Causes of uncertainty in China's net primary production over the 21st century projected by the CMIP5 Earth system models
Résumé
Net primary production is the initial step of the carbon cycle in which atmospheric CO2 is fixed by plants. The responses of net primary production (NPP) to climate change and CO2 are key processes that have the potential to significantly affect the climate–carbon feedback and future atmospheric CO2 levels. Understanding future NPP changes is important for China that became the world's largest CO2 emitter since 2006. Here, we analysed NPP changes in China under the four emission scenarios from 11 Earth system models participating in the Coupled Model Intercomparison Project Phase 5. We find a general increase of NPP over the 21st century under the four emission scenarios, with the large percentage increase in northwestern China and Qinghai–Tibetan Plateau. However, there is a large model spread in the increase of NPP at both country and local scales. We present a statistical approach to assess various processes to explain this large spread, and find that the large spread at the country level is predominantly attributed to inter-model difference in parameterization of CO2 fertilization effect within each emission scenario. But the parameterization of CO2 fertilization effect not always dominates over the model spread across China. When it comes to the local scale, the model spread can be significantly contributed by inter-model difference in parameterization of NPP responses to precipitation along with precipitation projection in northwestern China. Our findings provide the reasons for divergent responses of future NPP through process decomposition and are the first to pinpoint that the model process dominating over the uncertainty exhibits regional dependence.