Inferring CO2 fertilization effect based on global monitoring land-atmosphere exchange with a theoretical model

Rising atmospheric CO _2 concentration ([CO _2 ]) enhances photosynthesis and reduces transpiration at the leaf, ecosystem, and global scale via the CO _2 fertilization effect. The CO _2 fertilization effect is among the most important processes for predicting the terrestrial carbon budget and future climate, yet it has been elusive to quantify. For evaluating the CO _2 fertilization effect on land photosynthesis and transpiration, we developed a technique that isolated this effect from other confounding effects, such as changes in climate, using a noisy time series of observed land-atmosphere CO _2 and water vapor exchange. Here, we evaluate the magnitude of this effect from 2000 to 2014 globally based on constraint optimization of gross primary productivity (GPP) and evapotranspiration in a canopy photosynthesis model over 104 global eddy-covariance stations. We found a consistent increase of GPP (0.138 ± 0.007% ppm ^−1 ; percentile per rising ppm of [CO _2 ]) and a concomitant decrease in transpiration (−0.073% ± 0.006% ppm ^−1 ) due to rising [CO _2 ]. Enhanced GPP from CO _2 fertilization after the baseline year 2000 is, on average, 1.2% of global GPP, 12.4 g C m ^−2 yr ^−1 or 1.8 Pg C yr ^−1 at the years from 2001 to 2014. Our result demonstrates that the current increase in [CO _2 ] could potentially explain the recent land CO _2 sink at the global scale.