Mahatma Gandhi said: “Earth provides enough to satisfy every man’s need but not every man’s greed.” What is then the “right” lifestyle, which we all should adopt to be sustainable? It depends on how many people there are on the Earth. Sustainability does not depend only on lifestyle, but also on how many people share the biocapacity of the land. Here, the ecological footprint is used to calculate how humanity could live sustainably on different imaginary Earths with different population densities.
By Lucia Tamburino
The ecological footprint (hereafter EF) was introduced by Mathis Wackernagel in the 1990s as a measure of people’s total environmental impact . It takes into account the impact of all the goods consumed in a given region, including imports. The EF is directly comparable with biocapacity, as it is expressed in the same unit: gha (global hectare). Therefore, it provides a simple criterion to understand whether a country is sustainable: if its total EF (per capita EF x population) is below its biocapacity, the country is ecologically sustainable, otherwise not.
The EF is not an indicator of the wealth of a country or its citizens, but they are correlated. For this reason, it is often used in an interesting mental exercise proposed during Earth overshoot day , which helps to understand the impact of human lifestyles and the ecological limits of our planet: fix the population, take the per capita EF of a country of your choice, then calculate the number of Earths that would be necessary to achieve sustainability with that EF. This exercise allows us to answer the question: “How many Earths would we need to be sustainable, if all the people lived like… Norway? Rwanda?” or whatever country.
Another useful mental exercise is this: fix the Earth, take the population density of a country of your choice, then calculate the per capita EF that would be needed in order to achieve sustainability with that density. This allows us to answer the question: “How would we need to live to be ecologically sustainable, if the whole world had the population density of… Norway? Rwanda?” or whatever country. Despite its similarity with the previous exercise, I have never seen this one proposed anywhere, until now.
I just introduce a variation on the definition of population density: instead of considering the number of people per physical hectare in a country, I will consider the number of people per global hectare. This is a measure of how many people share the biocapacity of the country, expressed in biologically productive hectares (global hectares). I call this measure population biodensity. The reason for this choice is that global hectares are more suitable than physical hectares to express the capacity of a country to sustain its people. In table 1 and the other examples below, I use data from the Global Footprint Network, updated to 2016.
Imagine the world having the same population biodensity as Rwanda
If the whole world had the population biodensity of Rwanda, about 26.77 billion people would inhabit our planet. To be sustainable, the average per capita EF in this world could be at most 0.45 gha per person (see table 1).
Is it possible to survive with such a low per capita EF? Currently, there is no country in the world with such a low per capita EF, not even the poorest ones, like Congo, Mozambique or Yemen (respectively 0.7, 0.8, and 0.7 gha per person). Unless there were extraordinary (and probably unrealistic) advances in technology, people would be very poor, likely living in houses without electricity and without warm water, eating only cereals every day.
Even so, it would be hard to satisfy the food demands of almost 27 billion people with the current 4.9 billion hectares of cultivated land [2,3], even if it was entirely dedicated to food (and hence excluding cotton, biofuels, tobacco, etc.). Agriculture would have to be highly intensive, implying a large use of fertilizers, pesticides and GM varieties, and people would have to adopt a plant-based diet. A further expansion of agricultural land would likely occur, at the expense of natural ecosystems, with huge losses of biodiversity and ecosystem services, and a worsening of climate change. The last tropical forests would probably disappear. Many species of animals and plants that are already threatened would be extinguished, and many other would become threatened. Agriculture would face serious pest species – natural enemies that regulate them would be gone. The increase of water consumption would lead to a reduction of water reserves, which in many places are already overexploited . The consequent water scarcity would compromise agriculture productivity and hence food security, causing conflict, starvation and massive migrations [5, 6, 7].
Imagine the world having the same biodensity as Norway
If the whole world had the population biodensity of Norway, around 1.70 billion people would inhabit our planet. People could have an average per capita EF of 7.27 gha and humanity would still be sustainable.
No poverty, no malnutrition. Under current technology, there would be enough resources to ensure a comfortable life for everyone, without damaging the planet. Riots and violence are in a large extent caused by resource scarcity and rising food prices . Those would disappear, enhancing the conditions for a potentially more peaceful world.
Without dietary restrictions, less than 1 billion hectares of cultivated land would be enough to ensure food security. Deforestation would almost certainly stop, as it is driven mainly by agricultural expansion . The extension of the total cultivated land would depend on several factors; mainly on the extension of land dedicated to modern biofuels. Assuming a billion hectares for biofuels, we can estimate around 2 billion hectares of cultivated land. Thus, almost 3 billion hectares would be released for natural ecosystems, reversing recent trends of biodiversity decline. Forests in subtropical areas would recover, halting desertification. Crucial and sensitive ecosystems, like rainforests in Africa, Asia and Latin America, would also recover. All these increasing forests would mitigate climate change . Children could grow up happily and safely, with fewer threats in environments with less pollution. They could see and enjoy the beauty of nature.
Imagine something in between
Currently, we are 7.7 billion people, with a biodensity of 0.63 people per gha. To be sustainable at this biodensity, our per capita EF would have to be 1.7 gha, less than in Ghana, Botswana, Azerbaijan and other low/medium-income countries. Should we be all poor? People do not want this. Indeed, people recognize that developing countries must improve their material conditions. But this implies that their environmental impacts will increase.
Even under optimistic assumptions (perfect resource distribution, no greed, advanced technology), to ensure a decent life for all, we must achieve a per capita EF no lower than 2 and perhaps up to 3 gha per person (own estimates, unpublished data). Note that 2 gha per person is a very low value, less than in Namibia: Gandhi would not call someone greedy with a similar EF. Still, to be sustainable with this per capita EF (i.e., to limit our impact below the biocapacity of the Earth), we would have to be at most 6.08 billion (see fig. 1). Now we are 7.7 billion, and growing.
We do not need to reduce population as much as in the global-Norway scenario to achieve sustainability; something in between is enough. However, as shown in fig. 1, at least a certain degree of population decrease is necessary to achieve sustainability and ensure a decent life for all. Sustainability depends on a combination of consumption, technologies employed, and population numbers. It’s possible to be rich and unsustainable like the UK, or poor and sustainable like Congo. Contrary to Gandhi’s statement, it is also possible to be poor and unsustainable like Rwanda, or rich and sustainable like Norway. It is impossible to be rich, sustainable and—at the same time—highly densely populated. The numbers are clear. Any sustainable, acceptable and realistic scenario requires a stabilization (first) and then decrease of population, at the global level.
People advocating for ending population growth and fewer people globally are neither misanthropes cheering human extinction, nor racists who want to blame the poor for unsustainability, as we are often accused. We are people with a vision – a well-motivated vision of a sustainable, peaceful world that would be pleasant for all, from Norway to Rwanda and in every other country.
Moreover, we might achieve this world voluntarily, without coercion [10,11]. It would require time. During the transition, population would continue to grow due to demographic momentum. In any case, humanity must face serious environmental problems and dramatic resource scarcity, at least for a while: this is inescapable. Humanity could fail. If it fails, population would decrease anyway, not through a reduction of fertility rates, but through an increase of mortality rates, which is much worse, because it implies conflict, starvation, mass migration and other catastrophes. This nightmare scenario is a real possibility, even if today it is trendy to “think positive” and to selfishly ignore the risks that we are placing on future generations.
But, if humanity does succeed, our grandchildren could live under the best conditions that humanity has ever experienced in all its history. All the technology, facilities and advances that human progress has been able to achieve, in a sustainable, peaceful world with flourishing nature.
Can you imagine it? I can. You may say I’m a dreamer. But I’m not the only one.
 Wackernagel, Mathis, and William Rees. Our ecological footprint: reducing human impact on the earth. Vol. 9. New Society Publishers, 1998.
 FAO, The future of food and agriculture trends and challenges, Report, Food and Agriculture Organization of the United Nations (FAO) (2017).
 Dubois, Olivier. The state of the world’s land and water resources for food and agriculture: managing systems at risk (chapt. 1). Earthscan, 2011.
 Varis, Olli, Marko Keskinen, and Matti Kummu. “Four dimensions of water security with a case of the indirect role of water in global food security.” Water Security 1 (2017): 36-45.
 Abel, Guy J., et al. “Climate, conflict and forced migration.” Global environmental change 54 (2019).
 Brown, Lester R. “Could food shortages bring down civilization?.” Scientific American 300.5 (2009): 50-57.
 Diamond, Jared. Malthus in Africa: Rwanda’s Genocide in Collapse: How societies choose to fail or succeed (chapt. 10). Penguin, 2005.
 Natalini, Davide, Aled Jones, and Giangiacomo Bravo. “Quantitative assessment of political fragility indices and food prices as indicators of food riots in countries.” Sustainability 7.4 (2015).
 Bastin, Jean-Francois, et al. “The global tree restoration potential.” Science 365.6448 (2019).
 Weisman, Alan. Countdown: Our last, best hope for a future on Earth?. Hachette UK (2013).
 Robinson, W. C., Ross, J. A. The global family planning revolution: three decades of population policies and programs. The World Bank, 2007.