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Alteration of land use will continue to be the dominant driver of environmental change in the tropics for the next several decades. It can take the form of fundamental vegetation cover transformation, or of intensification of existing land use without substantial change in cover type. Atmospheric composition changes and resultant climate changes could become ecologically significant within the next century. Changes in atmospheric composition in the tropics are essentially the same as those in higher latitudes, despite differences in the source and sink strengths for trace gases. Such changes can affect the functioning of tropical ecosystems through several processes, principally those related to carbon and nutrient assimilation and their interactions. Atmospheric composition may also have an indirect affect on tropical ecosystems via its effects on the climate. Predicted temperature increases in the tropics are less extreme than at high latitudes, but could still be biologically significant, especially at the tropical margins. The structure and productivity of ecosystems of the subhumid and dry tropics are very sensitive to changes in water balance, which could be caused by a combination of changes in precipitation and temperature. It is presently not possible to predict rainfall changes at ecologically meaningful scales with any confidence.