Abstract
There is growing observational evidence that the level of sunlight received at the Earth's surface is not stable over the years but undergoes substantial decadal changes. These changes, known popularly as global dimming and brightening, are related to anthropogenic air pollution, as well as changes for cloudiness. Here, we summarize the evidence and discuss the implications of these variations in biospheric growth in general and agricultural applications in particular. Emphasis is placed on implications for forest canopies, grassland and agricultural crop production. Yet not only the total amount of sunlight that the biosphere receives is important but also the relative portion of the direct and diffuse light. Increasing air pollution and cloudiness can alter this partitioning towards a higher relative portion of diffuse light, in addition to a reduction in the total amount of sunlight. This diffuse light can enhance photosynthesis, particularly in tall and dense vegetation canopies, as it is distributed more effectively within the canopy compared with direct solar radiation, which is only accessible to the outermost (sunlit) layers. Here, we conclude that the growth of tall canopies such as forests or crop fields may be favoured under global dimming conditions owing to the enhanced diffuse light, despite the overall reduction in the total amount of sunlight. On the other hand, under the same dimming conditions, the yield of grassland, with its shallow open canopy, may be reduced, as it has little benefit from the enhanced diffuse fraction, while suffering from a loss in the total amount of sunlight for photosynthesis. Variations in solar radiation therefore may have largely varying effects on agricultural production depending on the structure of the crop canopy.