If we sustain the idea of the growth limits existence (by considering basically, in this case, the demographic variable), the evident implication of this statement is the existence of a population threshold, which is the last boundary for a sustainable relationship between the population and the environment.
This hypothetic maximum number of inhabitants has been called carrying capacity.
The carrying capacity concept has been developed in the seventies, as a result of the Malthusian ideas revision and has become the hot topic of many debates and definitions. According to most authors, the carrying capacity can be defined as ‘the maximum number of individuals that can indefinitely be supported in a given habitat, without any decrease of resource availability or access’.
As it can be deduced from the definition above, this maximum number of people is a quite relative concept, since it depends on the given level of natural resources consumption, which is strongly influenced by the cultural, productive and economic organisation of the societies, and also by the scale analysis.
Consequently, many authors have tried to establish this hypothetic maximal number of individuals which the planet can sustain, principally, in terms of food production. By having a quick glance at the main proposals, considering all of them, from the first studied estimations (the oldest ones is from 1679) to the most actual ones, we can see that the obtained results are really different from each other, since most of them take different start-point estimations on the real capacity of Earth to produce food, as it exceeds any expectations. Here are provided some examples of these estimations: 12300 million, according to UN; 50000-60000, according to FAO; and 97000, according to a Harvard University study.
For this reason, some authors criticise the validity of these estimations, since the resources are, as previously said, variable within time, therefore this hypothetic number is also variable. Another criticism on these estimations, provided from the demographic perspective, rejects the lack of the population individual characterisation: in the consulted literature, there is no reference on the sex and age distribution of this maximal number of individuals, although it is quite obvious that this distribution determines the environmental impact, consequently, it determines the carrying capacity too.