  There is ongoing interest in the global entropy production rate as a climate
diagnostic and predictor, but progress has been limited by ambiguities in its
definition; different conceptual boundaries of the climate system give rise to
different internal production rates. Three viable options are described,
estimated and investigated here, two of which -- the material and the total
radiative (here 'planetary') entropy production rates -- are well-established
and a third which has only recently been considered but appears very promising.
This new option is labelled the 'transfer' entropy production rate and includes
all irreversible processes that transfer heat within the climate, radiative and
material, but not those involved in the exchange of radiation with space.
Estimates in three model climates put the material rate in the range $27$-$48$
mW/m$^2$K, the transfer rate $67$-$76$ mW/m$^2$K, and the planetary rate
$1279$-$1312$ mW/m$^2$K.
  The climate-relevance of each rate is probed by calculating their responses
to climate changes in a simple radiative-convective model. An increased
greenhouse effect causes a significant increase in the material and transfer
entropy production rates but has no direct impact on the planetary rate. When
the same surface temperature increase is forced by changing the albedo instead,
the material and transfer entropy production rates increase less dramatically
and the planetary rate also registers an increase. This is pertinent to solar
radiation management as it demonstrates the difficulty of reversing greenhouse
gas-mediated climate changes by albedo alterations. It is argued that the
transfer perspective has particular significance in the climate system and
warrants increased prominence.
