Global warming, biodiversity loss, freshwater shortages, food crisis — there are many reasons to think that the planetary future looks rather grim for human societies. Is there any hope that things can turn out well? It is quite hard to remain optimistic, yet when looking at it from basic physics one can find a way forward. In this book chapter that has just been published online I looked at the issue of sustainability and the role of technology using our thermodynamic Earth system approach.
Human societies are fuelled by free energy that is generated by Earth system processes (red arrow) so when societies consume more, less is available for natural processes. The pathway of sustainable growth involves technology such as photovoltaics that can generate free energy more efficiently than natural processes (left blue arrow), thereby making the whole Earth a more powerful system (blue arrow in the middle). Source for images of hurricane and Earth: NASA.
The key to understand how this pathway to sustained future growth may look like is to recognise that global environmental problems are typically linked to energy. Global climate change is, quite clearly, related to energy. It is mostly caused by the human consumption of fossil fuels to meet our needs for primary energy. Consuming fossil fuels releases carbon dioxide, which builds up in the atmosphere, enhances the greenhouse effect, which causes global warming and associated climatic changes. The loss of biodiversity is closely associated with a loss of natural ecosystems, as humans use more land, and use it more intensively, to meet their demand for food. Food is chemical energy, mostly in form of carbohydrates, that is generated by photosynthesis. When more and more of the energy generated by photosynthesis is appropriated to human use – by using more land and by using land more intensively – less is available for the natural biosphere. So natural ecosystems degrade, unable to maintain their complex food webs and becoming unable to sustain high levels of diversity. Also the availability of freshwater is related to energy. There is surely plenty of water available on Earth, but it is mostly in form of seawater in the world’s oceans. The hydrologic cycle makes freshwater out of it, but it requires huge energy inputs during evaporation. This energy is released when vapour condenses in the atmosphere, precipitates, and provides freshwater to the land surface. Here, too, we have the same pattern: when human societies use more freshwater, less is available for the natural world.
This all seems quite closely related to the notion of limits to growth and planetary boundaries, natural limits of how much energy, food and water is made available by the natural Earth system to fuel and feed human societies. When human societies consume more, less is left for the natural world. This thinking even applies when societies shift to energy systems based on renewable energy. When, for instance, more wind turbines take more energy out of atmospheric motion, it leaves a less dissipative atmosphere behind, weakening its dynamics. It seems to lead to an inevitable conclusion: to become sustainable, human societies must consume less from the Earth’s environment to avoid degrading the Earth system too much. Or, in thermodynamic terms, it seems that more human activity implies more dissipation, making the rest of the Earth system less dissipative.
But there is another pathway. Some human-made technology has the potential to do things better and more efficient than what natural processes can achieve. Most importantly, humans invented photovoltaics. Photovoltaics converts the energy contained in sunlight directly into electricity, and it does so at a much higher efficiency than what photosynthesis can accomplish in natural ecosystems. While ecosystems operate at about 1-2% efficiency, photovoltaics can already achieve 20% and more. With this technology, humans contribute a novel, more efficient process to the Earth system that generates free energy – energy able to perform work and that fuels dissipative activities. This technology could cover currently unproductive areas, such as deserts, parking lots or rooftops. This energy could then not only meet current human energy demands, but it could also be used together with other technologies to generate more freshwater, for instance using membranes for seawater desalination instead of vaporizing water. Desalination by membranes requires a lot less energy than desalination by vaporization, which is the way that the natural hydrologic cycle does it. With this extra energy input from human technology, this could then enhance the hydrologic cycle. Overall, this could make the whole Earth system more productive, or, in thermodynamic terms, a more powerful and dissipative system.
This would, in principle, allow for future growth that would be sustainable, but it requires human-made technology. I am not taking a stance whether we should, or should not, have further growth. But I think it is critical to be aware of this pathway, because it may very well turn out that this path of further growth is unavoidable. Physical Earth systems clearly evolve and operate near their thermodynamic limits – the atmosphere operates at its state of maximum power, setting the magnitude of surface-atmosphere exchange, planetary heat transport and hydrologic cycling. It works as hard as it can, generates as much free energy in form of kinetic energy as possible, and hence, dissipates at the highest possible rate. The terrestrial biosphere, where the generation of chemical free energy by photosynthesis is intimately linked to gas exchange and evaporation, evolves to and operates at its limit as well, maximising its productivity given the natural means available. In the natural state, this free energy allows ecosystems to be as dissipative as possible. So why would we expect that human societies, with the ability of generating free energy by photovoltaics even more efficiently than the physical and biological world, would not do the same, enhancing free energy generation and dissipation further? This would clearly result in a different world in the future, resulting in what we called a type V planet of an advanced civilisation. And this pathway will not come automatically and without effort. But it may well be the only feasible option we will have.
Talk: Want to see this in form of a lecture? Join the talk I will be giving in the “Lectures for Future” series in Munich on 13 December 2021 (in German).
Reference: Kleidon A. (2022) Empowering the Earth System by Technology: Using Thermodynamics of the Earth System to Illustrate a Possible Sustainable Future of the Planet. In: Wilderer P.A., Grambow M., Molls M., Oexle K. (eds) Strategies for Sustainability of the Earth System. Strategies for Sustainability. Springer, Cham. https://doi.org/10.1007/978-3-030-74458-8_29
Earthsystem.org 
Thanks Axel,
Just earlier today I read about this in my U of Wisc. alumni magazine.
It referenced this use of Perovskite.
I thought the hope was quite extraordinary if they can really achieve those kind of efficiencies.
https://www.advancedoxford.com/oxford-pv-casestudy/
and
https://www.oxfordpv.com/
Best and Happy Holidays,
sak
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Hi Steve,
As always, thanks very much for sharing the links!
Apart from what’s currently possible, photovoltaics has also still a lot of room for future improvements. Even the currently best efficiencies are still way below the theoretical limits set by thermodynamics, which are above 70% (set by radiation, see e.g. here: or here: , and not to be confused with the Shockley-Queissler limit of single layer PV, see e.g., here: ).
Best and Happy Holidays to you too,
Axel
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It’s good that this idea that humans removing energy and water from natural processes takes away from those natural processes is still being discussed. We need more of that.
However, the idea that energy from photovoltaics is free energy is surely a huge over-simplification. The costs, both industrial and environmental, involved in constructing and operating those systems makes them anything but free, even if there are periods in some countries where most electrical energy can come from those sources.
Also, the notion that growth can be sustainable surely goes against not only thermodynamics but against the fact that we live on a finite planet. Frankly, I’m astounded that a scientist can use the term “sustainable growth” as though it could be a real thing. I hope that you didn’t quite mean that growth could be sustained indefinitely (which is what the word sustainable means, unless it is modified with some time period, e.g. “sustained for several years”).
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Thanks for the good points you raise!
Free energy is a physical term – it is energy without entropy, so it can be freely converted into other energy forms and is thus able to perform work. What you mention regarding costs is basically the need for such free energy to build solar panels. There is a very nice energy-based concept called Energy Return On Investment (EROI, see here ) which requires that there is more (free) energy gained from PV than expended. This is currently the case with PV production, and this ratio may even improve in the future due to technological innovations.
Growth can still be sustainable for a couple of generations. In fact, the exponential growth we have seen in the past decades could keep going for another hundred or twohundred years, and we would still not hit the limit of free energy that PV could generate. But even then, we are not at the limit, because one could deploy solar farms in space – an option that is not that far fetched (see e.g. here ), although it may sound like science fiction (in fact, the limit then would be set by what is known as the Dyson sphere, see here: ). But then, who would have imagined that we have high-speed trains, mobile phones and the internet twohundred years ago?
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Thanks, that makes things clearer. I’m familiar with EROI (sometimes termed EROEI) and have seen quite varied calculations for so-called renewable energy. It’s heavily dependent on what energy inputs are used. Still, energy inputs is one thing, resource inputs (all requiring energy, of course) are another, and pollution or habitat destruction is another.
So when you talk of sustainable growth, you’re thinking in terms of time periods. Ultimately, growth is unsustainable as it requires increasing inputs from a finite planet and increasing environmental destruction (from mining and refining the resources, as well as from building infrastructure). Solar farms in space also require resources though some of the infrastructure is located in space.
Thanks for the reference to the Dyson sphere though the appeal to technical change over recent centuries is disappointing. However, all of those examples could be thought of as continuations of the technological changes seen in the first half of the last century, rather than something completely new (imagine someone from 1900 being thrust into a 1960 world versus someone from 1960 being thrust into 2020 and trying to make sense of what they see).
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Yes, I completely agree that resources are needed. When we look at a well-engineered, optimized system like the natural biosphere, we note that recycling plays a major role. Most resources in rainforest (as an example of a highly productive system) are recycled, and these involve food webs, including fish migrations in the rivers, and fish-eating animals that live on land and that bring the nutrients back to the forest. What this, in the end, results in is that nutrients don’t limit the activity of the biosphere – but rather the physical constraints: water availability and “energy” (not quite, it actually is heating-driven transport, but that’s a different story). These physical constraints then result in well-defined biogeographical patterns across continents.
When we apply the same template to the human system, we would expect recycling to become more prominent as well (resulting in what sometimes is referred to as the circular economy). At some point, it is simply more efficient to recycle rather than to mine the resources. And, perhaps, human societies can be smart enough to come up with smart policies to encourage this transition before we feel the detrimental effects that would otherwise force us to do so. I am hesitant about the smartness of policy – I am not an expert in the field, and my feeling is that it is easy to overestimate the impacts of policy as well. In the end, whatever is needed, people need to change their behavior and habits, and ideally see a benefit in this change.
And, yes, when I talk about sustainable growth, I am thinking about the next decades, not eternity. I think it is an important distinction, because, quite frankly, I am pessimistic that we can simply say, let’s consume less and actually do this. I think it is simply unrealistic. I think more realistic is to focus on making growth sustainable.
Hope this clarifies a few things.
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