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

TLDR

Satellite measurements and radiative calculations show that Earth's outgoing longwave radiation is essentially linear with surface temperature over a wide range, implying that radiative forcing has the same impact in warmer and colder climates, yet the underlying reason and limits of this linearity remain unclear. The study introduces a simple semianalytical model to explain Earth's linear OLR as an emergent property of a condensable-gas-dominated greenhouse atmosphere. The model attributes linearity to a balance between increasing surface thermal emission and narrowing spectral windows, which ceases at high temperatures when continuum absorption eliminates the windows. The model elucidates Earth's longwave contribution to climate sensitivity and indicates that exoplanets with different condensable greenhouse gases may exhibit climate dynamics akin to Earth's.

Abstract

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 50 y ago, it is still unclear why this relation is valid and when it breaks down. Here we present 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. Our model provides a way of understanding the longwave contribution to Earth's climate sensitivity and suggests that extrasolar planets with other condensable greenhouse gases could have climate dynamics similar to Earth's.