Large-scale conversion of tropical forests into pastures or annual crops could lead to changes in the climate. We have used a coupled numerical model of the global atmosphere and biosphere (Center for Ocean-Land- Atmosphere GCM) to assess the effects of Amazonian deforestation on the regional and global climate. We found that when the Amazonian tropical forests were replaced by degraded grass (pasture) in the model, there was a significant increase in the mean surface temperature (about 2.5°C) and a decrease in the annual evapo-transpiration (30% reduction), precipitation (25% reduction), and runoff (20% reduction) in the region. The differences between the two simulations were greatest during the dry season. The deforested case was associated with larger diurnal fluctuations of surface temperature and vapor pressure deficit; such effects have been observed in existing deforested arms in Amazonia. The calculated reduction in precipitation was larger than the calculated decrease in evapotranspirat... Abstract Large-scale conversion of tropical forests into pastures or annual crops could lead to changes in the climate. We have used a coupled numerical model of the global atmosphere and biosphere (Center for Ocean-Land- Atmosphere GCM) to assess the effects of Amazonian deforestation on the regional and global climate. We found that when the Amazonian tropical forests were replaced by degraded grass (pasture) in the model, there was a significant increase in the mean surface temperature (about 2.5°C) and a decrease in the annual evapo-transpiration (30% reduction), precipitation (25% reduction), and runoff (20% reduction) in the region. The differences between the two simulations were greatest during the dry season. The deforested case was associated with larger diurnal fluctuations of surface temperature and vapor pressure deficit; such effects have been observed in existing deforested arms in Amazonia. The calculated reduction in precipitation was larger than the calculated decrease in evapotranspirat...