Response of Terrestrial Net Primary Production to Quadrupled CO(2) Forcing: A Comparison between the CAS-ESM2 and CMIP6 Models

SIMPLE SUMMARY: Terrestrial ecosystems, a key part of the Earth system, can absorb about 31% of CO(2) emissions generated by human activities, which slows increasing atmospheric CO(2) concentrations and global climate warming. The ways in which terrestrial net primary production responds to elevated CO(2) and associated climate change is a key topic of climate sciences. This study estimated the responses of net primary production to quadrupled CO(2) using 23 Earth system models, and analyzed the underlying causes of uncertainties among the models. The results showed that all of the models projected positive responses of net primary production, with an averaged magnitude of 44.78 PgC year(−1) and an uncertainty of ±20.93 PgC year(−1). The uncertainty came from many sources, and this study emphasizes the role of different projections in CO(2)-induced climatic anomalies and different climate sensitivities. These results provide a reliable estimation of global net primary production response to quadrupled CO(2), and point out that more understanding and improvements are needed for Earth system models to represent both physical climatic processes and the terrestrial carbon cycle. ABSTRACT: Terrestrial net primary production (NPP) is a key carbon flux that changes with rising atmospheric CO(2) and CO(2)-induced climate change. Earth system models are commonly used to investigate these NPP changes because of their fundamentally trustworthy ability to simulate physical climate systems and terrestrial biogeochemical processes. However, many uncertainties remain in projecting NPP responses, due to their complex processes and divergent model characteristics. This study estimated NPP responses to elevated CO(2) and CO(2)-induced climate change using the Chinese Academy of Sciences Earth System Model version 2 (CAS-ESM2), as well as 22 CMIP6 models. Based on CMIP6 pre-industrial and abruptly quadrupled CO(2) experiments, the analysis focused on a comparison of the CAS-ESM2 with the multi-model ensemble (MME), and on a detection of underlying causes of their differences. We found that all of the models showed an overall enhancement in NPP, and that CAS-ESM2 projected a slightly weaker NPP enhancement than MME. This weaker NPP enhancement was the net result of much weaker NPP enhancement over the tropics, and a little stronger NPP enhancement over northern high latitudes. We further report that these differences in NPP responses between the CAS-ESM2 and MME resulted from their different behaviors in simulating NPP trends with modeling time, and are attributed to their different projections of CO(2)-induced climatic anomalies and different climate sensitivities. These results are favorable for understanding and further improving the performance of the CAS-ESM2 in projecting the terrestrial carbon cycle, and point towards a need for greater understanding and improvements for both physical climatic processes and the terrestrial carbon cycle.