Temporal evolution of the aerosol radiative forcing due to changing emissions of individual aerosol species and their precursors over the Indian region as estimated using a global climate model CAM6.

Aerosols are known to play an important role in regulating the Earth’s energy budget, by directly interacting with solar and terrestrial radiation and modifying the cloud properties. Radiative forcing is commonly used as an index for quantifying such imbalances in the Earth’s radiation budget by any factor. Quantifying aerosol radiative forcing is an initial step towards understanding the response of the Earth’s climate system to changes in emissions of aerosols from anthropogenic sources from pre-industrial (year 1850) to present day.

Our present study is designed to understand implications of changing emissions of aerosols over the historical period (1985-2014) on the evolution of aerosol and cloud radiative forcing using a state of art global chemistry-climate model named CAM6. The estimates of evolution of aerosol radiative forcing and its decomposition into direct radiative forcing (DRF or ERF_ARI), cloud radiative forcing (CRF or ERF_ACI) and surface albedo radiative forcing (SARF) on a global scale with special emphasis over the Indian region is being investigated. For this purpose, simulations are performed by CAM6 model for the 30-year period from 1985 to 2014 with model meteorology nudged towards the ERA5 reanalysis data using CMIP6 global emission inventory. We are trying to understand the implications of changing emissions of aerosols on the estimates of ERF_ARI, ERF_ACI and SARF to understand the contribution of each pathway through which changes in emissions of aerosols from PI to PD perturb the radiation budget of the earth-atmosphere system. We follow the methodology of Ghan et al. (2012) and use various combinations of additional radiative diagnostics with neglected absorption and scattering of aerosols and clouds along with all sky fluxes of shortwave (SW) and longwave (LW) radiation at top of the atmosphere (TOA) to decompose the total aerosol radiative forcing into ERF_ARI, ERF_ACI, and SARF.

Our results show that although the overall effect of changing emissions aerosols and their precursors from anthropogenic sources is to produce a negative radiative forcing at the top of atmosphere (TOA) thereby resulting in cooling over the south Asian region, we find that the aerosol-radiation interaction (ari) leads to warming while aerosol-cloud interaction (aci) results in cooling over the same region. The results from our CAM6 simulations show that the annual mean shortwave aerosol direct radiative forcing (DRF or ERF_ARI) averaged across the Indian land mass due to major aerosol species has increased from 0.46 W/m² to 0.76 W/m² during the 30-year period from 1985 to 2014. More results with greater details on the contribution of individual aerosol towards the temporal evolution of ERF_ARI and ERF_ACI will be presented.

 

Keywords: aerosols, Radiative forcing, ARI, ACI, CAM6