Projections of changes in ecosystem productivity under 1.5 °C and 2 °C global warming

The ecosystem plays an important role in mitigating global warming through carbon assimilation. In addition, climate change has both positive and negative impacts on ecosystem productivity. To combat climate change, the Paris Agreement has set the goal of limiting global warming well below 2 °C with an ultimate target of 1.5 °C above preindustrial levels. It remains unclear how ecosystem productivity will respond to these warming targets. Here, we project global changes in gross (GPP) and net (NPP) primary productivity under 1.5 °C/2 °C warming using a dynamic vegetation model coupled with a state-of-art climate model. We use site-level observations to validate the present-day simulations and the multimodel ensemble to validate the future projections. We identify the main drivers of ecosystem changes by distinguishing between the impacts of CO2 fertilization, fast climatic responses to CO2 radiative perturbation, and slow climatic feedbacks to ocean warming due to the large heat capacity. Compared with the present day, global GPP increases by 11.5% (20.9%) and NPP increases by 14.8% (27.1%) under the 1.5 °C (2 °C) warming scenario with similar magnitudes of change in the tropics and subtropics. Changes in the leaf area index mainly follow those of GPP/NPP but with more significant enhancement in boreal regions. CO2 fertilization causes the dominant contributions to GPP changes over 83% of land areas. The slow responses to ocean warming cause negative changes in GPP/NPP and partly offset regional CO2 fertilization over the tropics, where the climate is projected to be drier due to atmosphere-ocean feedback. The fast climatic responses to CO2 radiative perturbation further decrease GPP/NPP but with a moderate magnitude. The projection suggests that ecosystem productivity will increase in a warmer climate but with large spatial heterogeneity.