Quantifying the response of terrestrial carbon fluxes to future climate change: Results from CMIP5 Earth System Models simulations

Quantification of the terrestrial ecosystem feedback is crucial for better prediction of future global climate-carbon cycle. However, previous studies using coupled climate-carbon models show large uncertainties of terrestrial carbon-climate feedbacks. While the largest uncertainty in feedbacks is induced by the models' responses to radiative and physiological CO2, recent studies demonstrate that the N cycle and land cover change could have significant impacts. Here, we explore the responses of carbon fluxes between atmosphere and land to various forcing using five experiments (i.e., historical, RCP45, RCP85, esmFdbk2, esmFixClim2) from 14 CMIP5 Earth System Models (ESMs). The simulated modern global net primary productivity (NPP) ranges from 46.1 Pg C yr-1 in CCSM4 to 90.8 Pg C yr-1 in MPI-ESM-LR, with an ensemble mean of 68.6 Pg C yr-1. Eleven of the fourteen ESMs substantially overestimate current global NPP as calculated from MOD17A3 dataset (53.5 ± 1.7 Pg C yr-1). All models predicted an increase of global NPP in both RCP concentration scenarios. With N limitation, CCSM4 and NorESM1-M simulated the lowest current NPP and future relative increase of NPP in RCP concentration scenarios. A comparison of the coupled and uncoupled CO2-climate experiments indicates that while the increase of NPP north of 45°N is a combined effect of CO2 fertilization and global warming, the increase of global NPP in the RCP4.5 concentration scenario is dominated by CO2 fertilization. In general, the magnitude of NPP increases caused by CO2-induced warming and/or CO2-fertilization in each model is significantly correlated with its simulated modern NPP value. Similar to NPP, the increase of terrestrial heterotrophic respiration carbon fluxes (RH) and fire emission carbon fluxes (fFire) in RCP concentration scenarios are tightly correlated with their modern intensity among the CMIP5 ESMs. Consequently, the simulated response of net ecosystem productivity (NEP = NPP - RH - fFire) in RCP scenarios is correlated with its modern magnitude. Despite different magnitudes, all ESMs predict that the land acts as a sink of carbon in both RCP scenarios without land cover change. In contrast, when including land cover change, two and five ESMs suggest that the land will become a source of carbon with the RCP4.5 and RCP8.5 scenarios, respectively. Thus, future land cover change, which is very uncertain, is consistently predicted to contribute significantly to the terrestrial carbon-climate feedback. In sum, our results suggest that (1) an accurate representation of the modern terrestrial carbon fluxes is critical to constrain future carbon-climate feedbacks and (2) in addition to CO2-fertilization and CO2-induced warming, the N cycle and land cover change are important for predicting future carbon-climate interactions.