Ship-based CO2 flux estimates of the contemporary air-sea flux of CO2 showed that the Southern Ocean (south of 35oS) plays an important role as a significant carbon sink, with a net uptake at the rate of −0.8 ~ −1.0 Pg C/year (Takahashi et al., 2009; Landschützer et al., 2014) largely consistent with climate model-based estimates (e.g., Nevison et al., 2016). However, a recent study based on measurements from biogeochemical profiling floats estimated a much smaller net CO2 uptake rate of −0.08 Pg C/year for the Southern Ocean south of 35oS (Gray et al., 2018). This is because ship-based CO2 flux estimates underestimate CO2 outgassing in the poorly observed area around Antarctica where carbon-rich Circumpolar Deep Waters upwell to the surface ocean, particularly in the winter season. A new study published in Science used aircraft-based measurements of the vertical atmospheric CO2 gradient to provide a revised estimate of –0.53 Pg C/year for the Southern Ocean south of 45oS. The new estimate indicates stronger summertime uptake and weaker wintertime outgassing compared to the recent profiling float-based estimate and confirms the role of the Southern Ocean as a significant carbon sink.
Figure 4 from Long et al., (2021): Observationally based estimates of Southern Ocean air-sea fluxes. (A) The seasonal cycle of air-sea CO2 flux south of 45°S estimated from aircraft campaigns (black points, labels), plotted at the center of the 90-day window for which the emergent flux constraint was calibrated. Whiskers show the standard deviation derived from propagating analytical and statistical uncertainties; the black line shows a two-harmonic fit used to estimate the annual mean flux. The colored lines give the seasonal cycle from atmospheric inversion systems as well as the neural network extrapolation of the Surface Ocean CO2 Atlas (SOCAT) pCO2 observations and profiling float observations from the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project. Fluxes are averaged over the period 2009–2018, except for the three neural network–based flux estimates incorporating SOCCOM observations, which are averaged over the period 2015–2017. (B) Annual mean flux estimated in this study (leftmost bar) including uncertainty (whisker), along with the mean and standard deviation (whiskers) across the inversion systems shown in (A) as well as the surface-ocean pCO2-based methods; averaging time periods are noted in the axis labels (both SOCAT flux estimates were derived using neural network training over the full observational period).
Long, M. C., Stephens, B. B., McKain, K., Sweeney, C., Keeling, R. F., Kort, E. A., … & Wofsy, S. C. (2021). Strong Southern Ocean carbon uptake evident in airborne observations. Science, 374(6572), 1275-1280. https://doi.org/10.1126/science.abi4355
Takahashi, T., Sutherland, S. C., Wanninkhof, R., Sweeney, C., Feely, R. A., Chipman, D. W., et al. (2009). Climatological mean and decadal change in surface ocean pCO2 and net sea-air CO2 flux over the global oceans. Deep Sea Research, Part II, 56(8-10), 554-577. https://doi.org/10.1016/j.dsr2.2008.12.009
Landschützer, P., Gruber, N., Bakker, D. C. E., & Schuster, U. (2014). Recent variability of the global ocean carbon sink. Global Biogeochemical Cycles, 28(9), 927-949. https://doi.org/10.1002/2014GB004853
Nevison, C. D., Manizza, M., Keeling, R. F., Stephens, B. B., Bent, J. D., Dunne, J., et al. (2016). Evaluating CMIP5 ocean biogeochemistry and Southern Ocean carbon uptake using atmospheric potential oxygen: Present-day performance and future projection. Geophysical Research Letters, 43, 2077-2085. https://doi.org/10.1002/2015GL067584
Gray, A., Johnson, K. S., Bushinsky, S. M., Riser, S. C., Russell, J. L., Talley, L. D., et al. (2018). Autonomous biogeochemical floats detect significant carbon dioxide outgassing in the high-latitude Southern Ocean. Geophysical Research Letters, 45, 9049-9057. https://doi.org/10.1029/2018GL078013