LAUSR

The CO_2 seasonal cycle amplitude (SCA) in the Northern Hemisphere has increased since the 1960s—a feature attributed mainly to enhanced vegetation activity along climate warming and CO_2 increase. We identified a temporal change in the sign of the correlation between SCA and air temperature (T) from positive to negative around the year 2000 at most Northern Hemisphere ground stations, consistent with signals from satellite column CO_2 measurements since the mid‐2000s. Further, we explored potential causes of this change using net biome productivity estimates from three atmospheric inversions for the period 1980–2015. The change in the SCA‐T relationship is primarily attributable to changes in the net biome productivity‐T relationship: positive correlations weakened in the spring in the high latitudes, confirming a limit to the “warmer spring‐bigger carbon sink” mechanism; negative correlations diminished in the autumn/winter in the mid‐to‐high latitudes, challenging the “warmer winter‐larger carbon release” assumption and highlighting the complexity of carbon processes outside the peak growing season.