Another Take on the Airborne Fraction of Anthropogenic CO2
Reference Le Quere, C., Raupach, M.R., Canadell, J.G., Marland, G., Bopp, L., Ciais, P., Conway, T.J., Doney, S.C., Feely, R.A., Foster, P., Friedlingstein, P., Gurney, K., Houghton, R.A., House, J.I., Huntingford, C., Levy, P.E., Lomas, M.R., Majkut, J., Metzl, N., Ometto, J.P., Peters. G.P., Prentice, I.C., Randerson, J.T., Running, S.W., Sarmiento, J.L., Schuster, U., Sitch, S., Takahashi, T., Viovy, N., van der Werf, G.R. and Woodward, F.I. 2009. Trends in the sources and sinks of carbon dioxide. Nature Geoscience 10.1038/ngeo689. In a study of the sources and sinks of anthropogenic CO2, the international team of Le Quere et al. (2009) -- comprised of thirty-one scientists from seven different countries -- developed "a global CO2 budget for each year during 1959-2008 and analyzed the underlying drivers of each component." In this undertaking, they say "the global increase in atmospheric CO2 was determined directly from measurements, CO2 emissions from fossil fuel combustion were estimated on the basis of countries' energy statistics, [while] CO2 emissions from land-use change were estimated using deforestation and other land-use data, fire observations from space, and assumptions on the carbon density of vegetation and soils and the fate of carbon." For the time evolution of the land and ocean CO2 sinks, however, they used "state-of-the-art models on which [they] imposed the observed meteorological conditions of the past few decades." So what did they find? Le Quere et al. report that "between 1959 and 2008, 43% of each year's CO2 emissions remained in the atmosphere on average," and that "the rest was absorbed by carbon sinks on land and in the oceans." However, they state that "in the past 50 years, the fraction of CO2 emissions that remains in the atmosphere each year has likely increased, from about 40% to 45%," stating "models suggest that this trend was caused by a decrease in the uptake of CO2 by the carbon sinks in response to climate change and variability."
Although their findings can be interpreted as indicating that either the growth in the ocean's or land's uptake rate of anthropogenic CO2 (or both) may have decreased with time - as they and others have suggested - they temper this conclusion with the caveat that "changes in the CO2 sinks are highly uncertain." In this regard, for example, it should be noted that the time evolution or temporal histories of the anthropogenic CO2 sinks in Le Quere et al.'s analysis were obtained from models; and they themselves admit that "sink processes not considered in current models may be contributing to the observed changes." And, of course, these "sink processes" would obviously be doing so in unknown ways that are not included in their calculations. In addition, the work of Khatiwala et al. (2009), which covers a much longer span of time, reveals the existence of multidecadal variability that in a short half-century context could be interpreted as a trend, but in a much longer two-and-a-half century timeframe is readily recognized as being an oscillatory variation about an essentially constant mean.
Additional Reference Khatiwala, S., Primeau, F. and Hall, T. 2009. Reconstruction of the history of anthropogenic CO2 concentrations in the ocean. Nature 462: 346-349.
