Reconciling human demands with planetary boundaries

By Lucia Tamburino | 7 September 2022
The Overpopulation Project


Human population grows, Earth doesn’t. A certain environmental impact is needed to satisfy basic human needs. When multiplied by a huge population, even a small average individual impact can become high, overshooting planetary boundaries. Achieving both environmental sustainability and high human development is hence a challenge. Can we win this challenge without considering the population factor?

To be sustainable, humanity should not consume in a year more resources than our planet is able to produce yearly. Human population grows, the planet doesn’t.

It’s like we had to share a cake every year among more and more people. Of course, it’s important that the cake is fairly distributed: if some people take too much for themselves, there is not much left for all the others. But even if it is perfectly equally distributed, the single slice of cake to which each is entitled becomes smaller and smaller when population grows. Eventually, it can happen that the slice becomes not bigger than a few crumbs.

In a recent work titled “Reconciling a positive ecological balance with human development: A quantitative assessment,” published in the scientific journal Ecological Indicators, Giangiacomo Bravo and I estimated the minimum “slice of cake,” namely the minimum environmental impact that each individual must have in order to achieve a good level of human development [1]. We called this threshold τ and we estimated it by relating country-level ecological footprint (global hectares per capita, gha/cap) with the UN’s Human Development Index (HDI). The UN regards a HDI of 0.7 as providing a level of high development, while 0.8 represents a very high level. Fitting a curve to the most recent data, we found that an HDI of 0.7 corresponds to 2.14 gha/cap. This is higher than the currently available planetary biocapacity of 1.6 gha/cap, meaning that even perfect redistribution could not sustainably lift everyone out of poverty, using our current technology mix. Technology plays an important role because it helps to exploit resources efficiently: if we improve technology in the future, we could conceivably meet human needs with fewer resources, bringing τ down. Nevertheless, technology is technology, it’s not magic: it can diminish our impact, but it cannot reduce it to zero [2,3,4]. When multiplied by a huge population, even small individual impacts lead to a high total impact, which can lead to overshooting planetary boundaries.

The question is: going forward, is achieving environmental sustainability possible by focusing only on minimizing individual impacts? Or must limiting population play a role too?

In our work, my colleague and I addressed exactly this question. More specifically, we combined the threshold τ with a new ecological indicator – the eco-balance. Eco-balance is the extent to which the biocapacity of a country, region or the world exceeds the sum of all its people’s eco-footprints. That is, if eco-balance is negative, the population is currently unsustainable.

Would humanity be sustainable if resources were distributed equally among all the people of the world and bringing the impact of each person to the threshold τ needed for a decent quality of life? By analysing data, we found the answer is no, it wouldn’t. Even under this ideal and idyllic scenario, the total impact of the whole population would overshoot planetary boundaries for sustainable use of the biosphere (see Figure 1). The eco-balance of the Earth would significantly improve compared with the current unequal use of resources, which shows that reducing individual impacts is certainly important and useful. Nevertheless, it would not be sufficient. The eco-balance of the Earth would improve, but still remain negative. To make it positive, we could reduce our individual impacts even more, but this would go below the threshold τ, undermining human welfare. To reconcile both, human development and ecological sustainability, we also need to reduce the global human population.

The good news is that we can reduce population without coercion. Unfortunately, there is great ignorance on the topic: many people equate population policies with coercion because they only know the example of the one-child policy in China, or the forced vasectomies in India in 1975-76. But actually there are many counterexamples of population policies that were both effective and voluntary: from Indonesia to Iran, from Thailand to Tunisia, and in many other countries. Not only were they able to reduce fertility rates without violating human rights, but they also improved living conditions, especially for women and children [5,6,7].

The bad news is that demographic policies cannot provide a quick fix. If fertility rates immediately dropped below replacement rate, that could help solve all our environmental problems in the medium to long term. However in the short term, population would keep increasing, due to demographic momentum (due to disproportionately large numbers of young people who will grow up and form families). This means that whatever we do, we must be prepared to face at least a transition period with a large and increasing population. Therefore, we need to take all the possible countermeasures to limit environmental damages and catastrophic consequences. How to face this period is an interesting issue, but it is outside the scope of our work.

In our paper, we also performed sustainability analyses at the country level and what we found is pretty interesting. Forget the stereotype of poor countries that are ecologically sustainable and rich countries that are not: reality is more complex and nuanced. As you can see in Figures 2 and 3, there are also rich countries that are sustainable and poor countries that aren’t, something that has been recently highlighted also in another work [8]. Moreover, a relatively poor country like India has an eco-balance that is even worse than the eco-balance of US, and poor countries like Iraq and Jordan are among the countries with the worst eco-balances overall, together with the richest countries of the Arabian Gulf. This suggests that population plays an important role in sustainability.

Depending on their situations, countries would need to emphasize different things in order to reconcile ecological sustainability and human development (Figure 4). For example, low-income and middle-income countries cannot ethically reduce their individual impact because in many cases it is already under the threshold τ. The best way they have is hence to reduce their populations, which can result in a double positive effect: reducing environmental impact and increasing human well-being.

Rich countries like the Arabian Gulf oil states, the US, Japan and European countries can instead reduce their individual impacts by decreasing consumption and/or adopting better technologies. However, for many of them, this would not be sufficient to become environmentally sustainable, unless they crossed the threshold τ. This means that they also need to reduce their populations to become sustainable.

The good news is that in many rich countries, fertility rates are below replacement rate, which implies that those countries are spontaneously heading into a population decline in the future (unless immigration prevents that). In some of these countries, the population has already begun to stabilize or slightly decline. The bad news is that this decline is usually perceived as a problem, and instead of enhancing this trend, politicians try to counter and reverse it. Yes, it is true that a population decline, like any change, requires some effort to adapt to it. But this change could prove relatively easy to manage without dramatic costs. For example, Japan is still one of the richest and most innovative countries in the world, even though its population has been (slightly) decreasing for more than 10 years [9].

Let us conclude by noting that the numeric results of our work depend on the specific estimate of τ, or acceptable per capita environmental demands. With lower values for τ, less population reduction would be required to achieve sustainability (the smaller the slice, the more people can eat the cake). However, our proposed τ threshold is already lower than the world average, and it is much lower than the per capita impact for all the Arabian Gulf countries, the US, all countries in Europe and almost all countries in Latin America, and even some middle-income country in Africa, such as Botswana and Namibia [10]. Realistically acceptable values for τ are probably higher, not lower.

Future technological advances may reduce the per capita impact needed to achieve high levels of human development, leading to lower values for τ. However, technology can be a double-edged sword, having unintended ecological impacts. Nevertheless, population is projected to grow in the future, which means that new technologies, that could be enough to achieve sustainability today, may be not enough tomorrow, with a larger population. The main message of our work is that there are no independently sustainable levels of population, technology use, or per-capita consumption, but only sustainable combinations of technology, consumption, population, and available biocapacity. With a huge population, we will be very unlikely to reconcile ecological sustainability and human development, even with smart technology.

To sum up, possible combinations are:

  1. Large population and high human development → then we cannot be ecologically sustainable and this situation cannot last
  2. Large population and ecological sustainability → then a large part of the population cannot achieve a high level of human development.
  3. High human development and ecological sustainability → then population needs to diminish.

The fourth combination – large population, ecological sustainability and high human development – is not an option on a finite planet.


[1] L. Tamburino, G. Bravo “Reconciling a positive ecological balance with human development: A quantitative assessment.” Ecological Indicators, 129 (2021), p. 107973.

[2] R. Goodland “The concept of environmental sustainability.” Annual Review of Ecology and Systematics, 26 (1995), pp. 1-24

[3] K.W. Knight, E.A. Rosa “The environmental efficiency of well-being: A cross-national analysis.” Social Science Research, 40 (2011), pp. 931-949, 10.1016/j.ssresearch.2010.11.002 URL:

[4] V. Smil Grand Transitions: How the Modern World Was Made. Oxford University Press, Oxford (2021)

[5] C. Wolf, W.J. Ripple, E. Crist “Human population, social justice, and climate policy.” Sustainability Science (2021), pp. 1-4

[6] J.N. O’Sullivan “Synergy between population policy, climate adaptation and mitigation.” Pathways to a Sustainable Economy, Springer (2018), pp. 103-125

[7] G . Sedgh, L.S. Ashford, R. Hussain “Unmet need for contraception in developing countries: examine women’s reasons for not using a method.” Technical Report. The Guttmacher Institute (2016)

[8] M. Wackernagel, L.Hanscom, P. Jayasinghe, D. Lin, A. Murthy, E. Neill, and P. Raven. “The importance of resource security for poverty eradication.” Nature Sustainability (2021): 1-8.

[9] Dutta, S., Lanvin, B., Wunsch-Vincent, S., 2020. “GLOBAL INNOVATION INDEX 2020 Who Will Finance Innovation?” Technical Report. Cornell University, INSEAD, and WIPO (World Intellectual Property Organization).

[10] Global Footprint Network. “Ecological Deficits and Reserves.”

Reprinted with permission from Frank Götmark – Project leader of The Overpopulation Project (TORP); Professor, Animal ecology and Conservation Biology, University of Gothenburg.

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