Linearity of outgoing longwave radiation: From an atmospheric column to
global climate models
The linearity of the global-mean outgoing longwave radiation (OLR) with
surface temperature has important implications for climate sensitivity.
Global climate models robustly produce a 1.9 W/m2/K of global-mean
longwave clear-sky (LWCS) feedback. This number is consistent with
idealized single-column atmospheric models. However, there is
considerable spatial variation in the LWCS feedback including negative
values over tropical oceans known as the “super-greenhouse effect”
which is compensated by larger values in the subtropics/extratropics.
Therefore it is unclear how the idealized model results are relevant for
the global-mean LWCS feedback in comprehensive climate models. Here we
show with a simple analytical theory and model output that the
compensation of this spatial variability to produce a robust global-mean
feedback can be explained by two facts: 1) when conditioned upon
free-tropospheric column relative humidity (RH), the LWCS feedback is
independent of RH, and 2) the global histogram of column RH is largely
invariant under warming.