How many people can the world sustain?

Overpopulation-  It is possible to estimate an optimal human population size based on various criteria and assumptions. Here, we do not deal with the lower bound of the human population (the minimum viable population) as we are certainly well above that limit. Concerning the upper bound, we have to consider the carrying capacity of the Earth for homo sapiens. The carrying capacity for any species is the maximum number of individuals that can be indefinitely supported at a given consumption level by a given environment. For humans, the estimates differ substantially, ranging from less than 1 billion to more than 1,000 billion people, depending on the average consumption, technology and other factors. Around two thirds of the estimates fall in the range of 4 billion to 16 billion persons, and the median value is about 10 billion -the size that the UN expects in 2055 under its median variant projection.

However, "maximum" is not the same as "optimal." Apart from constraints due to carrying capacity, other criteria should be considered. We can define the optimal population size as the size that produces the best results according to explicit goals or targets. Targets chosen in the well-known study of Daily et al.include sufficient wealth, access to resources, universal human rights, preservation of biodiversity and cultural diversity, and support for intellectual, artistic and technological creativity. Estimating the amount of energy to satisfy these human needs while keeping ecosystems and resources intact, they calculated the optimal population size in the vicinity of 1.5 - 2 billion people.

Another study estimated optimal population size based on the minimal land needed for food production (0.5 hectare per person) and soil conservation - resulting in a population size of 3 billion people. Naturally, these results depend strongly on the assumed per capita consumption considered to satisfy needs for everyone.  In a third study, Pimentel et al. considered a comfortable consumption based on European living standard and a sustainable use of natural resources, suggesting only 2 billion people as appropriate size.

In a recent article, "Sustainable welfare and optimum population size", Lianos and Pseiridis attempt to estimate optimal population size using an objective criterion designed to assure that human resource use does not deplete Earth's natural capital. ...

The authors calculate the maximum gross world product, (the combined gross national product of all the countries in the world), the production of which would leave the natural capital of the Earth and other species' populations intact. In order not to exceed this maximum GWP, but keep a comfortable European average per capita GWP level ($11.000), we should reduce our population to 3.1 billion. If we wish to keep population at 7 billion, the per capita product must be radically reduced to $4.950, from the current $16.100. From this it is clear that the current situation cannot be sustained in the long run, and one way or another, further decline in the ecological footprint-biocapacity ratio is needed.