Cropland biophysical impacts on land surface temperature show diurnal differences across tropical Africa

Africa has undergone the largest cropland expansion worldwide over the past two decades, yet its biophysical impacts on land surface temperature (Ts), particularly its diurnal variations, remain elusive. Here, we quantify the diurnal cycles of Ts differences (ΔTs) between croplands and nearby grasslands over tropical Africa using 17 years of geostationary satellite observations and a space-for-time substitution approach. We find that croplands consistently cool the surface at night relative to surrounding grasslands, while their daytime effects depend strongly on hydroclimatic conditions, with daytime ΔTs being negative in more arid regions and positive in less arid regions. Analyses of biophysical mechanisms suggest that cropland-induced changes in turbulent heat fluxes are the dominant drivers of ΔTs, primarily resulting from differences in leaf area index (ΔLAI). During daytime in less arid regions, reduced cropland turbulent heat fluxes, resulting either directly from decreased ΔLAI or indirectly via surface albedo-induced changes in surface radiative energy, lead to surface warming. In all other cases, enhanced cropland turbulent heat fluxes drive surface cooling. These findings provide a mechanistic understanding of how cropland expansion alters local climate across hydroclimatic gradients and highlight the potential risk of intensified daytime warming in less arid regions associated with cropland expansion.