Which are the main drivers of global dimming and brightening?

The Global Dimming and Brightening (GDB) phenomenon plays an important role in the Earth's climate, with clouds and aerosols being the major drivers. This study investigates GDB causes by quantifying the contributions of changes in clouds, aerosols, water vapor and ozone to the surface solar radiation (SSR) changes during 1984–2018. To this aim, radiative transfer calculations were performed by the FORTH-RTM (Foundation for Research and Technology-Hellas Radiative Transfer Model) on a monthly basis and 0.5°x0.625° spatial resolution using modern and improved datasets for clouds and aerosols. Validation against high-quality ground measurements confirmed RTM's reliability. Results show a global mean brightening of 0.88 Wm⁻²decade⁻¹ from 1984 to 2018, stronger over land (2.57 Wm⁻²decade⁻¹) than oceans (0.19 Wm⁻²decade⁻¹). Globally, changes in clouds (especially middle-level cloud amount (CA) and high-level cloud optical thickness (COT)) were the main GDB drivers. However, the contribution of aerosol optical depth (AOD) changes was remarkable over specific land areas with strong anthropogenic activity, such as Europe, India and East China. In the 80's and 90's changes in AOD were the main GDB driver, subsequently in the 2000s high-level cloud optical thickness contributed the most to GDB followed by the AOD changes, while finally in the 2010s both clouds and AOD had comparable contributions. Over land, AOD had a comparable contribution to GDB with that of clouds whereas the contribution of aerosols' asymmetry parameter (AP) and single scattering albedo (SSA), water vapor and ozone was quite small or insignificant.