Towards Determining the Contrail Cirrus Efficacy

Contrail cirrus has been emphasized as the largest individual component of aircraft climate impact, yet respective assessments have been based mainly on conventional radiative forcing calculations. As demonstrated in previous research work, individual impact components can have different efficacies, i.e., their effectiveness to induce surface temperature changes may vary. Effective radiative forcing (ERF) has been proposed as a superior metric to compare individual impact contributions, as it may, to a considerable extent, include the effect of efficacy differences. Recent climate model simulations have provided a first estimate of contrail cirrus ERF, which turns out to be much smaller, by about 65%, than the conventional radiative forcing of contrail cirrus. The main reason for the reduction is that natural clouds exhibit a substantially lower radiative impact in the presence of contrail cirrus. Hence, the new result suggests 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 direct simulations of the surface temperature response to contrail cirrus are available. Such simulations are needed in order to confirm the power of ERF for assessing contrail cirrus efficacy.