718 research outputs found

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

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    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

    The Maximum Entropy Production Principle: Its Theoretical Foundations and Applications to the Earth System

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    The Maximum Entropy Production (MEP) principle has been remarkably successful in producing accurate predictions for non-equilibrium states. We argue that this is because the MEP principle is an effective inference procedure that produces the best predictions from the available information. Since all Earth system processes are subject to the conservation of energy, mass and momentum, we argue that in practical terms the MEP principle should be applied to Earth system processes in terms of the already established framework of non-equilibrium thermodynamics, with the assumption of local thermodynamic equilibrium at the appropriate scales

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

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    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

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

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    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

    Working at the limit: A review of thermodynamics and optimality of the Earth system

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    Optimality concepts related to energy and entropy have long been proposed to govern Earth system processes, for instance in the form of propositions that certain processes maximize or minimize entropy production. These concepts, however, remain quite obscure, seem contradictory to each other, and have so far been mostly disregarded. This review aims to clarify the role of thermodynamics and optimality in Earth system science by showing that they play a central role in how, and how much, work can be derived from solar forcing and that this imposes a major constraint on the dynamics of dissipative structures of the Earth system. This is, however, not as simple as it may sound. It requires a consistent formulation of Earth system processes in thermodynamic terms, including their linkages and interactions. Thermodynamics then constrains the ability of the Earth system to derive work and generate free energy from solar radiative forcing, which limits the ability to maintain motion, mass transport, geochemical cycling, and biotic activity. It thus limits directly the generation of atmospheric motion and other processes indirectly through their need for transport. I demonstrate the application of this thermodynamic Earth system view by deriving first-order estimates associated with atmospheric motion, hydrologic cycling, and terrestrial productivity that agree very well with observations. This supports the notion that the emergent simplicity and predictability inherent in observed climatological variations can be attributed to these processes working as hard as they can, reflecting thermodynamic limits directly or indirectly. I discuss how this thermodynamic interpretation is consistent with established theoretical concepts in the respective disciplines, interpret other optimality concepts in light of this thermodynamic Earth system view, and describe its utility for Earth system science.</p

    Principals\u27 Instructional Leadership in Title I Schools: A Closer Look

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    The students in Title I schools remain the most vulnerable in our education system. Principals in these schools must be exceptional and well prepared. However, principals have been trained in a universal approach that is not sufficient for those who lead schools with high poverty rates as well as culturally and linguistically diverse learners. The purpose of this mixed-methods descriptive study was to gain insight from principals about instructional leadership in Title I schools. Thirty-two principals described their perceptions about the preparation, supports, and challenges necessary to develop instructional leadership, including cultural proficiency for Title I schools. The findings in this study highlight the complexity of the principal role with a specific focus on Title I schools. While principals reported positive experiences as well as support from their leadership preparation programs and school districts, neither was sufficient to fully prepare them to be instructional leaders in Title I schools. Recommendations include a comprehensive, cohesive district coaching and mentoring program that considers elements necessary to build well-prepared and exceptional leaders for Title I schools

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

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    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 m2^{-2}, which matches rates inferred from observations of about 2.1 - 2.5 W m2^{-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 m2^{-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
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