We introduced an adjoint technique to study sensitivity of the Earth's radiation budget (ERB) to cloud cover, water vapor, atmospheric temperature, and the Earth's surface temperature. This technique allowed us to calculate the partial derivatives of a defined ERB function with respect to all of the above variables on the model grid points by a single calculation of the adjoint model. We employed the radiation model of the National Centers for Environmental Prediction medium-range forecasting system to perform such an adjoint sensitivity analysis. For clear sky the absorbed shortwave radiation (ASR) was much more sensitive to the water vapor in the middle troposphere at high latitudes in the summertime hemisphere than at the tropical and subtropical areas. The outgoing longwave radiation (OLR) was 1 order of magnitude more sensitive to water vapor mixing ratios in the upper troposphere than to those in the middle and lower troposphere. In the tropics, more than 80% of a perturbation increase in the Earth's surface emission was trapped by the clear-sky column atmosphere, while only about 60--70% is trapped at middle and high latitudes. Rapidly decreasing latitude bands of trapping were found within the subtropics, and the semipermanent troughs over the central oceans displayed a significant effect. For the case of cloudy sky the ASR was more sensitive to low clouds than to middle and high clouds. The most sensitive low clouds tended to be located at midlatitudes rather than in the tropical regions. The OLR, as expected, was most sensitive to high clouds and displayed similar sensitivity to middle clouds. The net ERB was most sensitive to low clouds at midlatitudes in the summer hemisphere. The relationship between the results obtained and the greenhouse effect over tropical oceans as well as the influence of clouds on the climate change were explored, and some model deficiencies were discussed. ¿ 1998 American Geophysical Union