Global carbon budgets estimated from atmospheric O<sub>2</sub>∕N<sub>2</sub> and CO<sub>2</sub> observations in the western Pacific region over a 15-year period

Abstract. Time series of the atmospheric O2∕N2 ratio and CO2 mole fraction of flask samples obtained from the National Institute for Environmental Studies' (NIES's) flask sampling network are presented. The network includes two ground sites, Hateruma island (HAT; 24.05∘ N, 123.81∘ E) and Cape Ochiishi (COI; 43.17∘ N, 145.50∘ E), and cargo ships regularly sailing in the western Pacific. Based on temporal changes in fossil-fuel-derived CO2 emissions, global atmospheric CO2 burden and atmospheric potential oxygen (APO), which were calculated from the observed O2∕N2 ratio and CO2 mole fraction according to APO = O2+1.1×CO2, we estimated the global carbon sinks of the ocean and land biosphere for a period of more than 15 years. In this carbon budget calculation, we adopted a correction for the time-varying ocean O2 outgassing effect with an average of 0.54 PgC yr−1 for 2000–2016. The outgassing effect, attributed mainly to global ocean warming, was evaluated under the assumption that the net ocean gas flux is proportional to the change in the ocean heat content for the 0–2000 m layer. The resulting oceanic and land biotic carbon sinks were 2.6±0.7 and 1.5±0.9 PgC yr−1, respectively, for a 17-year period (2000–2016) and 2.4±0.7 and 1.9±0.9 PgC yr−1, respectively, for a 14-year period (2003–2016). Despite the independent approaches, the sink values of this study agreed with those estimated by the Global Carbon Project (GCP) within a difference of about ±0.4 PgC yr−1. We examined the carbon sinks for an interval of 5 years to assess the temporal trends. The pentad (5-year) ocean sinks showed an increasing trend at a rate of 0.08±0.02 PgC yr−2 during 2001–2014, while the pentad land sinks showed an increasing trend at a rate of 0.23±0.04 PgC yr−2 for 2001–2009 and a decreasing trend at a rate of -0.22±0.04 PgC yr−2 during 2009–2014. Although there is good agreement in the trends of the pentad sinks between this study and that of GCP, the increasing rate of the pentad ocean sinks of this study was about 2 times larger than that of GCP.