Efficacy of Solar irradiance, Methane and Black carbon aerosols

Abstract

In climate science literature, the concept of radiative forcing is used to infer the equilibrium temperature change that would occur as a result of changes in the forcing agents such as greenhouse gases and aerosols. Radiative forcing is defined as the energy imbalance that occurs at the top-of-atmosphere because of the imposed changes in the forcing agents. It would be a useful quantity to compare the importance of various forcing agents if the climate system response depends on the radiative forcing and not on the forcing agents. However, recent studies show that the climate system response depends on the forcing agents to some extent. In order to address this issue, the concept of ‘efficacy’ of forcing agents was introduced. Efficacy is the ratio of the equilibrium global mean surface temperature change per unit forcing from an agent to the equilibrium temperature change per unit carbon-dioxide (CO2) forcing from the same initial climate state. An efficacy of one implies that two forcing agents that induces same radiative forcing would produce same global mean temperature response while a non-unity efficacy would indicate different temperature response for same radiative forcing. Past studies investigated the efficacy of various forcing agents but a detailed assessment of the climate processes responsible for the efficacy to be different from one is lacking. Using a global climate model, Community Atmosphere Model version 5 (CAM5) developed by the National Centre for Atmospheric Research (NCAR), we estimate the efficacy of three climate forcing agents - solar irradiance, methane (CH4) and black carbon (BC) aerosols and investigate the physical mechanisms responsible for a differing efficacy of these forcing agents compared to CO2. We also investigate and compare the hydrological cycle response to CO2 and these forcing agents. To compute the radiative forcing and efficacy, we adopt the Hansen’s prescribed-sea-surface-temperature (SST) and Gregory’s regression approaches. The recent Intergovernmental Panel on Climate Change (IPCC 2013) finds the radiative forcing estimated using these methods to be better predictor of global mean surface temperature change compared to other radiative forcing definitions. This study is the first modelling study which provides a comprehensive assessment of the physical mechanisms responsible for a non-unity efficacy for solar irradiance, methane and black carbon aerosols. It also highlights the importance of including the efficacy factor in the “forcing and response” relationship for an improved estimate of surface temperature change due to a particular forcing agent. Further investigation using a multi-model intercomparison framework would permit an assessment of the robustness of the results shown in this study.