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Recent increase in oceanic carbon uptake driven by weaker upper-ocean overturning

Abstract

The ocean is the largest sink for anthropogenic carbon dioxide (CO2), having absorbed roughly 40 per cent of CO2 emissions since the beginning of the industrial era1,2. Recent data show that oceanic CO2 uptake rates have been growing over the past decade3,4,5,6,7, reversing a trend of stagnant or declining carbon uptake during the 1990s8,9,10,11,12,13,14. Here we show that ocean circulation variability is the primary driver of these changes in oceanic CO2 uptake over the past several decades. We use a global inverse model to quantify the mean ocean circulation during the 1980s, 1990s and 2000s, and then estimate the impact of decadal circulation changes on the oceanic CO2 sink using a carbon cycling model. We find that during the 1990s an enhanced upper-ocean overturning circulation drove increased outgassing of natural CO2, thus weakening the global CO2 sink. This trend reversed during the 2000s as the overturning circulation weakened. Continued weakening of the upper-ocean overturning is likely to strengthen the CO2 sink in the near future by trapping natural CO2 in the deep ocean, but ultimately may limit oceanic uptake of anthropogenic CO2.

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Figure 1: Upper-ocean overturning circulation during the last three decades.
Figure 2: Global and regional decadal variability of the oceanic CO2 sink.
Figure 3: Simplified conceptual diagram illustrating how changes in upper-ocean overturning circulation have affected the oceanic CO2 sink.

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Acknowledgements

T.D. acknowledges support from a University of California Regents Junior Faculty Fellowship, and from NASA grant NNX16A122G. M.H. acknowledges support from Australian Research Council grant DP120100674. F.P. acknowledges funding from the National Science Foundation award OCE 1436992. We thank all of the scientists who collected the oceanographic tracer data used in this study.

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Contributions

All authors conceived this study. T.D. performed the model simulations and analysed the data. T.D. wrote the manuscript with input from M.H. and F.P.

Corresponding author

Correspondence to Tim DeVries.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Atlantic Ocean upper-ocean overturning circulation.

Same as Fig. 1 (transports in Sv), but for circulation in the Atlantic Ocean north of 35° S. Net transports across 200 m and 1,000 m are shown to the right of each diagram for each period. Also shown is the upper-mid ocean (UMO) transport (northward ocean transport integrated from the surface to 1,000 m) at 26° N, for comparison with previous studies of Atlantic Meridional Overturning Circulation variability21. Uncertainties as in Fig. 1.

Extended Data Figure 2 Global meridional overturning circulation variability over three decades.

Colours show the global ocean meridional overturning circulation (MOC) above 1,800 m for the 1980s (a), the 1990s (b), and the 2000s (c). Positive numbers represent clockwise circulation, and negative numbers counterclockwise circulation. The contour interval is 4 Sv. The black contours superimposed on b and c show the difference between the MOC in each decade and the prior decade. Negative differences are shown as dashed contours, positive differences as solid contours, with a contour interval of 3 Sv. Contours are drawn only where the absolute difference is greater than or equal to 3 Sv.

Extended Data Figure 3 Air–sea fluxes, transports, and storage rates of natural CO2.

Same as Fig. 2, but for natural CO2 only. Units are Pg C yr−1.

Extended Data Figure 4 Air–sea fluxes, transports, and storage rates of anthropogenic CO2.

Same as Fig. 2, but for anthropogenic CO2 only. Units are Pg C yr−1.

Extended Data Figure 5 Comparison of circulation-induced and solubility-induced changes in CO2 flux.

For the decades of the 1990s (a) and 2000s (b), green arrows and lettering show the zonally integrated anomalous air–sea CO2 flux (Pg C yr−1, negative values indicate ocean uptake) due to circulation variability, calculated by comparing the air–sea CO2 flux within each decade to what it would have been had circulation remained unchanged from the previous decade. Uncertainties were calculated as in Fig. 2. Magenta arrows and lettering show the zonally integrated anomalous air–sea CO2 flux due to temperature-driven changes in solubility, calculated by comparing simulations with constant SST and decadally varying SST. Uncertainties (one standard deviation) were calculated by propagating two different SST reconstructions in our suite of five different circulation models (see Methods).

Extended Data Figure 6 Comparison of SOCOM and assimilation model oceanic CO2 uptake.

Curves show the interannually-varying global sea-air CO2 flux (negative into ocean) from the models participating in the SOCOM intercomparison15. Symbols with error bars are the decadally averaged sea–air CO2 fluxes estimated from our data-assimilated ocean circulation model. One version of our assimilation model includes the effects of circulation as captured by the decadally averaged circulation model (black diamonds, as presented in main text), while the other additionally includes solubility effects due to decadal SST changes (red star, Extended Data Fig. 5). Uncertainties calculated as in Fig. 2 and Extended Data Fig. 5. Data are from refs 4, 5, 15, 24 and 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63.

Extended Data Figure 7 The distribution of observations used to constrain the data assimilation model.

a, Spatial distribution of CFC (CFC-11 and CFC-12) observations during each assimilation time period. b, Distribution of CFC-11 (blue) and CFC-12 (yellow) observations over time, binned at 2-year intervals (where n is the number of observations within each interval). The printed percentages in each assimilation time period indicate the percentage of model grid points that have a CFC-11 or CFC-12 observation during that period. c, Distribution of paired temperature and salinity observations over time, in 2-year bins. Observations before 1980 are included in the 1980–1982 bin.

Extended Data Figure 8 Model-data residuals for potential temperature.

Zonally averaged model-data residuals for the Atlantic (left) and Pacific (right) basins for the three assimilation time periods. Contour interval is 0.25 °C. Positive values indicate that modelled potential temperature is higher than observed (obs).

Extended Data Figure 9 Model-data residuals for CFC-11.

Zonally averaged model-data residuals for the Atlantic (left) and Pacific (right) basins for the three assimilation time periods. Contour interval is 0.25 pmol kg−1. Positive values indicate that modelled CFC-11 is higher than observed.

Extended Data Figure 10 Comparison of circulations tailored separately for each decadal period to piecewise constant and to unchanging circulations.

The quality of a circulation is quantified by using it to propagate CFC-11 and computing the R2 between modelled and observed CFC-11 concentrations for each region and time period. Colours indicate different circulations. The black R2 values for a given time period were calculated using the circulation that was data-assimilated for that period and held steady throughout the CFC propagation. Green R2 values are obtained if we change the circulation on the fly from decade to decade, as we did for the CO2 simulations. Red R2 values are obtained if the circulation in the 1990s (2000s) was held constant at the 1980s (1990s) levels. The green R2 values are much higher than the red R2 values, verifying that a decadally varying circulation fits the observed CFC concentrations much better than an unchanging circulation.

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DeVries, T., Holzer, M. & Primeau, F. Recent increase in oceanic carbon uptake driven by weaker upper-ocean overturning. Nature 542, 215–218 (2017). https://doi.org/10.1038/nature21068

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