Revisiting the Global Budget for Atmospheric Potential Oxygen

Observations of atmospheric oxygen and CO₂ can be combined into the tracer atmospheric potential oxygen (APO), which largely cancels variations from terrestrial biosphere exchange while retaining strong sensitivity to fossil fuel burning, the uptake of anthropogenic CO₂ by the ocean, and outgassing of both O₂ and CO₂ due to ocean warming. Our understanding of the global budget of APO therefore tests our knowledge of the linkages between carbon, oxygen, and heat in the earth system. Resplandy et al. (Scientific Reports, 9, 20244, 2019) showed that with some simplifying assumptions one could solve the APO budget for the component of the APO trend which is due to ocean warming, which places a constraint on ocean heat uptake. Here we revisit the global APO budget posed by Resplandy et al. and make corrections to fossil fuel component of the APO trend for the global refinement of metals, revised inventories, and cement carbonation. These corrections are significant but approximately cancel in sum, which yields an ocean warming term of similar magnitude to that found by Resplandy et al., continuing to support estimates of OHC change at the upper range, although the uncertainty on this term is large. Alternately, one can also solve for the APO budget for the fossil fuel component, using ocean heat uptake based on ocean temperature measurements and the heat/APO scale factor from Resplandy et al. This approach yields a correction factor on reported global fossil-fuel emissions of 0.95 +/- 0.13, thus supporting reported estimates to within their uncertainties. We discuss the implications of these findings and future strategies to reduce the overall uncertainty in the APO budget.