Towards uncovering the origin of efficacy differences for different radiative forcing contributions related to transport emissions

Abstract

The total climate impact of any transport sector consists of several individual contributions from, e.g., various trace gases, aerosols, or cloudiness changes. Ranking such contributions according to their quantitative importance in terms of radiative forcing requires that each component has the same efficacy, i.e., that for all forcings the global mean surface temperature response is linked to the radiativ forcing with the same climate sensitivity parameter. Experience has shown that this crucial assumption is not always true. The physical reasons of different efficacies for different forcing mechanisms are still insufficiently explored. Here, we investigate the potential of radiative feedback analysis to explain efficacy variations. As an example, we have analysed equilibrium climate change simulations forced by a radiative forcing from CO2 increase and by a radiative forcing from enhanced NOx and CO surface emissions (via ozone changes). We find that forcing dependent efficacies can be traced back to a forcing dependence of certain feedback mechanisms, like the stratospheric temperature feedback or the cloud feedback. However, for a successful analysis it is important to comply with certain methodological constraints and to scale the forcing to a level that allows to separate statistically significant feedback changes from the natural variability. The latter is found to differ substantially for the various global radiative feedback parameters.