This assessment by the Environmental Effects Assessment Panel (EEAP) of the United Nations Environment Programme (UNEP) provides the latest scientific update since our most recent comprehensive assessment (Photochemical and Photobiological Sciences, 2019, 18, 595–828). The interactive effects between the stratospheric ozone layer, solar ultraviolet (UV) radiation, and climate change are presented within the framework of the Montreal Protocol and the United Nations Sustainable Development Goals. We address how these global environmental changes affect the atmosphere and air quality; human health; terrestrial and aquatic ecosystems; biogeochemical cycles; and materials used in outdoor construction, solar energy technologies, and fabrics. In many cases, there is a growing influence from changes in seasonality and extreme events due to climate change. Additionally, we assess the transmission and environmental effects of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is responsible for the COVID-19 pandemic, in the context of linkages with solar UV radiation and the Montreal Protocol.

Many field experiments have indicated that litter decomposition in semi-arid areas may be partly or fully controlled by photodegradation. We devised a study to test our hypothesis that light exposure makes plant litter more degradable. Dry, senescent, aboveground plant litter from Miscanthus x giganteus was exposed to light including ultraviolet (UV) radiation for various lengths of time from 0 to 289 days. Weight loss was measured after exposure and appeared to be modest and did not increase with time of exposure. The litter of the longest and shortest exposure time as well as controls were then incubated with soil and moisture for 35 days and CO 2 and N 2 O production were measured. The longest exposed litter degraded much faster than any other treatment during incubation with moisture, about twice as fast as the unexposed control. The shortest exposed however, degraded only slightly faster than the unexposed control. This suggests that increasing litter degradability is a more important mechanism for photodegradation than direct light-induced mass loss. N 2 O production from decomposition of the exposed litter was high in the beginning, suggesting that nitrogen may be released quickly. The mechanism is probably that light exposure leaves the nitrogen in plant litter easily available to microbial utilisation upon wetting. Such a mechanism might play an important role for nutrient cycling in semi-arid areas.

Black carbon (BC), the residue from burning with insufficient oxygen supply, is assumed to be very stable in the environment. Here we present a simple model for BC movement and decomposition in soils based on the assumption that BC consists of two fractions with different turnover time, and that BC can move in the environment as well as decompose. Decomposition rate was calibrated against laboratory data, whilst a recent field experiment was used to calibrate losses from downward movement through the soil profile. Losses by erosion are still poorly quantified, but mass balance indicates that they may be one of the most important fluxes. The model was able to acceptably predict CO 2 production from BC as well as BC left in the soil at the end of the experiment, although BC in the subsoil was underestimated. The model was sensitive to erosion rate (varied ±50%), moisture and temperature response function on a 100-year time scale. The model was not sensitive to the decomposition rate of the stable pool on a 100 year time scale, but it was very sensitive to that on a millennial time scale. Implications and directions for future research are discussed.

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