What was done
In light of their independent assessment of the absorption of anthropogenic CO2 by the global ocean, and knowing the historical record of fossil fuel use and cement production, as well as the concomitant change in the atmosphere's CO2 content, Khatiwala et al. were able to calculate, as a residual, the net absorption of anthropogenic CO2 by earth's land surface since the inception of the Industrial Revolution.

What was learned
A graphical representation of the researchers' results is presented in the figure below.

The evolution of anthropogenic-produced carbon originating from fossil fuel use and cement production (FC), together with the similar evolution of the three anthropogenic carbon sinks - atmosphere (A), ocean (O) and land (L) - between AD 1765 and 2005. Adapted from Khatiwala et al. (2009).

What it means
Of primary concern to us in this particular analysis of the three scientists' findings, is the fact that their results indicate that "the terrestrial biosphere was a source of anthropogenic carbon until the 1940s, roughly in line with previous model-based estimates, after which it turned into a sink of anthropogenic CO2." In addition, their more detailed analyses indicate that between the 1980s and 2000s, the strength of the global land sink grew from a value of 0.3 Pg C per year to 1.1 Pg C per year, while the independent estimates of the IPCC's Fourth Assessment Report for the same time interval were 0.3 Pg C per year to 1.3 Pg C per year. Hence, it would appear that the terrestrial biosphere is truly beginning to "flex its muscles," as the aerial fertilization and transpiration-reducing effects of the ongoing rise in the air's CO2 content are beginning to manifest their tremendous ability to enhance both the amount and vitality of the planet's CO2-sequestering vegetation.