Weak enhancement of carbon fixation in the Northern Hemisphere terrestrial ecosystems by aerosol emissions during 1980–2014 with the Community Earth System Model

The effects of aerosols on carbon sequestration in sunlit and shaded leaves of vegetation cover are well understood; however, it is difficult to quantify these effects on gross primary productivity (GPP) with existing observational data. In this study, the Community Earth System Model (CESM) is used to quantify the effects of aerosol emissions on the GPP of sunlit and shaded leaves in Northern Hemisphere terrestrial ecosystems from 1980 to 2014. The results revealed that aerosol emissions caused an increase of approximately 2.96 Pg C (approximately 4.8 %) in the cumulative GPP in the Northern Hemisphere. Carbon fixation from sunlit leaves (C.sun) contributed to a net increase of 3.33 Pg C, while carbon fixation from shaded leaves (C.sha) led to a net decrease of 0.37 Pg C from 1980 to 2014. The influence of aerosol emission changes on terrestrial GPP exhibited significant spatial heterogeneity after the 1980s, as carbon fixation from shaded leaves (25.8 %) contributed to an increase in GPP, whereas carbon fixation from sunlit leaves (38.6 %) led to a decrease in GPP. In addition, aerosol-induced vapour pressure deficit (VPD) dominated the changes in Northern Hemisphere GPP (42.5 %), while soil moisture (SM) and diffuse photosynthetically active radiation (PAR_dif) were the most important climatic factors, accounting for approximately 33 % and 25.5 %, respectively. Our results indicate that the effects of VPD on GPP should be adequately considered when assessing ecosystem responses to future climate conditions.