World population size is currently estimated as 7.4 billion people. It is growing by about 1 billion every 12 years, and it is expected to double from 4 to 8 billion between 1974 and 2023 (United Nations projections). It only reached 1 billion in about 1804 and took a further 119 years to reach 2 billion. Recent increases have been unprecedented and have raised concerns about their effect on the sustainability of social, economic and environmental systems. Changes in population size and structure have the potential to disrupt global stability from the individual level, such as the impact on intergenerational relations within families, to national fiscal and to global environmental systems. There are also political and strategic implications. Even if population growth is not the prime driver, larger population size is likely to magnify challenges arising from other underlying causes, with varying impacts in different parts of the world.
Global population change
Global population size and structure can change only through births and deaths. Around 1950, life expectancy at birth was 47 years; by 2015 this had increased to 71 years. At present, life expectancy is increasing at about 3 years every decade, and this is likely to continue to increase in the foreseeable future. A woman would have 5 children on average with the fertility rates of 1950, but that figure had been halved by 2015. Future declines are likely to be less substantial, with a value of about 2 children per woman expected by 2100. Lower fertility is associated with development, both because couples are increasingly able to decide on their preferred number of children, and because lower infant and child mortality means they do not require so many births to reach that goal. This demographic transition represents a remarkable achievement and major global success but have also been responsible for the rapid growth in global population size noted above.
Global population growth has been generally smooth, increasing to a maximum annual rate of 0.9% around 1965, before falling back to 0.5% currently, and it is expected to decline towards 0 over this century. However, these figures mask considerable differences between different parts of the World, differences that can potentially lead to major tensions. Growth rates have been lowest in the most developed regions. Initially, growth rates were higher in all parts of the rest of the World, but recently growth rates in middle-income countries have tended to converge towards those of the more developed countries (in Europe, North America, Japan and Australasia),with high growth being particularly concentrated in the least developed regions, primarily sub-Saharan Africa together with a small number of other countries such as Afghanistan. In the middle of the 20th century, the population of Africa was less than half the size of Europe. By 2025, it is expected to be twice that of Europe, and by 2100 there are expected to be as many people aged 15 to 24 in Africa as the total European population.
The share of the world’s population in the more developed regions was 32% in 1950, 17% in 2015 and is likely to fall to 13% in 2050, when numbers would be in absolute decline, as is already the case for countries such as Japan. In contrast, the populations of the least development countries are likely to account for 20% of the global population in 2050, compared to just 8% in 1950. The impact of a variable on the global system may be given by the equation I = P A T, where impact, I, depends on population size (P), affluence (A), and technology (T). Of these 3 variables, population is by far the most predictable, for example, the 95% confidence interval for world population size in 2050 is between 9.3 and 10.2 billion people based on United Nations probabilistic population projections that have been generally accurate at the global level (although less so at the national level). Affluent nations use more resources for a given population size even allowing for the fact that they can deploy technologies that may mitigate environmental and other impacts. For example, average CO2 emissions per person in the least developed countries is only 3% of the value of a person living in an OECD country, or 4% of a person living in China in 2011 (World Bank). This suggests that emissions would rise at a lower rate than population growth, which is concentrated in countries with relatively low emissions. However, since less affluent nations want to become richer, additional resources will be required, a need reinforced by population growth. Growing populations place particular pressure on resources such as water for domestic use, industrial expansion and for irrigation to meet additional global food needs.
In the second half of the 20th century, the proportion of children - those aged under 15 - fell from about 27% to 17% of the total population in the more developed regions, largely due to sharp drops in fertility. Other regions showed an initial pronounced increase due to higher fertility and improvements in infant and child mortality in the immediate post-war period. After that, they started to decline, although the year when the proportion of children falls below 25% is likely to be delayed for 50 to 100 years. The proportion of the population aged 15 to 64, conventionally defined as working age, has been higher in more developed regions, but middle-income countries are now catching up. A reduction in fertility leads to a one-off increase in the proportion of young adults since the succeeding generations will be smaller. This "demographic dividend" increases the proportion of the population that is economically active, and that "dividend" helped development in East and South-East Asia in the 1980s. A similar trend is expected in the least developed countries, but the benefits will only accrue if there are sufficient jobs available, since a large numbers of disaffected, unemployed youth may have a negative rather than positive impact on development.
The proportions of older people, those aged 65 and over, are increasing everywhere or are very likely to do to so in future, but with a substantial lag in the least developed countries, where there is little indication of population aging to date. However, middle-income countries are starting to age and at a faster rate than was the case for more developed regions. The proportions of older people, those aged 65 and over, are increasing in more developed regions, although the post-war baby boom of the 1950s and 1960s led to an increase in births, which tended to offset population aging in those periods. However, in more recent decades, fertility has dropped substantially in most parts of the World. In some European countries such as Spain, Italy and Greece, the fertility level is below 1.4 children per woman, a level which if continued in the absence of migration would mean that each generation would be succeeded by a generation only about 2/3 its current size. The impact of fertility decline on population aging has been reinforced by improving longevity. The potential dependency ratio, the number of potential workers defined as those aged 20-64 for each person aged 65 or over, was 7.5 in the most developed countries in 1950, but half that number in 2015, and it is likely to be about two by 2050. Potential support values in medium-income countries have also fallen sharply but are about double those of the most developed countries at each corresponding year. Aging requires additional resources to be transferred from workers to non-workers. While the proportion of child dependents has fallen, resource needs for older people are increasing substantially. These resources include pensions and healthcare, mainly from the formal sector, and personal care from the informal sector, especially close family members. Within the older population, the "old old," those aged 80 and over, form an increasing fraction. These groups are much heavier users of health and social care services and thereby add pressure to the budgets of these sectors. Changing population structures and wider social and economic trends put pressure on both sectors. Lower fertility and changing economic and social systems that require high geographic mobility mean that younger relatives may no longer live close to their parents. Traditionally, women are particularly likely to provide personal care, but more people are childless, younger women are increasingly less likely to marry and women with formal labor market responsibilities may be less willing or able to provide care for parents-in-law. These aging trends are inexorable and highly predictable, and the British Office for Budget Responsibility identified the costs of population aging as the largest single challenge to the sustainability of the United Kingdom’s fiscal system. Policy responses to population aging in high-income countries have been muted in part because they are frequently politically unpopular. These policy responses can include less generous pension arrangements and, in particular, extension of working lives for current workers who had expected to retire from the labor force at ages similar to their predecessors.
In less developed countries, reductions in mortality in the 1950s tended to keep populations younger, since improvements were concentrated among infants and children. Highly effective public health interventions such as vaccinations and the treatment of communicable diseases were relatively straightforward to implement.
Reduction in fertility and increase of the older population
Reduction in fertility means that the number of young people is reduced, thereby increasing the proportion of the older population. Mortality improvement is increasingly concentrated at older ages, so the number of older people increasing rapidly. This trend reflects success in controlling both unwanted fertility and mortality. However, the sharper the decline in fertility and mortality, the more rapid the increase in population aging. As a consequence, the challenges associated with population aging are more substantial in medium-income countries where pension and healthcare schemes are less developed.
The implications of decisions at a particular time can directly affect population trends for up to a century or more in the future, with indirect effects taking even longer. This is well illustrated by China, a country that experienced a number of important changes, and is the largest country in the world and thus has the greatest impact on overall global values. After the Communist revolution in 1948, the government took the view that people were the nation’s greatest asset and that population growth should be encouraged by increasing fertility. Therefore, in the 1950s and 1960s, the fertility rate was about 6 children per woman, although the effect on population growth was offset to some extent by the catastrophic Great Leap Forward of the 1960s that led to the death of about 45 million Chinese people (estimates vary). The government reversed its pro-natal policy in 1980, introducing a "one-child family" policy to restrict population growth, and fertility fell to 2.5 children per woman. The Chinese government was efficient in achieving its objectives by mobilizing all levels of society, sometimes using questionable methods. At the same time, life expectancy at birth had increased by 20 years in the previous two decades, leading to a substantial demographic dividend and one of the most rapid examples of population aging ever experienced at that time. While there are currently about seven people aged 29 to 64 for each person aged 65 or over in China, compared with 3.5 in Europe, both are expected to have a value of two by 2050.
More recently, the Chinese government has recognized there are a range of problems associated with low fertility societies. In a true one child society, no one has a brother, sister, aunt or uncle. With greater longevity, a 1 – 2 - 4 system becomes established, with each child having 2 parents and 4 living grandparents, but no other relatives. However, low fertility in China is not simply a consequence of government pressure. It appears that Chinese parents have adapted to low fertility, which has become institutionalized, so that even though restrictions are relaxed, they are reluctant to return to earlier patterns. In one of the most developed parts of China, Shanghai region, with a population of 30 million people, women have an average of 0.7 children, about half the level of low fertility parts the World such as Southern Europe. China’s experience emphasizes that demographic changes, planned or unplanned, will have very long-term effects. China’s current challenges with population aging arise in part from the pro-natalist policies of the 1950s and 1960s (large numbers of people currently entering older age groups) and the 1980s and 1990s (deficit of working age people). These trends are compounded by the experience of mortality and migration over the period. China was enormously successful in reducing mortality. Between the 1950s and 2015, life expectancy at birth increased by around 43 years, whereas its neighbor Russia increased life expectancy by only 11 years in the same period. International migration has not been a major factor in Chinese population trends, but China experienced the largest level of migration in human history. It is estimated that about 150 million people, mainly young adults, moved from rural areas to expanding economic areas around the coast starting in the early 1980s. This left older people in rural areas, sometimes without the family that was their main source of support, and, in any case, over time there are increasingly fewer young people to provide support.
China is an extreme case, but similar trends of high fertility in the early post World War II period were followed by historically very low levels in the later 20th century. These trends were also combined with improved longevity and more long-distance migration in high-income countries as well, leading to similar but possibly less extreme consequences.
Population pressure has always been a potential cause of conflict, with expanding populations attempting to acquire additional resources (sometimes by appropriation of goods or land rather than by colonization). High population growth means more troops were available, and a higher proportion of young men in a population is itself a factor associated with violence and conflict both internally and externally. Countries with a perceived population deficit may advocate pro-natalism such as France did following its defeat in the Franco-Prussian war, a result of fear of German expansionism following France’s defeat. More recently both Turkey and Iran have adopted pro-natalist policies in a region of considerable instability. India is likely to overtake China as the World's largest country around 2025, near the time China’s population is expected to start declining in absolute terms. India already has 30% more men than China aged 20 to 24, the group likely to be in military forces.
A rapidly expanding population has both a greater incentive and greater ability to appropriate resources by force, and that may in turn encourage neighboring countries to invest in more sophisticated arms as a response to a perceived or actual threat. Similar points hold not only for conflict between states but also for internal conflicts which form an increasing fraction of violence throughout the World. These challenges are compounded when young men form a large fraction of population, especially if they do not feel that they share improved conditions. Population pressures are particularly likely to exacerbate tensions in areas like the Middle East and Africa where they can place increased demand on scarce resources such as water.
International migration has become an increasing focus of the political and social agenda in recent years. From a global perspective, the numbers involved are not large; only about 3% of the World’s population are international migrants. However, migration is particularly unpopular in European countries. A recent poll for the International Organization for Migration found 7.5% of Europeans in favor of increased migration and 52% in favor of reduced migration, and this was the period before large-scale flows of migrants, many from Syria, Iraq and Afghanistan in 2015. Migration is also a source of tension in other parts of World, such as South Africa, and it has substantial potential to cause disruption. Remittances may help to develop the local economy in sending countries, but they may lead to a loss of skilled personnel. Remittances may also exacerbate inequalities and be used for consumption rather than investment. For receiving countries, while employers may welcome sources of flexible, cheap and motivated labor, native workers may see competition for employment reducing their chances and conditions. Migrants are likely to benefit from migration, but disappointment is possible if they find that they do not quickly obtain the expected benefits of movement, and/or experience problems integrating to the new culture.
A lack of organizational structure
This brief summary has not been able to include closely related population issues such as urbanization that will often reinforce the potentially disruptive impact of demographic change. In addition, there are a number of potentially important areas that are very uncertain, those including large-scale population displacements due to conflict and/or the impact of climate change. Attention has concentrated on changes that are very likely to occur. For example, everyone who reaches age 65 in the next year is already alive, but it is also clear that planning for these trends has been lacking. It is tempting for governments to delay unpopular measures such as increasing pension ages, or for international funding to switch priorities from areas with major long-term benefits such as family planning to areas of contemporary concern such as reproductive rights or HIV/AIDS. However, population trends will continue to have a major effect on global sustainability.