arXiv subjects
Publications and source records attributed to Axel Kleidon.
Which technologies are currently booming in power generation? The answer can clearly be seen in the trends in global electricity generation data. Analysing this data using the theory of diffusion of innovations reveals how photovoltaics and wind power are gaining ground worldwide. Other technologies - especially coal and nuclear power - are being displaced, with Germany playing a pioneering role.
The energy transition is also about switching to electricity-based technologies such as heat pumps and electric mobility. They avoid heat as an intermediate step and are therefore much more efficient. This can significantly reduce the demand for primary energy in the future, which can then be fully covered by the expansion of renewable energies. Entropy and the maximum possible combustion temperature can be used to understand why combustion is so inefficient.
Life is a planetary feature that depends on its environment, but it has also strongly shaped the physical conditions on Earth, having created conditions highly suitable for a productive biosphere. Clearly, the second law of thermodynamics must apply to these dynamics as well, but how? What insights can we gain by placing life and its effects on planetary functioning in the context of the second law? In Kleidon (2010), I described a thermodynamic Earth system perspective by placing the functioning of the Earth system in terms of the second law. The Earth system is represented by a planetary hierarchy of energy transformations that are driven predominantly by incoming solar radiation, these transformations are constrained by the second law, but they are also modified by the feedbacks from various dissipative activities. It was then hypothesised that life evolves its dissipative activity to the limits imposed by this hierarchy and evolves feedbacks aimed at pushing these limits to higher levels of dissipative activity. Here I provide an update of this perspective. I first review applications to climate and global climate change to demonstrate its success in predicting magnitudes of physical processes, particularly regarding temperatures, heat redistribution and hydrological cycling. I then focus on the limits to dissipative activity of the biosphere. It would seem that the limitations by thermodynamics act indirectly by imposing limitations associated with transport and material exchange. I substantiate this interpretation and discuss the broader implications for habitability, the emergence and evolution of life, and the contemporary biosphere.
In the course of the energy transition, energy generation from nuclear power - through nuclear fission and perhaps in the future through nuclear fusion - is often proposed as an alternative or supplement to renewable energy sources. There are already good reasons why electricity generation from nuclear energy is significantly more expensive than other forms of generation, while increasing dryness as a result of climate change is generally calling into question the reliability of thermal power plants. Nuclear energy is therefore unlikely to play a role in a future energy supply that relies on low costs and reliability.
The warmer temperatures of global climate change strengthen the water cycle, evaporation and precipitation increase. But the extremes of heavy rain, floods, dry periods and droughts will also increase. How does this fit together? Simple physical considerations show which factors mainly regulate the strength of the water cycle in the Earth system, and how this determines water availability on land. This can be used to interpret the observed changes in the water balance in Germany and explain the increasing dryness in Germany.
The transition of our energy system to renewable energies is necessary in order not to heat up the climate any further and to achieve climate neutrality. The use of wind energy plays an important role in this transition in Germany. But how much wind energy can be used and what are the possible consequences for the atmosphere if more and more wind energy is used?
The wind blows stronger and more reliably over the sea than over land. Thus, offshore wind energy is expected to make a major contribution to the energy transition in Germany, especially in the German Bight. But what happens when a growing number of wind farms extract more and more wind energy from the atmosphere?
Energy scenarios and transition pathways need estimates of technical wind energy potentials. However, the standard policy-side approach uses observed wind speeds, thereby neglecting the effects of kinetic energy (KE) removal by the wind turbines that depletes the regional wind resource, lowers wind speeds, and reduces capacity factors. The standard approach therefore significantly overestimates the wind resource potential relative to estimates using numerical models of the atmosphere with interactive wind farm parameterizations. Here, we test the extent to which these effects of KE removal can be accounted for by our KE Budget of the Atmosphere (KEBA) approach over Kansas in the central US, a region with a high wind energy resource. We find that KEBA reproduces the simulated estimates within 10 - 11%, which are 30 - 50% lower than estimates using the standard approach. We also evaluate important differences in the depletion of the wind resource between daytime and nighttime conditions, which are due to effects of stability. Our results indicate that the KEBA approach is a simple yet adequate approach to evaluating regional-scale wind resource potentials, and that resource depletion effects need to be accounted for at such scales in policy applications.
Many aspects of anthropogenic global change, such as land cover change, biodiversity loss and the intensification of agricultural production, threaten the natural biosphere. These aspects seem somewhat disjunct and specific so that it is hard to obtain a bigger picture of what these changes imply and to distinguish beneficial from detrimental human impacts. Here I describe a holistic approach that provides such a bigger picture and use it to understand how the terrestrial biosphere can be sustained in the presence of increased human activities. This approach focuses on the free energy generated by photosynthesis, energy needed to sustain either the dissipative metabolic activity of ecosystems or human activities, with the generation rate being set by the physical constraints of the environment. We can then distinguish two kinds of human impacts on the biosphere: detrimental effects caused by enhanced human consumption of this free energy, and empowering effects that allow for more photosynthetic activity and therefore more dissipative activity of the biosphere. I use examples from the terrestrial biosphere to illustrate this view as well as global datasets to show how this can be estimated. I then discuss how certain aspects of human-made technology can act to enhance the free energy generation of the terrestrial biosphere, which can then facilitate sustaining the biosphere in times at which human activity increasingly shapes the functioning of the Earth system.
We seek to model the coupled evolution of a planet and a civilization through the era when energy harvesting by the civilization drives the planet into new and adverse climate states. In this way we ask if triggering "anthropocenes" of the kind humanity is experiencing now might be a generic feature of planet-civilization evolution. In this study we focus on the effects of energy harvesting via combustion and vary the planet's initial atmospheric chemistry and orbital radius. In our model, energy harvesting increases the civilization's population growth rate while also, eventually, leading to a degradation of the planetary climate state (relative to the civilization's habitability.) We also assume the existence of a Complex Life Habitable Zone in which very high levels of $CO_2$ are detrimental to multi-cellular animal life such as those creating technological civilizations. Our models show that the civilization's growth is truncated by planetary feedback (a "climate dominated anthropocene") for a significant region of the initial parameter space.
How much wind energy does the atmosphere generate, and how much of it can at best be used as renewable energy? This review aims to give first-order estimates and sensitivities to answer these questions that are consistent with those obtained from numerical simulation models. The first part describes how thermodynamics determines how much wind energy the atmosphere is physically capable of generating at large scales from the solar radiative forcing. The work done to generate and maintain large-scale atmospheric motion can be seen as the consequence of an atmospheric heat engine, which is driven by the difference in solar radiative heating between the tropics and the poles. The resulting motion transports heat, which depletes this differential solar heating and the associated, large-scale temperature difference. This interaction between the thermodynamic driver and the resulting dynamics leads to a maximum in the global mean kinetic energy generation rate of about 1.7 W m$^{-2}$, which matches rates inferred from observations of about 2.1 - 2.5 W m$^{-2}$ very well. The second part focuses on the limits of converting the kinetic energy of the atmosphere into renewable energy. The momentum balance of the lower atmosphere shows that at large-scales, only a fraction of about 26% of the kinetic energy can at most be converted to renewable energy, yielding a typical resource potential of about 0.5 W m$^{-2}$ per surface area. The apparent discrepancy with much higher yields of small wind farms can be explained by the spatial scale of about 100 km at which kinetic energy near the surface is being dissipated and replenished. I close with a discussion of how these insights are compatible to established meteorological concepts, inform practical applications, and can set the basis for doing climate science in a simple, analytical, and transparent way.
With the use of the appropriate technology, such as photovoltaics and seawater desalination, humans have the ability to sustainably increase their production of food and energy while minimising detrimental impacts on the Earth system.
The notion that the whole is more than the sum of its parts has a long tradition in science. This, of course, also applies to the Earth system. With its myriad of processes, spanning from purely physical to life and human activity, the Earth is a vastly complex system. It may thus seem that there is nothing simple and general to say because of this overwhelming complexity. What I want to show here is that by formulating the Earth as a thermodynamic system, one can identify general directions and infer simple functioning because thermodynamics imposes fundamental limits on the dynamics. At the center of this description are energy conversions and states of disequilibrium, which are at the core of the dynamics of Earth system processes, from convection cells to living organisms and human societies. They are linked to each other and interact by their exchanges of energy and mass, and ultimately affect how much of the input of low entropy solar radiation from the Sun is converted into free energy, energy able to perform work, before the energy gets re-emitted by the Earth as high entropy terrestrial radiation. The emergent thermodynamic behavior of the Earth then becomes simple because the dynamics evolve to and operate at thermodynamic limits. Such behavior of Earth system processes operating at the edge of their limit can then be linked to previously described holistic theories, such as the Gaia hypothesis, with similarities in the described emergent behavior. Such a thermodynamic view, however, can go further, as it can also be used to understand the role of human societies in the Earth system and the potential pathways to a sustainable future. Thermodynamics taken together with the energy conversions and interactions within the Earth system can thus provide a basis to understand why the whole Earth system is more, and simpler, than the sum of its spheres.
Photosynthesis converts sunlight into the chemical free energy that feeds the Earth's biosphere, yet at levels much lower than what thermodynamics would allow for. I propose here that photosynthesis is nevertheless thermodynamically limited, but this limit acts indirectly on the material exchange of water and carbon dioxide. I substantiate this interpretation using global observation-based datasets of radiation, photosynthesis, precipitation and evaporation. I first calculate the conversion efficiency of photosynthesis in terrestrial ecosystems and its climatological variation, with a median efficiency of 0.78% (n = 13445). The rates tightly correlate with evaporation (r2 = 0.89), which demonstrates the importance of the coupling of photosynthesis to material exchange. I then infer evaporation from the maximum material exchange between the surface and the atmosphere that is thermodynamically possible using datasets of solar radiation and precipitation. This inferred rate closely correlates with the observation-based evaporation dataset (r2 = 0.85). When this rate is converted back into photosynthetic activity, the resulting patterns correlate highly with the observation-based dataset (r2 = 0.56). This supports the interpretation that it is not energy directly that limits terrestrial photosynthesis, but rather the material exchange that is driven by sunlight. This interpretation can explain the very low, observed conversion efficiency of photosynthesis in terrestrial ecosystems as well as its spatial variations. More generally, this implies that one needs to take the necessary material flows and exchanges associated with life into account to understand the thermodynamics of life. This, ultimately, requires a perspective that links the activity of the biosphere to the thermodynamic constraints of transport processes in the Earth system.
We develop a classification scheme for the evolutionary state of planets based on the non-equilibrium thermodynamics of their coupled systems, including the presence of a biosphere and the possibility of what we call an agency-dominated biosphere (i.e. an energy-intensive technological species). The premise is that Earths entry into the Anthropocene represents what might be from an astrobiological perspective a predictable planetary transition. We explore this problem from the perspective of the solar system and exoplanet studies. Our classification discriminates planets by the forms of free energy generation driven from stellar forcing. We then explore how timescales for global evolutionary processes on Earth might be synchronized with ecological transformations driven by increases in energy harvesting and its consequences (which might have reached a turning point with global urbanization). Finally, we describe quantitatively the classification scheme based on the maintenance of chemical disequilibrium in the past and current Earth systems and on other worlds in the solar system. In this perspective, the beginning of the Anthropocene can be seen as the onset of the hybridization of the planet - a transitional stage from one class of planetary systems interaction to another. For Earth, this stage occurs as the effects of human civilization yield not just new evolutionary pressures, but new selected directions for novel planetary ecosystem functions and their capacity to generate disequilibrium and enhance planetary dissipation.
Convective and radiative cooling are the two principle mechanisms by which the Earth's surface transfers heat into the atmosphere and that shape surface temperature. However, this partitioning is not sufficiently constrained by energy and mass balances alone. We use a simple energy balance model in which convective fluxes and surface temperatures are determined with the additional thermodynamic limit of maximum convective power. We then show that the broad geographic variation of heat fluxes and surface temperatures in the climatological mean compare very well with the ERA-Interim reanalysis over land and ocean. We also show that the estimates depend considerably on the formulation of longwave radiative transfer and that a spatially uniform offset is related to the assumed cold temperature sink at which the heat engine operates.
The chemical composition of the earths atmosphere far from equilibrium is unique in the solar system and has been attributed to the presence of widespread life. Here I show that this perspective can be quantified using non-equilibrium thermodynamics. Generating disequilibrium in a thermodynamic variable requires the extraction of power from another thermodynamic gradient, and the second law of thermodynamics imposes fundamental limits on how much power can be extracted. When applied to complex earth system processes, where several irreversible processes compete to deplete the same gradients, it is easily shown that the maximum thermodynamic efficiency is much less than the classic Carnot limit, so that the ability of the earth system to generate power and disequilibrium is limited. This approach is used to quantify how much free energy is generated by various earth system processes to generate chemical disequilibrium. It is shown that surface life generates orders of magnitude more chemical free energy than any abiotic surface process, therefore being the primary driving force for shaping the geochemical environment at the planetary scale. To apply this perspective to the possible future of the planet, we first note that the free energy consumption by human activity is a considerable term in the free energy budget of the planet, and that global changes are closely related to this consumption of free energy. Since human activity and demands for free energy is going to increase in the future, the central question is how human free energy demands can increase sustainably without negatively impacting the ability of the earth system to generate free energy. I illustrate the implications of this thermodynamic perspective by discussing the forms of renewable energy and planetary engineering that would enhance overall free energy generation and thereby "empower" the future of the planet.