The Interaction Between Climate Forcing and Feedbacks

A Perturbed Parameter Ensemble (PPE) with the Community Atmosphere Model version 6 (CAM6) is used to better understand the sensitivity of aerosol forcing and cloud feedbacks to changes in model processes. Aerosol forcing through aerosol-cloud interactions is mostly negative (a cooling) due to shortwave radiation, while feedbacks are positive or negative in different regions due to contrasting longwave and shortwave effects. Both forcing and feedbacks are related to the mean climate state. Higher magnitude cloud radiative effects generally mean larger magnitude net negative forcing and larger magnitude net positive feedback. Aerosol forcing is broadly related to the susceptibility of clouds to drop number. Feedbacks also related to susceptibility, but to a lesser extent and in different regions to aerosol forcing. Aerosol forcing and cloud feedbacks are anti-correlated in the CAM6 PPE such that stronger negative forcing is associated with stronger positive feedbacks. Even the processes governing forcing and feedback sensitivity in the PPE are similar. These include the warm rain formation process, ice loss processes and deep convective intensity. A climate model is run many times with modified parameters to see how the parameters affect key aspects of climate change. The paper focuses on two aspects of climate change. First, the cloud response to aerosol particles tends to create a cooling, which partially offsets greenhouse gas warming, but the magnitude of the cooling is not well known. It varies a lot in the model when parameters are changed. Second, the paper examines the cloud response to surface temperature increases, called cloud feedbacks, which are the largest uncertainty in estimating the level of future climate change. Cloud feedbacks are also sensitive to parameters. The results show that the cloud feedbacks and aerosol forcing changes are similar but opposite in the model: the cooling and warming generally increase together. This occurs because they are linked to similar parameters, which indicate sensitivity to critical processes, including how rain forms, and how much ice is in the atmosphere. Parametric uncertainty of Aerosol Forcing and Cloud Feedbacks are large Aerosol Forcing and Cloud Feedbacks are related through cloud processes and depend on the mean state of clouds Warm rain formation and ice processes are critical sensitivities that couple forcing and feedback