Sustainable population

The concept of sustainable population concerns how human numbers interact with environmental limits, economic systems, and social equity. While human population size is linearly linked to ecological impact, empirical research shows that most variation in environmental pressure between countries and over time is explained by differences in consumption and technology use. ^[1][2][3]

Estimates of a sustainable population vary widely, depending on assumptions about technology, equity, and consumption levels. Some frameworks focus on numeric carrying capacity, while others emphasize changing systems: improving access to education and healthcare, reducing inequality, and shifting consumption norms.

Sustainability is increasingly viewed as a dynamic balance between human well-being and planetary boundaries, not a fixed population threshold.

Estimates

World population growth rate, 1950–2050, as estimated in 2011 by the U.S. Census Bureau, International Data Base. Although the rate of growth decreases, population continues to rise. In 2050 still growing by over 45 million per year

Map of countries and territories by fertility rate as of 2020^[update]

Sustainable population

Many studies have tried to estimate the world's sustainable population for humans, that is, the maximum population the world can host.^[4] A 2004 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 9.8 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".^[5] A 2012 United Nations report reviewed 65 different estimates of the Earth's maximum sustainable population and found that the most frequently cited figure was 8 billion.^[6][7] This aligns with a 2025 estimate of the actual global population, as reported in a recent demographic analysis.^[8]

Climate change, excess nutrient loading (particularly nitrogen and phosphorus), increased ocean acidity, rapid biodiversity loss, and other global trends suggest humanity is causing global ecological degradation and threatening ecosystem services that human societies depend on.^[9][10][11] Because these environmental impacts are all directly related to human numbers, recent estimates of a sustainable human population often suggest substantially lower figures, between 2 and 4 billion.^[12][13][14] Paul R. Ehrlich stated in 2018 that the optimum population is between 1.5 and 2 billion.^[15] Geographer Chris Tucker estimates that 3 billion is a sustainable number, provided human societies rapidly deploy less harmful technologies and best management practices.^[16] Other estimates of a sustainable global population also come in at considerably less than the current population of 8 billion.^[17][18][19] Rich countries with high usage of resources and high emissions over the years are claimed to see a decline. Whereas, low income countries with would become a focus for population growth as less natural resources such as water and food would be consumed.^[20]

A 2014 study published in the Proceedings of the National Academy of Sciences of the United States of America posits that, given the "inexorable demographic momentum of the global human population," efforts to slow population growth in the short term will have little impact on sustainability, which can be more rapidly achieved with a focus on technological and social innovations, along with reducing consumption rates, while treating population planning as a long term goal. The study says that with a fertility-reduction model of one-child per female by 2100, it would take at least 140 years to reduce the population to 2 billion people by 2153.^[21][22] The 2022 "Scientists' warning on population," published by Science of the Total Environment, states that "environmental analysts regard a sustainable human population as one enjoying a modest, equitable middle-class standard of living on a planet retaining its biodiversity and with climate-related adversities minimized," which is estimated at between 2 and 4 billion people.^[23]

Some scholars criticize the assumptions behind overpopulation estimates. Sociologist Jade Sasser, for example, argues that focusing on limiting global population can obscure the historical responsibility of high-income nations for ecological degradation.^[24]

But if current human numbers are not ecologically sustainable, the costs are likely to fall on the world’s poorest citizens, regardless of whether they helped cause the problem.^[25][26] In fact, countries that contribute the most to unsustainable production and consumption practices often have higher income per capita and slower population growth, unlike countries that have a low income per capita and rapidly growing populations.^[27]

According to a 2022 study published in Sustainable Development, a sustainable population is required for both preserving biodiversity and food security. The study says that falling fertility rates are linked to access to contraception and family planning services, and has little to no relation to economic growth.^[28]

World population

According to data from 2015, the world population is projected to reach 8.5 billion by 2030, up from the current 8 billion, to exceed 9 billion people by 2050, and to reach 11.2 billion by the year 2100.^[29] Most of the increase will be in developing countries whose population is projected to rise from 5.6 billion in 2009 to 7.9 billion in 2050. This increase will be distributed among the population aged 15–59 (1.2 billion) and 60 or over (1.1 billion) because the number of children under age 15 in developing countries is predicted to decrease. In contrast, the population of the more developed regions is expected to undergo only slight increase from 1.23 billion to 1.28 billion, and this would have declined to 1.15 billion but for a projected net migration from developing to developed countries, which is expected to average 2.4 million persons annually from 2009 to 2050.^[30] Long-term estimates in 2004 of global population suggest a peak at around 2070 of nine to ten billion people, and then a slow decrease to 8.4 billion by 2100.^[31]

However, these projections assume substantial improvements in contraceptive availability throughout the developing world and large decreases in desired family size (particularly in sub-Saharan Africa), which may or may not happen.^[32] Ultimately, all population projections should be interpreted with caution, as they depend on a range of assumptions and uncertain future developments.^[33] Particular care is needed to remember that future population size will depend on policy decisions and individual choices.^[34]

Systemic perspectives on sustainable population

Some researchers and policy advocates argue that sustainable population should be seen not as a fixed number but as a system condition where human well-being and ecological integrity can coexist. In this view, population is linked to consumption, technology, and resource distribution.^[35]

High-income countries contribute disproportionately to global emissions despite slow population growth, while low-income countries with higher fertility have low per capita impact.^[36] This challenges the adequacy of approaches focused solely on population reduction.

A systems approach emphasizes reducing total resource use through innovation, equity, and shifting consumption norms.^[37] From this perspective, sustainability is viewed as a balance between human needs and planetary limits, not a fixed population cap.^[38]

A recent report from the Institute for Futures Studies highlights the need to assess sustainable population in relation to climate, technology, inequality, and consumption, emphasizing systemic approaches over fixed numerical thresholds.^[39]

Carrying capacity

Urbanization in Seattle, Washington, United States

Talk of economic and population growth overshooting the limits of Earth's carrying capacity for humans is popular in environmentalism.^[40] The potential limiting factor for the human population might include water availability, energy availability, renewable resources, non-renewable resources, heat removal, photosynthetic capacity, or land availability for food production.^[41] Or, as current trends suggest, the limiting factors might involve ecosystems’ ability to absorb human pollution, as with climate change, ocean acidification, or the toxification of rivers and streams.^[42][11][43] The applicability of carrying capacity as a measurement of the Earth's limits in terms of the human population has been questioned, since it has proved difficult to calculate or predict the upper limits of population growth.^[40] The concept of carrying capacity has often been cited in arguments advocating population stabilization since the mid-20th century.^[44]

The application of the concept of carrying capacity for the human population, which exists in a non-equilibrium, has been criticized for not successfully being able to model the processes between humans and the environment.^[40][45] In popular discourse the concept is often used vaguely in the sense of a "balance between nature and human populations".^[45]

Critiques of fixed population estimates in carrying capacity

Systemic perspectives on sustainability have informed critical reassessments of the concept of carrying capacity. Some scholars question the usefulness of defining a sustainable population by a fixed number. Joel E. Cohen argues that Earth's human carrying capacity depends on multiple interacting factors, including technology, living standards, inequality, and cultural values.^[46] He notes that estimates of carrying capacity have varied widely and often reflect political or normative assumptions rather than scientific consensus.^[47] Rather than seeking a single numerical limit, Cohen suggests focusing on improving human well-being within environmental boundaries—through technological innovation, voluntary fertility reduction, and fairer resource distribution.

Some ecological economists, including Nicholas Georgescu-Roegen and Herman Daly, have argued that Earth's long-term carrying capacity may decline over time due to the depletion of non-renewable resources. This perspective reinforces the case for limiting both consumption and population growth to ensure sustainability.^[48]

See also

• Population ageing
• Population growth
• Human overpopulation
• Intergenerational equity
• Overshoot (population)