No increase is detected and modeled for the seasonal cycle amplitude of δ¹³C of atmospheric carbon dioxide

Abstract. Measurements of the seasonal cycle of δ¹³C(CO₂) provide information on the global carbon cycle and the regulation of carbon and water fluxes by leaf stomatal openings on ecosystem and decadal scales. Land biosphere carbon exchange is the primary driver of δ¹³C(CO₂) seasonality in the Northern Hemisphere. We use isotope-enabled simulations of the Bern3D-LPX Earth System Model of Intermediate Complexity and fossil fuel emission estimates with a model of atmospheric transport to simulate local atmospheric δ¹³C(CO₂). Unlike the observed growth of the seasonal amplitude of CO₂ at northern sites, no significant temporal trend in the seasonal amplitude of δ¹³C(CO₂) is detected at most sites, consistent with the insignificant model trends. Comparing the preindustrial and modern periods, the modeled small amplitude changes at northern sites are linked to the near-equal increase of background atmospheric CO₂ and the seasonal signal of the net atmosphere-land δ¹³C flux in the northern extratropical region, with no long-term temporal changes in the isotopic fractionation by C₃ plants. The good data-model agreement in the seasonal amplitude of δ¹³C(CO₂) and its decadal trend provides implicit support for the regulation of stomatal conductance by C₃ plants towards intrinsic water use efficiency to grow proportionally to atmospheric CO₂ over recent decades. Disequilibrium fluxes contribute little to the seasonal amplitude of the net land isotope flux north of 40° N but contribute near-equally to the isotopic flux associated with growing season net carbon uptake in tropical and Southern Hemisphere ecosystems, pointing to the importance of monitoring δ¹³C(CO₂) over these ecosystems. We propose to apply seasonally-resolved δ¹³C(CO₂) observations as a novel constraint for land biosphere models and underlying processes for improved projections of the anthropogenic carbon sink.