The Variability of Air-sea O2 Flux in CMIP6: Implications for Estimating Terrestrial and Oceanic Carbon Sinks

The measurement of atmospheric O-2 concentrations and related oxygen budget have been used to estimate terrestrial and oceanic carbon uptake. However, a discrepancy remains in assessments of O-2 exchange between ocean and atmosphere (i.e. air-sea O-2 flux), which is one of the major contributors to uncertainties in the O-2-based estimations of the carbon uptake. Here, we explore the variability of air-sea O-2 flux with the use of outputs from Coupled Model Intercomparison Project phase 6 (CMIP6). The simulated air-sea O-2 flux exhibits an obvious warming-induced upward trend (similar to 1.49 Tmol yr(-2)) since the mid-1980s, accompanied by a strong decadal variability dominated by oceanic climate modes. We subsequently revise the O-2-based carbon uptakes in response to this changing air-sea O-2 flux. Our results show that, for the 1990-2000 period, the averaged net ocean and land sinks are 2.10 +/- 0.43 and 1.14 +/- 0.52 GtC yr(-1) respectively, overall consistent with estimates derived by the Global Carbon Project (GCP). An enhanced carbon uptake is found in both land and ocean after year 2000, reflecting the modification of carbon cycle under human activities. Results derived from CMIP5 simulations also investigated in the study allow for comparisons from which we can see the vital importance of oxygen dataset on carbon uptake estimations.