Oceanic Uptake of Anthropogenic CO₂

• Project Description
• Papers from this lab
• Bibliography (from a 1992 proposal)

Oceanic Uptake of Anthropogenic CO₂

The oceans role in moderating the build-up of atmospheric CO₂ is clearly important but difficult to accurately determine. Box-diffusion and 3-D ocean general circulation models predict similar rates of CO₂ uptake of about 2±0.6 Gt C/yr (Siegenthaler and Sarmiento, 1993). The error in the model predicted uptake rates depends primarily on the uncertainty in the model's parameterization of upper ocean mixing, which can be significant, as illustrated by the 0.4 Gt C/yr difference in ocean uptake rates predicted by the Hamburg and GFDL 3-D circulation models (Maier-Reimer and Hasselman, 1987; Sarmiento et al, 1992). Both of these 3-D models have problems accurately simulating the penetration of bomb ¹⁴C into the oceans (Toggweiler et al., 1989), the tracer often used to calibrate ocean mixing rates in carbon models.

The low estimates of oceanic CO₂ uptake (<1 Gt C/yr) derived from the meridional gradient in atmospheric CO₂ and an atmospheric transport model (Tans et al., 1990) shook geochemist's confidence in the ocean models. The low uptake rates derived by Tans et al, however, have been recalculated to higher levels (~1.8 Gt C/yr) by correcting for river input, cooler ocean surface skin temperatures and atmospheric CO oxidation (Siegenthaler and Sarmiento, 1993). The error in the oceanic CO₂ uptake rate derived from this approach is "difficult to estimate" (Siegenthaler and Sarmiento, 1993). It is advantageous, therefore, to determine oceanic CO₂ uptake using as many approaches as possible in order to narrow the acceptable range.

We determined the oceanic CO₂ uptake rates during the last 20 years by constructing atmospheric CO₂ and ¹³CO₂ budgets (Quay et al., 1992; Quay et al., submitted). Our approach used the measured changes in the concentration and ¹³C of atmospheric CO₂ and the depth-integrated ¹³C/¹²C of the oceanic dissolved inorganic carbon between 1970s and 1990s to directly determine the net oceanic CO₂ uptake. This approach, which does not require estimates of rates of ocean mixing or air-sea gas exchange, yielded net CO₂ uptake rates of 1.2±1.2 Gt C/yr for the ocean and 1.0±1.0 Gt C/yr for the terrestrial biota. Tans et al (1993) modified our atmospheric CO₂ and ¹³CO₂ budget approach by replacing the measured change in the depth-integrated ¹³C/¹²C of the ocean with an estimate of the air-sea ¹³C/¹²C disequilibrium, i.e., the difference between the measured ¹³C/¹²C of the atmospheric CO₂ and that expected if the atmosphere was in isotopic equilibrium with the surface ocean. Quay et al. (submitted) calculated an oceanic CO₂ uptake rate of 1.2±1.0 Gt C/yr using this approach.

Atmospheric CO₂ and ¹³CO₂ budgets, when coupled with an atmospheric transport model, are an important method to estimate oceanic and biospheric uptake of CO₂ on shorter temporal and spatial scales, e.g., the "double deconvolution" approach used by Keeling et al. (1989) and others. This method relies on the different isotopic effect that net photosynthesis and air-sea gas exchange have on the ¹³C/¹²C of atmospheric CO₂ in order to distinguish between oceanic and biospheric CO₂ uptake. Recently Battle et al. (2000) determined the oceanic and biospheric CO₂ uptake rates of 2.0±0.6 and 1.4±0.8 Gt C yr^-1 based on six years of atmospheric CO₂, ¹³C/¹²C and O₂ concentration measurements between 1991 and 1997.

Presently the most uncertainty in the calculated CO₂ uptake rates lies primarily in our poor knowledge of the ¹³C/¹²C inventory change and air-sea disequilibrium, because of the lack of ocean ¹³C/¹²C measurements (Quay et al., 1992; Tans et al., 1993), and secondarily in the isotopic disequilibrium between the atmosphere and land biota. Ciais et al. (submitted) found that the error in the oceanic ¹³C/¹²C disequilibrium yielded a ~0.4 Gt C/yr uncertainty in oceanic uptake whereas the error in atmosphere-biota disequilibrium yielded an uncertainty of ~0.1 Gt C/yr when applying the "double deconvolution" approach between 1990 and 1992. These results underscore the need for accurate ocean-wide ocean ¹³C/¹²C measurements.

Systematic high quality ocean-wide ¹³C/¹²C measurements were made in our laboratory on samples collected during NOAA/OACES cruises and at the WHOI AMS laboratory as part of the WOCE ¹⁴C measurement program in the 1990s. In total, approximately 25,000 ¹³C/¹²C measurements were made during the OACES and WOCE cruises. These oceanic ¹³C/¹²C data are necessary to fully utilize the weekly ¹³C/¹²C measurements of atmospheric CO₂ being made by NOAA at ~25 stations worldwide. These recent ¹³C/¹²C data provide better estimates of ¹³C/¹²C invitatory changes at -65±33 per mil m decade^-1 (Fig. 1), air-sea isotopic disequilibrium at 0.60±0.10 per mil (Fig. 2) and surface ocean ¹³C/¹²C change of -0.16±0.02 per mil decade^-1 (Fig. 3). Oceanic uptake rates of anthropogenic CO₂ of 1.9±0.4 Gt C yr^-1 in the 1980s were estimated from an ocean box-diffusion model that was constrained by the measured change in ¹³C/¹²C surface and inventory changes.

Papers discussing these results:

Quay, P.D., R. Sonnerup, T. Westby, J. Stutsman and A. McNichol. (submitted). Anthropogenic changes of the ¹³C/¹²C of dissolved inorganic carbon in the ocean as a tracer of CO₂ uptake. Global Biogeochem. Cycles

Kortzinger, A., P.D. Quay and R.E. Sonnerup. (submitted). The relationship between anthropogenic CO₂ and the ¹³C Suess effect in the North Atlantic Ocean. Global Biogeochem. Cycles

Sonnerup, R.E., P.D. Quay and A.P. McNichol. 2000. The Indian Ocean ¹³C Suess effect. Global Biogeo. Cycles 14: 903-916.

Sonnerup, R.E., P.D. Quay, A.P. McNichol, J.L. Bullister, T.A. Westby and H.L. Anderson. 1999. Reconstructing the oceanic ¹³C Suess effect. Global Biogeo. Cycles 13: 857-872.

Zhang, J., P.D. Quay and D.O. Wilbur.1995. Carbon isotope fractionation during gas-water exchange and dissolution of CO₂. Geoch. Cosmo. Acta 59: 107-114.

Quay, P.D., B. Tilbrook and C.S. Wong (1992) Oceanic uptake of fossil fuel CO₂: Carbon-13 evidence. Science 256: 74-79.