Background
The authors write that "although approximately one-half of total CO2 emissions is at present taken up by combined land and ocean carbon reservoirs (Schimel et al., 2001)," coupled climate/carbon-cycle models "predict a decline in future carbon uptake by these reservoirs, resulting in a positive carbon-climate feedback."

What was done
In an effort to shed more light on the subject, Ballantyne et al. used "global-scale atmospheric CO2 measurements, CO2 emission inventories and their full range of uncertainties to calculate changes in global CO2 sources and sinks during the past fifty years."

What was learned
The five U.S. scientists say their mass balance analysis shows that "net global carbon uptake has increased significantly by about 0.05 billion tonnes of carbon per year and that global carbon uptake doubled, from 2.4 ± 0.8 to 5.0 ± 0.9 billion tonnes per year, between 1960 and 2010."

What it means
In the concluding paragraph of Ballantyne et al.'s Nature article, they state that "although present predictions indicate diminished C uptake by the land and oceans in the coming century, with potentially serious consequences for the global climate, as of 2010 there is no empirical evidence that C uptake has started to diminish on the global scale." In fact, as their results clearly indicate, just the opposite appears to be the case, with global carbon uptake actually doubling over the past half-century.

Reference
Schimel, D.S., House, J.I., Hibbard, J.I., Bousquet, P., Ciais, P., Peylin, P., Braswell, B.H., Apps, M.J., Baker, D., Bondeau, A., Canadell, J., Churkina, G., Cramer, W., Denning, A.S., Field, C.B., Friedlingstein, P., Goodale, C., Heimann, M., Houghton, R.A., Melillo, J.M., Moore III, B., Murdiyarso, D., Noble, I., Pacala, S.W., Prentice, I.C., Raupach, M.R., Rayner, P.J., Scholes, R.J., Steffen, W.L. and Wirth, C. 2001. Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems. Nature 414: 169-172.