Terrestrial biospheric and oceanic CO2 uptakes estimated from long-term measurements of atmospheric CO2 mole fraction, δ13 C, 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-Alesund, 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 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 uptake 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 inter-annual variability in association with El Nino events. On the other hand, the oceanic uptake increased secularly with less inter-annual variability during 1996–2013.