River runoff from the world's major rivers is an important part of the hydrologic cycle. Runoff changes in response to global greenhouse-induced warming will have impacts in many areas, including agriculture, water resources, and land use. A global atmospheric model is used to calculate the annual river runoff for 33 of the world's major rivers for the present climate and for a doubled CO2 climate. The model has a horizontal resolution of 4° × 5°, but the runoff from each model grid box is quartered and added to the appropriate river drainage basin on a 2° × 2.5° resolution. The computed runoff depends on the model's precipitation, evapotranspiration, and soil moisture storage. For the doubled CO2 climate, the runoff increased for 25 of the 33 rivers, and in most cases the increases coincided with increased rainfall within the drainage basins. There were runoff increases in all rivers in high northern latitudes, with a maximum increase of 47%. At low latitudes there were both increases and decreases ranging from a 96% increase to a 43% decrease. The effect of the simplified model assumptions of land-atmosphere interactions on the results is discussed.