Impact of different ERA reanalysis data on GPP simulation

The performance of the terrestrial biosphere models (TBMs) is limited by the accuracy of climate forcing data. As the reanalysis products of the European Centre for Medium-Range Weather Forecasts (ECMWF), ERA-Interim and ERA5 data are widely used in the simulation of terrestrial carbon budgets and reveal their responses to climate change. However, the discrepancy between simulated carbon budgets driven by ERA-Interim and ERA5 on a global scale has not been evaluated. In this study, driven by ERA-Interim and ERA5, we conducted two simulations by a TBM, BEPS (Boreal Ecosystem Productivity Simulator), to investigate the differences of simulated Gross primary productivity (GPP) in temporal trends and spatial patterns and to identify the differences in climate factors resulted in the spreads of simulated GPPs. We found that by 2015, the relatively stable difference of simulated GPP by ERA-Interim and ERA5 was about 3.55 Pg C yr(-1). Since 2016, the differences of simulated GPP increased gradually and peaked in the last year of our simulation in 2018 at 13.16 Pg C yr(-1). This significant difference in GPP was due to the changes of GPP in the Amazon Basin, Congo Basin and South Asia, where tropical forests and tropical savannahs & grasslands were widely distributed. In these regions, the GPP of ERA5 in total was at least 3.0 Pg C yr(-1) lower than that of ERA-Interim after 2016. The difference of GPP in such regions was the main reason why ERA5 and ERA-Interim GPP showed different interannual variability. And less precipitation and higher temperature of ERA5 in tropical regions mainly results in the reduction of GPP compared with the results driven by ERA-Interim. Our results highlight challenges in using ERA5 and ERA-Interim to evaluate responses of ecosystem to climate change and provide implications for reducing the uncertainty of climate forcing data in simulating terrestrial carbon cycle.