The quasi-linear relation between planetary outgoing longwave radiation and surface temperature: a climate footprint of radiative and non-radiative processes

The slope of the quasi-linear relation between planetary outgoing longwave radiation (OLR) and surface temperature (TS) is an important parameter measuring the sensitivity of the Earth’s climate system. The primary objective of this study is to seek a general explanation for the quasi-linear OLR-TS relation that remains valid regardless of the strength of the atmospheric window’s narrowing effect on planetary thermal emission at higher temperatures. The physical understanding of the quasi-linear OLR-TS relation and its slope is gained from observation analysis, climate simulations with radiative-convective equilibrium and general circulation models, and a series of online feedback suppression experiments. The observed quasi-linear OLR-TS relation manifests a climate footprint of radiative (such as the greenhouse effect) and non-radiative processes (poleward energy transport). The former acts to increase the meridional gradient of surface temperature and the latter decreases the meridional gradient of atmospheric temperatures, causing the flattening of the meridional profile of the OLR. Radiative processes alone can lead to a quasi-linear OLR-TS relation that is more steeply sloped. The atmospheric poleward energy transport alone can also lead to a quasi-linear OLR-TS relation by rerouting part of the OLR to be emitted from a warmer place to a colder place. The combined effects of radiative and non-radiative processes make the quasi-linear OLR-TS relation less sloped with a higher degree of linearity. In response to anthropogenic radiative forcing, the slope of the quasi-linear OLR-TS relation is further reduced via stronger water vapor feedback and enhanced poleward energy transport.