Sensitivity of global gross primary production to elevated CO2

Terrestrial ecosystems have accounted for more than half of the global carbon sink during the past decades and offset 25%-30% of current anthropogenic CO₂ emissions. The projected increase in CO₂ concentration will depend on the magnitude of terrestrial plants’ feedback to CO₂: i.e. the sensitivity of plant carbon uptake in response to elevated CO₂, and the strength of the CO₂ fertilization effect (CFE) in a changing (and warming) environment. Projecting vegetation responses to future increases in CO₂ concentration under climate change is a major uncertainty, as ecosystem models, field experiments and satellite-based models show large disagreements. In this study, using a recently developed, parameter-sparse model (the ‘P model’), we assess the sensitivity of GPP to increasing CO₂ under idealized conditions, in comparison with other vegetation models and field experiments. We investigate the impact of two central parameters, the ratio of J_max to V_cmax (at a common temperature) and the curvature of the light response curve, on the sensitivity of GPP to CO₂. We also quantified the spatial-temporal trend of CFE using the β factor, defined as the percentage increase in GPP in response to a 100-ppm increase in atmospheric CO₂ concentration over a defined period. We show how modelled β has changed over the satellite era, and infer the possible effect of climatic variables on changes of CFE from spatial patterns of the modelled trend in β.