nuclear energy represented 6 % and hydroelectricity 2 %. This
8 % was the non organic contribution to the energy balance. The remaining 12 %
consisted of biomass, i.e. organic vegetable sources (Table 1.1 ). If the waste uti-
lized in order to produce energy is excluded, the rest of this 12 % was composed of
food for humans and working animals (today a marginal source of power), and

rewood, an important item of consumption only in developing countries.

3

For the terms “ micro- ” and “ macro-inventions ” see Mokyr ( 1990 ).