Thermodynamic efficiency and entropy production in the climate system.

We present an outlook on the climate system thermodynamics. First, we construct an equivalent Carnot engine with efficiency eta and frame the Lorenz energy cycle in a macroscale thermodynamic context. Then, by exploiting the second law, we prove that the lower bound to the entropy production is eta times the integrated absolute value of the internal entropy fluctuations. An exergetic interpretation is also proposed. Finally, the controversial maximum entropy production principle is reinterpreted as requiring the joint optimization of heat transport and mechanical work production. These results provide tools for climate change analysis and for climate models' validation.