  We present a new outlook on the climate system thermodynamics, studying some
of its macroscopic properties in terms of the 1st and 2nd laws of
thermodynamics. We review and clarify the notion of efficiency of the climate
system by constructing formally an equivalent Carnot engine with efficiency
eta, and show how the Lorenz energy cycle can be framed in a macro-scale
thermodynamic context. Then, by exploiting the 2nd 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
re-interpreted as requiring the joint optimization of heat transport and
mechanical work production. These results provide new tools for climate change
analysis and for climate models' validation.
