Human population has been growing continuously since the end of the Black Death, around the year 1350, although the most significant increase has been in the last 50 years, mainly due to medical advancements and increase in agricultural productivity. The rate of population growth has been declining since the 1980s. The United Nations has expressed concern on continued population growth in sub-Saharan Africa. Recent research has demonstrated that those concerns are well grounded. As of March 15, 2017 the world's human population is estimated to be 7.491 billion by the United States Census Bureau, and over 7 billion by the United Nations. Most contemporary estimates for the carrying capacity of the Earth under existing conditions are between 4 billion and 16 billion. Depending on which estimate is used, human overpopulation may or may not have already occurred. Nevertheless, the rapid recent increase in human population is causing some concern. The population is expected to reach between 8 and 10.5 billion between the years 2040 and 2050. In May 2011, the United Nations increased the medium variant projections to 9.3 billion for 2050 and 10.1 billion for 2100.
The recent rapid increase in human population over the past three centuries has raised concerns that the planet may not be able to sustain present or future numbers of 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. Other problems associated with overpopulation include the increased demand for resources such as fresh water and food, starvation and malnutrition, consumption of natural resources (such as fossil fuels) faster than the rate of regeneration, and a deterioration in living conditions. Wealthy but highly populated territories like Britain rely on food imports from overseas. This was severely felt during the World Wars when, despite food efficiency initiatives like "dig for victory" and food rationing, Britain needed to fight to secure import routes. However, many believe that waste and over-consumption, especially by wealthy nations, is putting more strain on the environment than overpopulation.
Most countries have no direct policy of limiting their birth rates, but the rates have still fallen due to education about family planning and increasing access to birth control and contraception. Only China had imposed legal restrictions on having more than one child. Extraterrestrial settlement and other technical solutions have been proposed as ways to mitigate overpopulation in the future.
Concern about overpopulation is an ancient topic. Tertullian was a resident of the city of Carthage in the second century CE, when the population of the world was about 190 million (only 3–4% of what it is today). He notably said: "What most frequently meets our view (and occasions complaint) is our teeming population. Our numbers are burdensome to the world, which can hardly support us.... In very deed, pestilence, and famine, and wars, and earthquakes have to be regarded as a remedy for nations, as the means of pruning the luxuriance of the human race." Before that, Plato, Aristotle and others broached the topic as well.
Throughout history, population growth has usually been slow despite high birth rates, due to war, plagues and other diseases, and high infant mortality. During the 750 years before the Industrial Revolution, the world's population increased very slowly, remaining under 250 million.
By the beginning of the 19th century, the world population had grown to a billion individuals, and intellectuals such as Thomas Malthus predicted that mankind would outgrow its available resources, because a finite amount of land would be incapable of supporting a population with a limitless potential for increase. Mercantillists argued that a large population was a form of wealth, which made it possible to create bigger markets and armies.
During the 19th century, Malthus's work was often interpreted in a way that blamed the poor alone for their condition and helping them was said to worsen conditions in the long run. This resulted, for example, in the English poor laws of 1834 and in a hesitating response to the Irish Great Famine of 1845–52.
The UN Population Assessment Report of 2004 projects that the world population will plateau by 2050 and will remain stable until 2300. A 2014 study published in Science challenges this projection, asserting that population growth will continue into the next century. Adrian Raftery, a University of Washington professor of statistics and sociology and one of the contributors to the study, says: "The consensus over the past 20 years or so was that world population, which is currently around 7 billion, would go up to 9 billion and level off or probably decline. We found there's a 70 percent probability the world population will not stabilize this century. Population, which had sort of fallen off the world's agenda, remains a very important issue." A more recent UN projection suggests the population could grow to as many as 15 billion by 2100.
The human 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. 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.
Around 8000 BCE, at the dawn of agriculture, the population of the world was approximately 5 million. The next several millennia saw a steady increase in the population, with very rapid growth beginning in 1000 BCE, and a peak of between 200 and 300 million people in 1 BCE.
The Plague of Justinian caused Europe's population to drop by around 50% between 541 and the 8th century. Steady growth resumed in 800 CE. However, growth was again disrupted by frequent plagues; most notably, the Black Death during the 14th century. The effects of the 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. The population of Europe stood at over 70 million in 1340; these levels did not return until 200 years later. England's population reached an estimated 5.6 million in 1650, up from an estimated 2.6 million in 1500. New crops from the Americas via the Spanish colonizers in the 16th century contributed to the population growth.
In other parts of the globe, China's population at the founding of the Ming dynasty in 1368 stood close to 60 million, approaching 150 million by the end of the dynasty in 1644. The population of the Americas in 1500 may have been between 50 and 100 million.
Encounters between European explorers and populations in the rest of the world often introduced local epidemics of extraordinary virulence. Archaeological evidence indicates that the death of around 90% of the Native American population of the New World was caused by Old World diseases such as smallpox, measles, and influenza. Europeans introduced diseases alien to the indigenous people, therefore they did not have immunity to these foreign diseases.
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. At the turn of the 20th century, the world's population was roughly 1.6 billion. By 1940, this figure had increased to 2.3 billion.
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. The rate of human population growth peaked in 1964, at about 2.1% per year. For example, Indonesia's population grew from 97 million in 1961 to 237.6 million in 2010, a 145% increase in 49 years. In India, the population grew from 361.1 million people in 1951 to just over 1.2 billion by 2011, a 235% increase in 60 years.
There is concern over the sharp population increase in many countries, especially in Sub-Saharan Africa, that has occurred over the last several decades, and that it is creating problems with land management, natural resources and access to water supplies.
The population of Chad has, for example, grown from 6,279,921 in 1993 to 10,329,208 in 2009. Niger, Uganda, Nigeria, Tanzania, Ethiopia and the DRC are witnessing a similar growth in population. The situation is most acute in western, central and eastern Africa. Refugees from places like Sudan have further strained the resources of neighboring states like Chad and Egypt. Chad is also host to roughly 255,000 refugees from Sudan's Darfur region, and about 77,000 refugees from the Central African Republic, while approximately 188,000 Chadians have been displaced by their own civil war and famines, have either fled to either the Sudan, the Niger or, more recently, Libya.
According to projections, the world population will continue to grow until at least 2050, with the population reaching 9 billion in 2040, and some predictions putting the population as high as 11 billion in 2050. By 2100, the population could reach 15 billion. 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.
In 2000, the United Nations estimated that the world's population was growing at the rate of 1.14% (or about 75 million people) per year and according to data from the CIA's World Factbook, the world human population currently increases by 145 every minute.
According to the United Nations' World Population Prospects report:The world population is currently growing by approximately 74 million people per year. Current United Nations predictions estimate that the world population will reach 9.0 billion around 2050, assuming a decrease in average fertility rate from 2.5 down to 2.0.
Almost all growth will take place in the less developed regions, where today's 5.3 billion population of underdeveloped countries is expected to increase to 7.8 billion in 2050. By contrast, the population of the more developed regions will remain mostly unchanged, at 1.2 billion. An exception is the United States population, which is expected to increase by 44% from 2008 to 2050.
In 2000–2005, the average world fertility was 2.65 children per woman, about half the level in 1950–1955 (5 children per woman). In the medium variant, global fertility is projected to decline further to 2.05 children per woman.
During 2005–2050, nine countries are expected to account for half of the world's projected population increase: India, Pakistan, Nigeria, Democratic Republic of the Congo, Bangladesh, Uganda, United States, Ethiopia, and China, listed according to the size of their contribution to population growth. China would be higher still in this list were it not for its one-child policy.
Global life expectancy at birth is expected to continue rising from 65 years in 2000–2005 to 75 years in 2045–2050. In the more developed regions, the projection is to 82 years by 2050. Among the least developed countries, where life expectancy today is just under 50 years, it is expected to increase to 66 years by 2045–2050.
The population of 51 countries or areas is expected to be lower in 2050 than in 2005.
During 2005–2050, the net number of international migrants to more developed regions is projected to be 98 million. Because deaths are projected to exceed births in the more developed regions by 73 million during 2005–2050, population growth in those regions will largely be due to international migration.
In 2000–2005, net migration in 28 countries either prevented population decline or doubled at least the contribution of natural increase (births minus deaths) to population growth.
Birth rates are now falling in a small percentage of developing countries, while the actual populations in many developed countries would fall without immigration.
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 of more than one million. If the trend continues, the world's urban population will double every 38 years, according to researchers. The UN forecasts that today's urban population of 3.2 billion will rise to nearly 5 billion by 2030, when three out of five people will live in cities.
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. One billion people, one-seventh of the world's population, or one-third of urban population, now live in shanty towns, which are seen as "breeding grounds" for social problems such as crime, drug addiction, alcoholism, poverty and unemployment. In many poor countries, slums exhibit high rates of disease due to unsanitary conditions, malnutrition, and lack of basic health care.
In 2000, there were 18 megacities – conurbations such as Tokyo, Beijing, Guangzhou, Seoul, Karachi, Mexico City, Mumbai, São Paulo, London and New York City – that have populations in excess of 10 million inhabitants. Greater Tokyo already has 35 million, more than the entire population of Canada (at 34.1 million).
According to the Far Eastern Economic Review, Asia alone will have at least 10 'hypercities' by 2025, that is, cities inhabited by more than 19 million people, including Jakarta (24.9 million people), Dhaka (25 million), Karachi (26.5 million), Shanghai (27 million) and Mumbai (33 million). Lagos has grown from 300,000 in 1950 to an estimated 15 million today, and the Nigerian government estimates that city will have expanded to 25 million residents by 2015. Chinese experts forecast that Chinese cities will contain 800 million people by 2020.
From a historical perspective, technological revolutions have coincided with population explosions. 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, resulting in subsequent population explosions. For example, the use of tools, such as bow and arrow, allowed primitive hunters greater access to 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. In short, similar to bacteria that multiply in response to increased food supply, humans have increased their population as soon as food became more abundant as a result of technological innovations.
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 fast enough, resulting in significant population growth. Prior to these changes, seven out of ten children died before reaching reproductive age, while today about 95% of newborns in industrialized nations reach adulthood.
Agriculture has been the main factor behind human population growth. 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.
Humans have always exploited the environment using the easiest, most accessible resources first. The richest farmland was plowed and the richest mineral ore mined first. Overpopulation will demand ever more creative means in which to satisfy these basic human needs.
Human psychology and the cycle of entrenched poverty, as well as the rest of the world's reaction to it, are also causative factors. Areas with greater burden of disease and warfare, contrary to popular belief, do not experience less population growth over the long term, but far more over a sustained period as poverty becomes further entrenched . This is because parents and siblings who have experienced calamitous conditions suffer from a kind of post traumatic stress syndrome about losing their family members and overcompensate by having "extra" babies. These extra babies and calamities fuel a vicious cycle, and only in the small minority of cases does it cease . As this cycle is compounded over generations, calamities such as disaster or war take on a multiplier effect. For example, the AIDS crisis in Africa is said to have killed 30 million to date, yet during the last two decades money and initiatives to lower population growth by contraception have been sidelined in favor of combating HIV, feeding the population explosion that we see in Africa today. In 1990, this continent's population was roughly 600 million; today it is over 1,050 million, 150 million more than if the HIV/AIDS crisis had never occurred.
Population growth rates between 1950 and 2012 range from a 0.5% increase in the case of Bulgaria to a more than 100 fold increase for the United Arab Emirates (from 79,050 to 8.5 million). Roughly half of all nations have quadrupled their populations since 1950.
The theory of demographic transition held that, after the standard of living and life expectancy increase, family sizes and birth rates decline. However, as new data has become available, it has been observed that after a certain level of development (HDI equal to 0.86 or higher) the fertility increases again. This means that both the worry that the theory generated about aging populations and the complacency it bred regarding the future environmental impact of population growth are misguided.
Factors cited in the old theory included such social factors as later ages of marriage, the growing desire of many women in such settings to seek careers outside child rearing and domestic work, and the decreased need of children in industrialized settings. The latter factor stems from the fact that children perform a great deal of work in small-scale agricultural societies, and work less in industrial ones; it has been cited to explain the decline in birth rates in industrializing regions.
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.
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 observed 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.
Some groups (for example, the World Wide Fund for Nature and Global Footprint Network) have stated that the carrying capacity for the human population has been 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. Humanity as a whole was using, by 2006, 40 percent more than what Earth can regenerate. 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". 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.
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. Some point out that a more refined method of assessing Ecological Footprint is to designate sustainable versus non-sustainable categories of consumption. However, if yield estimates were adjusted for sustainable levels of production, the yield figures would be lower, and hence the overshoot estimated by the Ecological Footprint method even higher.
Other studies give particular attention to resource depletion and increased world affluence.
In a 1994 study titled Food, Land, Population and the U.S. Economy, David Pimentel and Mario Giampietro estimated the maximum U.S. population for a sustainable economy at 200 million. And in order to achieve a sustainable economy and avert disaster, the United States would have to reduce its population by at least one-third, and world population would have to be reduced by two-thirds.
Many quantitative studies have estimated the world's carrying capacity for humans, that is, a limit to the world population. A meta-analysis of 69 such studies suggests a point estimate of the limit to be 7.7 billion people, while lower and upper meta-bounds for current technology are estimated as 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".
Humanity's overall impact on the planet is affected by many factors besides raw population. The lifestyle (including overall affluence and resource utilization) and the pollution (including carbon footprint) are equally important. In 2008, The New York Times stated that the inhabitants of the developed nations of the world consume resources like oil and metals at a rate almost 32 times greater than those of the developing world, who make up the majority of the human population.
Some problems associated with or exacerbated by human overpopulation and over-consumption are:Inadequate fresh water 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.
Depletion of natural resources, especially fossil fuels.
Increased levels of air pollution, water pollution, soil contamination and noise pollution. Once a country has industrialized and become wealthy, a combination of government regulation and technological innovation causes pollution to decline substantially, even as the population continues to grow.
Deforestation and loss of ecosystems that valuably contribute to the global atmospheric oxygen and carbon dioxide balance; about eight million hectares of forest are lost each year.
Changes in atmospheric composition and consequent global warming.
Loss of arable land and increase in desertification. Deforestation and desertification can be reversed by adopting property rights, and this policy is successful even while the human population continues to grow.
Mass species extinctions and contracting biodiversity from reduced habitat in tropical forests due to slash-and-burn techniques that sometimes are practiced by shifting cultivators, especially in countries with rapidly expanding rural populations; present extinction rates may be as high as 140,000 species lost per year. As of February 2011, the IUCN Red List lists a total of 801 animal species having gone extinct during recorded human history, although the vast majority of extinctions are thought to be undocumented. Biodiversity would continue to grow at an exponential rate if not for human influence. 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." Ehrlich, from Stanford, said population growth is one of the main drivers of the Earth's extinction crisis.
High infant and child mortality. High rates of infant mortality are associated with poverty. Rich countries with high population densities have low rates of infant mortality.
Intensive factory farming to support large populations. It results in human threats including the evolution and spread of antibiotic resistant bacteria diseases, excessive air and water pollution, and new viruses that infect humans.
Increased chance of the emergence of new epidemics and pandemics. 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.
Starvation, malnutrition or poor diet with ill health and diet-deficiency diseases (e.g. rickets). However, rich countries with high population densities do not have famine.
Poverty coupled with inflation in some regions and a resulting low level of capital formation. Poverty and inflation are aggravated by bad government and bad economic policies. Many countries with high population densities have eliminated absolute poverty and keep their inflation rates very low.
Low life expectancy in countries with fastest growing populations.
Unhygienic living conditions for many based upon water resource depletion, discharge of raw sewage 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.
Elevated crime rate due to drug cartels and increased theft by people stealing resources to survive.
Conflict over scarce resources and crowding, leading to increased levels of warfare.
Less personal freedom and more restrictive laws. Laws regulate interactions between humans. Law "serves as a primary social mediator of relations 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. It was even speculated by Aldous Huxley in 1958 that democracy is threatened due to overpopulation, and could give rise to totalitarian style governments.
David Attenborough described the level of human population on the planet as a multiplier of all other environmental problems. In 2013, he described humanity as "a plague on the Earth" that needs to be controlled by limiting population growth.
Many of these problems are explored in the dystopic science fiction film Soylent Green, where an overpopulated Earth suffers from food shortages, depleted resources and poverty and in the documentary "Aftermath: Population Overload".
Some economists, such as Thomas Sowell and Walter E. Williams argue that third world poverty and famine are caused in part by bad government and bad economic policies. Most biologists and sociologists see overpopulation as a serious threat to the quality of human life. Some deep ecologists, such as the radical thinker and polemicist Pentti Linkola, see human overpopulation as a threat to the entire biosphere.
Overpopulation has substantially adversely impacted the environment of Earth starting at least as early as the 20th century. 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". There are also economic consequences of this environmental degradation in the form of ecosystem services attrition. 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."
Says Peter Raven, former President of the American Association for the Advancement of Science (AAAS) in their seminal work AAAS Atlas of Population & Environment, "Where do we stand in our efforts to achieve a sustainable world? Clearly, the past half century has been a traumatic one, as the collective impact of human numbers, affluence (consumption per individual) and our choices of technology continue to exploit rapidly an increasing proportion of the world's resources at an unsustainable rate. ... During a remarkably short period of time, we have lost a quarter of the world's topsoil and a fifth of its agricultural land, altered the composition of the atmosphere profoundly, and destroyed a major proportion of our forests and other natural habitats without replacing them. Worst of all, we have driven the rate of biological extinction, the permanent loss of species, up several hundred times beyond its historical levels, and are threatened with the loss of a majority of all species by the end of the 21st century."
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. However, as developing countries with high populations become more industrialized, pollution and consumption will invariably increase.
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.
It is said that 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. In the long term these effects can lead to increased conflict over dwindling resources and in the worst case a Malthusian catastrophe.
Many studies link population growth with emissions and the effect of climate change.
It has been suggested 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 the empirical grounds. Both 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.
Population and warfare are dynamical variables, and if their interaction causes sustained oscillations, then we do not in general expect to find strong correlation between the two variables measured at the same time (that is, unlagged). Korotayev and Turchin have explored mathematically what the dynamical patterns of interaction between population and warfare (focusing on internal warfare) might be in both stateless and state societies. Next, they have tested the model predictions in several empirical case studies: early modern England, Han and Tang China, and the Roman Empire. Their empirical results have supported the population-warfare theory: that there is a tendency for population numbers and internal warfare intensity to oscillate with the same period but shifted in phase (with warfare peaks following population peaks).
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.
There are several solutions and mitigation measures that have the potential to reduce overpopulation. Some solutions are to be applied on a global planetary level (e.g., via UN resolutions), some to be applied on a local government, state, city, or other organization level, and some are to be applied on a family or an individual level. Some of the mitigations are ways to implement social norms. Overpopulation is an issue that threatens the state of the environment and therefore societies must make a change in order to reverse some of the environmental effects brought on by current social norms.
For example, in societies like China, the government has put policies in place that regulate the number of children allowed to a couple. Other societies have already begun to implement social marketing strategies in order to educate the public on overpopulation effects. "The intervention can be widespread and done at a low cost. A variety of print materials (flyers, brochures, fact sheets, stickers) needs to be produced and distributed throughout the communities such as at local places of worship, sporting events, local food markets, schools and at car parks (taxis / bus stands)."
Such prompts work to introduce the problem so that social norms are easier to implement. Certain government policies are making it easier and more socially acceptable to use contraception and abortion methods. An example of a country whose laws and norms are hindering the global effort to slow population growth is Afghanistan. "The approval by Afghan President Hamid Karzai of the Shia Personal Status Law in March 2009 effectively destroyed Shia women's rights and freedoms in Afghanistan. Under this law, women have no right to deny their husbands sex unless they are ill, and can be denied food if they do."
One option is to focus on education about overpopulation, family planning, and birth control methods, and to make birth-control devices like male and female condoms, pills and intrauterine devices easily available. Worldwide, nearly 40% of pregnancies are unintended (some 80 million unintended pregnancies each year). 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. In the developing world, some 514,000 women die annually of complications from pregnancy and abortion, with 86% of these deaths occurring in the sub-Saharan Africa region and South Asia. Additionally, 8 million infants die, many because of malnutrition or preventable diseases, especially from lack of access to clean drinking water.
Women's rights and their reproductive rights in particular are issues regarded to have vital importance in the debate.
"The only ray of hope I can see – and it's not much – is that wherever women are put in control of their lives, both politically and socially; where medical facilities allow them to deal with birth control and where their husbands allow them to make those decisions, birth rate falls. Women don't want to have 12 kids of whom nine will die." — David Attenborough
Egypt announced a program to reduce its overpopulation by family planning education and putting women in the workforce. It was announced in June 2008 by the Minister of Health and Population Hatem el-Gabali. The government has set aside 480 million Egyptian pounds (about $90 million US) for the program.
The business magnate Ted Turner proposed a "voluntary, non-imposed" one-child family scenario. A "pledge two or fewer" campaign is run by Population Matters (a UK population concern organisation), in which people are encouraged to limit themselves to small family size.
Overpopulation issue 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.
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, political opponents and ignorant men were also believed to 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 programmes for decades.
Urban designer Michael E. Arth has proposed a "choice-based, marketable birth license plan" he calls "birth credits". Birth credits would allow any woman to have as many children as she wants, as long as she buys a license for any children beyond an average allotment that would result in zero population growth. If that allotment was determined to be one child, for example, then the first child would be free, and the market would determine what the license fee for each additional child would cost. Extra credits would expire after a certain time, so these credits could not be hoarded by speculators. The actual cost of the credits would only be a fraction of the actual cost of having and raising a child, so the credits would serve more as a wake-up call to women who might otherwise produce children without seriously considering the long term consequences to themselves or society.
Another choice-based approach, similar to Arth's birth credits, 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.
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 three or four billion by 2100, and Asia could add another billion in the same period. Because the median age of Africans is so low (e.g. Uganda = 15 years old) birth credits would have to limit fertility to one child per two women to reach the levels of developed countries immediately. For countries with a wide base in their population pyramid it will take a generation for the people who are of child bearing age to have their families. An example of demographic momentum is China, which added perhaps 400,000 more people after its one-child policy was enacted. Arth has suggested that the focus should be on the developed countries and that some combination of birth credits and additional compensation supplied by the developed countries could rapidly lead to zero population growth while also quickly raising the standard of living in developing countries.
There are also natural diseases, such as chlamydia, gonorrhea and even some diseases that produce no physical discomfort at all (such as some specific adeno-associated viruses) that may also cause sterilization of males and/or females in the population. Whereas the natural occurrence of these diseases is not sufficient to significantly reduce the population problem, large-scale production of these diseases and human-induced release of such diseases could quite well be used as a mitigation measure.
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. Marshall Savage (1992, 1994) has projected a human population of five quintillion (5 x 1018) throughout the Solar System by 3000, with the majority in the asteroid belt. Freeman Dyson (1999) favours the Kuiper belt as the future home of humanity, suggesting this could happen within a few centuries. 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."
K. Eric Drexler, famous inventor of the futuristic concept of molecular nanotechnology, has suggested in Engines of Creation that colonizing space will mean breaking the Malthusian limits to growth for the human species.
It may be possible for other parts of the Solar System to be inhabited by humanity at some point in the future. Geoffrey Landis of NASA's Glenn Research Center in particular has pointed out that "[at] cloud-top level, Venus is the paradise planet", as one could construct aerostat habitats and floating cities there easily, based on the concept that breathable air is a lifting gas in the dense Venusian atmosphere. Venus would, like also Saturn, Uranus, and Neptune, in the upper layers of their atmospheres, even afford a gravitation almost exactly as strong as that on Earth (see colonization of Venus).
Many science fiction authors, including Carl Sagan, Arthur C. Clarke, and Isaac Asimov, 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". The problem for these authors is not the lack of resources in space (as shown in books such as Mining the Sky), but the physical impracticality of shipping vast numbers of people into space to "solve" overpopulation on Earth. However, Gerard K. O'Neill's calculations show that Earth could offload all new population growth with a launch services industry about the same size as the current airline industry.
The StarTram concept, by James R. Powell (the co-inventor of maglev transport) and others, envisions a capability to send up to 4 million people a decade to space per facility. A hypothetical extraterrestrial colony could potentially grow by reproduction only (i.e., without any immigration), with all of the inhabitants being the direct descendants of the original colonists.
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. Cities concentrate human activity within limited areas, limiting the breadth of environmental damage. But this mitigating influence can only be achieved if urban planning is significantly improved and city services are properly maintained.