Human overpopulation

World population has gone through a number of periods of growth since the dawn of civilization in the Holocene period, around 10,000 BCE. The beginning of civilization roughly coincides with the receding of glacial ice following the end of the last glacial period.[32]

It is estimated that between 1–5 million people, subsisting on hunting and foraging, inhabited the Earth in the period before the Neolithic Revolution, when human activity shifted away from hunter-gathering and towards very primitive farming.[33]

Farming allowed growth of populations in many parts of the world, include Europe, the Americas and China through the 1600s, occasionally disrupted by plagues or other crisis.[34] [35] For example, Black Death are thought to have reduced the world's population, then at an estimated 450 million, to between 350 and 375 million by 1400.[36] The population of Europe stood at over 70 million in 1340;[37] these levels did not return until 200 years later.[38] In other parts of the globe, China's population census at the founding of the Ming dynasty in 1368 indicated that the population stood close to 60 million, (though these figures are debated by some historians) approaching 150 million by the end of the dynasty in 1644.[39][40] The population of the Americas in 1500 may have been between 50 and 100 million.[41] Encounters between European explorers and other populations introduced local epidemics or other violence: for example, 90% of the Native American population of the New World through Old World diseases such as smallpox, measles, and influenza,[42] or the slave trade in Africa greatly damaged populations.[43]

After the start of the Industrial Revolution, during the 18th century, the rate of population growth began to increase. By the end of the century, the world's population was estimated at just under 1 billion.[44] At the turn of the 20th century, the world's population was roughly 1.6 billion.[44] Dramatic growth beginning in 1950 (above 1.8% per year) coincided with greatly increased food production as a result of the industrialization of agriculture brought about by the Green Revolution.[45] The rate of human population growth peaked in 1964, at about 2.1% per year.[46] By 1940, this figure had increased to 2.3 billion.[47] Each subsequent addition of a billion humans took less and less time: 33 years to reach three billion in 1960, 14 years for four billion in 1974, 13 years for five billion in 1987, and 12 years for six billion in 1999.[48]

External video
How Earth's Population Exploded: Bloomberg Quicktake

From a historical perspective, technological revolutions have coincided with population expansion. There have been three major technological revolutions—the tool-making revolution, the agricultural revolution, and the industrial revolution—all of which allowed humans more access to food; hence increasing the carrying capacity, resulting in subsequent population explosions.[49][50] For example, the use of tools, such as bow and arrow, allowed primitive hunters greater access to more high energy foods (e.g. animal meat). Similarly, the transition to farming about 10,000 years ago greatly increased the overall food supply, which was used to support more people. Food production further increased with the industrial revolution as machinery, fertilizers, herbicides, and pesticides were used to increase land under cultivation as well as crop yields. Today, starvation is caused by economic and political forces rather than a lack of the means to produce food.[51][52]

Significant increases in human population occur whenever the birth rate exceeds the death rate for extended periods of time. Traditionally, the fertility rate is strongly influenced by cultural and social norms that are rather stable and therefore slow to adapt to changes in the social, technological, or environmental conditions. For example, when death rates fell during the 19th and 20th century—as a result of improved sanitation, child immunizations, and other advances in medicine—allowing more newborns to survive, the fertility rate did not adjust downward, resulting in significant population growth. For example, until the 1700s, seven out of ten children died before reaching reproductive age.[53]

Agriculture has sustained human population growth and has been the main driving factor behind it. With more food supply, the population grows with it. This occurs most in regions which are fertile and capable of higher food production in contrast to infertile regions unable to support agricultural productivity on larger or any scales at all. This dates back to prehistoric times, when agricultural methods were first developed, and continues to the present day, with fertilizers, agrochemicals, large-scale mechanization, genetic manipulation, and other technologies.[54][55][56][57]

Humans have historically exploited the environment using the easiest, most accessible resources first. The richest farmland was plowed and the richest mineral ore mined first. Anne Ehrlich, Gerardo Ceballos, and Paul Ehrlich note that overpopulation is demanding the use of ever more creative, expensive and/or environmentally destructive means in order to exploit ever more difficult to access and/or poorer quality natural resources to satisfy consumers.[58]

Central Australian landscape. Australia is mostly unpopulated as a result of having amongst the least fertile soils in the world.[59][60]

Many people from a wide range of academic fields and political backgrounds—including agronomist and insect ecologist David Pimentel,[61] behavioral scientist Russell Hopfenberg,[62] anthropologist Virginia Abernethy,[63] ecologist Garrett Hardin,[64] science writer and anthropologist Peter Farb, journalist Richard Manning,[65]cultural critic and writer Daniel Quinn,[66] and anarcho-primitivist John Zerzan,[67]—propose that, like all other animal populations, human populations predictably grow and shrink according to their available food supply, growing during an abundance of food and shrinking in times of scarcity.

Proponents of this theory argue that every time food production is increased, the population grows. Most human populations throughout history validate this theory, as does the overall current global population. Populations of hunter-gatherers fluctuate in accordance with the amount of available food. The world human population began increasing after the Neolithic Revolution and its increased food supply.[68] This was, subsequent to the Green Revolution, followed by even more severely accelerated population growth, which continues today.

Critics of this theory point out that, in the modern era, birth rates are lowest in the developed nations, which also have the highest access to food. In fact, some developed countries have both a diminishing population and an abundant food supply. The United Nations projects that the population of 51 countries or areas, including Germany, Italy, Japan, and most of the states of the former Soviet Union, is expected to be lower in 2050 than in 2005.[69] This shows that, limited to the scope of the population living within a single given political boundary, particular human populations do not always grow to match the available food supply. However, the global population as a whole still grows in accordance with the total food supply and many of these wealthier countries are major exporters of food to poorer populations, so that, "it is through exports from food-rich to food-poor areas (Allaby, 1984; Pimentel et al., 1999) that the population growth in these food-poor areas is further fueled.[61]

Regardless of criticisms against the theory that population is a function of food availability, the human population is, on the global scale, undeniably increasing,[70] as is the net quantity of human food produced—a pattern that has been true for roughly 10,000 years, since the human development of agriculture. The fact that some affluent countries demonstrate negative population growth fails to discredit the theory as a whole, since the world has become a globalized system with food moving across national borders from areas of abundance to areas of scarcity. Hopfenberg and Pimentel's findings support both this[61] and Quinn's direct accusation that "First World farmers are fueling the Third World population explosion."[71]

As of February 10, 2021, the world's human population is estimated to be, 7.851 billion.[72] Or, 7,622,106,064 on 14 May 2018 and the United States Census Bureau calculates 7,472,985,269 for that same date[73] and over 7 billion by the United Nations.[74][75][76] The population is expected to reach between 8 and 10.5 billion between the years 2040[77][78] and 2050.[79] In 2017, the United Nations increased the medium variant projections[80] to 9.8 billion for 2050 and 11.2 billion for 2100.[81] The UN population forecast of 2017 was predicting "near end of high fertility" globally and anticipating that by 2030 over ⅔ of the world population will be living in countries with fertility below the replacement level[82] and for total world population to stabilize between 10 and 12 billion people by the year 2100.[83]

The rapid increase in world population over the past three centuries has raised concerns among some people that the planet may not be able to sustain the future or even present number of its inhabitants. The InterAcademy Panel Statement on Population Growth, circa 1994, stated that many environmental problems, such as rising levels of atmospheric carbon dioxide, global warming, and pollution, are aggravated by the population expansion.[84]

Map of population density by country, per square kilometer. (See List of countries by population density.)

The UN Human Development Report of 1997 states: "During the last 15–20 years, more than 100 developing countries, and several Eastern European countries, have suffered from disastrous growth failures. The reductions in standard of living have been deeper and more long-lasting than what was seen in the industrialised countries during the depression in the 1930s. As a result, the income for more than one billion people has fallen below the level that was reached 10, 20 or 30 years ago".

Youth unemployment is also soaring, with the economy unable to absorb the spiraling numbers of those seeking to enter the work force. Many young people do not have the skills to match the needs of the Egyptian market, and the economy is small, weak and insufficiently industrialized... Instead of being something productive, the population growth is a barrel of explosives.

— — Ofir Winter, an Egypt specialist at the Institute for National Security Studies[94]

As the world's population has grown, the percentage of the world's population living on less than $1 per day (adjusted for inflation) has halved in 20 years. The graph shows the 1981–2001 period.

The United Nations indicates that about 850 million people are malnourished or starving,[95] and 1.1 billion people do not have access to safe drinking water.[96] Since 1980, the global economy has grown by 380 percent, but the number of people living on less than 5 US dollars a day increased by more than 1.1 billion.[97]

Log-log graph of total fertility rate (TFR) vs. GDP (PPP) per capita with population size shown as bubble area, for all countries having population greater than 2 million (2016 estimates; 30 largest countries bold).[98][99][100]

High rates of infant mortality are associated with poverty. Rich countries with high population densities have low rates of infant mortality.[101][102] However, both global poverty and infant mortality has declined over the last 200 years of population growth.[18][19]

An industrial area, with a power plant, south of Yangzhou's downtown, China

Overpopulation has substantially adversely impacted the environment of Earth starting at least as early as the 20th century.[86] According to the Global Footprint Network, "today humanity uses the equivalent of 1.5 planets to provide the resources we use and absorb our waste".[103] There are also economic consequences of this environmental degradation in the form of ecosystem services attrition.[104] Beyond the scientifically verifiable harm to the environment, some assert the moral right of other species to simply exist rather than become extinct. Environmental author Jeremy Rifkin has said that "our burgeoning population and urban way of life have been purchased at the expense of vast ecosystems and habitats. ... It's no accident that as we celebrate the urbanization of the world, we are quickly approaching another historic watershed: the disappearance of the wild."[105] Scientists suggest that the overall human impact on the environment during the Great Acceleration, particularly due to human population size and growth, economic growth, overconsumption, pollution, and proliferation of technology, has pushed the planet into a new geological epoch known as the Anthropocene.[106][107]

Further, even in countries which have both large population growth and major ecological problems, it is not necessarily true that curbing the population growth will make a major contribution towards resolving all environmental problems.[108]

The Worldwatch Institute said in 2006 that the booming economies of China and India are "planetary powers that are shaping the global biosphere". The report states:

The world's ecological capacity is simply insufficient to satisfy the ambitions of China, India, Japan, Europe and the United States as well as the aspirations of the rest of the world in a sustainable way.[109]

According to Worldwatch Institute, if China and India were to consume as much resources per capita as the United States, in 2030 they would each require a full planet Earth to meet their needs.[110] In the long term these effects can lead to increased conflict over dwindling resources.[111]

Many studies link population growth with emissions and the effect of climate change.[112][113] The global consumption of meat is projected to rise by as much as 76% by 2050 as the global population surges to more than 9 billion, resulting in further biodiversity loss and increased GHG emissions.[114][115]

Human overpopulation, continued population growth, and overconsumption are the primary drivers of biodiversity loss and the 6th (and ongoing) mass species extinction.[117][118][119][120][121] Present extinction rates may be as high as 140,000 species lost per year due to human activity, such as slash-and-burn techniques that sometimes are practiced by shifting cultivators, especially in countries with rapidly expanding rural populations, which have reduced habitat in tropical forests.[122]

Sir David King, former chief scientific adviser to the UK government, told a parliamentary inquiry: "It is self-evident that the massive growth in the human population through the 20th century has had more impact on biodiversity than any other single factor."[123][124] Paul and Anne Ehrlich said population growth is one of the main drivers of the Earth's extinction crisis.[125]

The Global Assessment Report on Biodiversity and Ecosystem Services, released by IPBES in 2019, says that human population growth is a significant factor in biodiversity loss.[126] The report asserts that expanding human land use for agriculture and overfishing are the main causes of this decline.[127] The 2020 World Wildlife Fund's Living Planet Report posits that 68% of vertebrate wildlife has been lost since 1970 due to human actions, including overconsumption, population growth, global trade and intensive farming.[128][129]

Population growth increases levels of air pollution, water pollution, soil contamination and noise pollution.[130]

Air pollution is causing changes in atmospheric composition and consequent global warming[131][132][133] and ocean acidification.[134]

Ecological collapse refers to a situation where an ecosystem suffers a drastic, possibly permanent, reduction in carrying capacity for all organisms, often resulting in mass extinction. Usually, an ecological collapse is precipitated by a disastrous event occurring on a short time scale. Ecological collapse can be considered as a consequence of ecosystem collapse on the biotic elements that depended on the original ecosystem.[135][136]

The ocean is in great danger of collapse. In a study of 154 different marine fish species, David Byler came to the conclusion that many factors such as overfishing, climate change, and fast growth of fish populations will cause ecosystem collapse.[137] When humans fish, they usually will fish the populations of the higher trophic levels such as salmon and tuna. The depletion of these trophic levels allow the lower trophic level to overpopulate, or populate very rapidly. For example, when the population of catfish is depleting due to overfishing, plankton will then overpopulate because their natural predator is being killed off. This causes an issue called eutrophication. Since the population all consumes oxygen the dissolved oxygen (DO) levels will plummet. The DO levels dropping will cause all the species in that area to have to leave, or they will suffocate. This along with climate change, and ocean acidification can cause the collapse of an ecosystem.

The resources to be considered when evaluating whether an ecological niche is overpopulated include clean water, clean air, food, shelter, warmth, and other resources necessary to sustain life. If the quality of human life is addressed, there may be additional resources considered, such as medical care, education, proper sewage treatment, waste disposal and energy supplies. Overpopulation places competitive stress on the basic life sustaining resources,[138] leading to a diminished quality of life.[86]

David Pimentel has stated that "With the imbalance growing between population numbers and vital life sustaining resources, humans must actively conserve cropland, freshwater, energy, and biological resources. There is a need to develop renewable energy resources. Humans everywhere must understand that rapid population growth damages the Earth's resources and diminishes human well-being."[139][140]

These reflect the comments also of the United States Geological Survey in their paper "The Future of Planet Earth: Scientific Challenges in the Coming Century": "As the global population continues to grow...people will place greater and greater demands on the resources of our planet, including mineral and energy resources, open space, water, and plant and animal resources." "Earth's natural wealth: an audit" by New Scientist magazine states that many of the minerals that we use for a variety of products are in danger of running out in the near future.[141] A handful of geologists around the world have calculated the costs of new technologies in terms of the materials they use and the implications of their spreading to the developing world. All agree that the planet's booming population and rising standards of living are set to put unprecedented demands on the materials that only Earth itself can provide.[141] Limitations on how much of these materials is available could even mean that some technologies are not worth pursuing long term.... "Virgin stocks of several metals appear inadequate to sustain the modern 'developed world' quality of life for all of Earth's people under contemporary technology".[142]

On the other hand, some cornucopian researchers, such as Julian L. Simon and Bjørn Lomborg believe that resources exist for further population growth. In a 2010 study, they concluded that "there are not (and will never be) too many people for the planet to feed" according to The Independent.[143] Some critics warn, this will be at a high cost to the Earth: "the technological optimists are probably correct in claiming that overall world food production can be increased substantially over the next few decades...[however] the environmental cost of what Paul R. and Anne H. Ehrlich describe as 'turning the Earth into a giant human feedlot' could be severe. A large expansion of agriculture to provide growing populations with improved diets is likely to lead to further deforestation, loss of species, soil erosion, and pollution from pesticides and fertilizer runoff as farming intensifies and new land is brought into production."[144]

According to the Millennium Ecosystem Assessment, a four-year research effort by 1,360 of the world's prominent scientists commissioned to measure the actual value of natural resources to humans and the world, "The structure of the world's ecosystems changed more rapidly in the second half of the twentieth century than at any time in recorded human history, and virtually all of Earth's ecosystems have now been significantly transformed through human actions."[145] "Ecosystem services, particularly food production, timber and fisheries, are important for employment and economic activity. Intensive use of ecosystems often produces the greatest short-term advantage, but excessive and unsustainable use can lead to losses in the long term. A country could cut its forests and deplete its fisheries, and this would show only as a positive gain to GDP, despite the loss of capital assets. If the full economic value of ecosystems were taken into account in decision-making, their degradation could be significantly slowed down or even reversed."[146][147]

Another study was done by the United Nations Environment Programme (UNEP) called the Global Environment Outlook.[148]

Although all resources, whether mineral or other, are limited on the planet, there is a degree of self-correction whenever a scarcity or high-demand for a particular kind is experienced. For example, in 1990 known reserves of many natural resources were higher, and their prices lower, than in 1970, despite higher demand and higher consumption. Whenever a price spike would occur, the market tended to correct itself whether by substituting an equivalent resource or switching to a new technology.[149]

Overpopulation may lead to inadequate fresh water[96] for drinking as well as sewage treatment and effluent discharge.Some countries, like Saudi Arabia, use energy-expensive desalination to solve the problem of water shortages.[150][151]

Fresh water supplies, on which agriculture depends, are running low worldwide.[152][153] This water crisis is only expected to worsen as the population increases.[154] Water deficits, which are already spurring heavy grain imports in numerous smaller countries, may soon do the same in larger countries, such as China or India, if technology is not used.[155] The water tables are falling in scores of countries (including Northern China, the US, and India) owing to widespread overdrafting beyond sustainable yields. Other countries affected include Pakistan, Iran, and Mexico. This overdrafting is already leading to water scarcity and cutbacks in grain harvest. Even with the overpumping of its aquifers, China has developed a grain deficit. This effect has contributed in driving grain prices upward. Most of the 3 billion people projected to be added worldwide by mid-century will be born in countries already experiencing water shortages. Desalination is also considered a viable and effective solution to the problem of water shortages.[150][156]

Overpopulation together with water deficits could trigger regional tensions, including warfare.[157]

Percentages of the Earth's surface covered by water, dedicated to agriculture, under conversion, intact, and used for human habitation. While humans occupy only 0.05% of the Earth's total area, human effects are felt on over one-quarter of the land.

The World Resources Institute states that "Agricultural conversion to croplands and managed pastures has affected some 3.3 billion [hectares]—roughly 26 percent of the land area. All totaled, agriculture has displaced one-third of temperate and tropical forests and one-quarter of natural grasslands."[158][159] Forty percent of the land area is under conversion and fragmented; less than one quarter, primarily in the Arctic and the deserts, remains intact.[160] Usable land may become less useful through salinization, deforestation, desertification, erosion, and urban sprawl. The development of energy sources may also require large areas, for example, the building of hydroelectric dams. Thus, available useful land may become a limiting factor. By most estimates, at least half of cultivable land is already being farmed, and there are concerns that the remaining reserves are greatly overestimated.[161]

Some countries, such as the United Arab Emirates and particularly the Emirate of Dubai have constructed large artificial islands, or have created large dam and dike systems, like the Netherlands, which reclaim land from the sea to increase their total land area.[162][163] Some scientists have said that in the future, densely populated cities will use vertical farming to grow food inside skyscrapers.[164] The notion that space is limited has been decried by skeptics, who point out that the Earth's population of roughly 6.8 billion people could comfortably be housed an area comparable in size to the state of Texas, in the United States (about 269,000 square miles or 696,706.80 square kilometres).[165]

Some scientists argue that there is enough food to support the world population,[166][167] and some dispute this, particularly if sustainability is taken into account.[168] A 2001 United Nations report says population growth is "the main force driving increases in agricultural demand" but "most recent expert assessments are cautiously optimistic about the ability of global food production to keep up with demand for the foreseeable future (that is to say, until approximately 2030 or 2050)", assuming declining population growth rates.[169] However, the observed figures for 2016 show an actual increase in absolute numbers of undernourished people in the world, 815 million in 2016 versus 777 million in 2015.[170] The FAO estimates that these numbers are still far lower than the nearly 900 million registered in 2000.[170]

Growth in food production has been greater than population growth.

The amounts of natural resources in this context are not necessarily fixed, and their distribution is not necessarily a zero-sum game. For example, due to the Green Revolution and the fact that more and more land is appropriated each year from wild lands for agricultural purposes, the worldwide production of food had steadily increased up until 1995. As world population doubled from 3 to 6 billion, daily calorie consumption in poor countries increased from 1,932 to 2,650, and the percentage of people in those countries who were malnourished fell from 45% to 18%. This suggests that Third World poverty and famine are caused by underdevelopment, not overpopulation.[171] However, others question these statistics.[172]

Percentage of population suffering from undernourishment by country, according to United Nations statistics.

The Food and Agriculture Organization of the United Nations states in its report The State of Food Insecurity in the World 2018 that the new data indicates an increase of hunger in the world, reversing the recent trend. It is estimated that in 2017 the number of undernourished people increased to 821 million, around 11 per cent of the world population. The FAO states: "Evidence shows that, for many countries, recent increases in hunger are associated with extreme climate events, especially where there is both high exposure to climate extremes and high vulnerability related to agriculture and livelihood systems."[170]

Although the proportion of "starving" people in sub-Saharan Africa has decreased, the absolute number of starving people has increased due to population growth. The percentage dropped from 38% in 1970 to 33% in 1996 and was expected to be 30% by 2010.[172] But the region's population roughly doubled between 1970 and 1996. To keep the numbers of starving constant, the percentage would have dropped by more than half.[146][173]

Food security will become more difficult to achieve as resources run out. Resources in danger of becoming depleted include oil, phosphorus, grain, fish, and water.[174][175] The British scientist John Beddington predicted in 2009 that supplies of energy, food, and water will need to be increased by 50% to reach demand levels of 2030.[176][177] According to the Food and Agriculture Organization (FAO), food supplies will need to be increased by 70% by 2050 to meet projected demands.[178]

Overpopulation causes crowding, and conflict over scarce resources, which in turn lead to increased levels of warfare.[179]

It has been suggested[180] that overpopulation leads to increased levels of tensions both between and within countries. Modern usage of the term "lebensraum" supports the idea that overpopulation may promote warfare through fear of resource scarcity and increasing numbers of youth lacking the opportunity to engage in peaceful employment (the youth bulge theory).

The hypothesis that population pressure causes increased warfare has been recently criticized on statistical grounds. Two studies focusing on specific historical societies and analyses of cross-cultural data have failed to find positive correlation between population density and incidence of warfare. Andrey Korotayev, in collaboration with Peter Turchin, has shown that such negative results do not falsify the population-warfare hypothesis.[181]

Furthermore, they have demonstrated that in the agrarian societies the rates of change of the two variables behave precisely as predicted by the theory: population rate of change is negatively affected by warfare intensity, while warfare rate of change is positively affected by population density.[181][182][183]

Human population, its prevailing growth of demands of livestock and other domestic animals, has added overshoot through domestic animal breeding, keeping and consumption, especially with the environmentally destructive industrial livestock production.[186]

• Loss of arable land and increase in desertification.[187] Deforestation and desertification can be reversed by adopting property rights, and this policy is successful even while the human population continues to grow.[188]
• Increased chance of the emergence of new epidemics and pandemics.[189] For many environmental and social reasons, including overcrowded living conditions, malnutrition and inadequate, inaccessible, or non-existent health care, the poor are more likely to be exposed to infectious diseases.[190]
• Starvation, malnutrition[95] or poor diet with ill health and diet-deficiency diseases (e.g. rickets). However, rich countries with high population densities do not have famine.[91]
• Low life expectancy in countries with fastest growing populations.[191] Overall life expectancy has increased globally despite of population growth, including countries with fast-growing populations.[18]
• Unhygienic living conditions for many based upon water resource depletion, discharge of raw sewage[192] and solid waste disposal. However, this problem can be reduced with the adoption of sewers. For example, after Karachi, Pakistan installed sewers, its infant mortality rate fell substantially.[193]
• Elevated crime rate due to drug cartels and increased theft by people stealing resources to survive.[194]
• Less personal freedom and more restrictive laws. Laws regulate and shape politics, economics, history and society and serve as a mediator of relations and interactions between people. The higher the population density, the more frequent such interactions become, and thus there develops a need for more laws and/or more restrictive laws to regulate these interactions and relations. It was speculated by Aldous Huxley in 1958 that democracy is threatened by overpopulation, and could give rise to totalitarian style governments.[195] However, over the last 200 years of population growth, the actual level of personal freedom has increased rather than declined.[18]

According to projections, the world population will continue to grow until at least 2050, with the population reaching 9 billion in 2040,[197][198] and some predictions putting the population as high as 11 billion in 2050.[199] The median estimate for future growth sees the world population reaching 8.6 billion in 2030, 9.8 billion in 2050 and 11.2 billion by 2100[200] assuming a continuing decrease in average fertility rate from 2.5 births per woman in 2010–2015 to 2.2 in 2045–2050 and to 2.0 in 2095–2100, according to the medium-variant projection.[200] Walter Greiling projected in the 1950s that world population would reach a peak of about nine billion, in the 21st century, and then stop growing, after a readjustment of the Third World and a sanitation of the tropics.[201]

Babies per woman (total fertility) in USA, Russia, China, Nigeria; 1800-2018

The theory of demographic transition held that, after the standard of living and life expectancy increase, family sizes and birth rates decline. Some research has suggested that fertility tends to increase again at very high levels of development, producing a "J"-shaped relationship between development and fertility,[202] but those increases have not been shown to be sustained.[203]

Many countries have high population growth rates but lower total fertility rates because high population growth in the past skewed the age demographic toward a young age, so the population still rises as the more numerous younger generation approaches maturity. "Demographic entrapment" is a concept developed by Maurice King, Honorary Research Fellow at the University of Leeds, who posits that this phenomenon occurs when a country has a population larger than its carrying capacity, no possibility of migration, and exports too little to be able to import food. This will cause starvation. He claims that for example many sub-Saharan nations are or will become stuck in demographic entrapment, instead of having a demographic transition.[204]

For the world as a whole, the number of children born per woman decreased from 5.02 to 2.65 between 1950 and 2005. A breakdown by region is as follows:

• Europe – 2.66 to 1.41
• North America – 3.47 to 1.99
• Oceania – 3.87 to 2.30
• Central America – 6.38 to 2.66
• South America – 5.75 to 2.49
• Asia (excluding Middle East) – 5.85 to 2.43
• Middle East & North Africa – 6.99 to 3.37
• Sub-Saharan Africa – 6.7 to 5.53

Excluding the theoretical reversal in fertility decrease for high development, the projected world number of children born per woman for 2050 would be around 2.05. Only the Middle East & North Africa (2.09) and Sub-Saharan Africa (2.61) would then have numbers greater than 2.05.[205]

Some groups (for example, the World Wide Fund for Nature[206][207] and Global Footprint Network) have stated that the yearly biocapacity of Earth is being exceeded as measured using the ecological footprint. In 2006, WWF's "Living Planet Report" stated that in order for all humans to live with the current consumption patterns of Europeans, we would be spending three times more than what the planet can renew.[208] Humanity as a whole was using, by 2006, 40 percent more than what Earth can regenerate.[209] However, Roger Martin of Population Matters states the view: "the poor want to get rich, and I want them to get rich," with a later addition, "of course we have to change consumption habits,... but we've also got to stabilise our numbers".[210] Another study by the World Wildlife Fund in 2014 found that it would take the equivalent of 1.5 Earths of biocapacity to meet humanity's current levels of consumption.[211]

But critics question the simplifications and statistical methods used in calculating ecological footprints. Therefore, Global Footprint Network and its partner organizations have engaged with national governments and international agencies to test the results—reviews have been produced by France, Germany, the European Commission, Switzerland, Luxembourg, Japan and the United Arab Emirates.[212] Some point out that a more refined method of assessing Ecological Footprint is to designate sustainable versus non-sustainable categories of consumption.[213][214]

Many studies have tried to estimate the world's carrying capacity for humans, that is, the maximum population the world can host.[215] A meta-analysis of 69 such studies from 1694 until 2001 found the average predicted maximum number of people the Earth would ever have was 7.7 billion people, with lower and upper meta-bounds at 0.65 and 98 billion people, respectively. They conclude: "recent predictions of stabilized world population levels for 2050 exceed several of our meta-estimates of a world population limit".[216]

Advocates of reduced population, often put forward much lower numbers. For example, Paul R. Ehrlich stated in 2018 that the optimum size of the global human population is between 1.5 and 2 billion.[217] Geographer Chris Tucker estimates that 3 billion is a sustainable number.[218]

Critics of overpopulation criticize the basic assumptions associated with these estimates. For example, Jade Sasser believes that calculating a maximum of number of humanity which may be allowed to live while only some, mostly privileged European former colonial powers, are mostly responsible for unsustainably using up the Earth is wrong.[219]

Urban areas with at least one million inhabitants in 2006. 3% of the world's population lived in cities in 1800, rising to 47% at the end of the twentieth century.

In 1800 only 3% of the world's population lived in cities. By the 20th century's close, 47% did so. In 1950 there were 83 cities with populations exceeding one million; but by 2007 this had risen to 468 "agglomerations".[220] If the trend continues, the world's urban population will double every 38 years. In 2007 UN forecasted that urban population would rise to three out of five or 60% by 2030 and an increase in urban population from 3.2 billion to nearly 5 billion by 2030.[221] As of 2018 55% live in cities and UN predicts that it will be 68% by 2050.[222]

The increase will be most dramatic in the poorest and least-urbanised continents, Asia and Africa. Projections indicate that most urban growth over the next 25 years will be in developing countries.[223] One billion people, one-seventh of the world's population, or one-third of urban population, now live in shanty towns,[224] which are seen as sources of social problems such as unemployment, poverty, crime, drug addiction, alcoholism, and other social ills. In many poor countries, slums exhibit high rates of disease due to unsanitary conditions, malnutrition, and lack of basic health care.[225]

A family planning placard in Ethiopia. It shows some negative effects of having more children than people can care for.

One option according to some activists is to focus on education about overpopulation, family planning, and birth control methods, and to make birth-control devices like male and female condoms, contraceptive pills and intrauterine devices easily available. Worldwide, nearly 40% of pregnancies are unintended (some 80 million unintended pregnancies each year).[228] An estimated 350 million women in the poorest countries of the world either did not want their last child, do not want another child or want to space their pregnancies, but they lack access to information, affordable means and services to determine the size and spacing of their families. In the United States, in 2001, almost half of pregnancies were unintended.[229] In the developing world, some 514,000 women die annually of complications from pregnancy and abortion,[230] with 86% of these deaths occurring in the sub-Saharan Africa region and South Asia.[231] Additionally, 8 million infants die, many because of malnutrition or preventable diseases, especially from lack of access to clean drinking water.[232]

Women's rights and their reproductive rights in particular are issues regarded to have vital importance in the debate.[233] This incentive, however, has been questioned by Rosalind Pollack Petchesky. Citing his attendance of the 1994 Cairo conference, he reported that overpopulation and birth control were being diverted by feminists into women's rights issues, mostly downplaying the overpopulation issue as only one minor matter of many others; most of these focusing on women's rights. Upon his observation, he argued this was forging many faults and distractions on the main problem of human overpopulation and how to solve it.[234]

Several scientists (including e.g. Paul and Anne Ehrlich and Gretchen Daily) proposed that humanity should work at stabilizing its absolute numbers, as a starting point towards beginning the process of reducing the total numbers. They suggested the following solutions and policies: following a small-family-size socio-cultural-behavioral norm worldwide (especially one-child-per-family ethos), and providing contraception to all along with proper education on its use and benefits (while providing access to safe, legal abortion as a backup to contraception), combined with a significantly more equitable distribution of resources globally.[235][236] In the book "Evolution Science and Ethics in the Third Millennium", Robert Cliquet and Dragana Avramov also point out that the one (and a half)-child-per-family ethos is certainly a good one and that we should reduce the world population so that it is no larger than 1 to 3 billion.[237]

Population planning that is intended to reduce population size or growth rate may promote or enforce one or more of the following practices, although there are other methods as well:

• Greater and better access to contraception
• Reducing infant mortality so that parents do not need to have many children to ensure at least some survive to adulthood.[238]
• Improving the status of women in order to facilitate a departure from traditional sexual division of labour.
• One-Child and Two-Child policies, and other policies restricting or discouraging births directly.
• Family planning[239]
• Creating small family "role models"[239]
• Tighter immigration restrictions

Overpopulation can be mitigated by birth control; some nations, like the People's Republic of China, use strict measures to reduce birth rates. Religious and ideological opposition to birth control has been cited as a factor contributing to overpopulation and poverty.[240]

Sanjay Gandhi, son of late Prime Minister of India Indira Gandhi, implemented a forced sterilization programme between 1975 and 1977. Officially, men with two children or more had to submit to sterilization, but there was a greater focus on sterilizing women than sterilizing men. Some unmarried young men and political opponents may also have been sterilized. This program is still remembered and criticized in India, and is blamed for creating a public aversion to family planning, which hampered government programs for decades.[241]

Another choice-based approach is financial compensation or other benefits (free goods and/or services) by the state (or state-owned companies) offered to people who voluntarily undergo sterilization. Such compensation has been offered in the past by the government of India.[242]

In 2014 the United Nations estimated there is an 80% likelihood that the world's population will be between 9.6 billion and 12.3 billion by 2100. Most of the world's expected population increase will be in Africa and southern Asia. Africa's population is expected to rise from the current one billion to four billion by 2100, and Asia could add another billion in the same period.[243]

Various scientists and science fiction authors have contemplated that overpopulation on Earth may be remedied in the future by the use of extraterrestrial settlements. In the 1970s, Gerard K. O'Neill suggested building space habitats that could support 30,000 times the carrying capacity of Earth using just the asteroid belt, and that the Solar System as a whole could sustain current population growth rates for a thousand years.[244] Marshall Savage (1992, 1994) has projected a human population of five quintillion (5 × 10¹⁸) throughout the Solar System by 3000, with the majority in the asteroid belt.[245] In Mining the Sky, John S. Lewis suggests that the resources of the solar system could support 10 quadrillion (10¹⁶) people. In an interview, Stephen Hawking claimed that overpopulation is a threat to human existence and "our only chance of long-term survival is not to remain inward looking on planet Earth but to spread out into space."[246] K. Eric Drexler has suggested in Engines of Creation that settling space will mean breaking the Malthusian limits to growth for the human species.

Many science fiction authors, including Carl Sagan, Arthur C. Clarke,[247] and Isaac Asimov,[248] have argued that shipping any excess population into space is not a viable solution to human overpopulation. According to Clarke, "the population battle must be fought or won here on Earth".[247] The problem for these authors is not the lack of resources in space (as shown in books such as Mining the Sky[249]), but the physical impracticality of shipping vast numbers of people into space to "solve" overpopulation on Earth. However according to calculations by Gerard K. O'Neill all new population growth could be facilitated with a launch services industry about the same size as the current airline industry.[250]

Despite the increase in population density within cities (and the emergence of megacities), UN Habitat states in its reports that urbanization may be the best compromise in the face of global population growth.[251] Cities concentrate human activity within limited areas, limiting the breadth of environmental damage.[252] But this mitigating influence can only be achieved if urban planning is significantly improved[253] and city services are properly maintained.

Paul Ehrlich pointed out in his book The Population Bomb (1968) argues that rhetoric supporting the increase of city density as a means of avoiding dealing with the actual problem of overpopulation to begin with and rather than treating the increase of city density as a symptom of the root problem, it has been promoted by the same interests that have profited from population increase e.g. property developers, the banking system, which invests in property development, industry, municipal councils etc.[254] Subsequent authors point to growth economics as driving governments seek city growth and expansion at any cost disregarding the impact it might have on the environment.[255]

The argument of overpopulation has been criticized as racist since control and reduction of human population is often focused on the global south, instead of on overconsumption and the global north.[262]

Elon Musk is a vocal critic of the idea of overpopulation. According to Musk, proponents of the idea are misled by their immediate impressions from living in dense cities.[263] Because of the negative replacement rates in many countries, he expects that by 2039 the biggest issue will be population collapse, not explosion.[264] Jack Ma expressed a similar opinion.[265]

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• Ehrlich, Paul R.; Ehrlich, Anne H. (9 January 2013). "Can a collapse of global civilization be avoided?". Proceedings of the Royal Society B. 280 (1754): 20122845. doi:10.1098/rspb.2012.2845. PMC 3574335. PMID 23303549.   Comment by Prof. Michael Kelly, disagreeing with the paper by Ehrlich and Ehrlich; and response by the authors
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