Global terrestrial ecosystem has functioned as a strong carbon (C) sink since 1960s, and this C sink has demonstrated large interannual variability (IAV). However, whether tropical or semi-arid areas have contributed the most to the IAV of global land C sink and the environmental drivers behind such large IAV changes remain controversial. In this study, we used the CLM4.5 model to investigate relative contributions of tropical terrestrial ecosystems to the changes of the global land C sink. Model results show that tropical forests dominate the trend and IAV of net biome productivity (NBP) in two periods, with the contribution of 42% and 38% in 1960–2008 and 40% and 51% in 1980–2008, followed by the semi-arid area, grasslands and croplands. We also found that regional contributions to both the trend and IAV of global NBP have a significant positive correlation with regional NEP mean. Compared to Tropical Asia and Africa, Tropical South America is the largest contributor (51% and 59%) to the trend and variability of tropical NBP. This large contribution of tropical forests was simulated to be caused by its stronger sensitivity to the temperature variation than the precipitation change. Results further indicate that temperature in tropical regions plays a greater role than precipitation in semi-arid regions on the IAV of global land C uptake. Regional contributions to the changes of global land C sink depend on the region classification, study period, and how the responses of carbon processes to environmental factors are represented in the model. More evaluation efforts are needed to verify the simulated land C feedbacks and to reduce prediction uncertainty in the regional and global C sink.