2.1 - The overpopulation issue is neglected in most efforts to make biosphere sustainable

Human population has grown fast since the industrial revolution surpassing 7.7 billion (Figure 1a) and continues to grow even faster. 

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A limited number of authors call the attention on this issue as a major obstacle to sustainability. Conspicuous among them is the milestone analysis of Wynes and Nicholas (2017) who denounce that the most effective individual action towards sustainability (having one fewer child) is generally ignored. Crist et al.(2017), Bongaarts and O’Neill (2018), Kuhlemann (2018), Abegão (2019), Hedberg (2020),) and Wolf et al. (2021) also denounce that the issue of overpopulation has been neglected and openly defend population degrowth (a non-compulsory one). Recently, Bradshaw et al. (2021) and Chure et al. (2021) have stressed the size of human population as a critical factor of biosphere’s unsustainability.

Equations 1 and 2  and Figure 2 show that the impact of humankind on biosphere depends not only on the high “impact per person” but also on the growing “number of persons”. Thus, “total impact” can be reduced by reducing any of the two independent variables of the equations, or both (Hedberg 2020). Nevertheless, most actions proposed to achieve sustainability insist in addressing only “impact per person”, and this message is repeated in report after report transforming it into an unquestionable (wrong) axiom.

The last report of the World Resources Institute (Searchinger et al. 2019) states that a reduction of the fertility rates to reach a replacement level is required to achieve sustainability, but the report foresees to achieve replacement levels only by 2050 with almost 10 billion people on Earth (population degrowth is not mentioned). The World Scientists’ Warning of a Climate Emergency (Ripple et al. 2020, 2021) goes a bit further and states that “the world population must be stabilized and, ideally, gradually reduced” but gives no numbers and the need for a not so “gradual” reduction is not considered.

Overpopulation is plainly ignored in several key documents addressing sustainability. For example, the IPCC reports on Climate Change and Land (IPCC 2019a) and Ocean and Cryosphere in a Changing Climate (IPCC 2019b), and the FAO report on Overcoming Water Challenges in Agriculture (FAO 2020) mention that population growth results into unprecedented rates of use of natural resources, increase of emissions from agriculture production, higher demand for food and water, etc. but do not address (not to say recommend) population degrowth as a potential way to face these problems. The same occurs with the Lancet Report on Health and Climate Change (Watts et al. 2018, 2019, 2020), the United Nations World Water Development Report (WWAP 2019) and the IPBES reports on biodiversity (IPBES 2019; IPBES-IPCC 2021). The influential Global Footprint Network (2020) states in its website that “the average Ecological Footprint per person worldwide needs to fall significantly” but the terms “overpopulation” and “population growth” appear only in old reports of its website while disappeared in the last ones.

Other documents such as the Living Planet Report of the World Wildlife Fund (WWF 2018, p.22) and the Encyclical “Laudato Si” (Pope Francis 2015) go further ahead and explicitly affirm that over-population is NOT the cause of the problem and that the single cause is over-consumption.

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2.2 - Meanwhile, human numbers continue to grow fast

The global fertility-rate has decreased to about 2.4 children-per-woman and most demographers forecast that this decreasing trend may continue during the next decades. But meanwhile almost no change is seen in the population growth-per-year due to the high absolute numbers (Figure 1b). World population continues to grow at about 220,000 persons per day; this is a running fact, not a forecast. The most accurate estimations forecast 2 billion additional people in 30 years from now (United Nations, Department of Economic and Social Affairs, Population Division, 2019). Figure 1b shows that a small reduction of the birth rate to less than 2 child per woman would drastically change the expected future numbers, but even in this case the population would reach almost 9 billion by 2050.

2.3 - The burden of another 4 billion people would be added to biosphere in 30 years from now

Bongaarts and O’Neill (2018) defend the need to address the overpopulation issue but mistakenly estimate that a burden of 4 billion people will be added to biosphere only by 2100, while these numbers would be already reached by 2050 (Figure 3). There are today 2 billion indigents in the world and their numbers are growing (Tables 1 & 2, Figure 4). These people should get out of indigence (a most desirable goal) and then will start to consume more food, water, energy, medicines, clothes… and their impact on biosphere will increase accordingly. This burden of the already existing 2 billion indigents should be added to biosphere even if population would stop to grow. Then it will be added to the burden of the 2 billion “new” people expected by 2050 if population continues to grow as forecasted.

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2.4 - The “impact per person” should be reduced by 70% in 30 years from now to achieve sustainability

The Global Footprint Network (2020) calculates how many Earths would be needed to make biosphere sustainable. The calculation for 2019 is 1.75 Earths and if this is correct (the calculation sounds very reasonable) the “total impact“ of humankind should be NOW reduced by about 40% in order to achieve sustainability (1 Earth). But if human population continues to grow at the expected rates (Figure 1b) biosphere should provide proper services to additional 4 billion people by 2050 (Figure 3) and then ca 3 Earths would be required. Thus, the reduction in “impact per person” to achieve sustainability should be almost 70% in 30 years from now.  

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2.5 - It is not feasible to reduce the “impact per person” by 70% at global level

The abrupt increase of the “impact per person” in the last century is due not only to overconsumption of superfluous and throw-away products. Much of it is due to the consumption of goods and services that today most people consider essential: a varied diet, safe water, sanitation, housing, heating, proper clothes, personal hygiene, health services, education, security, sport, support to the elderly, a bit travelling, even plain entertainment. Besides, poor people hope for a better life, that means to increase consumption, not necessary of superfluous things but just of basic needs.

Of course there exist overconsumption, avoidable pollution and avoidable overuse of natural resources, but no analysis indicates that stopping them may reduce “impact per person” by 70% or even close to that. The contrary is shown by the analysis of Wynes and Nicholas (2017) who calculated the contribution of individual actions to reduce the emissions of CO2 in the “consumerist” developed countries (Figure 5). Their analysis indicates that except for “having one fewer child”, the contribution of all the other actions together is very small (ca only 10% reduction in CO2 emissions). It must be stressed that the analysis by Wynes and Nicholas refers to the energy demand of wealthy countries, while the expected reduction in CO2 emission of the “green actions” in the under-developed countries will be even more negligible. There are no similar evaluations covering other impacts such as exploitation of natural resources and pollution, but basic knowledge indicates that for almost any type of impact the sum of all the proposed “green” actions to reduce “impact per person”  falls much below 70%.

In spite of the search effort performed in this review, it has not been possible to find even one analysis showing that the total impact of all stressors together can be reduced by 70% at global level, neither close to that, nor even by half of that. The increase in “total impact” by population growth is much higher than the reduction that can be obtained by cutting down “impact per person” (Figures 5 and 6). The most effective action towards sustainability (in fact, the single effective one) by far remains “having one fewer child”.

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2.6 - The fertility rate should be reduced to ca 1.5 children per woman to achieve sustainability

As stated above, if population continues to grow at the expected rate (Figure 1b) “total impact” should be reduced by almost 70% in order to achieve sustainability in 30 years from now. If all the reduction in “total impact” is to be obtained only by population degrowth, a global fertility rate of ca 1.5 children per woman would be required. This global fertility rate seems very difficult to achieve in the short-term, but there are countries where it is already a reality: Finland, Japan, Spain, Germany, Italy (Table 3) Greece, Cyprus, Croatia, Ukraine. A one child family is not unthinkable (Kuhlemann 2018) while a global reduction of “impact per person” by 70% in 30 years from now seems plainly impossible, no analysis indicates that this is feasible.