The optimum population of the world is the one that is most likely to
make the option of a good quality of life available to everyone
everywhere, both now and in the future. Establishing a consensus about
the size of such a population would be an important step towards
achieving it.

Estimates of an optimum involve three main steps.

First, estimate the maximum (carrying capacity) assuming a specified
lifestyle. The main criteria are the maintenance of biodiversity, the
availability of freshwater, and the availability of land--for
agriculture, forestry and artificial systems but above all for the
conversion of energy. (In applying the criteria, there are always two
questions to ask: 'What is the maximum amount of consumption that the
biosphere can stand?' and 'What is an adequate share of such
consumption per person?')

Second, convert the maximum (two to three
billion) into an optimum by applying a far wider range of criteria,
including personal liberty, mobility, recreation and political
representation.

Third, consider just two criteria (economies of scale
and technological innovation) in order to ensure that the optimum (one
to two billion) has not fallen below the minimum (half to one
billion). The estimates are so low because of the need for a huge
increase in median per capita consumption if everyone is to have the
option of an adequate material standard of living. Opinion-formers are
likely not to take much notice of such estimates, but it is probable
that minds will be concentrated by an energy shock some time during
the next decade. Achieving an optimum world population will not solve
the world's major problems, but it would make them solvable.