ALBERT

Description: The identification of the existence of decadal trends in Surface Solar Radiation (SSR), also known as Dimming (negative) and Brightening (positive), has significantly contributed in the last few decades to the understanding of the changes in the radiative balance in the atmosphere. Various studies were dedicated to pointing out and identifying reasons for SSR trends in different regions of the world, especially in first world countries. In this context, however, the aerosol-cloud interactions represent a major uncertainty. While previous studies have highlighted the individual contributions of aerosols and clouds to Global Dimming and Brightening (GDB), accounting for their interactions has been found to be a challenging task. In this study we present observational SSR trends in different parts of the world, and with the use of additional data (clear-sky SSR, cloud cover, AOD, etc.) we estimate contributions from each component. The results are discussed in the context of the conceptual framework on the role of aerosol and clouds in Dimming and Brightening, thus the focus is given on the contrast between GDB at stations in polluted and in pristine areas. Results indicate an enhancement of the trends by aerosol-cloud interactions in pristine regions, such as the Pacific Ocean and the Alps. Results from polluted areas, such as South and East Asia are also discussed. Challenges and uncertainties are further highlighted. The study is part of efforts to better understand the causes of GBD, an important step towards addressing its impacts and feedbacks on past and future climates.

Description: Decadal changes in observed surface solar radiation (SSR), coined dimming and brightening, are of interest for a number of reasons ranging from energy balance considerations to long-term planning of photovoltaic energy production. Decadal trends in anthropogenic aerosol have been advocated as one of the main drivers for the observed SSR phenomenon. Given the historical emission scenario, coupled climate models seem, however, unable to reproduce the observed SSR trends: their magnitudes, onset times, and lengths. In order to get more insight into these discrepancies, we analyse data from the historical simulations within the Coupled Model Intercomparison Project - Phase 6 (CMIP6). For any single model, we compare different ensemble members, which by design are different realizations (initial conditions) with identical forcings (boundary conditions). Thus any differences within the ensemble members are due to internal variability. We examine aerosol emissions, aerosol optical thickness (AOD), all-sky and clear-sky SSR on a per grid box level. It is intriguing to find that given identical emission scenarios, the onsets of the strongest AOD trends differ across ensemble members, as do the onsets of strongest SSR trends, differing among ensemble members by 10 years or more. Another highlight of the study is that SSR (all-sky and clear-sky) across different ensemble members varies less in certain locations (e.g. Europe and N. Asia) and greatly in others (Australia, Africa, S. America), possibly due to stronger forcing and less natural variability in the former regions, while the opposite in the latter regions.

Description: Various studies have examined internal variability of annual mean surface solar radiation. The variability was statistically characterized and a close link with decadal scale trends was demonstrated. The latter were shown to regionally bear an imprint from low frequency modes of sea surface temperature variability. Here we extend this analysis to further components of Earth's energy balance.