Official Version

Key Points

• The Earth’s Infrared Background (EIB) is defined as isotropic variability implied by the fluctuation–dissipation theorem

• The EIB is modeled using a stochastically forced energy balance climate model, yielding a red background consistent with observations

• The EIB provides a null hypothesis for separating atmospheric signals from random variability in outgoing longwave radiation

Abstract

Much of the outgoing longwave radiation (OLR) emitted to space can be described as a noisy “background” of random variability. A rigorous characterization of this background provides an objective null spectrum and enables the isolation of atmospheric phenomena within OLR observations, such as waves and storms. Previously, the background has only been considered in the Tropics. Here we study the background on global, subannual scales and focus on its physical origins. We define the background as isotropic fluctuations implied by the fluctuation‐dissipation theorem in response to internal atmospheric variability on small spatiotemporal scales. We use a stochastically forced energy balance climate model that generates a red spectrum in space and time, consistent with observations. By fitting the model to OLR measurements from satellites, we find that the background fluctuations have an upper bound of about 400 km and 2.5 days on their spatiotemporal (de)correlations, between meso‐scale and synoptic‐scale weather.