Earth's Infrared Background

Like Johnson noise, where the thermal fluctuations of charge carriers in a resistor lead to current fluctuations, the internal variability of Earth's atmosphere leads to fluctuations in the Outgoing Longwave Radiation (OLR) emitted to space, creating "Earth's Infrared Background" (EIB). We identify the background with spatially isotropic, random variability consistent with the fluctuation-dissipation theorem. It allows one to distinguish it from atmospheric variability on preferred spatiotemporal scales, including the annual/seasonal/daily cycles, waves, storms, and other coherent modes. Thus, like the Cosmic Microwave Background (CMB), where anisotropies in the microwave radiation represent features of interest in the Universe, anisotropies in the OLR represent features of interest in Earth's atmosphere. Unlike the CMB, which represents a snapshot of the early Universe, the EIB represents Earth's climate in (quasi-) steady state and is therefore described by its spatiotemporal variability. By fitting the OLR from satellite observations to a stochastically forced $\varepsilon$nergy balance climate model, we find that the EIB consists of random fluctuations with a standard deviation of 26.5 W m$^{-2}$, compared to the mean 240 W m$^{-2}$ flux emitted to space. These fluctuations have a red spectrum in both space and time with an upper bound of 400 km and 2.5 days on their spatiotemporal decorrelation, between meso-scale and synoptic-scale weather.