Climate‐induced oceanic oxygen fluxes: Implications for the contemporary carbon budget

Abstract

Atmospheric O 2 concentrations have been used to estimate the ocean and land sinks of fossil fuel CO 2. In previous work, it has been assumed that the oceans have no long‐term influence on atmospheric O 2. We address the validity of this assumption using model results and observations. Oceanic O 2 fluxes for the 1860–2100 period are simulated using a coupled climate model in which is nested an ocean biogeochemistry model. Simulated oceanic O 2 fluxes exhibit large interannual (±40 Tmol yr−1) and decadal (±13 Tmol yr−1) variability, as well as a net outgassing to the atmosphere caused by climate change (up to 125 Tmol yr−1 by 2100). Roughly one quarter of this outgassing is caused by warming of the ocean surface, and the remainder is caused by ocean stratification. The global oceanic O 2 and heat fluxes are strongly correlated for both the decadal variations and the climate trend. Using the observed heat fluxes and the modeled O 2 flux/heat flux relationship, we infer the contribution of the oceans to atmospheric O 2 and infer a correction to the partitioning of the ocean and land CO 2 sinks. After considering this correction, the ocean and land sinks are 1.8 ± 0.8 Pg C yr−1 and 0.3 ± 0.9 Pg C yr−1, respectively, for the 1980s (a correction of 0.1 from ocean to land) and are 2.3 ± 0.7 Pg C yr−1 and 1.2 ± 0.9 Pg C yr−1, respectively, in the 1990–1996 period (a correction of 0.5 from land to ocean). This correction reconciles the 1990s ocean sink estimated by the Intergovernmental Panel on Climate Change Third Assessment Report with ocean models.