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°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 (eCO₂) 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 eCO₂ (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 eCO₂ are associated with higher ecosystem respiration due to the increase in carbon storage caused by eCO₂ 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.