Clear-sky CO2 forcing is known to vary significantly over the globe, but
the state dependence which controls this is not well understood. Here we
construct a quantitatively accurate analytical model for
spatially-varying CO2 forcing, which depends only on surface and
stratospheric temperatures as well as column relative humidity. This
model shows that CO2 forcing is primarily governed by
surface-stratosphere temperature contrast, with the corollary that the
meridional gradient in CO2 forcing is largely due to the meridional
surface temperature gradient. The presence of H2O modulates this forcing
gradient, however, by substantially reducing the forcing in the tropics,
as well as introducing forcing variations due to spatially-varying
column relative humidity.