Contrail Cirrus Climate Impact: From Radiative Forcing to Surface Temperature Change

Abstract

Contrail cirrus has been regarded as the most important individual component of aviation-induced global climate impact, a conclusion prompted by radiative forcing estimates from a variety of models. However, there have been indications of a reduced effective radiative forcing of contrail cirrus with respect to its instantaneous radiative impact, as well as indications of a reduced contrail efficacy to force surface temperature changes. Here, we present a set of global climate model simulations driven by either upscaled contrail cirrus or a CO2 increase, first with prescribed and then with interactive sea surface temperature, yielding self-consistent results for forcings, feedbacks, and climate sensitivity. If contrail cirrus and CO2 induce the same classical (stratosphere adjusted) radiative forcing, we find the contrail cirrus effective radiative forcing reduced to about 55% compared to that from CO2, qualitatively confirming previous results. The surface temperature response per unit effective radiative forcing (the climate sensitivity parameter) is also smaller (reduced to about 40%) for contrail cirrus. In total, the simulations indicate an efficacy value as low as 0.21 for contrail cirrus, with an estimated statistical uncertainty between 0.10 and 0.32, while consolidated knowledge to quantify the respective systematic uncertainty is currently lacking. Our results indicate a much smaller relative contrail cirrus impact on global warming than classical or even effective radiative forcing estimates suggest. The analysis of radiative adjustments and feedbacks reveals a major role of natural clouds in driving the differences in the response behavior. We discuss consequences of the results for aviation climate impact assessments and promising further research directions.