Contrary to previous projections, it now appears that the world's population is unlikely to stop growing this century. There's at least an 80% chance that between 9.6 and 12.3 billion humans will inhabit the Earth by 2100 — and much of this increase will happen in Africa.
The new report, which is an extension of a 2013 study conducted by the United Nations, was led by demographers and statisticians at the University of Washington. It's the first U.N. population report to use Bayesian methods — a modern form of statistics that combines all available information to generate more accurate predictions.
Unlike the previous analysis, which depended too heavily on expert opinion, the new technique used statistical methods to combine government data and expert forecasts to such things as mortality rates, fertility rates, and international migration. It's therefore more statistically driven, which will in turn allow demographers to quantify the predictions and offer a confidence level that's more useful in planning.
According to the new analysis, most of the anticipated growth will be in Africa, where population is expected to quadruple from around 1 billion today to 4 billion by the end of the century. The primary reason is that birth rates in sub-Saharan Africa haven't been declining as quickly as expected. By the end of this century, there's an 80% chance that the population of Africa will be somewhere between 3.5 and 5.1 billion people.
Graph: UN 2012 world population projection (solid red line), with 80% prediction interval (dark shaded area, 95% prediction interval (light shaded area), and the traditional UN high and low variants (dashed blue lines).
As for other regions of the globe, they're expected to experience less change. Asia, currently at 4.4 billion, is expected to peak around 5 billion in 2050 and then begin to decline. Populations in North America, Europe, the Caribbean, and Latin America are projected to stay below 1 billion each.
Graph: UN 2012 population projections by continent.
In addition, the ratio of working age people to older people is unlikely to decline in all countries — even those that current have young populations.
The authors of the study suggest some important policy implications, along with some caveats. From the report:
Rapid population growth in high-fertility countries can create a range of challenges: environmental (depletion of natural resources, pollution), economic (unemployment, low wages, poverty), health (high maternal and child mortality), governmental (lagging investments in health, education and infrastructure), and social (rising unrest and crime).
Among the most robust empirical findings in the literature on fertility transitions are that higher contraceptive use and higher female education are associated with faster fertility decline. These suggest that the projected rapid population growth could be moderated by greater investments in family planning programs to satisfy the unmet need for contraception, and in girls' education. It should be noted, how-ever, that the UN projections are based on an implicit assumption of a continuation of existing policies, but an intensification of current investments would be required for faster changes to occur. It should also be noted that the projections do not take into account potential negative feedback from the environmental consequences of rapid population growth. The addition of several billion people in Africa could lead to severe resource shortages which in turn could affect population size through unexpected mortality, migration or fertility effects.
Which, of course, could be offset by advances is life extension technologies.
Read the entire study at Science: "World population stabilization unlikely this century".