CMIP5 and CMIP6: contrasting perspectives on Nigeria's climate transformation

This study extensively compares CMIP5 and CMIP6 models in simulating historical and projected annual and seasonal climate variability over Nigeria. Thirteen Global Climate Models (GCMs) from both CMIPs were considered and compared for two future projections of the radiative concentration pathways (RCP 4.5 and 8.5) and that of shared socioeconomic pathways (SSP2-4.5 and 5-8.5). This study delves deeper into climate modeling than any previous CMIP model performance comparison by analyzing the CMIP's mean and median multimodel ensemble (MME), projection uncertainties, and spatial climate variability. The results indicated that CMIP6 models and their MMEs exhibited higher Kling-Gupta Efficiency (KGE) than CMIP5 models for rainfall, maximum temperature (Tmax), and minimum temperature (Tmin), indicating an improvement in CMIP6 models compared to their predecessors. CMIP6 models cluster closely, reflecting consensus, while CMIP5 models widely disperse, leading to bias and high-centered root-mean-square difference values, indicating inconsistency. The spatial pattern of CMIP6 MME simulation closely aligns with reference data, showing improved rainfall and temperature estimate reliability. RCP-4.5 and RCP-8.5 projected a Tmax rise of 1.8 degrees C and 4.12 degrees C, while SSP2-4.5 and SSP5-8.5 projected a rise of 3.2 degrees C and 1.12 degrees C by 2100. Tmin rises are projected 2.1 degrees C and 3.9 degrees C for RCP-4.5 and RCP-8.5, and 2.3 degrees C and 3.8 degrees C for SSP2-4.5 and SSP5-8.5. In the case of rainfall, CMIP5 MME projected a decrease in rainfall by -10% and - 12% for RCP-4.5 and RCP-8.5, while CMIP6 MME projected an increase by 16% and 23% for SSP2-4.5 and SSP5-8.5.