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.