Effective radiative forcing and rapid adjustments of contrail cirrus

Contrail cirrus is regarded to be the largest contributor to aviation climate impact and thus plays an important role in developing strategies towards limiting aviation induced climate change. So far the climate impact relevance of contrails and contrail cirrus was estimated in terms of stratosphere adjusted radiative forcing. It has been shown that the efficacy of stratosphere adjusted radiative forcing of linear contrails in forcing surface temperature is significantly reduced. Thus stratosphere adjusted radiative forcing may be a questionable metric for assessing the climate impact. Here, we present results from global climate model simulations, designed to determine the effective radiative forcing of contrail cirrus, as effective radiative forcing is now considered as a superior metric for inter-comparing contributions to a total climate impact (in this case, for aviation). Effective radiative forcing is found to be significantly lower than the corresponding stratospheric adjusted radiative forcing. For a CO2 increase forcing of comparable magnitude, the reduction of effective radiative forcing in comparison to the stratosphere adjusted forcing is much smaller. Thus the climate impact to be deduced from our simulations is reduced compared to existing radiative forcing estimates. In a subsequent feedback analysis the rapid radiative adjustments, which are physically responsible for the reduced effective radiative forcings, have been determined. A large negative cloud adjustment, due to a loss of natural cirrus cover, is found to be the main driver of the substantial reduction in the contrail cirrus case.