The flux of heat from human activities such as commercial energy use, renewable source combustion, and the human metabolism has been incorporated into a variable resolution global weather forecast model, along with improved urban surface roughness and radiative properties. Sensitivity studies of these changes were used to show that the addition of anthropogenic heat improves the accuracy of surface air temperature forecasts. The addition of urban surface radiative properties has a secondary effect on the forecast temperature, and the addition of urban surface roughness changes has a minimal effect. Comparisons between observed and forecast boundary layer heights suggest that this parameter is poorly predicted by the model employed here, but that the impact of anthropogenic heating is likely to be a substantial increase in PBL heights over urban regions. Decreased atmospheric stability is also evidenced by comparisons of the diffusion constants for heat and moisture between the original and modified models, which show increases from a factor of 2 to a factor of 16 near the surface, depending on the size of the city. An examination of the effects of spatial averaging on heat flux suggests that significant sub-gridscale anthropogenic heating effects may occur, and implies that the results of the current simulations represent lower bounds. The simulations suggest that anthropogenic heat flux has a large local impact, with important implications for simulations of air-quality and radiative balance on regional and global scales. Crown Copyright (c) 2006 Published by Elsevier Ltd. All rights reserved.