Atmospheric mass balance analyses suggest that terrestrial carbon (C) storage is increasing, partially abating the atmospheric [CO2 ] growth rate, although the continued strength of this important ecosystem service remains uncertain. Some evidence suggests that these increases will persist owing to positive responses of vegetation growth (net primary productivity; NPP) to rising atmospheric [CO2 ] (that is, CO2 fertilization'). Here, we present a new satellite-derived global terrestrial NPP data set, which shows a significant increase in NPP from 1982 to 2011. However, comparison against Earth system model (ESM) NPP estimates reveals a significant divergence, with satellite-derived increases (2.8 ± 1.50%) less than half of ESM-derived increases (7.6±1.67%) over the 30-year period. By isolating the CO2 fertilization effect in each NPP time series and comparing it against a synthesis of available free-air CO2 enrichment data, we provide evidence that much of the discrepancy may be due to an over-sensitivity of ESMs to atmospheric [CO2 ], potentially reflecting an under-representation of climatic feedbacks and/or a lack of representation of nutrient constraints. Our understanding of CO2 fertilization effects on NPP needs rapid improvement to enable more accurate projections of future C cycle-climate feedbacks; we contend that better integration of modelling, satellite and experimental approaches offers a promising way forward.
Bibliographical noteFunding Information:
This work was supported by the US Geological Survey Ecosystems Mission Area, US Department of Energy Terrestrial Ecosystem Sciences Program (Award no. DE-SC-0008168), the US Geological Survey John Wesley Powell Center for Analysis and Synthesis, and the NASA Earth Observing System MODIS project (grant no. NNX08AG87A). Y.Y.L. is supported by an Australian Research Council DECRA Fellowship (project number DE140100200). We also acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modelling groups (listed in Supplementary Table 1 of this paper) for producing and making available their model output. For CMIP, the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US Government. All data presented in this analysis are publicly available; satellite estimates are available at the NTSG data portal (http://www.ntsg.umt.edu/data); CMIP5 experimental scenario data are available at the ESGF data portal (http://esgf.llnl.gov).
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