What limits photosynthesis? Identifying the thermodynamic constraints of the biosphere within the Earth system

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.Comment: accepted for publication in Special Issue "21st European Bioenergetics Conference", Biochimica et Biophysica Acta - Bioenergetic

Physical limits of wind energy within the atmosphere and its use as renewable energy: From the theoretical basis to practical implications

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.Comment: 52 pages, 10 figures, 2 tables. Accepted for publication in Meteorologische Zeitschrift (Contributions to atmospheric sciences), https://www.schweizerbart.de/submit/metz/index.php/met

Earth as a Hybrid Planet - The Anthropocene in an Evolutionary Astrobiological Context

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.Comment: Accepted for publication in the journal Anthropocen

Empowering the Earth system by technology: Using thermodynamics of the Earth system to illustrate a possible sustainable future of the planet

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.Comment: 11 pages, 6 figures. Book chapter submitted to "Violated Earth - Violent Earth: From a viscous cycle to a sustainable world", M. Grambow, M. Molls, K. Oexle, P. Wilderer (eds). submitted to Springer Publishers. 14 February 202

Geographic variation of surface energy partitioning in the climatic mean predicted from the maximum power limit

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.Comment: 17 pages, 3 figures, 2 table

Enhancing the efficiency of terrestrial biosphere model simulations by reducing the redundancy in global forcing data sets

Data sets of climatic variables and other geographic characteristics are becoming available in increasingly higher resolutions, resulting in substantial computing burdens for simulation models of the terrestrial biosphere. But by how much do higher resolutions of forcing data actually contribute to higher accuracy in model predictions? I investigated this question using the Cramer-Leemans climatology as an example for a high resolution forcing data set and a model of net primary productivity (NPP). I first used cluster analysis to reduce the complete grid of the climatology to a few grid points, each representative of regions with similar values. A global map of NPP was reconstructed by using the simulated values of the representative grid points for the respective regions. I then compared the reconstructed map of NPP to the one obtained from all grid points. The results show that a high accuracy in simulating the high resolution pattern and magnitude can be achieved by only considering a comparatively small subset of representative grid points. What this suggests is that, while high resolution data sets provide the necessary means to determine the typical regions, they do not add much accuracy to the overall outcome of model simulations because they contain many grid points with similar values. By reducing this redundancy, the methodology used here allows model simulations to be considerably more computing-time efficient while still retaining the accuracy in predicted quantities

Simulated impact of global climatic change on the geographic distribution of plant diversity

Elevated concentrations of atmospheric carbon dioxide (pCO2) are likely to lead to substantial warming in the coming century with altered hydrological regimes, thereby affecting the distribution of plant species. Here I use an individual-based modeling approach to plant diversity to estimate the impact of global climatic change on the geographic distribution of plant diversity. Differences in temperature, precipitation, and light use efficiency (to represent stimulation of photosynthesis due to higher pCO2) are used in isolation and in combination in order to investigate the role of these drivers. I find that the general warming associated with elevated pCO2 leads to profoundly different responses of simulated diversity in temperature-limited and tropical environments. While the growing season is lengthened in northern latitudes and therefore enables more plant growth strategies to be successful, elevated autotrophic respiration rates lead to higher mortality during plant establishment in the tropics, therefore reducing the range of successful plant growth strategies. The overall impact of elevated pCO2 on plant diversity will clearly be a combination of various factors. What these model results nevertheless point out is that global climatic change may alter plant diversity patterns disproportionally by reducing the overall success of plant establishment

Sustaining the Terrestrial Biosphere in the Anthropocene

Many aspects of anthropogenic global change, such as shifts in land cover, the loss of biodiversity, and the intensification of agricultural production, threaten the natural biosphere. The implications of these specific aspects of environmental change are not immediately obvious; therefore, it is hard to obtain a bigger picture of what these changes imply and distinguish the beneficial from the detrimental, where human impact is concerned. In this paper, I describe a holistic approach that allows us to obtain such a bigger picture and use it to understand how the terrestrial biosphere can be sustained in the presence of increased human activity. This approach places particular emphasis on the free energy generated by photosynthesis—energy that is required to sustain both the dissipative metabolic activity of ecosystems and human activities (with the generation rate being restricted by the physical constraints of the environment). Thus, one can then identify two types of human influence on the biosphere and their resulting consequences: the detrimental effects caused by enhanced human consumption of this free energy and the beneficial effects that allow for more photosynthetic activity and, therefore, more dissipative activity within the biosphere...

Hyperdiffusion, maximum entropy production, and the simulated equator-pole temperature gradient in an atmospheric general circulation model

Hyperdiffusion is used in atmospheric General Circulation Models to account for turbulent dissipation at subgrid scale and its intensity affects the efficiency of poleward heat transport by the atmospheric circulation. We perform sensitivity simulations with a dynamic-core GCM to investigate the effects of different intensities of hyperdiffusion and different model resolutions on the simulated equator-pole temperature gradient. We examine the different simulations using entropy production as a measure of baroclinic activity and show that there is a maximum in entropy production. In comparison to the climate at a state of maximum entropy production, every other simulated climate at a given resolution leads to an increased equator-pole temperature gradient. We then demonstrate that maximum entropy production can be used to tune low-resolution models to closely resemble the simulated climate of a high-resolution simulation. We conclude that tuning a GCM to a state of maximum entropy production is an efficient tool to tune low-resolution climate system models to adequately simulate the equator-pole temperature gradient