21^st century surface UV radiation changes deduced from CMIP6 models: part I-evolution of major influencing factors

For a given solar elevation, the levels of solar ultraviolet radiation at the Earth's surface are determined by the amounts of ozone, aerosols, and clouds, as well as by the reflectivity of the surface. Here, we study the evolution of these factors for three selected decades in the period 1950-2100 using results from simulations with Earth-System models (ESMs) participating in the 6^th phase of the Coupled Model Intercomparison Project (CMIP6). The simulations for the future are based on three Shared Socioeconomic Pathways: SSP1-2.6, SSP3-7.0, and SSP5-8.5. The models were grouped according to whether they use prescribed ozone fields or interactive chemistry schemes for ozone, revealing significant differences in the absolute levels and variability of total ozone column between the two groups of models. From mid-twenty-first century onward, the ozone recovery is evident in both groups under SSP3-7.0 and SSP5-8.5, but not under SSP1-2.6. The changes in the aerosol optical depth show distinct geographical patterns that are related to their sources, either natural (i.e., dust, biomass burning) or anthropogenic (industrial activities). The aerosols are generally more abundant in 1990-2000 compared to 1950-1960, particularly over regions with industrial activity, with a reversal of this pattern in 2090-2100. Most of these patterns are present in all three pathways, but with different signs compared to 1990-2000 in some regions (i.e., Europe, North America). Over areas with strong natural sources, the aerosol optical depth (AOD) in 2090-2100 increases further under all pathways. The changes in surface reflectivity are important mainly at the end of the twenty-first century and occur predominantly at the high and polar latitudes of both hemispheres, with reductions relative to 1950-1960 of up to 45% due to sea ice retreat. The alterations in the attenuation of shortwave solar radiation by changing cloudiness (expressed in the form of the cloud modification factor, CMF) are more evident at high latitudes, with decreases in 2090-2100 over the Arctic ranging from -5% (SSP1-2.6) to -13% (SSP5-8.5) and smaller decreases of up to -5% in the vicinity of the Antarctic coast. The simulations of ozone, aerosols, surface reflectivity, and clouds for the recent past (2003-2012) were compared to the Copernicus Atmosphere Monitoring Service (CAMS) reanalysis data, showing for total ozone better agreement to models with interactive ozone chemistry. The model-derived AOD shows significant differences from CAMS in various regions worldwide, with up to 0.2 higher values across the northern hemisphere. Finally, the comparisons for surface reflectivity and cloud effects οn this decadal scale reveal a general agreement between models and observations over most of the globe. Thus, we conclude that the projected changes have a good basis in the recent past, suggesting they are realistic estimates of how factors influencing solar ultraviolet radiation may differ under climate change.