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.