Towards determining the efficacy of contrail cirrus

Abstract

Contrail cirrus has been emphasized as the largest individual component of aircraft climate impact, yet respective assessments are based mainly on conventional radiative forcing calculations. Recent climate model simulations have provided the first estimate of contrail cirrus effective radiative forcing (ERF), which turns out to be much smaller, by about 65%, than its conventional radiative forcing. The reason for the reduction is that natural clouds make up for a considerably lower radiative impact in the presence of contrail cirrus. ERF is generally regarded as a superior metric to indicate the efficacy of some forcing agent to induce surface temperature changes. Hence, the new result may suggest a smaller role of contrail cirrus in the context of aviation climate impact (including proposed mitigation measures) than assumed so far. However, any conclusion in this respect should be drawn carefully as long as no simulations of the global mean surface temperature response to contrail cirrus are available. As a next step, these are needed to confirm the power of ERF for assessing the efficacy of contrail cirrus.