Changes in Photosynthetically Active Radiation in Ukraine during 1961–2020 in the Context of Surface Radiation Budget Variability

Photosynthetically active radiation (PAR), the solar radiation absorbed by plants in the 380–710 nm wavelength range, represents a key component of the surface radiation budget and plays a central role in terrestrial carbon assimilation. Understanding long-term PAR variability is increasingly important in the context of regional climate change and shifts in radiative forcing that influence surface energy fluxes.In Ukraine, routine PAR measurements are not performed due to the absence of standard instrumentation. Therefore, the quantification of PAR and the assessment of its multi-decadal variability require indirect reconstruction methods. This study develops a long-term PAR database for the warm season (April–October) using actinometric observations and the components of the surface radiation balance. Direct, diffuse, and total PAR were calculated using established conversion coefficients applied to measured shortwave radiation components. Spatial and temporal patterns were analysed using statistical and cartographic methods.The primary study period is 1961–2020, complemented by several shorter sub-periods (1961–1990, 1991–2020, 1991–2000, 2001–2010, and 2011–2020) analysed comparatively to identify decadal and multi-decadal shifts in PAR components. Since the 1980s–1990s, consistent with global warming trends and associated radiative perturbations, the redistribution of solar radiation reaching the surface has been observed. These changes are linked to factors such as aerosol loadings, cloudiness variability, and large-scale circulation patterns, all of which affect the surface radiation budget.Results indicate that direct, diffuse, and total PAR exhibit pronounced spatial gradients, increasing from western and northwestern regions, including the Ukrainian Carpathians, toward the Southern Steppe and Crimea. An increase in direct solar radiation during 2001–2010 relative to 1991–2000, and again in 2011–2020 relative to 1991–2000, resulted in marked increases in direct PAR. Conversely, declines in diffuse solar radiation resulted in reduced diffuse PAR, while heterogeneous changes in total shortwave radiation produced corresponding fluctuations in total PAR.These findings highlight the sensitivity of PAR to long-term changes in the surface radiation budget and contribute to understanding how regional climate change is modifying the radiative environment that underpins terrestrial productivity.