Earth’s Infrared Background

Only a small fraction, about 15%, of the Outgoing Longwave Radiation (OLR) emitted to space from Earth originates directly at its surface. The remaining 85% is only emitted to space after having been absorbed and re-emitted by greenhouse gases and clouds. As such, OLR fluctuations represent the “footprints” left by these absorbers as they interfere with the infrared radiation on its way out of the atmosphere. Therefore, in addition to its role in determining the global energy budget, OLR also holds valuable information about atmospheric variability. Extracting this information necessitates a quantitative description of the random fluctuations associated with natural variability of the atmosphere, or the “Earth’s infrared background” (EIB). However, a consensual description of the background is still lacking. At least part of the reason is the fact that the governing physical principles have not yet been identified. In this work, we identify the EIB with random variability implied by the fluctuation-dissipation theorem in response to the internal variability of the atmosphere on small spatiotemporal scales, that is, by the weak (linear) response to random fluctuations in (quasi-) steady state. Intuitively, the suggested description of the background is the atmospheric analogue of Johnson noise in electric circuits, where thermal fluctuations of charge carriers in a resistor lead to small-but-measurable voltage fluctuations across its terminals. Consistent with prior assumptions, the resulting background spectrum is broad sense “red”, a first-order process in time and second-order process in space. By fitting the OLR from satellite observations to a stochastically forced εnergy balance climate model, we find that the EIB consists of random fluctuations with an upper bound of about 400 km and 2.5 days on their spatiotemporal decorrelation, between meso-scale and synoptic-scale weather.