Earth‘s outgoing longwave radiation linear due to H2O greenhouse effect

Synopsis

Satellite measurements and radiative calculations show that Earth‘s outgoing longwave radiation (OLR) is an essentially linear function of surface temperature over a wide range of temperatures (60 K). Linearity implies that radiative forcing has the same impact in warmer as in colder climates and is thus of fundamental importance for understanding past and future climate change. Although the evidence for a nearly linear relation was first pointed out more than 50y ago, it is still unclear why this relation is valid and when it breaks down. In this talk it will be presented a simple semianalytical model that explains Earth‘s linear OLR as an emergent property of an atmosphere whose greenhouse effect is dominated by a condensable gas. Linearity arises from a competition between the surface‘s increasing thermal emission and the narrowing of spectral window regions with warming and breaks down at high temperatures once continuum absorption cuts off spectral windows.