New Book Chapter:  Understanding the Earth as a Whole System: From the Gaia Hypothesis to Thermodynamic Optimality and Human Societies

“The whole is more than the sum of its parts” – this has been said by smart people throughout human history, from Aristotle to Gibbs.  But how does it apply to the vastly complex Earth system?  In the book chapter just published I describe how this focus on the whole combined with a thermodynamic formulation of the Earth system including life helps us to understand that the whole is more, and simpler, than the sum of its parts.  This is because complex, natural systems appear to work at their thermodynamic limit.  The emergent functioning may then very well share characteristics similar to those postulated by the controversial Gaia hypothesis of James Lovelock, which states that life regulates the Earth for its own benefit.

Figure 1: The Earth’s environment and life interact, making it a highly complex system. Is the whole Earth system more, and simpler, than the sum of its parts?

The Earth system is clearly a vastly complex system with a myriad of processes that interact with each other.  With this overwhelming complexity in mind, it seems impossible to condense its functioning into a simple scheme of how the planet works.  Life surely contributes quite a bit in increasing this complexity further.  Add humans, and complexity reaches even higher levels.  So how can “the whole” become more, and possibly even simpler, than the sum of its parts?

The Gaia hypothesis – from superorganisms to Earth system thermodynamics

An example for an approach to describe planet Earth and its biosphere in a simpler, holistic way started about 50 years ago, when James Lovelock proposed the Gaia hypothesis. This hypothesis essentially condenses life on Earth to a rather simple role: it regulates the Earth for its own benefit. It received substantial criticism, particularly among evolutionary biologists, who are rather uneasy with the notion of a “higher goal”. What is “beneficial”? And how would organisms know how to “regulate”?

Yet, when looking at Lovelock’s motivation, it contained quite a bit of thermodynamics: He described how the presence of life on Earth may be detectable from space by the chemical disequilibrium that life created in the planetary atmosphere. With Lynn Margulis he described that Earth, like life, is a dissipative system that maintains a state far from thermodynamic equilibrium, drawing upon the notion from Schrödinger on the living cell as a dissipative system. More complex dissipative systems, such as human bodies and mammals in general, even regulate their body temperatures, creating homeostatic conditions. So why should the Earth not function in a similar way? These shared thermodynamic characteristics led to the catchy, but somewhat unfortunate notion of Earth being a superorganism.

So there is quite a bit of thermodynamics at the roots of the Gaia hypothesis.  How can we take this further and put Gaia on a more rigid, thermodynamic basis to formulate a holistic view of the Earth system?  A basis that is physical and more quantitative, can be generalized and, more importantly, can be tested?  The book chapter just published describes such an extension, drawing on my earlier work (e.g., Kleidon 2010, Kleidon 2012), based on my paper way back from 2002 on the Gaia hypothesis (Kleidon 2002) that has, in fact, stimulated my interest in the thermodynamics of the Earth system (Kleidon 2004).

We can start by identifying four central ingredients of the Gaia hypothesis that relate to thermodynamics and Earth system functioning: 

#1. The notion of thermodynamic disequilibrium, no matter whether we deal with physical Earth system processes or living organisms.  These disequlibrium states need to be maintained to sustain through time.  

#2. The central notion of  interactions, particularly regarding the effects of life on the Earth’s environment, and how that shapes the conditions to make a living.  

#3. The notion of optimality associated with improving conditions to the extent possible.  

#4. The dominance of negative feedbacks that one needs for homeostasis to arise.

These four notions can be seen in many complex, thermodynamic systems. So let’s see how these play out and how these relate to “Gaian-like” behavior. And these characteristics, in turn, provide a much richer potential for testing Gaian-like behavior.

The missing ingredient: maximum power

One aspect that has not been utilized within this thermodynamic “background” of the Gaia hypothesis are energy conversions and the constraining role that thermodynamics has on these. This can nicely be illustrated by a heat engine – an abstract device that turns a difference in heat into work (this is also what a conventional power plant does). It is well established that the work output of the heat engine is constrained by thermodynamics, and performing work can create disequilibrium. A heat engine can, for instance, move or lift things, thereby creating disequilibrium in form of kinetic or potential energy. With this addition, we can then attribute optimal behavior to the operation at such a thermodynamic limit, and the resulting homeostatic behavior to the negative feedback that governs the operation at the limit. Gaian-like behavior would then simply reduce to the general behavior of complex, thermodynamic Earth systems that maximize power, that is, work over time.

A first, physical test for such thermodynamic behavior is the climate system. Atmospheric motion needs power to be sustained, and this power is generated out of differences in radiative heating and cooling. Interactions come into play because more motion transports more heat, thereby depleting the temperature difference needed to generate motion. When these interactions are included, it results in a maximum power limit, setting a constraint on the intensity of atmospheric motion and heat transport (Kleidon Renner 2013). This limit can be formulated in quite a basic way. We have worked with this approach in my group for some time, for vertical convection and land surface functioning (Kleidon et al. 2014), as well as for large-scale dynamics and limits to wind power. It works remarkably well, reproducing observations and sensitivities to global climate change as well (Kleidon Renner 2013, Kleidon Renner 2017). We can connect this outcome to the four characteristics described above. The disequilibrium in this case is reflected in the kinetic energy generated by the power from radiative heating differences. Interactions come into play because the generated motion transports heat (and other stuff, like moisture and CO2), thereby affecting the temperature difference from which the power is being derived from. Optimality is associated with maximization of power, reflected in an optimum heat flux and temperature difference associated with the maximum. And last, but not least, there are negative feedbacks associated with the maximization that result in atmospheric motion to evolve to this limit – even though this may not quite result in a homeostatic outcome.

We can next apply this picture to the biosphere. Thermodynamics and maximum power has long been applied to ecosystems, for instance by Alfred Lotka or the Odums. Yet, these papers are quite speculative, and it is not clear how these formulations can predict specific aspects of life in similar detail as the maximum power limit can predict aspects of climate system functioning.

But let’s start and go through this step-by-step. First, life is clearly characterized by thermodynamic disequilibrium. Life is made of biomass, and this can turned into heat by combusting it with oxygen into water and carbon dioxide. Hence, the thermodynamic disequilibrium of life is directly associated with is the disequilibrium between biomass (so-called reduced carbon, i.e. carbon compounds less oxidized than carbon dioxide) and atmospheric oxygen. This is at the center of a thermodynamic description: life builds up this disequilibrium out of the energy generated by photosynthesis, and the metabolic activities of producers and consumers turn this energy back into heat and carbon dioxide.

Figure 2: Net Primary Productivity (NPP) of the biosphere expressed in energy rates of W/m2, estimated from satellite data as in Field et al. (1998)

So which aspect of thermodynamics then limits the thermodynamic activity of the biosphere? As described in the chapter, the likely bottleneck for photosynthesis is actually not the thermodynamic conversion from sunlight to chemical energy, but rather the thermodynamic constraint on mixing, which supplies carbon dioxide and nutrients. For the marine biosphere, this constraint is well documented – the mid latitude oceans see strong winds and mixing, and comparatively high levels of marine productivity, while the subtropical oceans show very low productivity because of the lack of mixing.

For the terrestrial biosphere, this may hold as well. Here, the uptake of carbon dioxide is intimately linked to the water loss by evaporation. This loss can be estimated very well from thermodynamics and the maximum power limit (see also this blogpost). And tropical rainforests seem to maximize this exchange. Their highly heterogeneous canopies appear to maximize the leaf area through which gas exchange takes place. Deep-reaching root systems in the soil exploit soil moisture at times when the lack of precipitation would restrict the gas exchange of the plants. It would seem that the biosphere may also maximize its power, although by rather different means than the physical climate system.

We also have interactions: plants altering evaporation by facilitation the access to soil moisture (as one example), and we have optimality associated with the maximization of power.

Figure 3: Biotic activity feeds back to planetary conditions by causing changes in the radiative properties (e.g. surface absorption, greenhouse gas composition) that affect atmospheric mixing and material exchange, thus impacting biotic activity. (Figure 7 in book chapter).

At the planetary scale, we can even take these effects further to link this kind of behavior back to the Gaia hypothesis. By removing carbon dioxide, the biosphere alters the chemical composition of the atmosphere, and thereby the radiative environment. This had dramatic effects over the course of Earth’s history. Originally, it is thought that the Earth’s atmosphere had comparably high greenhouse gas concentrations and no oxygen. It was through the work done by photosynthesizing life that the massive thermodynamic disequilibrium in form of reduced carbon and atmospheric oxygen was built up over the course of Earth’s history. The levels of carbon dioxide (or methane) in turn affect the magnitude of the greenhouse effect, which alters the radiative forcing for atmospheric mixing as well as surface temperatures. So one could think of a feedback diagram based on thermodynamics (see Figure 3) by which life could in fact regulate the atmospheric conditions to maximize photosynthesis and the dissipative activity of the global biosphere. That, in turn, sounds very Gaian, doesn’t it?

Humans on a Gaian planet – a contradiction?

So how does human activity factor into this picture? We can formulate human societies also as a dissipative process. Energy is consumed in form of food by the metabolic activities of humans and their livestocks, and by socioeconomic activities using mostly fossil fuels. The food is taken from the products of photosynthesis – as described by the concept of Human Appropriation of Net Primary Productivity (HANPP, e.g., Vitousek et al. 1986, Haberl et al. 2014). And by consuming biomass and fossil fuels, human societies deplete the massive disequilibrium that is contained in the reduced carbon and atmospheric oxygen that was created by life over the course of Earth’s history. So currently, human activity clearly is a dissipative process. The first criterion of thermodynamic disequilibrium is certainly fulfilled.

Figure 4: Human activity as an Earth system process that either (a) consumes free energy from the Earth system, thereby weakening its dynamics, or (b) generates additional free energy by technology and thereby strengthens the dynamics of the Earth system. (Figure 8 in book chapter).

The further evaluation is difficult though, because at the moment, human activity is not sustainable.  At present, we only act to consume energy, not generating it like the heat engines of the climate system or photosynthesis.  Hence, our dissipative activity acts to deplete other planetary disequilibrium states.  These effects are clearly detrimental to the Earth system (as illustrated in Figure 4), as reflected by the effects of global warming (due to the combustion of fossil fuels and the depletion of the planetary chemical disequilibrium) or tropical deforestation (due to increased demand for food production).  

Yet, in principle, human effects could also be beneficial when certain technology comes into play that allows to generate more power, and to generate it more efficiently.  A primary example for this is photovoltaics.  It generates useful energy directly, in electric form, and it does so at a much higher efficiency than photosynthesis. If installed in desert areas, then a lot more useful energy can be generated than by natural means.  With this additional free energy, human societies could, in principle, generate more resources, from which the biosphere could benefit as well.  This could make the whole Earth system more powerful – literally, by producing more energy to fuel the dissipative activities of the climate system, the biosphere, as well as human societies.  Then, human societies may evolve in a similar direction towards maximizing power, similar to how the biosphere or natural systems maximize power, although by entirely different means.


This book chapter came out of a contribution to an interdisciplinary workshop on the occasion of Alexander von Humboldt’s 250’th anniversary.  I have been lucky to be part of this highly stimulating event and very thankful to the organizers, philosophers Peter Koenig and Oliver Schlaudt of the University of Heidelberg.  The book is published open access by Heidelberg University press and can be downloaded here.


Kleidon, Axel (2023). Understanding the Earth as a Whole System: From the Gaia Hypothesis to Thermodynamic Optimality and Human Societies. in: König, Peter und Schlaudt, Oliver (Eds.): Kosmos: Vom Umgang mit der Welt zwischen Ausdruck und Ordnung, Heidelberg: Heidelberg University Publishing, 417-446.


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