In the period 1975 to 2020, world population increased  at an average of 83 million per year.  The 2019 increase  was the difference between approximately 140 million births and 57 million deaths. The annual  emission of carbon dioxide from fossil fuel combustion, cement manufacture and deforestation is currently 40 billion metric tons, of which almost half remains airborne.  The two challenges are thus the stabilization of world population and the stabilization of atmospheric carbon dioxide concentration.  In the following it is shown that  it is not possible to reduce world population to a sustainable 3 billion.  It will peak  at  almost 10 billion if the peak is reached in 2070, 11 billion if it is reached in 2100, and  12 billion if it is reached in 2130. How steep the subsequent decline will be, and how long it will last, can only be guessed at.

Population growth creates the need for increasing food production. Cereals are the most important crops for food and livestock feed; globally, 45 percent of the cereal harvest is consumed by humans, and 35 percent by livestock; the remainder is used for other purposes, including seed, ethanol (for blending with gasoline), and alcoholic beverages (beer, whisky,  vodka and sake).   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; reactive nitrogen is essential for photosynthesis, DNA, proteins and enzymes.  Both chemist Fritz Haber and chemical engineer Carl Bosch were awarded the Nobel Prize.   Production of Haber-Bosch ammonia began in 1913, and consumption of nitrogen  fertilizer is now 109 million metric tons per year, of which 55 percent is applied to cereal plants.. Outside China, which uses coal, ammonia synthesis is based on natural gas.  

The second advance was the Green Revolution that began in 1965, after agronomist Norman Borlaug had bred varieties of dwarf wheat that are sturdier than traditional varieties and give higher yields in response to heavier applications of fertilizer. Borlaug was awarded the Nobel Peace Prize in 1970. The breeding of hybrid maize by Illinois farmer Lester Pfister in the 1930s paralleled the work of Borlaug. The International Rice Research Institute launched high-yielding semi-dwarf rice in 1966.

 Global cereal yield  rose from 1.16 metric tons per hectare in 1950 to 4 tons in 2018. In the same period, the harvested cereal area increased from 600 million hectares to 730 million. If the average annual yield increase in 1992 to 2017 of 52 kilograms  per hectare continues to 2050, the yield in that year would be 5.8 tons per hectare, thereby raising  production (assuming no change in the harvested cereal area) to 4.2 billion tons. The Population Reference Bureau projects the global population in 2050 at 9.88 billion, so that production per person would be 420 kilograms, 10 percent above  the current 380 kilograms.  The annual nitrogen application on the world cereal area (approx. 82 kg per hectare in 2020) would  rise to approx. 150 kg per hectare if there is no improvement in nitrogen use efficiency.     

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 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. Many soils are deficient in nutrients. Inadequate amounts of nitrogen, phosphorus and potassium are remedied by fertilizers, but other nutrients, needed in small or trace amounts (they include  calcium, magnesium, sulphur, iron, boron, manganese, zinc, copper, molybdenum and nickel) are seldom applied. As agronomist Lloyd Evans put it: “These nutrients are the Davids which can render the Goliaths of nitrogen, phosphorus and potassium helpless in both plants and animal husbandry”. .

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 to global sea level rise.

If the ratio of a country’s population to its arable area does not exceed   2 persons per arable hectare , its population  could be fed adequately without nitrogen fertilizer or grain imports. If this ratio were the average on the world’s 1.6 billion arable hectares, world population would be 3.2 billion. Reducing world population to this number would  mean that China would have to reduce its 1400 million population to 250 million, and India its population of 1400 million to 340 million. Japan has 27 persons per arable hectare and Egypt 31; these two countries would have to reduce their populations even more drastically. By 2050 , the population of Japan is projected by the PRB to be reduced by 13 percent to 110 million, and  the.population of Egypt to increase by 56 percent to 158 million.

The “2 per arable hectare” criterion cannot be applied to countries with extremely small arable areas, such as Israel, Singapore, Kuwait,  the United Arab Emirates and Iceland. Countries that have less than two persons per arable hectare include Russia, Ukraine, Canada, Australia and Argentina. The ratio in the United States is just over 2, and the world average is certain to reach 5 in 2023. Major population reductions would require over a century in which fertility was one child per woman. The fertility rate in South Korea fell to 0.98 in 2018, the world’s lowest; if this fertility rate continues, the proportion of the population in the 65+ age-group would rise to over 40 percent after two generations. Few countries will follow South Korea’s example, or even that of Japan, which has a fertility rate of 1.3. It is obvious that a reduction of world population to 3 billion by means of fertility decline is impossible.  

The second global challenge is the rising atmospheric carbon dioxide concentration.   In 1906, the Swedish scientist Svante Arrhenius calculated that a doubling of the concentration (then 300 parts per million) would cause a rise of 2.1 degrees Celsius in global average surface temperature. Recent estimates vary, but it is probable that the rise would exceed 2 degrees C, and virtually certain that it would exceed 1.5 degrees, generally considered the “safe” limit.   It is most unlikely that the concentration can be stabilized at less than double the pre-industrial level of 280 parts per million. The rise in sea level in the period 2005 – 2100 has been estimated at 650 mm, with an uncertainty of 120 mm either way. The rise could ultimately reach  3 m, as it did in the Eemian interglacial 130,000 years ago.

 Stabilizing the CO2 concentration  at 450 ppm  would mean leaving most of the recoverable fossil fuel reserves underground; consuming all the recoverable  fossil fuel reserves as currently estimated would bring the concentration up to over 600 ppm.. In 2018, fossil-fueled power plants generated 61 percent of the global generation of 25,000 terawatt-hours. In 2050, fossil electricity has been optimistically projected to account for  43 percent of a global generation of 44,000 terawatt-hours.     This means that fossil-based electricity generation in 2050 will exceed that in 2018. Fossil fuels have enabled humanity to achieve scientific and technological advances undreamed of prior to the Industrial Revolution. There is a price to pay; there is no free lunch.

9. November 2020.