Twenty-first century surface UV radiation changes deduced from CMIP6 models. Part II: effects on UV index and plant growth weighted irradiance.

This paper investigates the evolution of changes in surface ultraviolet (UV) radiation globally, emphasizing the significant impacts of key factors influencing its variability, i.e., total column ozone, aerosols, clouds, and surface reflectivity. Simulations of UV radiation were performed by the UVSPEC radiative transfer model and span from the mid-twentieth century to the end of the twenty-first century. Input data were derived from eleven Earth System Models that participated in the 6th Phase of the Coupled Model Intercomparison Project (CMIP6). The UV Index (UVI) simulations for the late twentieth century indicate an increase in UVI levels relative to the 1950s in the Southern Hemisphere's mid and high latitudes, attributed to ozone depletion, and decreases in southeastern Asia due to increases in aerosols. Projections of changes in UVI for the last decade of the twenty-first century were derived for three Shared Socioeconomic Pathways (SSPs): SSP1-2.6, SSP3-7.0, and SSP5-8.5. Under SSP1-2.6, the scenario with the lowest greenhouse gas (GHG) and aerosol emissions, UVI is projected to increase relative to the 1950s by up to 20% in Europe and North America and to decrease by as much as - 10% over tropical and polar regions. Under SSP3-7.0 and SSP5-8.5, scenarios with higher GHG and aerosol emissions, UVI changes are generally negative globally due to ozone recovery and increases in aerosol optical depth, while localized positive changes are found over Central and South America, Europe, Africa, and the Pacific and Indian Oceans. The changes in the biologically effective solar irradiance for plant growth exhibit similar geographical patterns to UVI with slight differences, due to weaker sensitivity to changes in ozone.