On the causes of trends in the seasonal amplitude of atmospheric CO 2

Abstract

No consensus has yet been reached on the major factors driving the observed increase in the seasonal amplitude of atmospheric CO 2 in the northern latitudes. In this study, we used atmospheric CO 2 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 CO 2 . We found significant ( p CO 2 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 CO 2 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 CO 2 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 CO 2 measured at Barrow, highlighting the role of these factors in regulating seasonal changes in the global carbon cycle.

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