Population decline

Population decline, also known as depopulation, is a reduction in a human population size. Throughout history, Earth's total human population has continued to grow, but projections suggest this long-term trend may be coming to an end. From antiquity until the beginning of the Industrial Revolution in early modern Europe, the global population grew very slowly, at about 0.04% per year. After about 1800 the growth rate accelerated to a peak of 2.1% annually during the mid-20th-century baby boom, but since then, due to the worldwide collapse of the total fertility rate, it has slowed to 0.9% as of 2023. The global growth rate in absolute numbers accelerated to a peak of 92.8 million in 1990, but has since slowed to 70.4 million in 2023.
Long-term projections indicate that the growth rate of the human population on the planet will continue to slow down, and that before the end of the 21st century it will reach growth zero. Examples of this emerging trend are Japan, whose population is currently declining at the rate of 0.5% per year, and China, whose population has peaked and is currently declining at the rate of about 0.2% per year. By 2050, Europe's population is projected to be declining at the rate of 0.3% per year. Population growth has declined mainly due to the abrupt decline in the global total fertility rate, from 5.3 in 1963 to 2.2 in 2023. The decline in the total fertility rate has occurred in every region of the world and is a result of a process known as demographic transition. To maintain its population, ignoring migration, a country on average requires a minimum fertility rate of 2.2 children per woman of childbearing age. However, most societies experience a drop in fertility to well below two as they grow wealthier.
Birth dearth, a closely related demographic phenomenon which refers to the declining fertility rates observed in many modern industrialized, affluent societies, affects countries and geographic regions that are currently experiencing the highest rates of declining populations, such as Western Europe, Japan, the Russian Federation, and South Korea. Populations in other industrialized countries, such as the United Kingdom and the United States, and developing, poorer regions of the world, including the Balkans, Central Asia, the Middle East, and Sub-Saharan Africa, are also being impacted. For instance, the tendency of women in wealthier countries to have fewer children is attributed to a variety of reasons, such as lower infant mortality and a reduced need for children as a source of family labor or retirement welfare, both of which reduce the incentive to have many children. Better access to education for young women, which broadens their job prospects, is also often cited by some demographers, journalists, and political economists.
[|Possible consequences] of long-term national population decline can be net positive or negative, both on world economy and individual countries themselves. If a country can increase its workforce productivity faster than its population decline, the results, in terms of its national economy, the quality of life of its citizens, and the environment, can be net positive. If it cannot increase workforce productivity faster than its population decline, the results can be negative. So far, [|national efforts to confront a declining population] to date have been focused on the possible negative economic consequences and have been centered on increasing the size and productivity of the workforce through various means.

Causes

A reduction over time in a region's population can be caused by sudden, adverse events such as economic crises, outbursts of infectious diseases, climate change, famines, poverty, war, existential risks due to social and economic inequalities, and wealth disparities; long-term demographic trends such as birth dearth, sub-replacement fertility, persistently low birth rates, high mortality rates, increase of drug abuse, violent crimes, and political violence; continued emigration of native citizens to foreign countries; persistent and unresolved societal issues, including low fertility rates, lack of cohabitation and/or domestic partnership between spouses, declining marriage rates, increase of annulment and divorce cases, marital abuse, abandonment of children, childbirth by a single person or single-person adoption; and lifestyle choices associated with being single, urbanization, individualism, celibacy, social isolation, unemployment, and voluntary childlessness.

Short-term population shocks

Historical episodes of short-term human population decline have been common and they have been caused by several factors.
High mortality rates caused by:

Disease: for example, the Black Death that devastated Eurasia, the arrival and spread of Old World diseases in the Americas during the European colonization, and the Spanish flu pandemic in the aftermath of World War I ;
Drug epidemic: for example, the current opioid epidemic in the United States that began in the late 1990s, according to the Centers for Disease Control and Prevention.
Famine: for example, the Great Irish Famine caused by the infection of potato crops by the blight in British-ruled Ireland, and the Great Chinese Famine caused by the Great Leap Forward in communist China ; the former caused the Irish population to decline by 20–25% between 1841 and 1871, while the latter caused approximately 25 millions of deaths among the Chinese people and is considered to be the largest or second-largest famine in recorded history.
War: for example, the devastating consequences of the Mongol invasion of Central and Eastern Europe during the Late Middle Ages, which may have reduced the population of medieval Hungary by 20–40%;
Social unrest: for example, the displacement and forced migration of millions of Syrian citizens caused by the Syrian Civil War, the inception of an ongoing, international humanitarian crisis;
A combination of these: the first half of the 20th century in Imperial Russia and the Soviet Union was marked by a succession of major wars, alongside famines and natural disasters, which caused large-scale population losses.

Some population declines result from indeterminate causes, such as the Late Bronze Age collapse, a period of societal collapse in the Mediterranean basin which has been described as the worst disaster in ancient history. More frequently, short-term population declines are caused by warfare, famines, and large-scale epidemics, such as the Great Irish Famine, whose proximate cause for the famine was the infection of potato crops by the blight throughout Europe during the 1840s. Impact on food supply by the blight infection caused 100,000 deaths outside Ireland, and influenced much of the social unrest that culminated in the Revolutions of 1848. The famine and its effects permanently changed the island's demographic, political, and cultural landscape, producing an estimated 2 million refugees and spurring a century-long population decline.
Less frequently, short-term population declines are caused by genocide and/or ethnic cleansing. For example, during the early 20th century, the percentage of Christians in the Middle East mainly fell as a result of the late Ottoman genocides: the Armenian, Greek, and Assyrian genocides which were committed by the Ottoman Turks and their allies, which caused millions of deaths and forced the surviving Christian populations to flee and emigrate to Iraq, Syria, North America, and Western Europe. It has been estimated that the Holocaust and other industrial-scale genocides which Nazi Germany perpetrated against European Jews, Romani people, Poles, Serbs, German citizens with disabilities, and many other groups caused more than 13 million deaths in Nazi-occupied Europe during World War II, while the population of Cambodia declined by 25% due to the Cambodian genocide, the large-scale executions which were carried out by the Khmer Rouge.
In the contemporary world, the AIDS pandemic, originated by the emergence and spread of the human immunodeficiency virus, and the COVID-19 pandemic, originated by the emergence and spread of the severe acute respiratory syndrome-related coronavirus, have caused short-term drops in fertility and significant excess mortality in a number of countries.

Long-term historic trends in world population growth

In spite of these short-term population shocks, world population has continued to grow. From around the 10th century BCE to the beginning of the early modern period, world population grew very slowly, around 0.04% per year. During that period, population growth was governed by conditions now labeled the "Malthusian trap".
After 1800, driven by increases in human productivity due to the Industrial Revolution, particularly the increase in agricultural productivity, population growth accelerated to around 0.6% per year, a rate that was over ten times the rate of population growth of the previous 12,000 years. This rapid increase in global population caused Malthus and others to raise the first concerns about overpopulation.
In the aftermath of World War I, birth rates in the United States and many European countries fell below replacement level. This prompted concern about population decline. The recovery of the birth rate in most Western countries around 1940 that produced the "baby boom", with annual growth rates in the 1.0 – 1.5% range, and which peaked during the period 1962–1968 at 2.1% per year, temporarily dispelled prior concerns about population decline, and the world was once again fearful of overpopulation. After 1968, the global population growth rate started a long decline. The Population Division of the United Nations Department of Economic and Social Affairs has reported that in the year 2023 it had dropped to about 0.9%, less than half of its peak between 1962 and 1968. Although still growing, the UN predicts that global population will level out around 2084, and some sources predict the start of a decline before then.
The principal cause of this phenomenon is the abrupt decline in the global total fertility rate, from 5.3 in 1963 to 2.2 in 2023, as the world continues to move through the stages of the demographic transition. The decline in the total fertility rate has occurred in every region of the world and has brought renewed concern for population decline, sparked by some demographers, journalists, politicians, and political economists. The era of rapid global population increase, and concomitant concern about a population explosion, has been short compared with the span of human history.