Estimates of the Global Clear-Sky Longwave Radiative Feedback Strength From Reanalysis Data

Abstract

We use atmospheric profiles from ERA5, JRA55, and MERRA2 between 1993 and 2023 to estimate Earth's global clear-sky longwave feedback strength on the seasonal and interannual timescale. Differences in the relationship of relative humidity with skin temperature prior to 2008 lead to interannual feedback strengths between 1.34 Wm⁻² K⁻¹ (JRA55) and 1.89 Wm⁻² K⁻¹ (MERRA2). Restricting the analysis to the last 16 years yields more consistent interannual estimates of 2.05 Wm⁻² K⁻¹ on average, which is larger than the overall seasonal estimate of 1.91 Wm⁻² K⁻¹. The mid-tropospheric drying causing this difference suggests a substantial influence of ENSO variability on the interannual timescale. This indicates a long-term feedback strength smaller than 2.0 Wm⁻² K⁻¹, which is already at the lower end of previous estimates; emphasizing the importance of accurate long-term RH measurements to reliably project Earth's clear-sky feedback strength.