The Bad Environmental News and Its Fundamental Cause
Recent years have brought a cascade of bad environmental news from around the world: melting glaciers and acidifying oceans; fires of unprecedented size and intensity; unusually numerous and severe tropical storms; record-breaking droughts; dying coral reefs and boreal forests; massive bird losses and insect die-offs; and much more. The news is grim and the trends suggest worse to come.
While the details and proximate causes vary, the underlying cause seems clear enough: an immense and rapidly growing human economy, serving the needs and wants of unprecedented numbers of people (Rees 2020). The bad environmental news combined with the past hundred years’ population explosion — from two to more than eight billion human beings — support a prima facie argument for global overpopulation. Yet many environmentalists resist this conclusion, preferring to define Earth’s human carrying capacity with reference to a suite of optimistic technological and managerial reforms (Richie 2024).
Given people’s lack of enthusiasm for limiting our numbers or our per capita economic demands, such an approach is tempting. But it is morally problematic to define a sustainable population based on ambitious policy reforms that might not happen. Given the potential dangers involved, a reasonable precaution instead demands we base such judgements on the consumption levels and production systems currently in place, or something close to them. After all, reforming these is difficult and current per capita environmental impacts are increasing, not decreasing (Figure 1).
Figure 1. Current status of indicators for nine planetary boundaries for safe human use of the biosphere, showing six of the nine boundaries transgressed (entering the red zone). Licensed under CC BY-NC-ND 3.0. Credit: "Azote for Stockholm Resilience Centre, based on analysis in Sakschewski and Caesar et al. 2025". Click on the image to enlarge.
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A New Definition of Overpopulation
Some environmentalists claim that current environmental problems can be addressed successfully without reducing human numbers, while others disagree. We need a plausible framework for deciding who is correct. Since judgements regarding population matters necessarily involve both ethical principles and empirical scientific claims, a useful definition of overpopulation must make both aspects explicit (Coole 2018).
Harming our descendants by degrading essential ecosystem services appears to be an important and preventable evil on any defensible approach to ethics (Rolston 2020). Philosophers employing a wide variety of ethical approaches have argued that extinguishing numerous other species is also an important and preventable evil (Nussbaum 2024). Stipulating the truth of these two ethical principles — it is wrong to seriously degrade future human generations’ necessary ecological support systems; it is wrong permanently to extinguish numerous other species — a working definition of overpopulation follows:
Human societies, or the world as a whole, are overpopulated when their populations are too large to preserve the ecosystem services necessary for future people’s wellbeing or to share the landscape fairly with other species.
The scientifically verifiable aspects of overpopulation come into play in a number of ways. These include which ecosystem services are necessary for societies to function well and whether they are being sustained; whether and how other species are being displaced or extinguished; and whether current environmental impacts can be successfully reduced solely through behavioral restraint or technological improvements (Stephens et al. 2023). Such empirical questions run from the relatively straightforward and fully proven to the highly speculative. Their answers always come with some degree of uncertainty; still, we must answer them as well as we can and use those answers to intelligently discuss what a sustainable population might be.
Combining these ethical and scientific aspects, we may stipulate a formal definition of global overpopulation.
The world is overpopulated if:
(1) People are degrading essential global ecosystem services in ways that could seriously harm current and future human generations; or
(2) People are displacing wild animals and plants so thoroughly that we threaten to cause a mass extinction event, permanently extinguishing a large percentage of Earth’s species; and
(3) Avoiding (1) or (2) (or both) would become significantly more likely with a smaller global human population.
Note that criteria (1) and (2) are a disjunction. Although there could be compelling scientific evidence for both (1) and (2), either, in conjunction with (3), should be enough to ground a judgment that Earth is overpopulated. That is because they both involve gross injustice: of current people against future human generations, or of humans against other species. Including (1) and (2) as possible bases for a charge of overpopulation is meant to accommodate both anthropocentric and less solipsistic ethical views.
Note that criterion (3) asks us to make a pragmatic judgment about political action in the real world, rather than demand certainty, or ask what might be possible in an ideal setting. In this it differs from much recent work by philosophers attempting to specify an optimal global population (e.g. Greaves 2019) and by scientists attempting to specify a maximal one (e.g. Bradshaw et al. 2026). Such attempts, engaging numerous complicated practical and theoretical issues, typically end in uncertainty and calls for “further study.” They thus provide no practical guidance regarding actual population policies.
In contrast, my proposed definition avoids unnecessary complexity yet preserves a solid grounding in ethics and science. It defines overpopulation pragmatically, without reference to optima or maxima (or the related concept of carrying capacity). Instead, it proposes that if there is convincing evidence that humanity is rapidly degrading the global environment and that fewer people would help us decrease the damage we are doing, then we are overpopulated.
An alternative formulation for criterion (3) might be this: avoiding severe ecological degradation or mass extinction (or both) is only possible with a smaller global human population. This approach to defining overpopulation is common, but it is not sufficiently precautionary. Because the costs of failure are so steep, we need population policies that enhance the likelihood of our creating just and sustainable societies, not merely policies that are compatible with these goals in theory.
After all, we cannot prove that successfully mitigating devastating climate change or averting mass species extinction must involve smaller human numbers. Who knows what technological or social changes may happen in the future? What we do know, today, is that the environmental news is not good. Arguably, the imminent threat of ecological overshoot causing vast harms and grave injustices demands greater care, humility and precaution than modern societies have shown up until now in their environmental policies. Hence a probabilistic and reality-based criterion (3).
Applying the Definition Globally
This is an abridged version of the original paper “A New Definition of Global Overpopulation, Explained and Applied” (Cafaro 2026). Having developed the preceding definition of global overpopulation in the first half of that paper, I go on to apply it in the second half, asking whether humanity currently is overpopulated at 8.2 billion people. The analysis focuses on the two defining global environmental problems of our time, climate change and biodiversity loss. It also considers global ecological overshoot generally, in the context of the planetary boundaries framework (Figure 1). I conclude that given current systems of economic production and consumption, the direction they are trending, and the grave existential threats these pose to humanity and other species, humanity is indeed overpopulated at 8.2 billion people. We thus have a moral obligation to take steps to reduce our numbers (Cafaro 2022).
The approach to defining overpopulation laid out here is cautious, non-ideological, and reality-based for a reason. The reason is that life is good. Current generations owe it to our children and grandchildren to pass on the necessary means to enjoy their lives: a healthy, flourishing biosphere. We also owe it to all the other species whose continued existence depends on human restraint.
Overpopulation threatens great suffering for billions of people and extinction for millions of species. These facts justify humane efforts to reduce human numbers, as a matter of justice between current and future generations, and between people and other species. Addressing population is only part of creating just and sustainable societies, of course (Crist et al. 2021). But it is a necessary part. While taking up population matters can be contentious and challenging, continuing to ignore them will likely prove much worse.
References
Bradshaw, C. et al. 2026. Global human population has surpassed Earth’s sustainable carrying capacity. Environmental Research Letters 21: 064023.
Cafaro, P. 2022. Reducing human numbers and the size of our economies is necessary to avoid a mass extinction and share Earth justly with other species. Philosophia 50: doi 10.1007/s11406-022-00497-w.
Cafaro, P. 2026. A New Definition of Global Overpopulation, Explained and Applied. Journal of Population and Sustainability 10: 17-51.
Coole, D. 2018. Should We Control World Population? Cambridge: Polity Press.
Crist, E. et al. 2021. Protecting half the planet and transforming human systems are complementary goals. Frontiers of Conservation Science 2: 761292.
Greaves, H. 2018. Optimal population size. In G. Arrhenius, K. Bykvist and T. Campbell (eds.), Oxford Handbook of Population Ethics Oxford University Press.
Nussbaum, M. 2024. Justice for Animals: Our Collective Responsibility. New York: Simon & Schuster.
Rees, W. 2020. Ecological economics for humanity's plague phase. Ecological Economics 169: 106519.
Richardson, K. et al. 2023. Earth beyond six of nine planetary boundaries. Science Advances 9: eadh2458.
Ritchie, H. 2024. Not the End of the World: How We Can Be the First Generation to Build a Sustainable Planet. New York: Little, Brown, Spark.
Rolston III, H. 2020. A New Environmental Ethics. London: Routledge.
Stephens, J. et al. 2023. The dangers of mainstreaming solar geoengineering: A critique of the National Academies report. Environmental Politics 32: 157–166.
