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

<jats:title>Abstract</jats:title> <jats:p>Rising atmospheric CO<jats:sub>2</jats:sub> concentration ([CO<jats:sub>2</jats:sub>]) enhances photosynthesis and reduces transpiration at the leaf, ecosystem, and global scale via the CO<jats:sub>2</jats:sub> fertilization effect. The CO<jats:sub>2</jats:sub> 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<jats:sub>2</jats:sub> 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<jats:sub>2</jats:sub> 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<jats:sup>−1</jats:sup>; percentile per rising ppm of [CO<jats:sub>2</jats:sub>]) and a concomitant decrease in transpiration (−0.073% ± 0.006% ppm<jats:sup>−1</jats:sup>) due to rising [CO<jats:sub>2</jats:sub>]. Enhanced GPP from CO<jats:sub>2</jats:sub> fertilization after the baseline year 2000 is, on average, 1.2% of global GPP, 12.4 g C m<jats:sup>−2</jats:sup> yr<jats:sup>−1</jats:sup> or 1.8 Pg C yr<jats:sup>−1</jats:sup> at the years from 2001 to 2014. Our result demonstrates that the current increase in [CO<jats:sub>2</jats:sub>] could potentially explain the recent land CO<jats:sub>2</jats:sub> sink at the global scale.</jats:p>