## 5 Conclusions  
Our ability to assess and quantify the role of terrestrial ecosystems in carbon cycle is largely dependent on how well we can interpret the NPP dynamics under prevailing climatic condition. The objective of this study was to study the role of terrestrial ecosystem in the carbon cycle based on the most recent GIMMS-NDVI data and simulated NPP by five land-atmosphere models of the TRENDY project for the period 1982-2012. The results showed that simulated NPP corresponded with the NDVI trends in a temporal sense, but less so in a spatial sense. The simulated NPP, especially the mean ensemble of five models, reproduced general vegetation dynamics represented by NDVI. The global NPP was estimated to be about 63 Pg C \(\cdot\) y\({}^{-1}\), with an increasing trend of 0.214 Pg C \(\cdot\) y\({}^{-1}\). Similarly, the global mean NDVI was estimated to be 0.33, with an increasing trend of 0.00041 y\({}^{-1}\). Overall, both NPP and NDVI showed higher positive trends in the Northern Hemisphere compared to the Southern Hemisphere. Higher trends of NPP and NDVI in boreal regions (dominated by forests) suggest increased vegetation cover, likely due to the earlier onset and longer growing seasons. Similarly, in Temperate regions, higher NPP and NDVI trends support the suggested role of precipitation, nitrogen deposition and nitrogen fertilizer applications. We observed some NPP and NDVI decreasing trends, both in the Southern Hemisphere (possibly due to the effect of water deficit and higher temperature on plant growth) and in the Northern Hemisphere (possibly due to combined effects of regional droughs and wildfires on plant growth). A latitudinal trends analysis showed higher variations over Tropical and Extra Tropics. Overall, this study showed a general agreement between temporal trends in NDVI and the NPP, both for ensemble and for individual models simulations. Some of the discrepancies between NPP and NDVI can be partly attributed to the uncertainties associated with NDVI measurements (e.g., multiple satellite sensors, atmospheric conditions and corrections) and models formulations and assumptions (e.g., land-cover classification, associated model parameterization and interpolation effects in climate data). The scientific insight obtained from the spatial and temporal analyses of NPP and NDVI, for multiple decades encompassed in this study, is guiding our continued research on the contribution of terrestrial ecosystems to the global carbon cycle using a combination of models and observations in data assimilation mode. Our ultimate goal is to determine the regional and global magnitude of sources and sinks of carbon in terrestrial ecosystems, and their inter-annual and longer-term variability and change due to prevailing climate conditions.  
This study was funded primarily by a Laboratory Directed Research and Development project sponsored by the Pacific Northwest National Laboratory of the U.S. Department of Energy. We acknowledge the contribution of by the TRENDY project teams for allowing us to use their simulated NPP data. We thank three anonymous reviewers for their valuable comments that led to further improvement of the manuscript. All authors contributed to the design of this research and writing of manuscript. [PERSON] and [PERSON] analyzed the data. [PERSON] and [PERSON] wrote the manuscript. [PERSON] and [PERSON] reviewed and offered suggestions for further improvements of the manuscript. The authors declare no conflict of interest.