On the causes of trends in the seasonal amplitude of atmospheric CO2

No consensus has yet been reached on the major factors driving the observed increase in the seasonal amplitude of atmospheric CO2 in the northern latitudes. In this study, we used atmospheric CO2 records from 26 northern hemisphere stations with a temporal coverage longer than 15 years, and an atmospheric transport model prescribed with net biome productivity (NBP) from an ensemble of nine terrestrial ecosystem models, to attribute change in the seasonal amplitude of atmospheric CO2. We found significant (p < .05) increases in seasonal peak-to-trough CO2 amplitude (AMP(P-T)) at nine stations, and in trough-to-peak amplitude (AMP(T-P)) at eight stations over the last three decades. Most of the stations that recorded increasing amplitudes are in Arctic and boreal regions (> 50 degrees N), consistent with previous observations that the amplitude increased faster at Barrow (Arctic) than at Mauna Loa (subtropics). The multi-model ensemble mean (MMEM) shows that the response of ecosystem carbon cycling to rising CO2 concentration (eCO(2)) and climate change are dominant drivers of the increase in AMP(P-T) and AMP(T-P) in the high latitudes. At the Barrow station, the observed increase of AMP(P-T) and AMP(T-P) over the last 33 years is explained by eCO(2) (39% and 42%) almost equally than by climate change (32% and 35%). The increased carbon losses during the months with a net carbon release in response to eCO(2) are associated with higher ecosystem respiration due to the increase in carbon storage caused by eCO(2) during carbon uptake period. Air-sea CO2 fluxes (10% for AMP(P-T) and 11% for AMP(T-P)) and the impacts of land-use change (marginally significant 3% for AMP(P-T) and 4% for AMP(T-P)) also contributed to the CO2 measured at Barrow, highlighting the role of these factors in regulating seasonal changes in the global carbon cycle.