On Short-Term Climate Adjustments Following Abrupt Cirrus Cloud Formation from ISSR

Cirrus clouds play a crucial role in regulating Earth’s radiation balance, with human activities altering their coverage and thus exerting a positive radiative forcing on the climate system. Among these, condensation trails left by planes, known as contrails, can persist into contrail cirrus which are nearly indistinguishable from natural cirrus. Their forcing constitutes most of the non-CO2 impact of the aviation sector, but its quantification remains very uncertain. Adjustments to radiative forcing are particularly poorly known, with estimates from a single climate model, where adjustments in natural cirrus counteract more than half of the initial radiative forcing.

We quantify the climate adjustments that follow abrupt, global transformation of water vapour in ice-supersaturated regions (ISSRs) into cirrus clouds. Using a new method based on ensembles of simulations with the French LMDZ atmospheric general circulation model, we analyse atmospheric and surface climate responses over a four-day period. We quantify the time constants of the ice water response, and its modulation of the initial radiative forcing. Preliminary results suggest that adjustments counteract 70% of the initial forcing within 4 hours.

Working with an simulation ensemble allows us to quantify a statistically significant behaviour of the adjustments and their time constants. Our findings enhance our understanding of the impact of contrails on climate and hold important implications for future climate modelling and prediction.