Comment on "The Ocean Sink for Anthropogenic CO₂"

One complication is that the ocean inventory of anthropogenic CO₂ is an incomplete measure of the change in the ocean carbon content. The term "anthropogenic CO₂," as used by Sabine et al., refers to the excess carbon dioxide that has accumulated in the ocean as a direct response to rising CO₂ levels since preindustrial times. Changes in carbon accumulation driven by processes within the ocean, such as warming (whether anthropogenic or otherwise), or changes in ocean stratification are not counted as "anthropogenic" CO₂. These contributions are certainly much smaller than the component driven by rising atmospheric CO₂ levels, but they are not necessarily negligible. A complete global carbon budget must therefore also include a term for these ocean-driven exchanges of CO₂.

It is relatively straightforward to estimate the direct effect of long-term warming on air-sea carbon exchanges (3, 4). For example, using the box-diffusion model (5) tuned for consistency to match the Sabine et al. estimate of 118 petagrams of carbon in the absence of warming, the net uptake in the presence of warming is found to be 105 Pg C. This is consistent with a warming correction of –13 Pg C (6–8). Warming releases CO₂ from the ocean primarily because CO₂ is less soluble in warmer water.

It is likely that warming over the past century has also influenced CO₂ exchange indirectly through increases in the density stratification of the upper ocean, thereby decreasing vertical mixing and increasing the trapping of nutrients and metabolic CO₂ in subsurface waters. There is no simple way to estimate this stratification effect, but based on the results of general circulation models (3, 4), the effect through 1994 is most likely on the order of +6 Pg C, offsetting the warming effect. Combining the warming and stratification effects thus leads to an estimate of –7 Pg C for the ocean-driven term, with an uncertainty that is not well constrained but probably is about ±10 Pg C.

Another complication is that the ocean inventory of anthropogenic CO₂ may not be determined reliably using the C* method in the presence of warming or other ocean variability. The C* method is a hybrid of two methods, the first applied in the upper ocean where the isopycnal layers are contaminated everywhere with anthropogenic CO₂ and the second applied in the deeper waters where the isopycnal layers are partially uncontaminated. In the upper ocean, the C* method principally relies on chlorofluorocarbon ventilation ages in combination with the known history of atmospheric CO₂. If a different atmospheric history were used, the estimate of anthropogenic CO₂ uptake in these water would change proportionally, which illustrates that the method is not a direct observation of carbon accumulation but effectively a model-dependent estimate, albeit a highly constrained one. This model assumes that the ocean circulation has remained steady with time and is therefore subject to error if circulation rates have changed. For example, the box-diffusion model indicates that if the vertical diffusion rates were 20% higher before 1980, the Sabine et al. estimate (1), which effectively projects modern diffusion rates over the entire period, would underestimate the actual uptake by 7 Pg C.

In the deeper waters, the C* method depends on the spatial gradients in the tracer C*, which is a mathematical function of total carbon, oxygen, nitrate, phosphate, silica, alkalinity, temperature, and salinity. The assumption is made that the gradient in C*, along a given isopycnal surface from the older uncontaminated waters to the younger contaminated waters, is a measure of anthropogenic CO₂. However, the gradient in C* can be produced not only by uptake of CO₂ but also by other processes, particularly air-sea exchanges of heat and O₂ (9). Based on the C* sensitivity to heat and O₂ (10) and plausible estimates of changes in heat and O₂ content of the deeper waters (11–16), corrections on the order of 3 to 5 Pg C are implied, with the heat and O₂ effects partly canceling. More work is needed to assess these corrections as well as the impact on variable circulation, which are complicated by our limited knowledge of global hydrographic changes before the late 20th century. Until these corrections are properly assessed, it seems appropriate to revise upward the uncertainty in the estimate of anthropogenic CO₂ uptake. A reasonable revision might be from ±19 to ±23 Pg C/year.

In summary, the Sabine et al. (1) estimate of anthropogenic CO₂ uptake should be combined with an additional term of about –7 ± 10 Pg C to account for air-sea CO₂ exchanges driven by warming and stratification. The error estimate on the anthropogenic contribution should furthermore be increased to about ±23 Pg C to reflect uncertainties associated with changes in ocean circulation, heat content, and O₂ content on the C* method. Combining these corrections yields an estimate of 111 ± 25 Pg C for the net oceanic uptake of CO₂ from 1800 to 1994. I hope that this comment will initiate a process to examine these corrections more closely.

Ralph F. Keeling Scripps Institution of Oceanography La Jolla, CA 92093–0244, USA E-mail: rkeeling{at}ucsd.edu

References and Notes

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In Science Magazine

TECHNICAL COMMENTS
Response to Comment on "The Ocean Sink for Anthropogenic CO₂"

Christopher Sabine and Nicolas Gruber (17 June 2005)
Science 308 (5729), 1743d. [DOI: 10.1126/science.1109949]
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RESEARCH ARTICLES
The Oceanic Sink for Anthropogenic CO₂

Christopher L. Sabine, Richard A. Feely, Nicolas Gruber, Robert M. Key, Kitack Lee, John L. Bullister, Rik Wanninkhof, C. S. Wong, Douglas W. R. Wallace, Bronte Tilbrook, Frank J. Millero, Tsung-Hung Peng, Alexander Kozyr, Tsueno Ono, and Aida F. Rios (16 July 2004)
Science 305 (5682), 367. [DOI: 10.1126/science.1097403]
 |  Abstract »  |  Full Text »  |  PDF »  |  Supporting Online Material »

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