Interannual variability of the terrestrial ecosystem carbon sink is substantially regulated by various environmental variables and highly dominates the interannual variation of atmospheric carbon dioxide (CO2) concentrations. Thus, it is necessary to determine dominating factors affecting the interannual variability of the carbon sink to improve our capability of predicting future terrestrial carbon sinks. Using global datasets derived from machine-learning methods and process-based ecosystem models, this study reveals that the interannual variability of the atmospheric vapor pressure deficit (VPD) was significantly negatively correlated with net ecosystem production (NEP) and substantially impacted the interannual variability of the atmospheric CO2 growth rate (CGR). Further analyses found widespread constraints of VPD interannual variability on terrestrial gross primary production (GPP), causing VPD to impact NEP and CGR. Partial correlation analysis confirms the persistent and widespread impacts of VPD on terrestrial carbon sinks compared to other environmental variables. Current Earth system models underestimate the interannual variability in VPD and its impacts on GPP and NEP. Our results highlight the importance of VPD for terrestrial carbon sinks in assessing ecosystems’ responses to future climate conditions.

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Part of the Northern Hemisphere has experienced widespread autumn cooling during the most recent decades despite overall warming, but how this contrasting temperature change has influenced the ecosystem carbon exchange remains unclear. Here, we show that autumn cooling has occurred over about half of the area north of 25° N since 2004, producing a weak cooling trend over the period 2004–2018. Multiple lines of evidence suggest an increasing net CO2 release in autumn during 2004–2018. In cooling areas, the increasing autumn CO2 release is due to the larger decrease of gross primary productivity (GPP) growth than total ecosystem respiration (TER) growth suppressed by cooling. In the warming areas, TER increased more than GPP because the warming and wetting conditions are more favourable for TER growth than GPP increase. Despite the opposite temperature trends, there has been a systematic increase in ecosystem carbon release across the Northern Hemisphere middle and high latitudes.

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