Oceanic and terrestrial biospheric CO2 uptake estimated from atmospheric potential oxygen observed at Ny-Ålesund, Svalbard, and Syowa, Antarctica

Simultaneous measurements of the atmospheric O₂/N₂ ratio and CO₂ concentration were made at Ny-Ålesund, Svalbard, and Syowa, Antarctica for the period 2001–2009. Based on these measurements, the observed atmospheric potential oxygen (APO) values were calculated. The APO variations produced by changes in the oceanic heat content were estimated using an atmospheric transport model and heat-driven air–sea O₂ (N₂) fluxes, and then subtracted from observed interannual variations of APO. The oceanic CO₂ uptake derived from the resulting ‘corrected’ secular trend of APO showed interannual variability of less than ±0.6 GtC yr⁻¹, significantly smaller than that derived from the ‘uncorrected’ trend of APO (±0.9 GtC yr⁻¹). The average CO₂ uptake during the period 2001–2009 was estimated to be 2.9±0.7 and 0.8±0.9 GtC yr⁻¹ for the ocean and terrestrial biosphere, respectively. By excluding the influence of El Niño around 2002–2003, the terrestrial biospheric CO₂ uptake for the period 2004–2009 increased to 1.5±0.9 GtC yr⁻¹, while the oceanic uptake decreased slightly to 2.8±0.8 GtC yr⁻¹.