Patrícia Dérer (2018): “ Targets chosen in the well-known study of Daily et al.3 [Daily, G. C., Ehrlich, A. H. & Ehrlich, P. R. Optimum human population size. Popul. Environ. A J. Interdiscip. Stud. 15, 469–475 (1994)] 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… 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 size5 [Pimentel, D. et al. Will Limited Land, Water, and Energy Control Human Population Numbers in the Future? Hum. Ecol. 38, 599–611 (2010)]… 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. This is the unitary value of the ecological footprint-biocapacity ratio (L). The ecological footprint measures the demand that human consumption places on the biosphere. Biocapacity represents the biosphere’s regenerative capacity; i.e., it measures the productivity of various ecosystems. Between 1961–2009, their ratio L increased dramatically [quasi-linearly versus time from 0.7 to 1.5]. In the beginning of this period, the world had a substantial ecological reserve. That disappeared after about 10 years and since then we have been operating in deficit mode. Today the demand for resources exceeds the available supply by 50 % (L=1.5). (Fig.1)… The authors calculate the maximum gross world product (GWP, 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 (L=1). 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 $16,1007. 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. (Fig. 2) [Lianos, T. P. & Pseiridis, A. Sustainable welfare and optimum population size. Environ. Dev. Sustain. 18, 1679–1699 (2016)]” (Patrícia Dérer, “What is the optimal, sustainable population size for humans?”, The Overpopulation Project, 25 April 2018: https://overpopulation-project.com/what-is-the-optimal-sustainable-population-size-of-humans/ ).
[Editor’s note: If we address the disaster of the steadily increasing value of the ecological footprint-biocapacity ratio (L) by returning 50% of arable land to wilderness, then the sustainable carrying capacity reduces to about 1 billion. Further the calculation ignores increasing loss of arable land through desertification, salinization, urbanization and drought. The GDP per capita in the Occupied Palestinian Territories is about $3,200].