Terrestrial biospheric and oceanic CO₂ uptakes estimated from long-term measurements of atmospheric CO₂ mole fraction, δ¹³C, and δ(O₂/N₂) at Ny-Ålesund, Svalbard

Systematic observations of CO₂ mole fraction, the isotopic ratio δ¹³C of CO₂, and oxygen to nitrogen ratio (δ(O₂/N₂)) in the atmosphere have been carried out at Ny-Ålesund, Svalbard, since 1991, 1996, and 2001, respectively. The CO₂ mole fraction shows a clear seasonal cycle superimposed on a secular increase with an average rate of 2.0 ppm yr⁻¹ for the period of 1996–2013. On the other hand, δ¹³C and δ(O₂/N₂) decrease secularly at an average rate of −0.020‰ yr⁻¹ for 1996–2013 and −19.9 per meg yr⁻¹ for 2001–2013, respectively. Based on the observed secular trends of the CO₂ mole fraction and δ(O₂/N₂), the average CO₂ uptake during 2001–2013 was estimated to be 1.6 ± 0.8 and 2.3 ± 0.5 GtC yr⁻¹ for the terrestrial biosphere and the ocean, respectively. By using the observed CO₂ and δ¹³C, the corresponding CO₂ uptakes of 1.3 ± 0.6 and 2.6 ± 0.5 GtC yr⁻¹ were obtained for the same period. The estimates from the two methods are in good agreement with each other. The terrestrial biospheric CO₂ uptake derived by the latter method showed large interannual variability in association with El Niño events. On the other hand, the oceanic uptake increased secularly with less interannual variability during 1996–2013.