Non-uniform seasonal warming regulates vegetation greening and atmospheric CO ₂ amplification over northern lands

The enhanced vegetation growth by climate warming plays a pivotal role in amplifying the seasonal cycle of atmospheric CO ₂ at northern lands (>50° N) since 1960s. However, the correlation between vegetation growth, temperature and seasonal amplitude of atmospheric CO ₂ concentration have become elusive with the slowed increasing trend of vegetation growth and weakened temperature control on CO ₂ uptake since late 1990s. Here, based on in situ atmospheric CO ₂ concentration records from the Barrow observatory site, we found a slowdown in the increasing trend of the atmospheric CO ₂ amplitude from 1990s to mid-2000s. This phenomenon was associated with the paused decrease in the minimum CO ₂ concentration ([CO ₂ ] _min ), which was significantly correlated with the slowdown of vegetation greening and growing-season length extension. We then showed that both the vegetation greenness and growing-season length were positively correlated with spring but not autumn temperature over the northern lands. Furthermore, such asymmetric dependences of vegetation growth upon spring and autumn temperature cannot be captured by the state-of-art terrestrial biosphere models. These findings indicate that the responses of vegetation growth to spring and autumn warming are asymmetric, and highlight the need of improving autumn phenology in the models for predicting seasonal cycle of atmospheric CO ₂ concentration.