Large divergence of satellite and Earth system model estimates of global terrestrial CO2 fertilization

Atmospheric mass balance analyses suggest that terrestrial carbon (C) storage is increasing, partially abating the atmospheric [CO2] growth rate(1), although the continued strength of this important ecosystem service remains uncertain(2-6). 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')(5-8). Here, we present a new satellite-derived global terrestrial NPP data set(9-11), 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(12-15), 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(16-20) and/or a lack of representation of nutrient constraints(21-25). 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.