The global terrestrial carbon sink has increased proportionally with increasing fossil fuel CO2 emissions since the 1960s, acting as key negative feedback and mitigating climate change. A consensus among multiple independent observations suggests that intensifying terrestrial biosphere activities was dominated by increased GPP. However, global estimates of GPP, its interannual variability (IAV), and long-term trend remain highly uncertain. Carbonyl sulfide (OCS) was identified as a novel photosynthetic tracer and has been utilized to constrain GPP fluxes, as it diffuses from the atmosphere to photosynthetic enzymes along a shared pathway with CO2. However, current estimates of terrestrial OCS fluxes (and GPP fluxes) consider only the stomatal control but omit the internal mesophyll control of OCS (and CO2) diffusion. This project explicit incorporated a mechanistic mesophyll diffusion module into a land surface model (LSM), i.e., NCAR Community Land Model (CLM), and found that the global OCS fluxes would be underestimated by 22% annually and the seasonal length of OCS uptake would be greatly shortened in the boreal fall especially in the mid- to high-latitude. Moreover such “process” omission in LSM cannot be compensated by “parameter” adjustment. We further estimated global GPP fluxes diagnosed from plant OCS uptake, which is 160.7 Pg C/yr in 2000-2010, consistent with independent estimates inferred from soil respiration based and 13C isotope based estimates but higher than global GPP products upscaled from eddy covariance towers.
