On the basis of simulations of the fifth phase of the Coupled Model Intercomparison Project (CMIP5), we estimate the responses of net primary production (NPP) and net ecosystem production (NEP) to increases in atmospheric CO2 concentration and climate changes at global and regional scales.

The modeled NPP and NEP show significantly positive trends of approximately 0.4PgC/yr2 and 0.09PgC/yr2, respectively, in response to the rising atmospheric CO2. However, adverse trends of the two variables are driven by climate changes at the global scale. Regarding the spatial pattern, these decreases are mainly located in the tropical and temperate regions. Thus, the terrestrial carbon sink is accelerated not only by rising atmospheric CO2 concentration but also by the global warming at high latitude and altitude regions such as Tibet, Alaska, and Greenland.

Although the simulations indicate increases in NPP and NEP owning to CO2 fertilization effect, the strength of these trends significantly differs from that in the CMIP5 models. The enhanced trend in terrestrial carbon sink simulated by the Max Planck Institute Earth System Model at base resolution (MPI�ESM�LR) is shown to be approximately 47 times larger than that by the Community Earth System Model�Biogeochemistry (CESM�BGC) considering the CO2 fertilization effect. Differences in the modeled responses of NPP and NEP are attributed to the various processes of the land surface component accounting for the nitrogen limitation effect and plant function types (PFTs). In addition, we detected differences between CMIP5 models in accelerated terrestrial carbon loss forced by global warming of 6.0TgC/yr2 in CESM�BGC and 52.7TgC/yr2 in MPI�ESM�LR. Such divergence is responsible to some extent for the differences in simulated climate between the CMIP5 models, such as the difference in temperature of approximately 1.4K.