Trend and Statistical Analysis of Photosynthetically Active Radiation (PAR) Across the Three Major Climatic Zones in Nigeria for 41 Years (1984–2024)

Photosynthetically Active Radiation (PAR, 400–700 nm) is a key driver of photosynthesis, biomass formation, and crop yield, especially in the tropics where rainfed agriculture prevails. The present research examines the long-term variability and trends of PAR over Nigeria's three predominant climatic zones Humid Tropical Rainforest, Tropical Savanna, and Semi-Arid Sahel over a period of 41 years (1984–2024). NASA satellite-derived PAR data were examined using non-parametric statistical techniques such as Mann-Kendall trend detection test, Sen's slope estimator for rate estimation, and distribution-based analyses like kernel density estimation (KDE) and boxplots. Results show a statistically significant downward trend in PAR in all zones with Sen's slope ranging from –0.16 to –0.44 W/m² per year. The Humid Zone recorded the lowest PAR values due to constant cloud cover and excess rainfall, while the Tropical Savanna recorded mid-values, which were suitable for diverse cropping systems. The Semi-Arid Zone recorded the highest PAR levels but also with a significant decline, together with short wet seasons and high evapotranspiration. These declines in PAR have immediate consequences on agricultural productivity, ecosystem resistance, and food security, especially for staple crops like maize, sorghum, and millet that are radiation-sensitive. This study provides the first multi-decadal Nigeria-specific analysis of PAR trends for various climatic zones, and it offers important information to inform agricultural planning, climate adaptation, and policy development. Increased ground-level monitoring, adoption of climate-smart agriculture, and regional collaboration are recommended in an effort to mitigate the adverse impacts of reducing solar radiation on agriculture and livelihoods.