  In this paper we exploit two equivalent formulations of the average rate of
material entropy production in a planetary system to propose an approximate
splitting between contributions due to vertical and eminently horizontal
processes. Our approach is based only upon 2D radiative fields at the surface
and at the top of atmosphere of a general planetary body. Using 2D fields at
the top of atmosphere alone, we derive lower bounds to the rate of material
entropy production and to the intensity of the Lorenz energy cycle. By
introducing a measure of the efficiency of the planetary system with respect to
horizontal thermodynamical processes, we provide insight on a previous
intuition on the possibility of defining a baroclinic heat engine extracting
work from the meridional heat flux. The approximate formula of the material
entropy production is verified and used for studying the global thermodynamic
properties of climate models (CMs) included in the PCMDI/CMIP3 dataset in
pre-industrial climate conditions. It is found that about 90% of the material
entropy production is due to vertical processes such as convection, whereas the
large scale meridional heat transport contributes only about 10%. The total
material entropy production is typically 55 mWK-1m-2, with discrepancies of the
order of 5% and CMs' baroclinic efficiencies are clustered around 0.055. When
looking at the variability and co-variability of the considered thermodynamical
quantities, the agreement among CMs is worse, suggesting that the description
of feedbacks is more uncertain.
