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Reflection by polar S snow and ice cover

Reflection by tropical convective clouds

Evapotranspiration

Figure 9.3 Conceptual diagrams of (top) planetary albedo and (bottom) land surface albedo versus surface temperature and evapotranspiration, respectively. Source: Reprinted with kind permission of Springer Science and Business Media from: A. Kleidon and K. Fraedrich (2004) Biotic entropy production and global atmosphere-biosphere interactions. In A. Kleidon and R. D. Lorenz (eds) Non-equilibrium Thermodynamics and the Production of Entropy: Life, Earth, and Beyond, pp. 173-89. Heidelberg: Springer Verlag. Fig. 14.2. ©2004 Springer Verlag, Heidelberg.

Evapotranspiration

Figure 9.3 Conceptual diagrams of (top) planetary albedo and (bottom) land surface albedo versus surface temperature and evapotranspiration, respectively. Source: Reprinted with kind permission of Springer Science and Business Media from: A. Kleidon and K. Fraedrich (2004) Biotic entropy production and global atmosphere-biosphere interactions. In A. Kleidon and R. D. Lorenz (eds) Non-equilibrium Thermodynamics and the Production of Entropy: Life, Earth, and Beyond, pp. 173-89. Heidelberg: Springer Verlag. Fig. 14.2. ©2004 Springer Verlag, Heidelberg.

cloud and snow cover. It has a crucial role in the overall rate of entropy production for the Earth, and small changes in planetary albedo can dwarf the contribution of other processes to the overall planetary entropy budget. Figure 9.3 shows how planetary albedo and land surface albedo exhibit a minimum with respect to surface temperature and land surface evapotranspiration. When planetary albedo is low, absorption of solar radiation is maximal (ice, snow, and clouds reflect a minimal amount back to space) and therefore so is entropy production (Figure 9.3(a)). Now, the surface temperature relates to the strength of the atmospheric greenhouse, which in turn the biota partly determines through its effects on biogeochemical cycles. In consequence, many possible states with a range of global mean temperatures satisfy the constraints of global energy and carbon balances. Similarly, the energy- and water-budget constraints on land surface processes allow many potential states with differing rates of evapotranspiration, ranging from a bare surface to a fully vegetated one (Figure 9.3(b)). Therefore, applying the principle of maximum entropy production to these two cases means that the state of maximum entropy production is the most likely macroscopic state of the atmosphere-biosphere system. It is worth noting here the superficial resemblance between Volk's Wasteworld and the maximum entropy model. In a sense, maximum entropy production corresponds with the largest amount of 'waste' being produced (that is, the largest degradation of free energy). However, the Earth itself is far from being in a Wasteworld state (Kleidon 2004). 'Waste' does not accumulate in the Earth system - it returns into space as long-wave radiation, and the degradation of free energy is (usually) not a 'waste' of energy insofar as it permits the performance of work and the building of order. Therefore, a maximum in entropy production corresponds to the largest amount of available work. In that sense, the metaphor of a 'Wasteworld' would miss the crucial point that the Earth supports life (Kleidon 2004).

Kleidon's (2004) work supports his earlier idea that biotic effects tend to enhance GPP: when viewed as a dissipative process with sufficient degrees of freedom, biotic activity evolves toward a state of maximum entropy production. Kirchner's (2002) and Volk's (2002) counterclaims - that Gaian feedbacks can evolve by natural selection, but so can anti-Gaian feedbacks, and that Gaia is (probably) build from free by-products - both mean that biotic effects would have a random character and would not proceed in a particular direction. In the context of entropy production, these arguments suggest that biotic effects that boost entropy production are as likely to evolve as biotic effects that decrease entropy production. However, a lack of direction in biotic effects would contradict the maximum entropy production hypothesis. Kleidon (2004) argued that the energetics of biological evolution suggests that biotic effects would act in a definite direction, noting that as long ago as 1922 Alfred Lotka wrote:

It has been pointed out by Boltzmann that the fundamental object of contention in the life-struggle, in the evolution of the organic world, is available energy. In accord with this observation is the principle that, in the struggle for existence, the advantage must go to those organisms whose energy-capturing devices are most efficient in directing available energy into channels favorable to the preservation of the species.

(Lotka 1922a, 147)

If this should be true, then natural selection would favour those organisms that increased the degradation of free energy, or equivalently, entropy production. Lotka (1922b) also argued that natural selection tends to maximize the energy flux through the system, so far as is compatible with the constraints to which the system is subject. This assessment squares with Roderick Dewar's (2003) general interpretation of maximum entropy production from information theory: non-equilibrium systems with sufficient degrees of freedom evolve to a state of maximum entropy production given certain constraints on system functioning. Kleidon (2004), in effect, projects Lotka's reasoning to a global scale:

For photosynthetic life, those individuals which can absorb more solar radiation and convert it into organic compounds do not just enhance their own evolutionary benefit, but they would also tend to enhance total absorption of solar radiation of the Earth system. At the planetary scale, this would then imply that the energy flux through the biota is maximized by minimizing the planetary albedo, which, at the same time, leads to a maximization of entropy production of the whole Earth system.

(Kleidon 2004, 307)

Another attractive aspect of the maximum entropy production hypothesis is its avoidance of teleology. Critics of the Gaia hypothesis claim that it gainsays natural selection as an evolutionary process because it comes with teleological baggage (e.g. Kirchner 1989, 2002). In Lotka's (1922a, 1922b) interpretation, natural selection leads to life's evolving towards the 'goal' of maximizing the energy flux through the system (or closely associated, maximizing entropy production). However, this 'goal seeking' is not teleological - it is an evolution towards more probable states along the lines of the statistical interpretation of thermodynamics (Kleidon 2004). In like manner, the maximum entropy hypothesis does not entail teleology; it is a convenient short cut to describe the macroscopic behaviour of a non-equilibrium system in steady state (with sufficient degrees of freedom). It further implies that a system producing maximum entropy in a steady state will respond to external perturbations through negative feedbacks (that is, changes that will sustain the state of maximum entropy production), which is redolent of the homeostatic mechanisms claimed for the Gaia hypothesis.

A crucial point in the present discussion is how maximum entropy states relate to the Gaia hypothesis. Kleidon and Fraedrich (2004) contend that if biotic maximum entropy production is an emergent property of atmosphere-biosphere interactions in a steady state, then that has significant ramifications to understanding how the climate system adapts to environmental change. The sensitivity of a simple coupled climate-carbon cycle model to a prescribed external change in solar output of solar radiation illustrates this point (Kleidon 2004). The model implements the line of reasoning described above for biotic maximum entropy production and carbon cycling (Figure 9.3(a)). When a change in solar luminosity (which was 70 per cent of today's value some 4 billion years ago) forces the model, and under the assumption that biotic activity adjusts to a maximum in entropy production, then the resulting simulated surface temperature is insensitive to these changes. The resulting evolution of atmospheric carbon dioxide concentrations associated with the state of maximum entropy production is roughly in line with reconstructions of the past evolution of the atmospheric greenhouse. Importantly, the homeostatic outcome of this simple model is akin to the Gaia hypothesis, which postulates that the biosphere maintains environmental homeostasis. There is a fundamental difference between the two, however: the emergent state of the atmosphere-biosphere system in the example results from maximum entropy production as a physical selection principle, with biotic processes seen as additional degrees of freedom for processes in the climate system. The notion that environmental homeostasis may follow from the biosphere's adjusting to maximum entropy production when conditions change demands substantiation, perhaps with further modelling studies using process-based simulation models of the biosphere and the Earth system. Nevertheless, the perspective prosecuted by Kleidon and Fraedrich (2004) seems to be 'a promising path to appreciate the role of biodiversity in the functioning of the Earth system from a fundamental, thermodynamic perspective and to understand the ability of the Earth system to adapt to global changes' (Kleidon and Fraedrich 2004, 187).

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