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Application of Atmospheric O2/N2 Measurements for Determining Global Carbon Sinks

R. Keeling, E. Morgan and S. Walker

Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92037; 858-366-3376, E-mail: rkeeling@ucsd.edu

The Scripps O2 program has carried out precise measurements of the trend in atmospheric O2/N2 ratio from 1990 to present at ~10 sites around the globe. These records can be combined with the measured global trend in CO2 to separately resolve the global land and ocean carbon sinks. In effect, the CO2 measurements constrain the sum of the land and ocean sinks, while the O2 measurements provide a means to separate this sum into land and ocean components (Figure 1). The additional constraint from O2/N2 leverages the fact that the land sink involves net photosynthesis, which produces O2, while the ocean sink mostly involves carbonate reactions, which do not produce O2. The change in O2/N2 ratio effectively determines the change in the total atmospheric O2 inventory because changes in N2 are very small. The measurements since 1990 document an increasing sink for CO2 from the ocean combined with a relatively steady global land sink for CO2. The uncertainties are dominated by uncertainty in fossil-fuel burning and by uncertainty in a correction term for the O2 budget, involving warming-driven outgassing of O2 from the oceans. Despite these uncertainties, the measurements provide a precise constraint on the ocean sink on decadal time scales, while also resolving considerable additional in short-term variability in global air-sea O2 and CO2 exchanges via the tracer atmospheric potential oxygen (APO ~ O2+1.1CO2). The ocean sink estimated from O2/N2 measurements has recently been compared with other observational and model-based estimates of the ocean CO2 sink by the Global Carbon Project (GCP) (Friedlingstein, Earth. Sys. Sci. Data, 2023). The O2/N2-based estimate is generally in good agreement with other observational methods, although the comparison is complex because the methods are not necessarily measuring exactly same quantity. This presentation will discuss these differences and discuss opportunities to provide annual updates of the global sink estimates from a range of NOAA-based observational products, including O2/N2 measurements. 

Figure 1

Figure 1. Global land and ocean carbon sinks over the period 2013-2022 as constrained by the measured long-term trends in atmospheric O2/N2 ratio and CO2. The constraint on the combined ocean and land sink from the measured trend in CO2 and known fossil-fuel emissions is show by the gray line, with 1 sigma bounds shown by dashed lines.  The additional constraint from the measured trend in O2/N2 constrains the joint probability to lie within the blue circle, which is tangent to the gray dashed lines.  The O2/N2 trend provides a tighter constraint on the ocean sink than the land sink because uncertainty in fossil-fuel emissions project more strongly onto the land than the ocean sink.