Ocean warming and increased stratification of the upper ocean caused by global climate change will likely lead to declines in dissolved O2 in the ocean interior (ocean deoxygenation) with implications for ocean productivity, nutrient cycling, carbon cycling, and marine habitat. Ocean models predict declines of 1 to 7% in the global ocean O2 inventory over the next century, with declines continuing for a thousand years or more into the future. An important consequence may be an expansion in the area and volume of so-called oxygen minimum zones, where O2 levels are too low to support many macrofauna and profound changes in biogeochemical cycling occur. Significant deoxygenation has occurred over the past 50 years in the North Pacific and tropical oceans, suggesting larger changes are looming. The potential for larger O2 declines in the future suggests the need for an improved observing system for tracking ocean O2 changes.

http://www.blogueaqlpa.com/wp-content/uploads/2013/11/Keeling-et-al.-2010-Ocean-deoxygenation-in-a-warming-world.pdf

Ocean Deoxygenation in a Warming World

[1] Atmospheric O2 concentrations have been used to estimate the ocean and land sinks of fossil fuel CO2. In previous work, it has been assumed that the oceans have no long-term influence on atmospheric O2. We address the validity of this assumption using model results and observations. Oceanic O2 fluxes for the 1860–2100 period are simulated using a coupled climate model in which is nested an ocean biogeochemistry model. Simulated oceanic O2 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 O2 and heat fluxes are strongly correlated for both the decadal variations and the climate trend. Using the observed heat fluxes and the modeled O2 flux/heat flux relationship, we infer the contribution of the oceans to atmospheric O2 and infer a correction to the partitioning of the ocean and land CO2 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.

/pdf/climate-induced-oceanic-oxygen-fluxes-implications-for-the-c8pxwyqor4.pdf

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

Measurements of atmospheric O2/N2 ratio and CO2 concentration are presented over the period 1989‐2003 from the Scripps Institution of Oceanography global flask sampling network A formal framework is described for making optimal use of these data to estimate global oceanic and land biotic carbon sinks For the 10-yr period from 1990 to 2000, the oceanic and land biotic sinks are estimated to be 19 ± 06 and 12 ± 08 Pg C yr −1 , respectively, while for the 10-yr period from 1993 to 2003, the sinks are estimated to be 22 ± 06 and 05 ± 07 Pg C yr−1, respectively These estimates, which are also compared with earlier results, make allowance for oceanic O2 and N2 outgassing based on observed changes in ocean heat content and estimates of the relative outgassing per unit warming For example, for the 1993‐2003 period we estimate outgassing of 045 × 10 14 mol O2 yr −1 and 020 × 10 14 mol N2 yr −1 , which results in a correction of 05 Pg C yr −1 on the oceanic and land biotic carbon sinks The basis for this oceanic outgassing correction is reviewed in the context of recent model estimates The main contributions to the uncertainty in the global sinks averages are from the estimates for oceanic outgassing and the estimates for fossil fuel combustion The oceanic sink of 22 Pg C yr −1 for 1993‐2003 is consistent, within the uncertainties, with the integrated accumulation of anthropogenic CO2 in the ocean since 1800 as recently estimated from oceanic observations, assuming the oceanic sink varied over time as predicted by a box-diffusion model

http://bluemoon.ucsd.edu/publications/manning/ManningandKeeling2006.pdf

Global oceanic and land biotic carbon sinks from the Scripps atmospheric oxygen flask sampling network

[1] Carbon budgets inferred from measurements of the atmospheric oxygen to nitrogen ratio (O2/N2) are revised considering sea-to-air fluxes of O2 and N2 in response to global warming and volcanic eruptions. Observational estimates of changes in ocean heat content are combined with a model-derived relationship between changes in atmospheric O2/N2 due to oceanic outgassing and heat fluxes to estimate ocean O2 outgassing. The inferred terrestrial carbon sink for the 1990s is reduced by a factor of two compared with the most recent estimate by the Intergovernmental Panel on Climate Change (IPCC). This also improves the agreement between calculated ocean carbon uptake rates and estimates from global carbon cycle models, which indicate a higher ocean carbon uptake during the 1990s than the 1980s. The simulated decrease in oceanic O2 concentrations is in qualitative agreement with observed trends in oceanic O2 concentrations.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001GB001746

Revision of the global carbon budget due to changing air-sea oxygen fluxes

We present a method for testing the performance of global ocean carbon cycle models using measurements of atmospheric O2 and CO2 concentration. We combine these measurements to define a tracer, atmospheric potential oxygen (APO ≈ O2 + CO2), which is conservative with respect to terrestrial photosynthesis and respiration. We then compare observations of APO to the simulations of an atmospheric transport model which uses ocean-model air-sea fluxes and fossil fuel combustion estimates as lower boundary conditions. We present observations of the annual-average concentrations of CO2, O2, and APO at 10 stations in a north-south transect. The observations of APO show a significant interhemispheric gradient decreasing towards the north. We use air-sea CO2, O2, and N2 fluxes from the Princeton ocean biogeochemistry model, the Hamburg model of the ocean carbon cycle, and the Lawrence Livermore ocean biogeochemistry model to drive the TM2 atmospheric transport model. The latitudinal variations in annual-average APO predicted by the combined models are distinctly different from the observations. All three models significantly underestimate the interhemispheric difference in APO, suggesting that they underestimate the net southward transport of the sum of O2 and CO2 in the oceans. Uncertainties in the model-observation comparisons include uncertainties associated with the atmospheric measurements, the atmospheric transport model, and the physical and biological components of the ocean models. Potential deficiencies in the physical components of the ocean models, which have previously been suggested as causes for anomalously large heat fluxes out of the Southern Ocean, may contribute to the discrepancies with the APO observations. These deficiencies include the inadequate parameterization of subgrid-scale isopycnal eddy mixing, a lack of subgrid-scale vertical convection, too much Antarctic sea-ice formation, and an overestimation of vertical diffusivities in the main thermocline.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97GB03500

Testing global ocean carbon cycle models using measurements of atmospheric O2 and CO2 concentration

Molecular oxygen in the atmosphere is coupled tightly to the terrestrial carbon cycle by the processes of photosynthesis, respiration, and burning. This dissertation examines different aspects of this coupling in four chapters. Chapter 1 explores the feasibility of using air from sand dunes to reconstruct atmospheric O{sub 2} composition centuries ago. Such a record would reveal changes in the mass of the terrestrial biosphere, after correction for known fossil fuel combustion, and constrain the fate of anthropogenic CO{sub 2}.

Jeffrey Peck Severinghaus

Papers

Studies of the terrestrial O{sub 2} and carbon cycles in sand dune gases and in biosphere 2