A sustainable global population -and why we cannot achieve it


In the period 1975 – 2018, world population increased at an average of 83 million per year, and reached 7.6 billion in 2018. The increase in 2017 was the difference between approximately 145 million births and 62 million deaths. Despite population growth, the global average daily food supply per person rose from 2440 kilocalories in 1975 to 2940 kilocalories in 2015 (1). However, over 800 million people are undernourished and 300 million adults are obese.

Cereals are the most important crops for food and feed; globally, 45 percent of the cereal production is consumed by humans, and 35 percent by livestock. The remainder is used for industrial purposes, including ethanol, beer, whisky and vodka. The rise in world cereal production since the 1960s is mainly due to two technological advances. The first was Haber-Bosch ammonia synthesis, in which atmospheric nitrogen is fixed as ammonia (containing 82 percent nitrogen) which plants utilize for protein formation. Production of Haber-Bosch ammonia began in 1913, but did not begin to rise rapidly until the 1960s. The second advance was the Green Revolution that began in the mid-1960s, after agronomist Norman Borlaug had bred varieties of dwarf wheat that give higher yields in response to heavier applications of nitrogen, phosphorus and potassium fertilizer, pesticides and irrigation. The breeding and use of semi-dwarf rice and hybrid maize paralleled that of wheat.

The most striking achievement of chemical agriculture is the maize yield in the U.S., which rose from 2.5 tonnes per hectare (40 bushels per acre) in 1950 to 11.0 tonnes per hectare (175 bushels per acre) in 2016. The global cereal yield rose from 2.81 tonnes per hectare in 1992-96 to 3.91 tonnes in 2012-16 (2). Linear extrapolation of the 1992 – 2016 yield trend (52.3 kg per hectare per year) gives a yield of 5.73 tonnes per hectare in 2050. If the population in 2050 is taken as 9.85 billion (3), and the harvested cereal area remains 718 million hectares (as in 2016), production per person in 2050 would be 420 kg, 10 percent above the 2016 level of 382 kg; the uncertainty is about 10 percent either way. Assuming that the global average cereal yield without using nitrogen fertilizer is 1.6 tonnes per hectare, and that fertilizer increases grain yield by 30 kg per kg nitrogen applied, the global average nitrogen application on cereal crops, 80 kg per hectare in 2015, would be approximately 140 kg per hectare. If the incremental yield-nitrogen ratio rises to 35 by 2050, the nitrogen application would be 120 kg per hectare.

The success of the Green Revolution created three major ecological problems:

  1. Globally, about half the applied nitrogen is taken up by the crop plants; the remainder volatilizes in the form of ammonia and nitrous oxide (a powerful greenhouse gas) or leaches to groundwater, resulting in eutrophication (the formation of algae) in rivers, lakes and coastal waters; this creates “dead zones” in which fish cannot live.

  2. Applying nitrogen, phosphorus and potassium fertilizer to crops changes the balance between these nutrients and those needed in small or trace amounts; the latter include calcium, sulphur, magnesium, iron, manganese, copper, zinc, cobalt, boron and selenium.

  3. Approximately 40 percent of global irrigation water is obtained by pumping groundwater from tube wells; this has resulted in the depletion of aquifers and the lowering of groundwater levels, thereby contributing 0.4 mm to the global sea level rise of 3.4 mm per year (4).

As population growth increases the need for fertilizer, it follows that population reduction would ultimately solve the ecological problems. Unfortunately, human nature is such that global population reduction is not feasible. The reasons for this are given in the following.

In 1950, France had a population of 42 million and 20 million hectares of arable land, i.e. 2 persons per arable hectare. The nitrogen fertilizer application on cereals was negligible, and cereal production per person was about 400 kg per year, slightly higher than the present world average. If the ratio of population to arable land were 2 persons per hectare on the world’s 1.6 billion arable hectares, world population would be 3.2 billion. Reducing world population to this size would mean reducing the global average fertility rate (currently 2.5 children per woman) to 1.5 by 2050 and holding it at that level until 2200. The proportion of the population in the 65+ age-group would rise to 35 percent. Such a drastic change in the age distribution would mean raising the pensionable age to 70 years or more.

Adopting and enforcing a population limit for each country would be an insurmountable obstacle, as Charles Galton Darwin pointed out in 1952 (5). To lower the global average to 2 inhabitants per arable hectare, countries such as Canada, Russia, Australia and Argentina would not be required to reduce their populations, but would not be permitted to reach 2 inhabitants per arable hectare; they would be obliged to have a grain surplus for export to countries that need grain imports. China and India would each have to reduce its population to roughly 300 million; the combined population of the two countries would then be 20 percent of the world population instead of the present 35 percent (6). The relative population reductions in Japan and Egypt, which have 30 and 33 inhabitants per arable hectare respectively, would be much greater (6).

The population of China is projected to peak at 1.45 billion around 2030 and decline to one billion by 2100. This is partly a result of the so-called one-child policy launched in 1979 (in reality a 1.5-child policy). It was replaced by a two-child limit in 2016, but the fertility rate remains 1.6. Japan has a population of 126 million and a fertility rate of 1.4; the population is projected to decline to 102 million in 2050 and 60 million in 2100. These projected long-term declines are likely to be halted by pro-natalist policies based on the advice of growth-obsessed economists who believe that population decline results in a shortage of labour. A world population peak of at least 10 billion is almost inevitable, and this would make 70 percent of the world’s population dependent on Haber-Bosch ammonia. This is not sustainable, but there is no solution in sight. As a sustainable population cannot be attained by fertility decline alone, a mortality rise is highly probable. We can only guess when.

Bernard Gillan is an independent researcher with a degree in Engineering, based in Copenhagen, Denmark. He is the author of several papers on demography and population


  1. FAOSTAT data.

  2. World Bank data.

  3. Population Reference Bureau. World population data sheet 2018.

  4. Konikov, L.F. 2011. Contribution of global groundwater depletion since 1900 to sea-level rise. Geophysical Research Letters, 38; L17401.

  5. Darwin, C.G. 1952. The next million years. Hart-Davis, London.

  6. Lionos, T.P., A. Pseiridis. 2016. Sustainable welfare and optimum population size. Environment, Development and Sustainability, 18(6), 1679 – 1699. According to the authors, the optimum population of the world is 3.1 billion, and the populations (in millions) of the ten most populous countries are:

China 253, India 341, United States 326, Indonesia 88, Brazil 156, Pakistan 43, Nigeria 79, Bangladesh 17, Russia 249, Japan 9.2. The figure for Egypt is 7.4.


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