Population is a dynamic field. There have been significant changes in birth rates and the population trajectories of countries and continents in recent years. Global population is still rising by more than 80 million a year, however, and is most likely to continue rising for the rest of this century unless we take action.
Current world population
How we got here
Until the time of Napoleon, there were less than 1 billion people on Earth at any one time. Since the Second World War, we have been adding a billion people to the global population every 12-15 years. Our population is more than double today what it was in 1970.
Where we could be going next
Every two years, the United Nations makes projections for future population growth. In 2017, its main, "median" projection was a population of 9.8bn in 2050 and 11.2bn in 2100. Because many factors affect population growth, it makes a range of projections depending on different assumptions. Within its 95% certainty range, the difference in population in 2100 from the highest to lowest projection is almost 4bn people - more than half the population we have today.
The second graph above shows the UN's projected population if, on average, there was half-a-child less or half-a-child more per family than in the median projection.
This shows the enormous difference in total numbers that arise from just very small variations in family size. If we can achieve that modest reduction in number of children born, we will have more than 3bn people fewer by 2100 - a lower population than we have today.
(Source: United Nations World Population Prospects 2017)
Where population growth will occur
More than half of the people added to the world's population over the rest of the century will be in sub-Saharan Africa. This is a reflection of four main factors. First, although it is falling, fertility rate (family size) remains high in most African countries. Second, sub-Saharan Africa has a very young population - its average age in 2018 is just 18 years old. That means that many people are entering their childbearing years. Thirdly, people are living longer in Africa. Fourthly, people tend to have children young, meaning there are more generations alive at any one time.
These figures regarding populations of different continents do not reflect any assumptions about future migration, however. Climate change, poverty and population pressures themselves will lead to a highly mobile global population, with Africa likely to be the largest source of emigrants.
Fertility rates and population
'Total fertility rate' - broadly speaking, the number of children a woman is likely to have in her lifetime - gives an indication of how family size is changing (birth rate - number of babies born per thousand population - is pushed up when there are large numbers of young people in a population). A TFR of 2.1 is "replacement rate" - a population with that TFR will stabilise in time.
- The current global TFR is 2.5.
- Fertility rates are lower today than 50 years ago in every country in the world, except for the Democratic Republic of Congo.
- Fertility rates are expected to fall worldwide, to the point where no country is expected to have fertility of more than five births per woman by 2050.
- Today, 43 countries with populations of at least 1 million have fertility of 4 or more births per woman, 30 have fertility that is falling but is still between 2.5 and 3.9 births.
- 33 have fertility that has dropped to about replacement level relatively recently.
(Source: United Nations Population Fund, 2018)
Populations are also affected by death rates, net migration and the proportion of people of childbearing age, however. That is why populations of countries with significant falls in fertility (such as Nigeria) continue to grow. In particular, where birth rates have recently been high, when the babies born in that period reach childbearing age they increase the number of families, even though the size of their families is smaller than in the previous generation. This is called 'demographic momentum' and means that the impact of changes in fertility normally take decades to be reflected in population.
Many countries with less than replacement rate are also growing, usually as a result of net migration. Immigration increases numbers of people directly but also by increasing the birth rate. This is usually because migrants tend to be younger people of working age so more likely to have children than the existing population, and because in some cases, they come from countries or cultures with traditionally higher fertility rates and family sizes.
Bringing down fertility
Fertility rates tend to decrease when countries become more affluent, when women are more empowered, when children stay in education for longer and when, crucially, people are able and choose to use good, modern family planning.
More than 200 million women currently have an unmet need for family planning - meaning they do not want to get pregnant but are not using moden contraceptive methods. Research published by the Guttmacher Institute in 2016 shows that this is sometimes because they are unable to access family planning but more commonly because of concerns about side effects or other health impacts, a perception (often mistaken) that they do not need them and, in nearly a quarter of cases, because the women or those close to them opposed contraceptive use. Contraception provision must be accompanied by appropriate education and support to be effective.
Another important factor in uptake of contraception is desired family size. Religious, cultural and social influences all play a part in that, as do economic and political factors. Where people cannot rely on the state to support them, they tend to have larger families to ensure they have children who can support them. Where child mortality is high, people also seek to have more children. The "value" of women and girls may be judged by the number of children they have (not just in places where women are not empowered) and traditions valuing larger families are often internalised. In Niger, for instance, women's stated desired family size is 10 children, although their actual fertiity rate is just over seven.
Effective family planning programmes, such as that in Thailand, have also addressed desired family size.