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

Masahito Ueyama,Takashi Hirano,Tomo’Omi Kumagai,Joon Kim,Dennis Baldocchi,Yukiko Matsuura ,Minseok Kang,Luca Belelli Marchesini,Takeshi Ohta,Yukio Kosugi ,Jason Beringer,Takashi Machimura,Kentaro Takagi,Keisuke Ono,Satoru Takanashi,Kazuhito Ichii,Andrej Varlagin,Yasuko Mizoguchi,Taku M Saitoh ,Hideki Kobayashi,Lutz Merbold,M Syndonia Bret‐Harte ,Takanori Shimizu,Eugénie Euskirchen ,Yukio Yasuda

doi:10.1088/1748-9326/ab79e5

Abstract

Rising atmospheric CO2 concentration ([CO2]) enhances photosynthesis and reduces transpiration at the leaf, ecosystem, and global scale via the CO2 fertilization effect. The CO2 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 CO2 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 CO2 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 [CO2]) and a concomitant decrease in transpiration (−0.073% ± 0.006% ppm−1) due to rising [CO2]. Enhanced GPP from CO2 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 [CO2] could potentially explain the recent land CO2 sink at the global scale.

Highlights

Atmospheric CO2 concentrations [CO2] have risen at a rate of 2.16 ± 0.09 ppm yr−1 in recent decades due to human activities (Le Quéré et al 2018) and will continue to increase unless emission reductions occur (Anderson et al 2019)
The current study focused on C3 photosynthesis, because C4-dominated ecosystems were limited in our data set and a much smaller CO2 fertilization effect is expected in C4 ecosystems than C3-dominated ecosystems (Collatz et al 1992)
Since the upscaled CO2 fertilization effects did not differ in terms of the different input data, we showed the upscaled results based on the combination of the CRU/National Centers for Environmental Prediction (NCEP) and MODIS leaf area index (LAI) data as our best guess

Summary

Introduction

Atmospheric CO2 concentrations [CO2] have risen at a rate of 2.16 ± 0.09 ppm yr−1 in recent decades due to human activities (Le Quéré et al 2018) and will continue to increase unless emission reductions occur (Anderson et al 2019). Rising [CO2] has increased photosynthesis at the leaf level (Norby et al 2005, Wenzel et al 2016) and gross primary productivity (GPP) at the ecosystem scale (Wenzel et al 2016). This process, known as the CO2 fertilization effect, arises because the current [CO2] is too low to saturate the carboxylation in the leaf, and limits photosynthesis. FACE data sets have improved vegetation models representing the ecosystem response to rising [CO2] (Medlyn et al 2015), the mechanisms of the CO2 fertilization effect incorporated into state-of-the-art earth system models has been a major source of uncertainty in climate projections through the internal feedbacks between photosynthesis and other related processes (e.g. respiration, biomass allocation, and mortality) (Booth et al 2012, Wenzel et al 2016, Friedlingstein et al 2014, Churkina et al 2009)

Methods

Results

Conclusion