Another Take on the Airborne Fraction of Anthropogenic CO₂

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.