Flux Calculations

Our LDEO carbon products are available in terms of fCO2, however it is common for a user to have an interest in the associated flux calculated from these ocean values. Various approaches are used to estimate the ocean carbon sink but here we focus on the net sea–air CO2 flux across the sea–air interface which provides a direct estimate of the contemporary flux. The standard practice is to estimate this net flux of CO2 across the air–sea interface from surface ocean CO2 products using a bulk parameterization. When accomplishing this computation, other variables including sea surface temperature and wind speed are required. 

The air-sea flux bulk formula is described as:

Flux=kw·sol·pCO2−pCO2atm·(1−ice)

where kw is the gas transfer velocity, sol is the solubility of CO2 in seawater (in units of mol m−3 μatm−1), pCO2 is the partial pressure of surface ocean CO2 (in μatm), and pCO2atm (in μatm) represents the partial pressure of atmospheric CO2 in the marine boundary layer. The gas transfer velocity, k, is typically expressed as a function of wind speed and here we use a quadratic relationship (Wanninkhof 2014). To account for the seasonal ice cover in high latitudes, the fluxes are weighted by 1 minus the ice fraction (ice), i.e., the open ocean fraction. Fay & Gregor et al. (2021) provide a thorough discussion of the various parameters and uncertainties associated with specific choices made during the flux calculation. The SeaFlux product (Gregor & Fay et al. 2021 with updated versions: https://doi.org/10.5281/zenodo.4133802) provides all inputs to the calculation in a standardized format, beginning with year 1982 and extended each year as input datasets become available. Flux is often reported in moles of carbon per unit area per time (typically molC/m2/yr) locally or a global or regional flux estimate in PgC/yr. 

For each LDEO carbon product available here, we include a flux variable in the associated file. Referenced documentation provides specifics on flux calculation methodology for each product. Due to the longer time period covered by the LDEO-HPD product, the same inputs for flux calculation cannot be used, as only a few resources go back to the start year 1959. An overview of the differences is presented in the table below.

Table comparing inputs to flux calculation for 2 LDEO products

For the LDEO-HPD extension to the pre-observed period (pre 1989), Bennington et al. (2022) reported flux values calculated using the CarboScope water vapor corrected atmospheric pCO2 produced by Rödenbeck (2005). In subsequent updates, we take atmospheric value pCO2 from  NOAA’s marine boundary layer product starting for the year 1979. As described in the table above, we extend this back to 1959 using the Mauna Loa atmospheric pCO2 values corrected by the long term climatological difference between the Mauna Loa and global values reported by NOAA for their common years (1979 to present).  This approach allows our flux calculations to be updated annually using open-access resources. 

Ocean carbon flux estimates produced from fCO2 products such as those produced by LDEO can be compared directly with estimates from other products such as is done in Rödenbeck et al. 2015 and Fay et al. 2021. However, it is important to note that intercomparisons between these products and global ocean models, such as the hindcast models included in the Global Carbon Budget (Friedlingstein et al. 2023) require an adjustment for the river efflux of pre-industrial CO2. Recent global estimates of this adjustment range from 0.23 PgC/yr (Lacroix et al. 2020 to 0.78 PgC/yr (Resplandy et al., 2018) and also differ in their regional partitioning of where the efflux occurs. More information on this topic can be found in Fay & Gregor et al. (2021) Section 3.4 and in much more detail in Regnier et al. (2022).

 

Figure 3 from Bennington et al. 2022 showing estimated air-sea CO2 fluxes for 1959-2020

References

Bell, B., Hersbach, H., Berrisford, P., Dahlgren, P., Horányi, A., Muñoz Sabater, J., et al.: ERA5 monthly averaged data on single levels from 1979 to present. Copernicus Climate Change Service (C3S) Climate Data Store (CDS), 10, 252–266. https://doi.org/10.24381/cds. F17050d7, 2019.

Bell, B., Hersbach, H., Berrisford, P., Dahlgren, P., Horányi, A., Muñoz Sabater, J., et al.: ERA5 monthly averaged data on single levels from 1950 to 1978 (preliminary version). Copernicus Climate Change Service (C3S) Climate Data Store (CDS), https://cds.climate.copernicus-climate.eu/cdsapp#!/dataset/reanalysis-era5-single-levels-monthly-means-preliminary-back-extension?tab=overview, 2020.

Dickson, A. G., Sabine, C. L., and Christian, J. R. (Eds): Guide to best practices for ocean CO2 measurement. Sidney, British Columbia, North Pacific Marine Science Organization, 191 pp. (PICES Special Publication 3; IOCCP Report 8), https://doi.org/10.25607/OBP-1342, 2007.

Dlugokencky, E. J., Thoning, K. W., Lang, P. M., and Tans, P. P.: NOAA Greenhouse Gas Reference from Atmospheric Carbon Dioxide Dry Air Mole Fractions from the NOAA ESRL Car- bon Cycle CooperativeGlobal Air Sampling Network, data available at: https://www.esrl.noaa.gov/gmd/ccgg/mbl/data.php, 2019.

Fay, A. R., Gregor, L., Landschützer, P., McKinley, G. A., Gruber, N., Gehlen, M., Iida, Y., Laruelle, G. G., Rödenbeck, C., Roobaert, A., and Zeng, J.: SeaFlux: harmonization of air–sea CO2 fluxes from surface pCO2 data products using a standardized approach, Earth Syst. Sci. Data, 13, 4693–4710, https://doi.org/10.5194/essd-13-4693-2021, 2021.

Friedlingstein, P.,  et al.: Global Carbon Budget 2023, Earth Syst. Sci. Data, 15, 5301–5369, https://doi.org/10.5194/essd-15-5301-2023, 2023.

Good, S. A., Martin, M. J., & Rayner, N. A.: EN4: Quality controlled ocean temperature and salinity profiles and monthly objective analyses with uncertainty estimates. Journal of Geophysical Research: Oceans, 118(12), 6704–6716. https://doi.org/10.1002/2013JC009067, 2013.

Gregor L. & A.R. Fay et al: SeaFlux data set: Air-sea CO2 fluxes for surface pCO2 data products using a standardized approach. Zenodo doi: 10.5281/zenodo.4133802, 2021.

Lacroix, F., Ilyina, T., and Hartmann, J.: Oceanic CO2 outgassing and biological production hotspots induced by pre-industrial river loads of nutrients and carbon in a global modeling approach, Biogeosciences, 17, 55–88, https://doi.org/10.5194/bg- 17-55-2020, 2020.

Rayner, N. A., Parker, D. E., Horton, E. B., Folland, C. K., Alexander, L. V., Rowell, D. P., & Kaplan, A.: Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. Journal of Geophysical Research, 108(D14), 4407. https://doi.org/10.1029/2002JD002670, 2003.

Regnier, P., Resplandy, L., Najjar, R.G. et al.: The land-to-ocean loops of the global carbon cycle. Nature 603, 401–410. https://doi.org/10.1038/s41586-021-04339-9, 2022.

Resplandy, L., Keeling, R. F., Rödenbeck, C., Stephens, B. B., Khatiwala, S., Rodgers, K. B., Long, M. C., Bopp, L., and Tans, P. P.: Revision of global carbon fluxes based on a reassessment of oceanic and riverine carbon transport, Nat. Geosci., 11, 504–509, https://doi.org/10.1038/s41561-018-0151-3, 2018.

Rödenbeck, C., Keeling, R. F., Bakker, D. C. E., Metzl, N., Olsen, A., Sabine, C., and Heimann, M.: Global surface-ocean pCO2 and sea–air CO2 flux variability from an observation-driven ocean mixed-layer scheme, Ocean Sci., 9, 193–216, https://doi.org/10.5194/os-9-193-2013, 2013.

Wanninkhof, R.: Relationship between wind speed and gas exchange over the ocean revisited. Limnology and Oceanography: Methods, 12(JUN), 351–362. (ISBN: 1992101029) doi: 10.4319/lom.2014.12.351, 2014.