World population grew by almost 300% during the twentieth century; over the same time period, world per capita incomes grew by about 400%. This association of sizable increases in world population with large increases in per capita incomes should continue to the end of this century.
Forecasts of the world’s population only a few years in the future are generally quite accurate because the number of births and deaths during the next few years are largely determined by the existing distribution of the number of people at different ages. At the same time, forecasts of the population 50 or more years into the future are notoriously inaccurate because of difficulties in predicting changes over a long time period in birth rates, and to a much lesser extent, also in death rates.
In particular, one cannot take seriously the UN’s median forecast of 10 billion persons by the end of the century. For this forecast assumes that birth rates in high fertility countries will not decline during the next half-century. Since it is highly likely that these countries will raise significantly their per capita incomes and education of their women, their fertility rates will fall, probably drastically. If so, world population in 2100 would be well below 10 billion people.
However, for the sake of this discussion, I assume that the UN forecast is approximately correct, so that about 10 billion people will inhabit the earth by the end of this century. Posner mentions various likely benefits of a much larger population, such as greater demand for and supply of innovations in the medical and other sectors, and greater world specialization by skill.
Few countries have experienced sustained declines in their populations since the beginning of the 19th century. The substantial world growth in per capita incomes during the past 150 years has been associated with growing world populations. I believe that declining populations are bad for long run economic welfare. If I am correct, countries such as Russia, Japan, and Germany, with fertility rates that are far below replacement levels are likely to face an unattractive economic future unless either they take in enough immigrants to make up for their low fertility levels, or they have large increases in fertility rates.
Given the sharp rise in food prices during this first decade of the 21st century, it would appear difficult to feed adequately a much larger and richer world population. Yet, unlike say the production of copper, no natural limits sharply curtail the amounts of food that can be produced. Food output will expand with a growth in the amount of land devoted to food production-currently agriculture takes a small fraction of the world’s arable land. Also, the world can invest much more in fertilizers and in improving food technology, so that greater output can be squeezed out of each acre used to grow corn, wheat, soy, dairy, meats, and other foods.
Greater demand for water due to larger populations and greater wealth would make clean water scarcer. This could produce a water shortage unless countries began to price more efficiently the water used in agriculture and industry, by far the largest water users. With sensible prices, the available water should be sufficient to satisfy all essential water needs of a much larger world population.
An increase from 7 to 10 billion people on the planet will significantly raise population density in many parts of the world, and thereby increase the potential for severe outbreaks of communicable diseases. However, cities like Hong Kong show that it is possible even with current knowledge to control the spread of disease in densely populated communities. As knowledge of how to track and combat diseases greatly improves over time, the medical challenges created by densely populated areas should be reduced even further.
A larger population combined with growing per capita incomes would increase global warming and worldwide pollution. Although the severity of the global warming problem by the end of this century is not fully established by climate science, the world should be prepared to meet various worst-case climate scenarios. This would require the development of mitigation techniques that can be rather quickly ramped up in case global warming turns out to be a severe problem (see the analysis in Becker, Murphy, and Topel, “On the Economics of Climate Policy”, The B.E. Journal of Economic Analysis and Policy, Volume 10, number 2). Such technologies are certainly achievable by the end of the century with substantial private and public investments in developing new methods to capture and store various harmful gases emitted by fossil fuels.
If world population grew to 10 billion by the end of the century-an unlikely outcome- that would present considerable challenges. However, greater population would add real benefits as well, and I am inclined toward the view that the benefits will exceed the harm.