When all is said and done (del II)

För ca två år kom mycket tänkvärda saker, som också kort togs upp på TCS: ”Den livgivande Solen”.
Inlägget handlade egentligen om ”annorlunda” funderingar om Jordens olika ”balanser”, inklusive de energetiska (klicka för större bild).

Det som är ”annorlunda” borde egentligen inte vara det.
Vetenskapsområdet ”Irreversibel termodynamik”  – eller termodynamik för icke-jämviktssystem – har funnits mycket länge, och och är vad som teoretiskt sammanbinder bl.a. kopplade energiflöden och masstransport.
Likafullt jobbar ju bl.a. klimatvetenskapen i dag utifrån de ramverk av de statiska begrepp som ”jämvikt” och ”[strålnings]balans” som man vant sig vid. Frågan är hur stor denna illusion är?
När jag åter läser TCS-inlägget ser jag via en Google-sökning att Judith Curry tog upp samma sak ett halvår senare, i ett längre och mycket djupare inslag, på sin blogg. Jag minns dock inte att hennes inlägg skulle ha diskuterats på TCS. Det finns nu även en fungerande länk  till en fritt nedladdningsbar PDF-version av Kleidon’s artikel i Journal of Non-equilibrium Thermodynamics, vars Abstract inleds som följer:

• Abstract The Earth system is maintained in a unique state far from thermodynamic equilibrium, as, for instance, reflected in the high concentration of reactive oxygen in the atmosphere. The myriad of processes that transform energy, that result in the motion of mass in the atmosphere, in oceans, and on land, processes that drive the global water, carbon, and other biogeochemical cycles, all have in common that they are irreversible in their nature. Entropy production is a general consequence of these processes and measures their degree of irreversibility. The proposed principle of maximum entropy production (MEP) states that systems are driven to steady states in which they produce entropy at the maximum possible rate given the prevailing constraints…

Judith Curry har också sökt göra en sammanfattning av Kleidon’s artikel, med speciell tonvikt på sakfrågor som exempelvis återkopplingar i ”klimatsystemet” [som genomgående bör vara negativa] – ett utdrag:

• … One of the most important implications of MEP is that it implies that the associated thermodynamic processes react to perturbations with negative feedbacks in the steady state behavior. This follows directly from the maximization of entropy production, which essentially corresponds to the maximization of the work done and the free energy dissipated by a process, as explained above. Imagine that a thermodynamic flux at MEP is perturbed and temporarily reduced. This reduction in flux would result in a build-up of the thermodynamic force, e.g., temperature gradient in the case of poleward heat transport. In this case, the process would not generate as much kinetic energy as possible. The enhanced temperature gradient would then act to enhance the generation of kinetic energy, and thereby the flux, thus bringing it back to its optimal value and the MEP state. If the boundary conditions shape the optimum change, then a perturbation of the state would be amplified until the new optimum is reached, which could be interpreted as a positive feedback to the perturbation.
• What MEP states is that the functional relationship itself takes a shape that maximizes entropy production and thereby results in negative feedbacks. This maximization can be understood as the direct consequence of the system to achieve its most probable configuration of states, as in the case of equilibrium statistical mechanics.
• This discussion of feedbacks and MEP is quite different from the conventional treatment of feedbacks in climatology, which are usually based on temperature sensitivities. In the usual analysis, the total change in temperature ΔTtotal is expressed as the sum of the direct response of temperature to the change in external forcing (ΔT0) and the contribution of feedbacks (ΔTfeedbacks): ΔTtotal = ΔT0 + ΔTfeedbacks.If the total change in temperature is expressed as ΔTtotal = f · ΔT0, with f being the feedback factor, then a positive feedback is defined as f > 1, while a negative feedback is defined as f < 1. The feedback framework plays a very important role in the analysis of anthropogenic climatic change…

Judith Curry skriver avslutningsvis:  The 2nd law of thermodynamics is an underutilized piece of physics in climate science.  It is not a simple beast to wrestle with, but I think there are some important insights to gain. Optimality, self-organizing criticality, and nonlinearity are factors that are not adequately accounted for in traditional climate feedback analyses, and an entropy-based framework would be more consistent with the climate shifts that are actually observed.
De över 300 bloggkommentarerna är mycket läsvärda. Ja, kanske den nuvarande synen på ”klimatsystemet” är en återvändsgränd – döm själv.

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Peter S

Professor emeritus i Fysikalisk Kemi vid KTH. Klimatdebattör sedan 2003.