The important role of feedback processes for contrail cirrus climate impact

Contrail cirrus is regarded to be the largest contributor to aviation induced global warming based on classical radiative forcing and exceeds the corresponding climate impact of accumulated air traffic CO₂ emissions. However, recent studies indicate that the leading role of contrail cirrus declines when using more advanced climate metrics, such as the effective radiative forcing, or even disappears when considering the induced surface temperature change. 

Here we present results from climate model simulations to derive a fully self-consistent set of classical radiative forcings, effective radiative forcings and corresponding surface temperature changes for a contrail cirrus and CO₂ perturbation. The simulations were extensively evaluated by feedback analysis in order to determine the origin of the reduced efficacy of contrail cirrus to warm Earth’s surface. When switching from classical radiative forcing to effective radiative forcing the impact of contrail cirrus decreases by 45% relative to CO₂. Feedback analysis revealed a reduced formation of natural cirrus as the major reason, as contrail cirrus formation removes large parts of available ambient humidity. When looking at surface temperature change, the efficacy of contrail cirrus turned out to be reduced, even more, by 79% relative to CO₂. Again, cloud feedbacks were found to be the major reason for the different behavior between the contrail cirrus and CO₂ perturbation, however, in this case mainly triggered by decreasing low- and mid-level clouds in the CO₂ simulation. The efficacy reduction is also supported by a larger negative lapse rate feedback (change of the vertical temperature slope) which is the result of a temperature dipole formed by contrail cirrus, with strongest warming rates directly below the contrail cirrus cloud base and decreasing strength towards surface.