1 Introduction
The planetary boundary framework shows that the disruption of overarching, planetary-level systems are causing environmental changes that are taking us out of the stable environmental conditions of the Holocene. There are nine planetary boundaries—see Figure 10.1: the ‘big three’ (the climate system, the ozone layer, and the ocean); the four biosphere boundaries (biodiversity, land, fresh water, and nutrients); and the two aliens (novel entities and aerosols). Seven planetary systems have already been pushed beyond their critical limits: the climate system, biosphere integrity (biodiversity), the nitrogen and phosphorus cycles (nutrients), land use, freshwater and green water, ocean acidification, and novel entities (especially plastics).1 If these transgressions persist, the entire planet may be pushed into a new state that would be much less hospitable for human societies, not to mention other species.
The nine planetary boundaries (2023 update). See also the 2025 update: https://www.stockholmresilience.org/news--events/general-news/2025-09-24-seven-of-nine-planetary-boundaries-now-breached.html, “Seven of nine planetary boundaries now breached” (Planetary Health Check 2025).
LICENSED UNDER CC BY-NC-ND 3.0. (CREDIT: AZOTE FOR STOCKHOLM RESILIENCE CENTRE, BASED ON ANALYSIS IN RICHARDSON ET AL., 2023.)
Climate change and biodiversity loss are especially crucial. On their own, they are pushing the Earth system out of the Holocene state: “transitions between time periods in Earth history have often been delineated by substantial shifts in climate, the biosphere, or both.” Based on these observations, Earth system scientists suggest “a two-level hierarchy of boundaries, in which climate change and biosphere integrity should be recognized as core planetary
The Hothouse Earth pathway would be triggered by a cascade of tipping points in the climate system. Tipping points are self-perpetuating changes
The geographical distribution of global and regional tipping elements, color-coded according to the best estimate for their temperature thresholds, beyond which the element would likely be ‘tipped’
FIGURE DESIGNED AT PIK (UNDER CC BY LICENSE), BASED ON ARMSTRONG MCKAY ET AL., (2022)
Among the factors that contribute to the crossing of planetary boundaries, population growth plays a key role. World population has grown dramatically in recent decades, contributing substantially to the Great Acceleration: the
This chapter focuses on the impact of these demographic changes on the planetary boundaries of the climate system and biosphere integrity. Without neglecting the other major causes of climate change and biodiversity loss, such as economic growth, it investigates an essential but relatively marginalized aspect of the problem. It develops a normative approach that evaluates population policies and examines which policies can be justified in our context of planetary boundaries transgression, understanding population policies as government policies that influence fertility rates, such as educational policies, contraceptive availability, and taxes.
While green political theorists have been stressing for decades that truly sustainable societies require limits to population growth, this topic has only recently emerged in the philosophical literature.6 Whether they support rights-based or consequentialist approaches, more and more philosophers have begun addressing population growth and its impacts on the environment, especially with regards to climate change.7 As we will see below, while several climate ethics scholars such as Cafaro and Rieder are now indeed supporting population policies that reduce fertility, influential voices such as Caney and Heyward still tend to marginalize this mitigation measure in favor of other measures related to affluence levels and technological innovation.
A major reason why population growth reduction has only started to emerge in climate ethics scholarship over the last decade and remains marginalized is that this topic is unpopular. It raises many ethical and political problems and meets with strong social resistance.8 But the problems specific to our context of population growth in a world of more than 8 billion human beings cannot be ignored. The point here is to foster democratic debate on this controversial
The first section sets the framework for the analysis by explaining the contribution of the demographic factor to climate change and biodiversity loss. The second section critically examines an influential way of discussing population growth in the climate ethics literature: using the IPAT equation (Environmental impact = Population × Affluence × Technology) to over-emphasize the importance of “T” and downplay the role of “P.” The third section explores and evaluates population policies and proposes possible justifications for certain positive and negative incentive measures. The final section considers objections commonly made against these incentive policies.
The core claim is that all three components of the IPAT equation need to be addressed urgently and simultaneously if we are to take seriously the radical implications of the planetary boundary framework. Technological innovation and affluence are indeed key to reduce the environmental impact of our lifestyles and policies and reach sustainable societies, but they cannot substitute for a reduction in the world’s population.
2 Population Growth, Climate Change, and Biodiversity Loss
2.1 The Demographic Factor
Over the last decades, global GHG emissions have grown at a rate unprecedented in human history. Between 1970 and 2000, the average atmospheric concentration of GHG increased by 1.3% per year. Between 2000 and 2010, it increased by 2.2% per year.9 Average annual GHG emissions between 2010 and 2019 were higher than in any previous decade, even though the rate of growth slowed to 1.3% per year.10
Already in the AR5, the IPCC (2014, 8) stressed that population growth and climate change are directly linked: “Globally, economic and population growth continue to be the most important drivers of increases in CO2 emissions from fossil fuel combustion.” Indeed: “Between 2000 and 2010, both drivers outpaced
Population growth also plays a major role in biodiversity loss. According to the IPBES (2019, 5), there are five “direct drivers” of biodiversity loss: changes in land and sea use, overexploitation of species, climate change, pollution, and invasive species. Population growth is listed as an “indirect driver” of change, together with consumption patterns, trade, and technological innovation. Cafaro, Hansson, and Götmark (2022) reviewed over 30 recent studies that report a negative impact of excessive population on biodiversity. Population increases led to huge agricultural conversions of land for human use and phenomena such as urbanization and sprawl to accommodate human needs, which led to habitat loss; they also contributed to the overexploitation of species through overhunting, overfishing, and overharvesting; finally, they also caused the spread of invasive species, increased water and air pollution, and, as we saw above, contributed to increasing GHG atmospheric concentrations.
When discussing the importance of the demographic factor, it is important to avoid two pitfalls: (1) focusing only on population growth, isolating this factor from other drivers of environmental degradation; and (2) focusing only on developed or developing countries, leaving one of the two categories out of the analysis.
Regarding pitfall (1), population growth is not the only—and not necessarily the most important—driver of climate change and biodiversity loss. One of the key messages of the IPAT equation and its variations, such as the Kaya identity (Emissions = Population × GDP per capita × Energy used per unit of GDP × CO2 generated per unit of energy), is that a multicausal analysis is required to properly understand current environmental problems.
Take climate change. Although the number of people matters, other factors, such as income, wealth, diets, education, household size and geographical
This radically unequal allocation of per capita emissions at the global level implies that the reproductive choices of people in developed countries have a much greater environmental impact than the reproductive choices of people in developing countries (at least regarding carbon emissions and climate change).14 For instance, the total carbon footprint of a child born in the US is more than 160 times greater than the footprint of a child born in Bangladesh; it is 200 times greater than that of a child born in Niger, a country with a fertility rate of 7.6, the highest in the world.15
This leads us to pitfall (2). Given the highest carbon footprint associated with reproductive choices in developed countries, it might be tempting to focus on population policies exclusively in these countries. This would also avoid the problems associated with Malthus- and Hardin-inspired positions that place the overwhelming burden on poor countries in the name of environmental
A legitimate concern raised by the inclusion of the environmental impact of population growth in developing countries is that it can lead to unjust outcomes: one should also consider other crucial factors, for instance in terms of social justice, such as the fact that having large families might be a necessity for poor people to protect their fundamental interests. But if the objective is also to address social injustices, reducing population growth everywhere remains a relevant measure. Not only does high fertility perpetuate poverty by locking more people into poverty traps; it also leads to environmental degradation, which harms poor people disproportionately.18 Higher population paths could expose hundreds of million more people to climate impacts such as flooding, water stress, and drought over this century.19 These numbers would be much higher if we cross tipping points in the Earth system. For this reason, a applying sufficiency to population policies is a way of protecting basic needs, interests, or rights, both now and in the future.
2.2
How could population policies contribute to mitigating climate change and restoring biodiversity? Let us start with the individual level.20 Although most individual actions have an infinitesimal effect on the amount of GHG in the atmosphere, reproductive choices have a substantial effect on individual GHG emissions by creating a “carbon legacy.” Murtaugh and Schlax (2009) have quantified the impact of Americans’ reproductive choices on individual emissions. According to their calculations, each child an average American chooses to have will most likely offset all of the emissions reductions they have previously made, even if they make significant efforts to limit their carbon footprint. In one likely scenario, each child adds on average 9,441 metric tons of CO2 to an American’s individual emissions, a carbon legacy that represents almost six times the parent’s lifetime emissions.
Choosing to have one fewer child would save individuals in developed countries an average of 58.6 tons of CO2-equivalent (tCO2e) per year, which is considerable.21 Indeed, this action has by far the greatest effect on reducing someone’s individual carbon footprint. Other high-impact actions, such as living without a car, avoiding a transatlantic flight, and adopting a vegan diet, save respectively 2.4 tCO2e, 1.6 tCO2e, and 0.8 tCO2e per year. Actions commonly recommended by governments and public institutions, such as using a hybrid car, recycling, and using energy-efficient light bulbs, are all comparatively low-impact actions, saving less than 0.8 tCO2e annually.22
These empirical studies are based on a controversial notion: that of the carbon legacy. According to Murtaugh and Schlax, “a mother and father are each responsible for one half of the emissions of their offspring, and 1/4 of the emissions of their grandchildren”.23 A first difficulty with this notion is to find a fair
Another advantage of this alternative interpretation of the notion of carbon legacy is that it avoids the issue of the overdetermination of responsibility. Let us take three overlapping generations: generation A, B, and C. If we follow Murtaugh and Schlax’s interpretation, the responsibility for the grandchildren’s emissions (generation C) is overdetermined: their parents (generation B) are together responsible for 100% of their emissions, and their grandparents (generation A) for 50% of their emissions, leading to a total intergenerational responsibility of 150%. There is no such implication with the alternative interpretation.24
A second difficulty with the notion of carbon legacy is the uncertainty concerning the per capita emissions of children and grandchildren in the coming decades. Murtaugh and Schlax compare three possible scenarios relative to the 2005 baseline year, when global emissions were 4.31 metric tons of CO2 (tCO2) per person per year.25 The optimistic scenario is based on aggressive GHG mitigation measures that would reduce per capita annual emissions to 0.5 tCO2 by 2100; the constant scenario is based on maintaining emissions at 4.31 tCO2 per capita per year throughout the century; the pessimistic scenario assumes that no drastic measures to combat climate change will be taken over the century,
Murtaugh and Schlax base their calculation on the second scenario, which represents an intermediary between “optimistic” and “pessimistic” projections. They note that this scenario does not take into account the likely increase in global GHG emissions in the first part of the century, implying that this brings it closer to an optimistic scenario. However, this scenario also does not take into account the possibility of a gradual reduction in per capita emissions over the next decades caused by an energy transition at the global level.
Despite these difficulties with the normative and empirical assumptions on which the notion of carbon legacy relies, Murtaugh and Schlax’s approach has a double advantage. First, it gives a general idea of the impact that the decision to have a child can have in terms of additional GHG emissions in developed countries. Second, it shows that the emission reductions made possible by having one fewer child are substantial compared to most, if not all other actions. This remains the case even if one reduces the degree of responsibility of parents for their children’s and grandchildren’s emissions, and even if one chooses a more optimistic scenario for future GHG atmospheric concentrations.
2.3 The Global Level
Moving from the individual to the global level, O’Neill et al. (2010) have studied the effect of reduced population growth on future emissions, compared to the population growth projected as most likely by the dominant models. Under different scenarios of reduced population growth over the century, global GHG emissions are reduced by 16–29% by 2050, and by 37–41% by 2100. They conclude that reducing population growth would contribute significantly to the international effort not to exceed the 2°C limit for total average warming.26
Complementing the “wedge approach” by Pacala and Socolow (2004), where each wedge in a “stabilization triangle” represents 1 billion metric tons of carbon emissions avoided, Cafaro (2011, 208) calculated that holding global population to 8 billion in 2060 would provide about 3 wedges, meaning that it would prevent 3 billion metric tons of carbon from being released into to atmosphere. Reducing population growth would reduce more GHG emissions than the following five measures combined: doubling the fuel efficiency of the world auto fleet; halving the average mileage traveled per car; tripling the
Population decreases can also contribute to preserving other species or wild places and restoring biodiversity, in both developed and developing countries. Thanks to Europe’s population stabilization, within the past two decades, about 7.6 million hectares of agricultural land have gone out of production in Eastern Europe, which allowed wild species to recolonize many former agricultural areas. Ecological restoration can accelerate and lock in these trends.28 Over the past 50 years, successful national family planning programs in the Far East, South Asia, Latin America, Africa, and elsewhere have contributed to the conservation of biodiversity.29 Four major empirical studies calculated that a sustainable global population would probably not exceed 3 billion individuals.30 Even then, to be sustainable, societies would need to drastically improve existing modes of consumption and production. In addition, this number only considers Earth’s human carrying capacity; leaving enough habitat and resources to avoid a mass species extinction would drive the number for a sustainable global population even lower.
3 How to Interpret the IPAT Equation?
How have climate ethicists reacted to these more and more numerous empirical studies on the environmental impacts of population growth? I discuss here
According to Heyward (2012) and Caney (2020), population is certainly an important component of the IPAT equation, but they hold that we should focus first and foremost on the two other causes of disruption to the climate system: affluence and especially technology.
Heyward emphasizes the role of technological innovation in mitigation. Although dealing with population increase is, in her view, desirable, she focuses primarily on measures in developing countries and argues that the most important is to ensure compliance with the “collective duty to invest in technologies that could help accommodate a growing population”.31 Her main concern is that population reduction policies will lead to internalization, which holds people liable for the full costs of their reproductive choices. To avoid the risk of parents being forced to provide for their children’s welfare on their own, she favors technological solutions, such as increases in energy efficiency and in the use of renewable energy. She adds, “Another potentially beneficial form of technology is carbon dioxide removal (CDR)”.32
Caney also emphasizes technological factors such as increased energy efficiency, technology transfer, and the development of renewables and electric vehicles. He endorses the view of ecological liberalism, according to which “Persons should limit their environmental impacts in order to fulfill their responsibilities to future generations but can choose how to discharge this responsibility (selecting whether to do it through lowering consumption, enhanced use of clean technology, or limiting reproduction)”.33 This allows each person to fulfill their individual duty to reduce their carbon footprint as they see fit, in accordance with their life plan.
These positions have two main advantages: they avoid dealing with the controversial topic of population policy, which reduces their burden of justification in terms of practical solutions to climate change; and they appear to square reasonably well with basic moral intuitions in terms of individual rights and liberties. They however have two important limitations.
The first limitation is highlighted by the implications of the data discussed in the previous section. While the most recent empirical studies do mention
This brings us to the core claim of this chapter: the IPAT equation should not be used to emphasize any one of its three factors over the others; it should rather be used to fully consider each of them. Without an institutional framework to promote technological innovation, consumption reductions, and population reductions through education, regulation and incentives, we risk creating a far more dangerous world. Earth system scientists point out that such a world is not necessarily a scenario for the distant future: “the evidence from tipping points alone suggests that we are in a state of planetary emergency: both the risk and urgency of the situation are acute.” Indeed, “the intervention time left to prevent tipping could already have shrunk towards zero, whereas the reaction time to achieve net zero emissions is 30 years at best. Hence we might already have lost control of whether tipping happens”.35 In such a context of climate emergency, ecological liberalism, which allows each individual to choose their mitigation actions autonomously, is too permissive. If one is primarily concerned with preserving the basic liberties and wellbeing of individuals, it is actually necessary to rein in the excessive liberties some individuals and corporations have become used to in a context of economic deregulation to protect the rights and liberties of other individuals vulnerable to environmental impacts.
The second limitation is that these approaches do not sufficiently highlight the problems specific to technological measures. CDR, especially when deployed at large scale, poses major risks.36 First, since many CDR technologies are yet unproven, they might eventually prove technologically, economically,
Heyward is aware of (at least) some of these risks when she stresses that “such measures must be carried out in conjunction with programmes aimed at furthering global justice,”37 but there is no guarantee that global justice is compatible with CDR. Given the many ethical issues raised by large-scale CDR, this focus on technological innovation requires a stronger justification. Just like betting on automatic population stabilization, betting on new technologies that would allow humanity to keep growing indefinitely is dangerous: as recent history shows, new technological innovations are as likely to increase GHG emissions (e.g., more carbon-intensive datacenters and new fracking methods) as they are to help decrease them (e.g., wind turbines and solar panels).38
Energy efficiency also raises problems because of the rebound effect. Energy and material efficiency gains in the production, supply, or use of a commodity or service regularly lead to increases in their use or consumption. Technical innovations such as increased energy efficiency can lead users to consume more of either the same product or other products, thereby cancelling out the energy gain achieved through increased efficiency.39 The rebound effect is not systematic, and it is true that increased energy efficiency can lead to GHG emissions reductions, but this effect puts into question the idea that technological change can serve as a substitute for population policies.
Again, elements in the IPAT equation should not be considered as alternatives between which individuals can freely choose, but as complementary factors that both policies and individual behaviors should simultaneously incorporate.
4
Which population policies should be promoted to fairly address climate change and biodiversity loss? As Hickey, Rieder, and Earl (2016, 853) helpfully suggest, population policies can be located on a “coercion spectrum,” from policies with a low risk of coercion to policies with a high risk of coercion. While choice-enhancing measures such as improving education, promoting literacy, and improving women’s economic opportunities are desirable for both intrinsic (providing access to basic goods) and instrumental (reducing GHG emissions and limiting biodiversity loss) reasons, coercive policies such as forced abortions and mandatory sterilizations represent unacceptable violations of the human rights to autonomy and bodily integrity. Educational and empowerment policies are certainly part of the toolbox of measures to reduce population growth, especially as they contribute not only to climate justice, but also social and global justice by protecting crucial rights, such as the right to education of girls and women. At the same time, we cannot count solely on such measures, as they can be too slow to change fertility rates or too ineffective to motivate enough people to have fewer children. Moreover, this kind of measure has more impact in developing countries than in developed ones: fertility rates and family planning needs are higher in Asia and Africa than in the EU and the US.40
Population policies can rely on incentives to complement choice-enhancing measures. Incentives are located in the mid-range of the coercion spectrum, meaning that they can be justified, on the crucial condition that they are properly designed and implemented. Incentives can be defined as a cost or benefit aimed at deliberately changing the behavior of an agent or group of agents. Regarding population growth reduction, negative incentives discourage parents from having too many children, while positive incentives encourage them to have fewer.
Incentives are compatible with democratic societies insofar as they allow parents to make their own choices, while at the same time economically encouraging them to have small families. It is however important to distinguish between justifiable and unjustifiable incentives. Three relevant criteria are: the purpose pursued by the authority setting incentives up; the margin incentives leave for a voluntary response; and the side effects on the people who are subject to incentives.41
Other positive incentives are possible, such as reducing taxes for households with small families. In many countries, such as France, Switzerland, or the US, a pro-natalist culture determines procreative incentives, especially through taxation.44 The idea here is to reverse this incentive structure by encouraging couples to have fewer children. Making sterilization free for both men and women is also among the options to be considered. Giving a financial reward for sterilization would also be a possibility, but this measure should be implemented with caution. On the one hand, it is important not to set the amount of the reward too high, in order to avoid disadvantaged people reluctantly using it as an opportunity to obtain money. On the other hand, in a world of 8 billion people, facing dangerous climate change and mass species extinction, it is important not to set the amount of the reward too low, to avoid losing the incentivizing effect of the policy. An important point here is to respect the criterion of the margin left by incentives for a voluntary response. In general,
Negative incentives are more difficult to justify, as they incur costs rather than provide benefits. Given the scale of the population problem and the gravity and urgency of climate change and biodiversity loss, these options should however also be explored. One possible measure here would be a tax increase for families with too many children. Since larger families benefit more from tax-financed public services, including public education, it seems fair that smaller families pay less tax and larger families pay more. This is not to say that parents alone should provide for the welfare of their children: the system of social solidarity established through progressive taxation is one of the most important achievements of contemporary democracies in terms of social justice. Rather, the aim is to achieve a fair tax system in which those who benefit the most pay the most—while obviously continuing to take account of different levels of income and wealth in a progressive taxation system, to avoid further disadvantaging the least well-off. The idea here is not to “punish” parents for having more children; the idea is precisely to avoid any form of punishment by using incentives rather than coercive regulation.
A major difficulty here is determining who has “too many” children. In the context of developed countries, Overall (2013, 183) and Rieder (2016, 37) support the moral duty not to give birth to more than two children per couple, or more than one child per person. Their objective is that each person should limit themselves to the procreative replacement of their person by a new person. Conly (2016) believes that more than one child per couple would already be too many. I see three strong reasons for supporting a one-child family norm in developed countries: (1) fertility rates are often already below two births per women; (2) procreation choices have a comparatively higher environmental impact; and (3) procreation means are more widely available and more affordable. The situation is different in poor countries with high fertility rates: supporting a one-child family norm in such a context would be much less feasible (given the limited effectiveness of government policies and availability and affordability of contraceptive means) and desirable (given possible tensions with human rights). The fertility rate reductions made possible by population policies will take more time to reach the objective of a one-child family in this context. There is therefore no “one size fits all” approach at the global level.
It is however important to keep in mind that population growth reduction or even population stabilization over the 21st century will probably not be sufficient to put an end to the transgression of planetary boundaries. If we take seriously the studies that stress the need to reduce the world’s population to
5 Defending Incentive Policies
5.1 An Undue Inducement?
Incentive population policies raise many objections. I focus here on those challenges that are the most relevant within the framework of this paper. A first objection supports that encouraging people to have fewer children may reduce parental autonomy and go against their interests. Incentives are morally problematic when the benefits they provide are large enough to short-circuit individuals’ freedom of choice—this is the so-called “undue influence” argument.47
A first way to avoid this problem is to set the financial rewards at an amount that is not too high, as proposed above for the positive incentives for sterilization. But this would not be enough to avoid the objection altogether: even with a small amount, positive and negative incentives can limit the autonomy of agents by pushing them to have smaller families than they would like.
A second reply is that there is in fact already an incentive structure in place for reproductive choices. The pro-natalist culture of many developed and developing countries, as well as the institutions that support this culture,
The question, therefore, is not whether or not to incentivize, but how best to do so. In a context where population growth is contributing to the crossing of planetary boundaries, incentives for smaller families seem much more justified than incentives for larger families. All the more so if we take into account the fact that pronatalist policies are undermining the sexual and reproductive rights of women around the world, typically leading to abortion restrictions or prohibitions, and reduced access to sexual and reproductive healthcare.49
A third reply is that the kind of incentive policies supported in the previous section would not necessarily go against the interests of the parents. Empirical research shows that having fewer children, or not having any, does not reduce individuals’ quality of life.50 The life plan of many individuals includes starting a family, but people’s life plans evolve over time, not only according to autonomous choices, but also through the influence of moral and social norms and public policies. If pro-natalists norms and policies are replaced by small family norms and policies, the life plan of many individuals will evolve accordingly, without necessarily interfering with their interests—provided that population policies are properly designed and implemented so that they do not violate their individual rights. Which leads us to a second objection.
5.2 A Violation of the Individual Right to Procreate?
The idea of procreative liberty is widely held. Robertson (1996, 24) expresses this idea in the following way: “Procreative liberty should enjoy presumptive primacy when conflicts about its exercise arise because control over whether one reproduces or not is central to personal identity, to dignity, and to the meaning of one’s life.” The individual right to procreate is commonly seen as a means of protecting this individual freedom. This right is defined in the United
Raz (1984, 195) defines an individual right as follows: “‘X has a right’ if and only if X can have rights, and other things being equal, an aspect of X’s well-being (his interest) is a sufficient reason for holding some other person(s) to be under a duty.” Although having a child is not as fundamental an interest as someone’s interest in health, subsistence, or life, for many people having a child is a very strong interest. Most people wish to have one or more children, and their conception of a good life largely relies on this possibility. The right to procreate protects, among other things, the interest in having biological offspring and the interest in having a family.52 The desire to pass on one’s genes and the desire to participate in family life are two bases of the right to procreate, and any state intervention that seeks to limit this right, whether in the form of educational, incentive, or coercive policies, is often viewed with suspicion.
But what exactly does the right to procreate imply? For example, can I claim it to oppose a tax increase caused by the birth of my second child? The two fundamental interests protected by this right do not imply an absolute right to freely determine the size of one’s family. On the one hand, it is sufficient to have one child to transmit one’s genetic heritage. Having more children certainly increases the guarantee that this heritage will be transmitted over several generations, but as in the case of most rights, no absolute guarantee is given. On the other hand, it is sufficient to have one child to found a family. A flourishing family is not necessarily a large family, but a family where the relationship between parent(s) and child(ren) is based on values such as love, respect, and gratitude. As Conly (2016, 49–52) stresses, the joys of family intimacy, the unique bonds between people, the shared story are all possible with just one child.
It is true that having a (biological or adopted) child deeply changes the people who become parents and the meaning of their lives. But this does not imply that with only one child, or no child at all, these people would have no meaning in their lives, no identity, or would necessarily be unhappy. People do have a right to procreate that protects fundamental interests, but these interests can be met with one child and are therefore not jeopardized by the kind of population policy supported here.
5.3 More Children for People with Ecological Awareness?
A third objection focuses on the values transmitted by parents to their children. Population policies push people with a strong ecological awareness to have fewer children. This means there is a risk that the offspring of people with such awareness will be fewer in number than the offspring of people with no or low ecological awareness. According to this objection, values such as respect for the environment, love of nature, and mindfulness are transmitted from parents to children. For this reason, people with high ecological awareness should have more children than those with no or low ecological awareness to counterbalance the effects of people who are not concerned about the environment.54
A first response to this argument is that such behavior would contradict the values that ecologically aware parents uphold and seek to pass on to their children.55 If they want their behaviors to reflect their ethical commitments, ecologically aware people should not have multiple children, as this action contradicts their objective to reduce their overall ecological footprint.
A second response is that the argument is based on the problematic assumption that “children adopt many of the beliefs and practices of their parents”.56
The third reply draws on virtue ethics. A person who is aware of the existence of planetary boundaries and of the considerable pressure of human activities on these boundaries would not take Earth system science data lightly and would no doubt feel some responsibility. While they may well choose to have one biological child, maybe two, it is unlikely that they would choose to have more. They would probably see it as environmentally vicious behavior, showing a lack of consideration for the planet and for others. The virtue of temperance, for example, encourages people to live in relative moderation, to reduce their consumption, and to look after their carbon footprint. Given the excess of consumption and the additional GHG emissions inevitably caused by a new child, a temperate person would likely choose to have a small family and would be more likely to accept population policies.57
6 Conclusion
While population growth is not the sole cause of climate change and biodiversity loss, it contributes significantly to the increasing environmental impact of human societies. However, influential philosophical approaches continue to neglect or at least downplay the importance of the “P” component of the IPAT equation. The core claim of this chapter is that that all three components of the IPAT equation need to be addressed urgently and simultaneously. There is no justification to further avoid addressing the topic of population reduction, no matter how unpopular and controversial this topic can be. A central part of the argument for addressing population is that there is already a pronatalist incentive structure in place; that is, institutions already encourage individuals to make certain reproductive choices. There are compelling reasons in our
Increased population and affluence are two of the main drivers of climate change and biodiversity loss.58 Taking the planetary boundary framework seriously implies not only reducing population, but also reducing economic growth.59 Technological innovation and managerial efficiency will not be enough to avoid dangerous anthropogenic interference with the climate system; by themselves, they certainly cannot end massive global biodiversity loss.60 Avoiding dangerous climate change and a mass species extinction involves economic changes accompanied by ethically acceptable population policies. Sufficiency in affluence and technology are not sufficient; we also need sufficiency in population. Reducing excessive wealth and replacing unsustainable technologies with cleaner ones are certainly important measures to address climate change and biodiversity loss, but these measures will remain insufficient as long as they are not complemented by measures to reach a more sustainable global population size.
For this reason, it would be relevant for future research to articulate the IPAT equation with limitarianism, the distributive justice approach according to which “it is not morally permissible to have more resources than are needed to fully flourish in life”.61 Whereas sufficientarianism sets a lower threshold on the quantity of goods people should have to live a decent or flourishing life, limitarianism sets an upper threshold on the goods individuals should be allowed to possess. This approach would not only further justify the need to put an absolute limit on extreme wealth, linking this form of wealth with global poverty and inequalities, historical injustices, and threats to democracy.62 It would also allow governments implementing population policies and facing a
Acknowledgements
This chapter is an adapted, revised, and updated version of Bourban (2019). I am very grateful to Philip Cafaro and Lisa Broussois for their highly valuable feedback on previous drafts of the paper. I would also like to thank the editors of the volume as well as the audience of the Human Tech Colloquium at the University of Twente in October 2022 for their helpful comments and questions.
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O’Sullivan, J. (2018). “Synergy between Population Policy, Climate Adaptation and Mitigation.” In Pathways to a Sustainable Economy: Bridging the Gap between Paris Climate Change Commitments and Net Zero Emissions, edited by M. Hossain, R. Hales and T. Sarker, 103–125. Springer International Publishing.
OurWorldInData. (2022). “Per capita consumption-based CO₂ emissions, 2020.” accessed 21.11.2022. https://ourworldindata.org/grapher/consumption-co2-per-capita.
Ritchie, H. and Roser, M. (2019). “CO₂ and Greenhouse Gas Emissions.” https://ourworldindata.org/co2-and-other-greenhouse-gas-emissions.
Tamburino, L. and Bravo, G. (2021). “Reconciling a positive ecological balance with human development: A quantitative assessment.” Ecological Indicators 129: 107973. doi: https://doi.org/10.1016/j.ecolind.2021.107973.
J. Rockström et al., “A Safe Operating Space for Humanity,” Nature 461, no. 7263 (2009); W. Steffen et al., “Planetary Boundaries: Guiding Human Development on a Changing Planet,” Science 347, no. 6223 (2015); L. Persson et al., “Outside the Safe Operating Space of the Planetary Boundary for Novel Entities,” Environmental Science & Technology (2022); L. Wang-Erlandsson et al., “A Planetary Boundary for Green Water,” Nature Reviews Earth & Environment 3, no. 6 (2022). Planetary Health Check 2025: https://www.stockholmresilience.org/news--events/general-news/2025-09-24-seven-of-nine-planetary-boundaries-now-breached.html, “Seven of nine planetary boundaries now breached”.
Steffen et al., “Planetary Boundaries,” 744.
Steffen et al., “Trajectories of the Earth System in the Anthropocene,” Proceedings of the National Academy of Sciences 115, no. 33 (2018).
T. Lenton et al., “Climate Tipping Points—Too Risky to Bet Against,” Nature 575 (2019).
UN, “World Population Prospects 2022: Summary of Results,” (New York: UN DESA, 2022).
It is true that the field of population ethics, which focuses on the number, identity, and quality of life of future people (and animals), emerged in the 1980s (see especially Parfit [1986]); the impact of population growth on the environment however represents a much more recent subject of concern for philosophers.
In this perspective, it is interesting to compare Cafaro’s early review of the literature in 2012, which highlighted that “population policy has attracted relatively little attention from ethicists”, with his more recent review, which stresses that “philosophical ethicists have, over the past decade, taken up the issue of climate change and population policy and there are now numerous articles and books which explore the subject.” See Cafaro (2012, 45); Cafaro (2022, 1).
The way population growth has been overlooked in the last Summary for Policymakers of the Working Group III of the IPCC is a good illustration of this resistance: see Cafaro, chapter 12, this volume.
IPCC, “Summary for Policymakers,” in Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group Iii to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, ed. O. Edenhofer, R. et al. (Cambridge University Press, 2014), 6.
IPCC, “Summary for Policymakers,” in Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group Iii to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, ed. P. R. Shukla, et al. (Cambridge University Press, 2022), 10.
S. Dhakal et al., “Emissions Trends and Drivers,” in IPCC (2022), 4.
This is why policies to slow down economic growth should also be implemented to reduce the deterioration of the biosphere’s living conditions: see in particular the circular society model developed by Arnsperger and Bourg (2017) and the new environmental economics model supported by Laurent (2020).
J. Cohen, “Population and Climate Change,” Proceedings of the American Philosophical Society 154, no. 2 (2010); L. Jiang and K. Hardee, “How Do Recent Population Trends Matter to Climate Change?,” Population Research and Policy Review 30, no. 2 (2011).
It is important to stress that there are also important differences in per capita emissions within developed countries: not all citizens in developed countries are high GHG emitters, and there are more and more high GHG emitters in developing and emerging countries. See Ritchie and Roser (2019), OurWorldInData (2022).
P. Murtaugh and M. Schlax, “Reproduction and the Carbon Legacies of Individuals,” Global Environmental Change 19, no. 1 (2009), 18; T. Rieder, Toward a Small Family Ethic: How Overpopulation and Climate Change Are Affecting the Morality of Procreation (Springer, 2016), 31.
For a critical discussion of this type of approach, see Gardiner (2011, 443–456).
UN (2022).
J. O’Sullivan, “Synergy between Population Policy, Climate Adaptation and Mitigation,” in Pathways to a Sustainable Economy: Bridging the Gap between Paris Climate Change Commitments and Net Zero Emissions, ed. M. Hossain et al. (Springer International Publishing, 2018).
J. Dodson et al., “Population Growth and Climate Change: Addressing the Overlooked Threat Multiplier,” Science of The Total Environment 748 (2020).
This sub-section assumes that individual agents are to some degree responsible to mitigate their own emissions. It is now well recognized in the climate ethics literature that individuals have duties to at least take steps to mitigate their emissions, but climate ethics scholars still disagree on why they have such duties and the stringency of these duties (Fragnière 2016). The idea here is not to support that individuals are unconditionally responsible for their emissions, but that they have a responsibility to reduce their carbon footprint as long as this can be reasonably expected of them, based on factors such as their financial capacity and the availability and affordability of energy and transportation infrastructures; see Bourban (2022, 2023).
S. Wynes and K. Nicholas, “The Climate Mitigation Gap: Education and Government Recommendations Miss the Most Effective Individual Actions,” Environmental Research Letters 12, no. 7 (2017). Note that Wynes and Nicholas rely here on Murtaugh and Schlax (2009).
Wynes and Nicholas, “The Climate Mitigation Gap.”
Murtaugh and Schlax, “Reproduction,” 14.
Overdetermination of responsibility may be justified in some instances, but even if it were the case here, it would add a burden of justification to an argument that is already controversial enough: individuals’ reproductive choices come with a carbon legacy they have a responsibility to assume. If there is an alternative way of looking at carbon legacy that does not involve overdetermination of responsibility, then this alternative should be pursued, if only for pragmatic reasons.
Murtaugh and Schlax, “Reproduction,” 16.
These findings are confirmed by Rogelj et al. (2018, 327) and van Vuuren et al. (2018, 392), which stress the need to reduce population growth across the globe to at least 6.9 billion by 2100 if dangerous climate change is to be avoided.
Cafaro (2012, 47). Cafaro draws here on O’Neill et al. (2010). Of course, this does not mean that these other measures should not be implemented as well—with the exception of developing new nuclear power plants: see Bourban (2022). Since reducing population growth alone will not be sufficient to maintain global temperature increases well below 2°C, these additional actions are also important. However, they do not have an emission reduction effect comparable to reducing population growth, unless they are all implemented together. As the next section explains, all these measures should be implemented quickly and jointly.
Cafaro, Hansson, and Götmark “Overpopulation.”
W. Robinson and J. Ross, The Global Family Planning Revolution (The World Bank, 2007); R. Engelman and D. Johnson, “Removing Barriers to Family Planning, Empowering Sustainable Environmental Conservation: A Background Paper and Call for Action,” in Margaret Pyke Trust (2019).
T. Lianos and A. Pseiridis, “Sustainable Welfare and Optimum Population Size,” Environment, Development and Sustainability 18, no. 6 (2016); C. Tucker, “A Planet of 3 Billion,” Atlas Observatory (2019); P. Dasgupta, Time and the Generations: Population Ethics for a Diminishing Planet (Columbia University Press, 2019); L. Tamburino and G. Bravo, “Reconciling a Positive Ecological Balance with Human Development: A Quantitative Assessment,” Ecological Indicators 129 (2021).
C. Heyward, “A Growing Problem? Dealing with Population Increases in Climate Justice,” Ethical Perspectives 19, no. 4 (2012), 707.
Heyward, “A Growing Problem?”, 722.
S. Caney, “Human Rights, Population, and Climate Change,” in Human Rights and 21st Century Challenges: Poverty, Conflict, and the Environment, ed. D. Akande, et al. (Oxford University Press, 2020), 359.
Steffen et al., “Trajectories.”
Lenton et al., “Climate Tipping Points,” 595.
See e.g., K. Dooley and S. Kartha, “Land-Based Negative Emissions: Risks for Climate Mitigation and Impacts on Sustainable Development,” International Environmental Agreements: Politics, Law and Economics 18, no. 1 (2018).
Heyward, “A Growing Problem?” 732, n17.
Cafaro, “Climate Ethics and Population Policy.”
F.-D. Vivien, “Effet-Rebond,” in Dictionnaire De La Pensée Écologique, ed. D. Bourg and A. Papaux (PUF, 2015).
C. Hickey, T. N. Rieder, and J. Earl, “Population Engineering and the Fight against Climate Change,” Social Theory and Practice 42, no. 4 (2016), 855.
R. Grant, “Rethinking the Ethics of Incentives,” Journal of Economic Methodology 22, no. 3 (2015), 360.
A. Haakenstad et al., “Measuring Contraceptive Method Mix, Prevalence, and Demand Satisfied by Age and Marital Status in 204 Countries and Territories, 1970–2019: A Systematic Analysis for the Global Burden of Disease Study 2019,” The Lancet 400 (2022). This study focuses on unmet contraceptive needs for women; it is however also possible to calculate unmet needs by focusing on men or couples; see Stan Becker, “Measuring Unmet Need: Wives, Husbands or Couples?,” International Family Planning Perspectives 25, no. 4 (1999).
Conly, One Child, 117.
This does not mean that in pro-natalist cultures people make children because of tax benefits; it simply means that, in such cultures, there is a stronger economic and social pressure, especially on women, to make children.
Lianos and Pseiridis, “Sustainable Welfare and Optimum Population Size”; Tucker, “A Planet of 3 Billion”; Dasgupta, Time and the Generations: Population Ethics for a Diminishing Planet; Tamburino and Bravo, “Reconciling a Positive Ecological Balance with Human Development: A Quantitative Assessment.”
Rogelj et al., “Scenarios Towards Limiting Global Mean Temperature Increase Below 1.5°C,” 327; van Vuuren et al., “Alternative Pathways to the 1.5°C Target Reduce the Need for Negative Emission Technologies,” 392.
Hickey, Rieder, and Earl, “Population Engineering and the Fight against Climate Change,” 865–66.
T. Rieder, “Procreation, Adoption and the Contours of Obligation,” Journal of Applied Philosophy 32, no. 3 (2015).
M. Scigliano, “Welcome to Gilead: Pronatalism and the Threat to Reproductive Rights,” (Population Matters, 2021).
Hickey, Rieder, and Earl, “Population Engineering and the Fight against Climate Change,” 866.
UN, “Final Act of the International Conference on Human Rights: Teheran, 22 April to 13 May 1968,” ed. United Nations (1968), 4.
Conly, Only One Child, 39.
On the notion of “interspecies injustice”, see Wienhues (2020).
S. Wisor, “Is There a Moral Obligation to Limit Family Size?” Philosophy & Public Policy Quarterly 29, no. 3/4 (2009): 29.
Overall, Why Have Children? The Ethical Debate, 190–91.
Wisor, “Is There a Moral Obligation to Limit Family Size?” 29.
Rieder, Toward a Small Family Ethic, 58. Note that the first two responses to the objection also rely to some extent on virtue ethics. They draw on the virtue of integrity, which requires that we integrate our deep ethical commitments into our everyday actions. Another issue here is the one of measuring ecological awareness. One could argue that one’s ecological footprint could serve as a proxy for one’s degree of ecological awareness, but people with high ecological awareness may have very little control over their carbon footprint, just like people with low ecological awareness may have a small carbon footprint for other reasons. Virtue ethics allows to avoid this difficulty by focusing on the kind of motivation that drives people to have children or not to have them.
IPCC, “Summary for Policymakers”; Cafaro, Hansson, and Götmark, “Overpopulation Is a Major Cause of Biodiversity Loss.”
E. Crist et al., “Protecting Half the Planet and Transforming Human Systems Are Complementary Goals,” Frontiers in Conservation Science 2 (2021); H. Daly and J. Farley, Ecological Economics, Second Edition: Principles and Applications (Island Press, 2011).
W. Ripple et al., “World Scientists’ Warning of a Climate Emergency 2022,” BioScience 72, no. 12 (2022).
I. Robeyns, “Having Too Much,” in NOMOS LVI: Wealth. Yearbook of the American Society for Political and Legal Philosophy, edited by J. Knight and M. Schwartzberg (New York University Press, 2017), 1.
I. Robeyns, Limitarianism: The Case Against Extreme Wealth (Astra House, 2024).