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

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 W m−2 K−1 (JRA55) and 1.89 W m−2 K−1 (MERRA2). Restricting the analysis to the last 16 years
yields more consistent interannual estimates of 2.05 W m−2 K−1 on average, which is larger than the overall seasonal estimate of 1.91 W m−2 K−1. The mid-tropospheric drying causing this difference suggest a substantial influence of ENSO variability on the interannual timescale. This indicates a long-term feedback strength smaller than 2.0 W m−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.