Changes in atmospheric carbon dioxide have been proposed as a major regulator of climate during the last 570 million years. Continental weathering and its variation over time are hypothesized to be important for controlling atmospheric carbon dioxide. Continental weathering is altered by changes in total runoff as well as changes in the size and elevation of the land masses to be weathered. When paleogeographic information for the Phanerozoic (570 m.y. ago to present) is used in a global climate model, the model exhibits substantial variations in precipitation, evaporation, and runoff. Even with dramatically different land-ocean distributions, an increase in global surface temperature leads to an increase in global precipitation but not always an increase in global runoff. During the early Phanerozoic (514--342 Ma), when the continents are relatively small and widely distributed, runoff depends on continental evaporation and temperature. As the continents migrate into subtropical latitudes, global runoff decreases as global land temperatures and evaporation increase. As the continents shift to higher latitudes, global runoff increases as global land temperatures and evaporation decrease. During the middle to late Phanerozoic (306 Ma-present), when large continental land masses predominate, runoff depends on continental precipitation. Experiments with increased atmospheric CO2 for the middle Ordovician (458 Ma), when the paleocontinent of Gondwana was centered at subtropical latitudes, and the early Silurian (425 Ma), when Gondwana had shifted to middle and high latitudes, also point to a correlation between land mass location and runoff. Global runoff increases 15% with increased CO2 at 425 Ma but remains unchanged for 458 Ma, even though global mean temperatures and precipitation increase comparable amounts for the two time periods. These results imply that weathering feedback between temperature and runoff may be dependent on land-ocean configuration. ¿ American Geophysical Union 1995