Preprints
https://doi.org/10.5194/esd-2022-38
https://doi.org/10.5194/esd-2022-38
22 Aug 2022
 | 22 Aug 2022
Status: this preprint is currently under review for the journal ESD.

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

Axel Kleidon

Abstract. Optimality concepts related to energy and entropy have long been proposed in Earth system science, yet they remain obscure, seem contradictory regarding their goal to either maximize or minimize, and have so far only played marginal roles. This review aims to clarify the role of thermodynamics and optimality in Earth system science by showing that it plays a pivotal role in how, and how much, work can be derived from the solar forcing, and that this imposes a major constraint to 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 the 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, such as hydrologic cycling or biotic activity. 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.

Axel Kleidon

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on esd-2022-38', Anonymous Referee #1, 20 Sep 2022
    • AC1: 'Reply on RC1', Axel Kleidon, 06 Feb 2023
  • RC2: 'Comment on esd-2022-38', Remi Tailleux, 04 Oct 2022
    • AC2: 'Reply on RC2', Axel Kleidon, 06 Feb 2023
  • RC3: 'Comment on esd-2022-38', Jonas Nycander, 09 Oct 2022
    • AC3: 'Reply on RC3', Axel Kleidon, 06 Feb 2023

Axel Kleidon

Axel Kleidon

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Short summary
The second law of thermodynamics has long intrigued scientists, but which role does it play in the Earth system? This review shows that its main role is that it shapes the conversion of sunlight into work. This work can then maintain the dynamics of the physical climate system, the biosphere, as well as human societies. The relevance of it is that apparently many processes work at their limits, so that they become predictable by simple means.
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