The role of tropical forests in climate is most often expressed in terms of the carbon they keep out of the atmosphere if deforestation is avoided or the carbon they remove from the atmosphere as they grow. The direct role of forests, particularly in the tropics, in maintaining low surface temperatures and relatively high precipitation has been underappreciated. Recent studies in the Brazilian agricultural frontier indicate that tropical deforestation, for pasture and crop production, has led to significant regional climate change in the last 40 years of a scale much larger than that attributed to the carbon released from deforestation. Deforestation reduces net surface radiation and evapotranspiration, thus increasing sensible heat flux and land surface temperature. In Mato Grosso state, the temperature of the forested Xingu Indigenous Park is 3℃ cooler than the surrounding mosaic of pasturelands, croplands, and remaining forest fragments. In the neighboring state of Rondônia, rainfall has significantly decreased and the dry season lengthened as deforestation occurred. Numerical model studies strongly suggest that Brazil’s agricultural frontier will be much warmer and dryer in coming decades as greenhouse gas concentrations increase. Thus, in Brazil, it is becoming clear that, because of their capacity to moderate regional climate, preserving tropical forests will be a key component of mitigating exogenously driven future climate change.