About optimum populations
What is an optimum population exactly? We don't know with any certainty how many people the Earth can support.
An optimum population is one which the earth can support sustainably, while allowing everyone to enjoy a good quality of life.
The Global Footprint Network (GFN) has developed the concepts of biocapacity the amount of land available to provide for human needs, and ecological footprint the land needed to satisfy the consumption of different nations in a sustainable manner, including the biological capacity to absorb and mitigate the CO2 emissions that lead to global warming.
The present total ecological footprint of all nations is 2.6 gHa/cap (global hectares per person) but the total biocapacity is only 1.8 gHa/cap [GFN Ecological Footprint Atlas 2009]. This overshoot means that humanity is already using 1.4 times as much resource as is sustainably available. This is only possible for a short time. We are living on the natural "capital" of the planet rather than its renewable "income".
The situation is more serious than the overall figures indicate. Low income countries have a typical footprint of 1.0 gHa/cap but the average for high income countries is 6.1, of which the UK is typical at 6.12, ie the world population of 6.8 billion would need 3.4 planet Earths to achieve typical UK living standards. The United States has an even higher consumption footprint.
So what are the optimum populations for my country and the world?
The optimum (best) number of people is not and never will be an exact number. It is the approximate size of population that allows communities to sustain a decent standard of living, ie to be able to go on enjoying a satisfactory life style indefinitely into the future. If there are too few people, it is difficult for economies or culture to flourish. On the other hand, excessive numbers are unsustainable.
As higher material living standards consume more natural resources and have more environmental impact per person, the higher the standard of living, the smaller the sustainable population size.
Based on GFN figures for 2006, the then world population could live sustainably at the living standard of a middle income country such as Algeria or Ecuador. This is equivalent to 1.8 gHa/person, less than 30% of the average per capita consumption of the rich nations.
What is the relevance of an optimum population? Isn't carrying capacity a more scientific concept?
The two terms have different meanings. The carrying capacity (for any species) is the maximum number of individuals that could be supported sustainably and indefinitely at a given consumption level by a given environment. Optimum population means the best balance between the number of people and the quality of life that they may obtain, though it should not be viewed as an exact number.
Because carrying capacity refers to maximum sustainable population for a given environment, it doesn't take into account any margin to allow for changes in the environment. This is another reason why the 'best' number of people is almost always fewer than the maximum that the environment can support.
For more information, see The Sustainability of Human Populations: How Many People Can Live on Earth, Towards Sustainable and Optimum Populations and the latest overshoot by country figures.