Ivana Abramovic

Peter Rindt

The Scientific definition of sustainability

For a while now we have been trying to make sense of the question: What is sustainability exactly? After all, without a clear measurable definition, how can we ever know if we have done enough to make our society sustainable? Or too little? Would it even be possible to do too much? Much has been written about what sustainability entails: for example, the Sustainable Development Goals by the United Nations [1], the Doughnut Economy framework by Kate Raworth [2], and the concept of ecological footprint [3]. In this article, however, we dive into the science underneath.

A scientific definition

The science of sustainability is extensive and ever expanding and improving. It is vast. The 5th IPCC report attempts to summarize all relevant science, and that report alone consists of 4 parts totaling over 4000 pages! Yet, in all this work there is one definition that stands out:

Sustainable development is:

“Development that meets the needs of the present without compromising the ability of future generations to meet their own needs [4].”


“Development within the boundaries where humanity can safely operate.”

The first definition was formulated by the Brundtland Commission in 1987 [4]. We reformulated this definition into a version that fits closely with the work of Johan Rockström and his team, which we will discuss next. Essentially both these definitions say that human life should be sustained. Also, they both focus on a human point of view, which is not surprising if you consider that nature will probably be better off without any humans around.

Is this definition measurable? Well… sort of. We just need to answer one more question: what does it take to sustain humanity? Or, alternatively formulated: what are the boundaries within which humanity can safely operate?


The Earth in comfort mode

To understand the definition of sustainable development, it is important to understand another crucial concept: we must think of the Earth as a system (the Earth System). Simply imagine the Earth as a fancy car. Such a car has different operating modes: “comfort” or “sports” for example. You can change mode at a click of a button. The steering of the car suddenly responds differently, more gas is injected if you push the pedal, and if you take a speedbump it feels different because the stiffness of the suspension has been changed.

The Earth, and everything on it, can also operate in different modes, though, they cannot be changed so easily. In fact, there are usually many millions of years in between each mode change of the Earth. This may sound strange, but you are probably familiar with ice ages, and pre-historic ages right? In each age the Earth works in a different mode. During an ice age ocean and air currents flow differently, more light is reflected by ice caps, and forests and deserts exchange humidity with the air in different places. There is an entirely different climate.

Currently, we are in an age (mode) called the Holocene, which is the period of roughly the last ten thousand years. From a human point of view, the Holocene = comfort mode. During this period, the global temperature is remarkably constant. This has allowed for plants, animals, and civilizations to thrive (after all predictable weather is pretty essential for agriculture and feeding billions of people). Currently, human activity is threatening to push us out of the Holocene mode, and into another unknown mode. There is a high risk that in this new mode is not so comfortable, and that 7 billion people may no longer be able to survive.

Figure 1: Top, temperature of the polar ice cap surface. Note that the temperature has been remarkable stable for the last 10 thousand years. This is the “comfort mode” which has allowed modern civilization to thrive. This period is officially know as the Holocene, also indicated in the bottom part. Top image adapted from dandebat.dk. Bottom image adapted from the original by Ray Troll.

Figure 1: Top, temperature of the polar ice cap surface. Note that the temperature has been remarkable stable for the last 10 thousand years. This is the “comfort mode” which has allowed modern civilization to thrive. This period is officially know as the Holocene, also indicated in the bottom part. Top image adapted from dandebat.dk. Bottom image adapted from the original by Ray Troll.

Measurable limits

So, what are the boundaries within which humanity can safely continue to develop? What are the limits to the Earth’s comfort mode? The team of Johan Rockström and Will Steffen propose 9 limits: the planetary boundaries [5,6,7].  These planetary boundaries are the limits within which our climate and biosphere are stable (the biosphere includes all living beings). Most of these boundaries are also linked. For example, we must keep the flows of nitrogen and phosphorus (widely used in crop fertilizer) within bounds, because they can have a negative effect on biodiversity. The same is true for land system change. If too much land is changed from natural habitats into farmland for example, biodiversity decreases. The biosphere in turn is closely linked to the climate. If the Earth’s ecosystems largely disappears, the climate will change drastically, and vice versa.

In addition, Erling Holden and colleagues propose to add two societal limits [8]. These are limits in which our society itself is stable. Limits to inequality are desirable to reduce conflict [9]. A minimum level of human development is desirable because people often don’t worry about sustainability when they are primarily concerned with putting food on the table [10,11]. Moreover, both are simply the right thing to do from an ethical point of view. If our definition is to meet the needs of present and future generations, then we should do that for all.

You can see all the planetary boundaries and societal boundaries in figure 2 below. Note that this figure is slightly simplified. For the full details see the original publications by Steffen and Holden [7,8]. Also, we made one modification: for boundary 10, the minimum value of the Human Development Index (HDI, calculated from education, life expectancy, and income), has been raised to 0.7, while it was originally 0.63 [8]. The reason is that for a HDI of 0.7, a majority of people will acknowledge and accept problems such as manmade climate change [10]. Acknowledgement and acceptance of these problems is essential if we want to solve them.

Figure 2: Measurable definition of sustainability. The Planetary boundaries, number 1 to 9, as formulated by Johan Rockström, Will Steffen, and their team. In the safe zone (blue) risk of climate catastrophe and ecosystem collapse are small. In the danger zone (red) the risk is extremely high. The real boundary lies somewhere in the uncertainty zone (yellow), but we are not exactly sure where. Additionally the two societal boundaries formulated by Erling Holden and team, number 10 and 11 – Note that this figure is slightly simplified compared to the original. To see the full details see the original papers [7,8].

Crossing boundaries

Crossing a planetary boundary is a bit like bending dry spaghetti, see video 1. You know you can bend the spaghetti a little. For planetary boundaries, this is the safe zone (blue). You also know that if you bend the spaghetti more, it is going to break! You just don’t know exactly when. This is the uncertainty zone (yellow). Finally, you know that if you try to bend the spaghetti into a 180 degree angle it will for sure break. This is the danger zone where we know for sure there will be trouble (red). The biggest problem is that once you have broken the spaghetti, there is no way back. Crossing planetary boundaries causes irreversible changes.

We can also understand the planetary boundaries by looking back to our analogy of the Earth in comfort mode. We are certain that within the safe zone the risk of changing to another Earth System mode is small. In the danger zone on the other hand, there is extremely high risk of a mode change that will lead to catastrophic climate change and ecosystem collapse. The real limit lies somewhere in between, in the uncertainty zone, but we don’t know exactly where. For the societal boundaries, unfortunately no uncertainty zone is indicated in the scientific literature yet.

Usually, a little uncertainty is not problematic. As long as we can see the big picture. There is one boundary, however, where uncertainty is huge. This is also the boundary that we have crossed the farthest: biosphere intactness. Biosphere intactness (biodiversity) is currently measured via the extinction rate of species (to be exact: the amount of extinctions per year per one million species). Normally, the about 1 species in a million go extinct every year (and a new species evolves, keeping the total amount of species constant). Currently the safe zone is set at an extinction rate of maximum 10. The limit is higher than 10, but we are not sure what it is exactly…. therefore the uncertainty zone lies between 10 and 100. More research is needed about the roles biodiversity fulfills in the ecosystem, and exactly how much biodiversity is needed. That said, however, currently we are far into the danger zone! Life, outside humanity, is quickly declining. The extinction rate, although hard to estimate, it least one hundred times higher than normal, and possibly up to one thousand times [7].

Video 1: Planetary boundaries are a bit like spaghetti. We are sure we can safely bend them a bit. Next follows a region where the spaghetti just might break! We’re just not sure when exactly. If we bend too much however, we are sure that the spaghetti will break. At this point there is no going back. The spaghetti cannot be put back together. Crossing planetary boundaries has irreversible effects.

Two boundaries stand out

The rapid decline of our ecosystems is particularly alarming, because not all planetary and societal boundaries are equally important. When a boundary is crossed, it usually does not lead to immediate catastrophe. For example, if we convert too much natural land into suburbs that does not immediately cause the collapse of the entire ecosystem on Earth. It does however push us closer to crossing the two most important boundaries: the climate change boundary (measured by atmospheric CO2 concentrations), and the biosphere intactness boundary (measured by species extinction rate). When one of these boundaries is exceeded, we are at very high risk of leaving the Earth’s comfort mode. Unfortunately, we are about to exceed the climate change boundary, and we have already put the ecosystem boundary far behind us… all hands on deck.


This definition may be measurable ... but I still can't choose which peanut butter to buy!

Yes, that is an excellent point. This definition of sustainability will not yet help us in the us in the supermarket. But, recall the eso-x method. The eso-x method is to first have a measurable definition of a sustainable world. Next, we work out roadmaps to fulfill these criteria (some early roadmaps are already available in literature). Finally we can identify the companies which are following these roadmaps (with their activities and investments), and we buy peanut butter from these companies.

Now we have completed the first step and we can carry on with studying roadmaps. These roadmaps should fulfill all planetary and societal boundaries.

(It is probably best to update our definition of sustainability every few years, as science progresses.)

In conclusion

We set out to find a measurable definition of sustainable development. Such a definition is absolutely essential if we want to make any kind of realistic plan.

Although science is always progressing and changing, we have found this definition! The planetary and societal boundaries, shown in figure 2, give us measurable goals that we can strive to fulfill.

Next, it is up to us for find the plans and roadmaps that we need to meet these goals. Some strategies can be found in science already, and eso-x can help expand and improve these plans. For example, we could create a plan for optimal land use by linking the plans for sustainable energy production, and plans to preserve biodiversity. So let’s do this!

[1] United Nations – Department of Economic and Social Affairs, „The 17 Sustainable Development Goals,” [Online]. Available: sdgs.un.org/goals.

[2] K. Raworth, Doughnut Economics : Seven Ways to Think Like a 21st-century economist, London: Random House, 2017.

[3] Global Footprint Network, „Ecological Footprint,” [Online]. Available: https://www.footprintnetwork.org/. [accessed 26 10 2021].

[4] G. Brundtland, „Our Common Future; Report of the World Commision on Environment and Development,” United Nations, 1987.

[5] J. Rockström et al., „A safe operating space for humanity,” Nature, vol. 461, pp. 472-475, 2009.

[6] J. Rockström et al., „Planetary Boundaries: Exploring the Safe Operating Space for Humanity,” Ecology and Society, vol. 14, nr. 2, p. 32, 2009. 

[7] W. Steffen et al., “Planetary boundaries: Guiding human development on a changing planet,” Science, vol. 347, no. 6233, 2015.

[8] E. Holden et al., „Sustainable development: Our Common Future revisited,” Global Environmental Change, vol. 26, pp. 130-139, 2014.

[9] K. Bahgat et al., „Inequality and Armed Conflict: Evidence and Data,” Peace Research Institute Oslo (PRIO), 2017.

[10] UNDP, „Human Development Report 2011: Sustainability and Equity – A Better Future for All,” New York, 2011.

[11] UNDP, „Human Development Report 2020: The next frontier – Human development and the Anthropocene,” New York, 2020.

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