Quantifying CO2 fertilization effect on the increase of CO2 seasonal cycle amplitude at northern high latitudes

Atmospheric observations show an enhanced increase of atmospheric CO₂ seasonal cycle amplitude (SCA) in the northern high latitudes relative to lower latitudes.  This enhancement coincides with intensified temperature increase in this region, which was suggested to be the primary driver in causing the CO₂ SCA amplitude at northern high latitudes. Here, we present new analyses using long-term observations of CO₂, COS, and δ13CO2 to quantify the impact of CO₂ fertilization on the CO₂ SCA increase at northern high latitudes. We quantify this effect by directly estimating changes of ecosystem processes such as stomatal conductance of CO₂, photosynthetic CO₂ uptake, and the ratio of intercellular to ambient CO₂ mole fractions using COS and δ13CO2 observations. Our results suggest CO₂ fertilization plays an important role in enhancing the photosynthetic CO₂ uptake over this region that has exceeded the trend in ecosystem respiration and been one of the major factors driving the high-latitudes CO₂ SCA increase.