Interpreting Differences in Radiative Feedbacks From Aerosols Versus Greenhouse Gases

Experiments with seven Coupled Model Intercomparison Project phase 6 models were used to assess the climate feedback parameter for net historical, historical greenhouse gas (GHG) and anthropogenic aerosol forcings. The net radiative feedback is found to be more amplifying (higher effective climate sensitivity) for aerosol than GHG forcing, and hence also less amplifying for net historical (GHG + aerosol) than GHG only. We demonstrate that this difference is consistent with their different latitudinal distributions. Historical aerosol forcing is most pronounced in northern extratropics, where the boundary layer is decoupled from the free troposphere, so the consequent temperature change is confined to low altitude and causes low‐level cloud changes. This is caused by change in stability, which also affects upper‐tropospheric clear‐sky emission, affecting both shortwave and longwave radiative feedbacks. This response is a feature of extratropical forcing generally, regardless of its sign or hemisphere. Understanding how the Earth's surface temperatures change in accordance with the anomalous energy flow into the system due to changes in greenhouse gases (GHGs) or anthropogenic aerosols is vital for predicting future temperature change. New data have made it possible to better calculate how efficiently the planet responds to temperature change (so as to return to energy equilibrium) for historical aerosols and GHGs. We find that the Earth requires greater surface temperature changes under aerosol climate forcing than it does for GHGs in order to balance out incoming and outgoing energy into the Earth system. By comparing with experiments that prescribe energy changes only outside the tropics, we find that the lower efficiency of aerosols in damping the radiative imbalance is related to their being mainly located away from the equator, unlike GHGs which are generally well mixed throughout the globe. This forcing away from the equator is tied to the vertical distribution of temperature changes. This distribution affects how efficiently surface temperature change leads to balancing the incoming and outgoing energy into the Earth system. Surface temperature can thus change differently for the same global average forcings. Effective climate sensitivity is larger (feedback more amplifying) for historical anthropogenic aerosol than greenhouse‐gas (GHG) forcing in Coupled Model Intercomparison Project phase 6The key difference is that GHG forcing is global and aerosols are mainly extratropical (and aerosol hemispheric contrast unimportant)Extratropical forcing causes a shallower temperature response than tropical forcing, hence more amplifying cloud and lapse‐rate feedbacks Effective climate sensitivity is larger (feedback more amplifying) for historical anthropogenic aerosol than greenhouse‐gas (GHG) forcing in Coupled Model Intercomparison Project phase 6 The key difference is that GHG forcing is global and aerosols are mainly extratropical (and aerosol hemispheric contrast unimportant) Extratropical forcing causes a shallower temperature response than tropical forcing, hence more amplifying cloud and lapse‐rate feedbacks