Contrasting effects of CO₂ fertilization, land-use change and warming on seasonal amplitude of Northern Hemisphere CO₂ exchange

Continuous atmospheric CO₂ monitoring data indicate an increase in the amplitude of seasonal CO₂-cycle exchange (SCA_NBP) in northern high latitudes. The major drivers of enhanced SCA_NBP remain unclear and intensely debated, with land-use change, CO₂ fertilization and warming being identified as likely contributors. We integrated CO₂-flux data from two atmospheric inversions (consistent with atmospheric records) and from 11 state-of-the-art land-surface models (LSMs) to evaluate the relative importance of individual contributors to trends and drivers of the SCA_NBP of CO₂ fluxes for 1980-2015. The LSMs generally reproduce the latitudinal increase in SCA_NBP trends within the inversions range. Inversions and LSMs attribute SCA_NBP increase to boreal Asia and Europe due to enhanced vegetation productivity (in LSMs) and point to contrasting effects of CO₂ fertilization (positive) and warming (negative) on SCA_NBP. Our results do not support land-use change as a key contributor to the increase in SCA_NBP. The sensitivity of simulated microbial respiration to temperature in LSMs explained biases in SCA_NBP trends, which suggests that SCA_NBP could help to constrain model turnover times.