Linearity of outgoing longwave radiation: From an atmospheric column to global climate models

Abstract:

The constancy of the Earth's longwave clear-sky (LWCS) feedback largely enables the diagnosis of climate sensitivity within a linear framework. CMIP5 models robustly produce a global-mean LWCS feedback of 1.9 W/m2/K, consistent with idealized single-column atmospheric models under fixed relative humidity (Koll & Cronin, PNAS, 2018). However, in CMIP5 models 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 single-column model results are relevant for the global-mean LWCS feedback in comprehensive global climate models. Here we show that the aforementioned consistency is not a coincidence, but rather a result of analogous physics; the single column model result is due to fixed column RH, while in the CMIP5 models it is due to the histograms of free-troposphere RH being invariant under warming. Our results also imply that, in a global or even a tropical-mean context, the clear-sky super-greenhouse effect is constrained to disappear and thus has little impact on large-scale climate.