Background
Noting that about 40% of the yearly amount of CO2 that is released to the air by human activities has historically remained in the atmosphere - a quantity that has come to be known as the airborne fraction of anthropogenic CO2 (AF) - the authors state that several studies have recently highlighted the possibility that the world's oceans and terrestrial ecosystems may be gradually losing their ability to sequester such a large proportion of humanity's CO2 emissions, citing the work of Le Quere et al. (2007), Schuster and Watson (2007) and Canadell et al. (2007). And they state that if this possibility actually materializes, "in a way predicted by models, this could add another 500 ppm of CO2 to the atmosphere by 2100," which would greatly exacerbate the so-called "climate crisis" envisioned by Al Gore and the Intergovernmental Panel on Climate Change.

What was done
Working with data on CO2 emissions arising from fossil-fuel use, cement production and changes in land use, as well as atmospheric CO2 concentrations measured at Mauna Loa and the South Pole, plus those derived from Law Dome and Siple ice-core data, together with their associated uncertainties, Knorr constructed a new-and-improved history of AF values stretching all the way back to 1850.

What was learned
The UK researcher reports that "despite the predictions of coupled climate-carbon cycle models, no trend in the airborne fraction can be found [italics added]." Or as he writes in the concluding section of his study ...

What it means
"The hypothesis of a recent or secular trend in the AF cannot be supported on the basis of the available data and its accuracy," which indicates that not only has the global ocean been increasing its uptake rate of anthropogenic carbon in such a way as to successfully "keep up" with the rate at which the air's CO2 content has risen in response to historical anthropogenic carbon inputs - as has concurrently been demonstrated by Khatiwala et al. (2009) - so also have earth's terrestrial ecosystems been "keeping up" in this regard.

References
Canadell, J.G., Le Quere, C., Raupach, M.R., Field, C.B., Buitenhuis, E., Ciais, P., Conway, T.J., Gilett, N.P., Houghton, J.T. 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.

Khatiwala, S. Primeau, F. and Hall, T. 2009. Reconstruction of the history of anthropogenic CO2 concentrations in the ocean. Nature 462: 346-349.

Le Quere, C., Roedenbeck, C., Buitenhuis, E.T., Conway, T.J., Langenfelds, R., Gomez, A., Labuschagne, C., Ramonet, M., Nakazawa, T., Metzl, N., Gillett, N. and Heimann, M. 2007. Saturation of the Southern Ocean CO2 sink due to recent climate change. Science 316: 1735-1738.

Schuster, U. and Watson, A.J. 2007. A variable and decreasing sink for atmospheric CO2 in the North Atlantic. Journal of Geophysical Research 112: 10.1029/2006JC003941.