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Principles Of Non-equilibrium Thermodynamics

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The following claim “What are the processes that drive Earth towards non-equilibrium,” can be deconstructed into two key elements: How did Earth’s thermodynamic system evolve away from thermal equilibrium? And what processes drive it towards non-equilibrium? The present-day Earth system is notably unique when contrasted to its planetary neighbours. This uniqueness, in areas such as global cycling is result of an exclusive atmosphere that is far from thermal equilibrium. The atmospheric system encompasses a vast range of processes at different scales; from water and carbon cycles that drive biogeochemical processes, atmospheric concentration and composition, and ultimately the planetary energy balance. Furthermore, this exclusiveness is also imitated in the atmospheric concentration of molecular oxygen throughout the atmosphere and relatively low humidity. According to (source), it is likely that concentrations of atmospheric oxygen have increased over the millennia (millions of years?). The justification for such an observation can be can be explained by the principle of maximum entropy production (MEP). This is because these complex processes have unique interactions and feedbacks – making them irreversible process – thus producing entropy. Furthermore, a simple and holistic view of Earth’s evolutionary past, present and future system can be grasped using the understanding of non-equilibrium thermodynamics.

Research Question: How did Earth’s present thermodynamic system evolve away from thermal equilibrium and what processes drive it towards non-equilibrium?

Thermodynamic principles

At the core of maintaining planetary dynamics are the laws of thermodynamics, which set fundamental principles that govern all physical processes. At the core of thermodynamics are the first and second laws. Whilst the first law states that energy is conserved – the second law governs natural thermodynamic processes. For instance, processes such as the mixing matter, water evaporating, and organisms decomposing – if these processes were not to occur all matter would become a uniform mix of everything – which in turn would likely lead to a dead Earth. These three processes are scientifically described as being irreversible, as they occur spontaneously and cannot be returned to their initial state. Furthermore, these seemingly trivial examples can be governed by the laws of thermodynamics – more specifically the second law of thermodynamics which states:

“The total entropy of any system plus that of its environment increases (a) as a result of any natural processes.” (Physics Principles with applications)

Specifically, the second law is quantified in terms of entropy. Entropy can be described as the natural phenomenon of the movement of heat energy, more specifically energy moving from higher states to lower states (hot to cold). The process of entropy production refers to Gibbs Free Energy principle.

Gibbs free energy is formulated as the energy within a system (closed or open) that is capable of performing work. Within a system, the sum of enthalpy (the sum of internal energy and the product of pressure and volume) plus the constant temperature (measured in Kelvin) and entropy of a system. This can be expressed in the following formula:

G = U – TS + PV (b)

Where U is defined as the internal energy of a system, T is the absolute temperature, S is final entropy, P is absolute pressure and V final volume. Gibbs free energy is formulated to describe the thermodynamic potential that can be used to calculate the maximum of reversible work that could be performed by a thermodynamic system. Furthermore, the second law and entropy can be used to govern the total entropy of the universe:

∆Suniverse = ∆Ssystem + ∆S surrounding > 0 (c)

Where ∆S is defined as the change in entropy for a spontaneous process. Therefore, entropy of an isolated system never decreases. It always increases within an isolated system.

However, within the roots of planetary evolution is the principle of maximum entropy production. Maximum entropy production states:

The steady state of an open (non-isolated) thermodynamic systems with sufficient degrees of freedom are maintained in a state at which the production of entropy is (d) maximised given the constraints of a system.” (Special issue “what is maximum entropy production).

Thus, the maximum entropy is essential when understanding Earth’s thermodynamic system.

Earth’s Thermodynamic System

Earth’s current environmental system correspond to a state far from thermodynamic equilibrium. According to Catling if the Earth was in thermodynamic equilibrium there would be a high atmospheric concentration of carbon dioxide, extreme temperatures, large cloud cover and little dissipation of energy. However, these physical factors are clearly do not apply to contemporary Earth. Earth’s present conditions illustrate that 21% of the atmosphere is molecular oxygen – and if the state was at equilibrium there would be no reactive oxygen (source). Therefore, Earth’s system processes strongly interact with one another – and are therefore far from equilibrium.

Earth’s climate system is often characterised as an open system and not a closed system. However, if it was stated as an open system this would violate the second law of thermodynamics, by such of decreasing the total entropy. In simple terms, this would mean that heat is flowing from cold to hot. Generally, Earth is described as a closed system because it exchanges energy, not mass with another system. This is because energy enters the atmosphere in different forms of electromagnetic radiation. This energy is then radiated back into space as longwave radiation from the Earth. The flow of electromagnetic radiation is regulated by the atmosphere and the ozone layer. Therefore, in terms of mass – the ocean, land and atmosphere are described as closed thermodynamic systems where the entropy budget is always increasing (Hockeyschitck – why the earth is a closed system). Mathematically the entropy budget of a system can be stated as:

dS/dt = σ−NEE (e)

Where dS/dt is defined as the rate of entropy (S) of a system, σ is represents the entropy being produced by irreversible processes within a system and NÉE represents the net entropy exchange across a system (Kleidon reference).

For an effective and thorough understanding of the applications of maximum entropy production to Earth’s thermodynamic system – it is first vital to identify and describe the transformations and the cycling of energy and mass, and how these processes interact. However, the Earth’s thermodynamic system is complex – so atmospheric turbulence and global biotic activity is excluded (Entropy production by Earth System Proccess ref). It can then be examined how the current environmental conditions would look like closer and further away from thermal equilibrium and derive the trends that allow a system to evolve away from thermal equilibrium.

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Earth’s numerous system process do not function in isolation, as they strongly interact. For example, in order for a process to take place, the process must perform work to degrade sources of free energy – which in turn produces entropy.

Hydrological Cycle

The hydrological cycle, commonly known as the water cycle – formulates the movement of water throughout Earth. This cycle is driven by energy conversions associated with evaporation from a warmer surface into an unsaturated atmosphere and subsequent condensation of water at cooler temperatures in the atmosphere.

Based upon these two cycles – the absorption of radiation and the emotion of long wave radiation drives these two cycles away from thermal equilibrium.

Atmospheric Circulation

When Earth is often referred to as a heat engine, this refers to the atmosphere. This is because the atmosphere converts heat energy into mechanical energy. More specifically atmospheric circulation relates to the movement of air from areas on high and low pressure. Due to friction at the surface, atmospheric circulation is slowed down – therefore it requires a continuous input of work to continue the steady state of circulation. This continuous input is formulated from the contrasting temperature gradient between the equator and the poles – and therefore the transfer of heat from hot to cold results in the downgrading of energy and entropy production. Therefore, this atmospheric heat engine provides a mean to transport elements within Earth. This cycle, dehumidifies the atmosphere – which in turn drives the hydrological cycle away from thermal equilibrium. Henceforth, because the atmosphere continually performs work – the dissipated heat is then reradiated back out of the atmosphere into space in the form of long wave radiation.

So what drove Earth away from thermal equilibrium? Earth’s current thermodynamic state is far from a thermal equilibrium due to the global cycling of matter. According to Kliedon (ref), if the Earth was in thermodynamic equilibrium, it would look drastically different from its current state. Furthermore, due to atmospheric circulation acting as a heat engine to dehumify the atmosphere, it is likely that the atmosphere would be very cloudy, the hydrologic cycle would would essentially be gone because evaporation and precipitation rates would likely be zero. There would be little runoff, which effects the movement of matter between the Earth and the ocean – and therefore there is no geological cycling.In turn, this would likely result in a high atmospheric co2 concentration – resulting in high temperatures due to an accelerated greenhouse effect. Yet, through time the Earth has evolved away from these possible thermal equilibrium predictions, towards a state of higher geochemical cycling, less cloud cover, cooler surface temperatures and reduced concentrations of greenhouse gases such as co2.

Quality of Evidence:

A majority of the evidence collected to undergo this investigation is deemed valid and credible. For instance, a majority of the data was sourced from Axel Kleidon. Kleidon is a renowned author with publications such as Thermodynamic Foundations of the Earth System and Non-equilibrium Thermodynamics and the production of entropy. Kleidon has also written numerous scientific papers, for numerous journals such as the Philosophical Transactions of the Royal Society. These papers served as the foundation for this investigation.

Numerous books, and websites were also used to make sure that enough evidence can be collected and formulated to form this investigation. Textbooks from renowned physicist Douglas C. Giancoli were utilised to form a bases of thermodynamics principles – such as the laws, entropy, enthalpy, reversible and irreversible processes.

However, it must be noted that certain principles based within this investigation may not be valid. According to Kleidon, “…what has been described here is just a simple, qualitative reasoning that seems reasonable under the assumption that maximum entropy production is valid.”

Evaluation of the Claim:

The research question “How did Earth’s present thermodynamic system evolve away from thermal equilibrium and what processes drive it towards non-equilibrium?” was investigated and research was collected. The evidence suggests that Earth’s present thermodynamic system is far from equilibrium. This was investigated through an understanding of basic hydrological and atmospheric cycles to deduce the conclusion that these global interactions are the result of this non-equilibrium. However, conclusions such as how Earth’s thermodynamic system evolved away from thermal equilibrium is topic for future research and investigations.

Improvements to the Investigation:

There are possible limitations to this investigation, which can be improved with deeper research, and investigating more sources. Furthermore, principles such as that of maximum entropy production could be validated to improve the investigations validity. Henceforth, mathematical proofs could be used to prove such principles.

There are also many more areas of possible research that can be examined, that would provide a more insightful conclusions to the research question. Furthermore, aspects such as maximum entropy production, atmospheric turbulence, global biotic activity, Gaia hypothesis, and a more in depth understanding of non-equilibrium thermodynamics could be further investigated to gain more accurate evidence to support the research question.

Extensions to the Investigation:

The research question focused on a broad aspect of non-equilibrium thermodynamics and and Earth’s evolution away from thermal equilibrium. Such considerations based upon non-equilibrium thermodynamics can give us insight into Earth’s evolutionary past, how and why it functions the way it does. Not only will this provide scientists with more concise understandings of Earth’s past, but it will allow us to predict future trajectories – and possibly how human induced change, such as increase in greenhouse gases can effect this non-equilibrium system.


It is evident that enough sources have been analysed and formulated to investigate the research question, which asked: “How did Earth’s present thermodynamic system evolve away from thermal equilibrium and what processes drive it towards non-equilibrium? It was found that Earth’s thermodynamic was anything but equilibrium, due to a strong global cycling of matter. Whilst the research question wasn’t disproved – more research must be conducted to validate principles such as maximum entropy production and its future applications.

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Principles Of Non-equilibrium Thermodynamics. (2022, February 27). Edubirdie. Retrieved February 2, 2023, from
“Principles Of Non-equilibrium Thermodynamics.” Edubirdie, 27 Feb. 2022,
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