Revised estimates of ocean-atmosphere CO2 flux are consistent with ocean carbon inventory

Abstract

The ocean is a sink for ~25% of the atmospheric CO₂ emitted by human activities, an amount in excess of 2 petagrams of carbon per year (PgC yr^-1). Time-resolved estimates of global ocean-atmosphere CO₂ flux provide an important constraint on the global carbon budget. However, previous estimates of this flux, derived from surface ocean CO₂ concentrations, have not corrected the data for temperature gradients between the surface and sampling at a few meters depth, or for the effect of the cool ocean surface skin. Here we calculate a time history of ocean-atmosphere CO₂ fluxes from 1992 to 2018, corrected for these effects. These increase the calculated net flux into the oceans by 0.8-0.9 PgC yr^-1, at times doubling uncorrected values. We estimate uncertainties using multiple interpolation methods, finding convergent results for fluxes globally after 2000, or over the Northern Hemisphere throughout the period. Our corrections reconcile surface uptake with independent estimates of the increase in ocean CO₂ inventory, and suggest most ocean models underestimate uptake.

Fig. 1. Effect of near-surface temperature corrections.

Global air–sea flux calculated by interpolating SOCAT gridded data using a neural network technique, followed by the gas exchange equation applied to the ocean mass boundary layer. The net flux into the ocean is shown as negative, following convention. The uncorrected curve uses the SOCAT fCO₂ at inlet temperature as usually done. Correction of the data to a satellite-derived subskin temperature is shown, and the additional change in flux due to a thermal skin assumed to be cooler and saltier than the subskin by 0.17 K and 0.1 salinity units. Excludes the Arctic and some regional seas—ocean regions included are shown in Supplementary Fig. 2.

Fig. 2. Global ocean-atmosphere CO₂ fluxes 1992–2018.

Fluxes are integrated a globally, and b for northern and southern hemispheres, calculated using a standard gas exchange formulation (see Methods section) with the nine interpolation schemes for fCO₂ described in the text shown as colored lines: TS red, MLR green, FFN blue. The line styles indicate the spatial clustering schemes used (illustrated in Supplementary Fig. 1): solid, Landschützer SOM; dashed, latitudinal regions; dotted, Fay and Mckinley biomes. The standard method, SOM-FFN as described in Landschützer et al., is shown as a thicker blue line. Shading indicates one- and two-standard deviations of the nine methods around the mean (thick black line).

Fig. 3. Observation-based estimates of anthropogenic CO₂ uptake.

The black line is our standard case global ocean-atmosphere flux increased by −0.57 PgC Cyr⁻¹ to account for pre-industrial and Arctic fluxes as described in the text. The shading gives one and two standard deviations of estimates around this value, including the uncertainty in gas transfer rates as assessed by Woolf et al.. Red horizontal line and uncertainty is a recent estimate of the global inventory increase of anthropogenic carbon in the ocean between 1994 and 2007. Dashed lines: two previous estimates of global uptake based on surface data: blue dashed line from Landschützer et al., red dashed line from Rödenbeck et al., both as quoted in Le Quéré et al.. Both are increased by the pre-industrial flux correction and Landschützer et al. also increased by Arctic correction.