Do humans have free will? Or are our actions merely manifestations of a thermodynamic imperative? Or are both views right in their own ways?

Figure: The human imprint on Earth can easily be seen at night. Is this imprint simply a manifestation of thermodynamics? Image source: NASA/NOAA.

I really like to think that I am free to decide on what I like, want and what I do, manifestations of what one commonly refers to as free will. Or one can call it human agency, describing the concept that humans can make independent and conscious decisions. It comes into play when we talk about individual human beings.

Yet, when we talk about many, or even all human beings, that’s quite a different matter. At that scale, we talk about the collective behaviour of all humans, and we talk about a scale at which the interactions with the Earth system matter. These interactions are critical, as humans draw their food and other resources from the Earth system and thereby impact its functioning. How does free will of individual human beings manifest itself at the planetary scale?

This is where thermodynamics comes in. It describes what systems do, it describes the limits of their activity, and it describes how they evolve. What thermodynamics tells me is that human activity at the planetary level does not have complete freedom but rather follows a general thermodynamic direction, like any other process within the Earth system. I refer to this general thermodynamic direction of processes at large scales and the associated notion that processes evolve and operate at their thermodynamic limit as the “thermodynamic imperative”. It is similar to Wilhelm Ostwald‘s energetic imperative.

It seems that these two views exclude each other, but as I will explain in the following, they mutually belong together. The central link between these views is that humans, at their very core, require a source of free energy to be active, just like any other dissipative Earth system process. By withdrawing this free energy from the Earth system, this results in consequences that impact the ability of the Earth to generate energy, and this feeds back to human activity when dealing with the activity of all humans at the planetary scale.

I wish I could explain this perspective in just a few words, but I think this would loose the bigger picture to see that humans are part of a natural evolutionary succession of the dynamics of the Earth system. So I take a little longer. I first describe the relevance of thermodynamics and limits for purely physical processes, then the relevance to the biosphere before getting to human activity. I do this to see that human activity is a progression of processes that takes the Earth system to the next level of dissipative activities.

The atmosphere evolves to work as hard as it can. It sets an example to distinguish between individual influences and the collective behavior of the whole atmosphere.

I want to start with applying thermodynamics to purely physical processes of the climate system. Here, the application is most straightforward: Solar radiation heats the planet unevenly, this creates temperature differences from which mechanical work can be produced to accelerate air into motion, just like a heat engine. The resulting atmospheric motion then acts to level out the temperature differences. It involves energy conversions, from radiation to heat, and some heat into the kinetic energy associated with motion, which is turned back into heat by friction, and is eventually radiated away into space.

There is quite a history of research that shows that the atmospheric general circulation operates at its thermodynamic limit. This was first popularised by Garth Paltridge in the 1970’s and expressed by the so-called principle of Maximum Entropy Production (MEP, see e.g., the nice review paper by Hisashi Ozawa and co-authors). I prefer a more specific way to look at atmospheric dynamics and thermodynamic limits than MEP, using the limit of maximum power of an atmospheric heat engine. I find power, defined in physics as work being performed per time, to be easier to understand and more specific as it differentiates between the different types of energy that are being involved. The end result is nevertheless pretty much the same as MEP. Over the last few years, we have shown that with this assumption that the atmosphere operates at this limit, we can predict climate and climate sensitivities with extremely simple equations just based on physics. These predictions are as good as highly complex climate models (see, for example, here and here). The success of these applications implies that the atmosphere indeed works at its limit, working as hard as it can to generate motion and redistribute heat.

I do not want to get into the specifics of climate here, but rather focus on the more general aspect as to why our simple, parsimonious approach works so well. With simple I mean that one can calculate the estimates with pencil and paper on the back of an envelope, and with parsimonious I mean that we basically do not need empirical fudge factors, just physics and the assumption that the atmosphere works at its limit.

Atmospheric motion involves highly complex processes of fluid dynamics and turbulence, so why does our approach work? I like to use the term “emergent complexity” here. It is not that climate dynamics is per se simple, but rather that the dynamics are so complex that the ultimate limitation comes from thermodynamics. The many different whirls from a tiny turbulent eddy to a huge low pressure system in the mid-latitudes provide freedom to arrange the atmospheric flow in many different ways to transport heat from A to B so that a lack of options is not the problem. It is that, overall, thermodynamics limits how much power can maximally be generated to drive the whole atmospheric flow and feed the growth of the different whirls and pressure systems. This limit then constrains how much heat, in total, is being transported, and this limitation can be expressed and quantified in relatively simple terms. It does not lead us to better understand how an individual whirl evolves, but it rather helps us to understand how the whole atmosphere works, which then allows us to predict the mean climate and its sensitivity.

What does this imply for agency? Well, in the atmosphere, we do not deal with living organisms, but we do deal with individual whirls of motion that grow and decease. These whirls have their influences, and are what we recognise as weather. These dynamics are even described in terminology that is similar to how living organisms are treated – meteorology uses the term „cyclogenesis“ for the growth of a mid-latitude low pressure system. Yet, overall, the dynamics of the whole atmosphere are limited by thermodynamics. As the dynamics evolve to the thermodynamic limit, one can predict the atmosphere as a whole, or climate, with a comparatively simple thermodynamic limit. One needs the freedom of the many, individual whirls to arrange the flow, but the atmosphere as a whole nevertheless follows the thermodynamic imperative.

The biosphere pushes its environmental limitations to make it as productive as possible. So it works as hard as it can, given the natural constraints of the Earth system.

Thermodynamics also applies to life. The vast majority of life is fuelled by the chemical energy generated by photosynthesis. It uses sunlight to split water, and drive a chain of processes that transfers some of the energy contained in sunlight to create chemical free energy in form of carbohydrates and atmospheric oxygen. This energy is then used to sustain the metabolisms of the primary producers (mostly plants on land and phytoplankton in the sea). Some of it is then used to fuel food webs, composed of animals that recycle nutrients and make these again available to the producers, resulting in the dissipation of the chemical energy created by photosynthesis and associated nutrient cycling in ecosystems.

When we look at patterns of natural biospheric activity at the larger scale, then one sees mostly the physical limitations of the climate system (see Figure below). Marine producers are mostly limited by oceanic mixing, as mixing brings essential nutrients from the deeper ocean layers to the surface where the producers harvest the sunlight. Mixing happens primarily in the mid-latitudes, where the low pressure systems of the atmosphere blow over the ocean and mix it, so productivity is comparatively high for open-ocean environments. In addition, there are some other regions in which nutrients are either brought in by rivers or by so-called western boundary currents that also relate to atmospheric dynamics.

On land, the principal limitation is water. Plants loose water as they take up the carbon dioxide from the atmosphere to store the energy from sunlight in carbohydrates. As a consequence, the patterns of terrestrial productivity basically reflect patterns of water availability (see Figure). As plants loose water and evaporate it back in the atmosphere, they act to enhance precipitation over land (particularly during dry periods, where root systems can reach water in the deeper soil layers, allowing rainforests to keep transpiring and being productive). Yet, the enhancement of hydrologic cycling on land has its limits, and the terrestrial biosphere probably operates quite close to these limits. I tested this hypothesis with climate model simulations a long time ago, and the results indicate that on land, terrestrial productivity is close to the limits of hydrologic cycling.

The large-scale patterns of biotic activity (“net primary productivity”, in units of grams of carbon per square meter per year) reflect mostly physical limitations. This map was motivated by the paper by Field et al. to plot marine and terrestrial biotic activity together, but uses more recent data for ocean and land.

So how does this link to agency? Well, the biosphere is composed of a huge number of individual organisms, and these do not function identically, but exhibit a vast diversity in their functioning. Also, life evolves and constantly creates new variations, which are then filtered by natural selection for their success. And the individuals affect their local environment. Yet, as a whole, the activity of biosphere is constrained by its physical limitations. Evolution can make the biosphere more active, the effects can make the environment less limiting, but the limitations cannot be eliminated entirely. This should result in the biosphere to be as productive as possible given these limitations, representing a state of maximum chemical power (a notion already described by Alfred Lotka in the 1920‘s). The limitations then make the biosphere predictable by abiotic factors. This is a very well established notion, reflected for instance in biogeographical patterns such as biome distributions.

This notion shares the same pattern as the atmosphere: Individual entities exist and have their effect, and they are critical, as they allow the process to explore the path to the physical limits. Yet, the impact of the individual at the planetary scale cannot be seen directly. The collective activity of the whole biosphere thus evolved to and operates at the physical and thermodynamic limits of the Earth system. It is a second example that needs freedom at the individual level, but follows the thermodynamic imperative at the scale of the whole biosphere.

Human activity is a planetary thermodynamic process that evolves to its limits to maximise power. This does not mean, however, that human agency does not matter.

Humans need energy to sustain their metabolisms and their socioeconomic activity. This is a rather central aspect of their existence, and this basic need for energy makes human activity a thermodynamic process. This energy comes from the Earth in form of harvesting a part of the productivity of the biosphere in form of agriculture, and in form of fossil fuels, deposits of ancient biomass locked up by geologic processes over millions of years ago. So human activity is a thermodynamic process that consumes chemical free energy of the Earth system.

When we apply the thermodynamic imperative directly to this notion, this would lead to the prediction that human societies inevitably increase their energy consumption in the future, as described e.g., by Tim Garrett (e.g., here) and Andy Jarvis (e.g., here). Yet, does this mean that human agency does not matter?

In the two previous examples of the atmosphere and the biosphere, individual entities played a critical role in providing the means of the whole system to find and evolve to its respective thermodynamic limit. Equivalently, I think that a diversity in human action, which we could link to human agency, is likely a necessary requirement for humans at the planetary scale to sustainably increase their activity by converting more and more energy. Such diversity in agency can result, for instance, in different energy policies at the national scale. While the US heavily favours a fossil-based energy system, other countries, like Germany, experienced huge advances in their shift towards a more sustainable energy system based on renewable energy. The extent to which this difference in energy policy then feeds back to the level of human activity of the respective country will then decide in the long run which policy will be more successful. In concrete terms, once renewable energy provides energy to humans with less effort (i.e., cheaper) than fossil fuels, it will provide an evolutionary advantage to those societies that favor a renewable energy policy. The reason why this transition to renewable energy has not happened yet is probably because we are so used to using fossil fuels, with all its associated infrastructure, that it is currently still less effort to use fossil fuels rather than renewable energy.

It seems to me that the basic mechanism that is at play here is essentially the same as in the atmosphere or in the biosphere. Individual entities have diverse strategies of gaining and using their energy harvests, and the extent to which these strategies will grow or decease and become more dominant depends on how successful these are in increasing their energy harvests. It links to the notion of Alfred Lotka that in the end, the success should go to such strategies that can harvest the most energy sustainably from their environment, and this then represents a state of maximum power.

To put this into a more concrete picture of how such a sustainable increase in energy consumption of humans can look like: Humans can increase their energy harvests sustainably by shifting to solar energy. This is because human-made technology, such as photovoltaics, is already much more efficient in harvesting sunlight and generating free energy than leaves can. And the biosphere is more successful in generating chemical free energy from sunlight than abiotic processes are. So the increasing dominance of human activity and their technology could be seen as the emergence of a new class of planetary processes that allows the Earth system to evolve to the next level of energy conversions.

Human agency is a condition for the thermodynamic imperative.

So in summary, I do no think that free will, or human agency, are in contradiction with a thermodynamic imperative of an ever increasing rate of human energy consumption. Just as it is important to recognize that human free will is not independent of the Earth system (because humans need the Earth as an energy source), it is also important to recognize that an increase in human energy consumption is not an automatic, deterministic trend as a consequence of thermodynamics (because humans need to develop the degrees of freedom to allow for such a trend). It rather emerges from the diverse behavior of human agents that use their individual freedom and variations to find out how things work „better“. And things work „better“ when more energy can be generated sustainably.

At present, we are at a crossroads where the impacts of depleting fossil fuels threaten the future thermodynamic trajectory, in terms of depleted stocks of easily-accessible fossil fuels, and in terms of impacting the planetary environment, e.g., by global warming, deforestation, loss of biodiversity and so on. It requires a diverse set of human strategies to find out what works best to overcome these challenges.

What this perspective tells us, I think, is how the mechanism is likely to play out. It is critical to have the means of creating variations and a diversity of actions. These can generate different strategies for harvesting energy, and how well these work in harvesting energy then feed back to the growth of these strategies. This will then show us what the best way is into a sustainable future, and this future is likely one in which more energy will be converted sustainably by the Earth system through human technology, thus following the thermodynamic imperative.