Terrestrial ecosystems currently take up as much as one third of anthropogenic CO2 emitted annually to the earth¿s atmosphere. Part of this biospheric sink is the result of photosynthesis being larger than respiration because of the CO2 fertilization, but several modeling studies indicate that this sink is sensitive to changes in the CO2 concentration of the atmosphere and climate. Here we present results from an empirical evaluation of photosynthesis and carbon sink sensitivity to projected CO2 and climate change, using a large-scale rainforest mesocosm. The direct CO2 fertilization on photosynthesis in the rainforest in Biosphere 2 was measured over a wide range of CO2 concentrations (400-1200 ppmv). Responses at the whole mesocosm scale were similar to that observed at smaller scales in experiments with leaves and isolated enzymes. This provides strong support for widely used approaches for scaling from process-based studies to global biospheric responses. The measurements indicate furthermore that the sensitivity of photosynthesis decreases with increasing CO2 concentration and biological organization level. To explore the significance of different photosynthetic response functions for the development of the carbon sink, we simulated the carbon content of the global tropical rain forest using a simple one-box model. The model simulations based on our experimental data indicate that the carbon sink in world tropical rainforests will reach its maximum within the next few decades, then decrease and eventually become a source, even with moderate assumptions about turnover time of the biosphere and climate sensitivity to increases in the atmospheric CO2 concentration.