Temperature dependence of spring carbon uptake in northern high latitudes during the past four decades

In the northern high latitudes, warmer spring temperatures generally lead to earlier leaf onsets, higher vegetation production, and enhanced spring carbon uptake. Yet, whether this positive linkage has diminished under climate change remains debated. Here, we used atmospheric CO₂ measurements at Barrow (Alaska) during 1979-2020 to investigate the strength of temperature dependence of spring carbon uptake reflected by two indicators, spring zero-crossing date (SZC) and CO₂ drawdown (SCC). We found a fall and rise in the interannual correlation of temperature with SZC and SCC (R_SZC-T and R_SCC-T ), showing a recent reversal of the previously reported weakening trend of R_SZC-T and R_SCC-T . We used a terrestrial biosphere model coupled with an atmospheric transport model to reproduce this fall and rise phenomenon and conducted factorial simulations to explore its potential causes. We found that a strong-weak-strong spatial synchrony of spring temperature anomalies per se has contributed to the fall and rise trend in R_SZC-T and R_SCC-T , despite an overall unbroken temperature control on net ecosystem CO₂ fluxes at local scale. Our results provide an alternative explanation for the apparent drop of R_SZC-T and R_SCC-T during the late 1990s and 2000s, and suggest a continued positive linkage between spring carbon uptake and temperature during the past four decades. We thus caution the interpretation of apparent climate sensitivities of carbon cycle retrieved from spatially aggregated signals.