Recent modifications to MODTRAN3, a 2 cm^-1 resolution radiative transfer model, have permitted its transformation into a full flux divergence algorithm. It is now possible to calculate heating and cooling rates arising from both thermal and solar sources. The thermal calculations have been validated against benchmark line-by-line (LBL) calculations provided as an outgrowth of the InterComparison of Radiation Codes used in Climate Models (ICRCCM), organized by Elligson and colleagues. The MODTRANS comparisons for separate species (H₂O, CO₂, and O₃) were all of sufficient quality, usually falling within the range of LBL comparisons, to warrant the extension to other species. Preliminary studies of the energy balance of the atmosphere and effects on global warming due to increased levels of both natural and man-made contaminants are now possible.