Background
The authors indicate that the ratio of CO2 accumulating in the atmosphere to the CO2 flux into the atmosphere due to human activities -- which is known as the airborne fraction (AF) -- is central to predicting changes in earth's surface temperature due to greenhouse gas-induced warming; and they note that this ratio has remained remarkably constant over the past five decades. Now, however, they report that Canadell et al. (2007) and Raupach et al. (2008) claim to have detected a long-term increasing trend in the airborne fraction that they interpret as being indicative of "a decreasing trend in the efficiency of the ocean and land carbon sinks."

What was done
Gloor et al. report that Knorr (2009) had already challenged Canadell et al. and Raupach et al. with respect to their detection of a positive AF trend, "arguing that given the noise in the data, the trend is not detectable." Thus, they challenged the second claim of Canadell et al. and Raupach et al., i.e., their contention that a positive AF trend is indicative of a decreasing planetary carbon sink efficiency by investigating "the question of what controls trends and decadal scale variations in CO2 airborne fraction using simple linear models describing the evolution of an atmospheric perturbation in CO2."

What was learned
The three researchers determined, first of all, that there is no one-to-one association between positive trends in CO2 flux to the atmosphere (due to fossil fuel emissions and changes in land use) and negative trends in earth's carbon sink efficiency. Secondly, they found that in order to detect trends in sink efficiencies from the time course of fossil fuel-derived CO2 emissions and temporal changes in land use, "it is necessary to disentangle the spin-up time and fossil fuel growth rate variation signatures in the airborne fraction from signatures due to other causes." And when they make the pertinent calculations for fossil-fuel and land-use changes, they say they "do indeed find a positive trend in the residuals," but they argue that this trend "is not statistically significant after correcting for known events such as the temporal distribution of the extrinsic forcings and likely omissions in the emissions (particularly from land-use change)," further noting that their analysis suggests that "trends in airborne fraction are not a very good diagnostic to detect changes in carbon sink efficiency because variations in the signal are complex and the signal-to-noise ratio is small."

What it means
When all is finally said and done, Gloor et al. conclude that "atmospheric data, if analyzed adequately, do not yet reveal a statistically significant signal," which suggests that the long-sought answer to the burning question -- of whether the carbon sink efficiency of the earth is either increasing, decreasing or remaining relatively constant -- is still, so to speak, up in the air.

So don't count the biosphere out yet!

References
Canadell, J.G., Le Quere, C., Raupach, M.R., Field, C.B., Buitenhuis, E.T., Ciais, P., Conway, T.J., Gillett, N.P., Houghton, R. and Marland, G. 2007. Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks. Proceedings of the National Academy of Sciences USA 104: 18,866-18,870.

Knorr, W. 2009. Is the airborne fraction of anthropogenic CO2 emissions increasing? Geophysical Research Letters 36: 10.1029/2009GL040613.

Raupach, M.R., Canadell, J.G. and Le Quere, C. 2008. Anthropogenic and biophysical contributions to increasing atmospheric CO2 growth rate and airborne fraction. Biogeosciences 5: 1601-1613.