Background
The authors write that "the land and ocean act as a sink for fossil-fuel emissions, thereby slowing the rise of atmospheric carbon dioxide concentrations (Ballantyne et al., 2012)." And they say that "although the uptake of carbon by oceans and terrestrial processes has kept pace with accelerating CO2 emissions until now, atmospheric CO2 concentrations exhibit a large variability on inter-annual timescales (Le Quere et al., 2013)," which are "considered to be driven primarily by terrestrial ecosystem processes dominated by tropical rainforests (Cox et al., 2013.)." But is this latter conclusion correct?

What was done
In an effort designed to explore this intriguing question, Poulter et al. employed (1) a terrestrial biogeochemical model, (2) atmospheric CO2 inversion and (3) global carbon budget accounting methods to investigate the evolution of the terrestrial carbon sink over the past 30 years, with a focus on the underlying mechanisms responsible for the exceptionally large land carbon sink reported in 2011 by Le Quere et al. (2012).

What was learned
The international team of 13 researchers, who come from six different countries, determined that (1) "our three terrestrial carbon sink estimates are in good agreement and support the finding of a 2011 record land carbon sink," that (2) "the global carbon sink anomaly was driven by growth of semi-arid vegetation in the Southern Hemisphere, with almost 60% of carbon uptake attributed to Australian ecosystems, where prevalent La Niña conditions caused up to six consecutive seasons of increased precipitation," and that (3) "since 1981, a 6% expansion of vegetation cover over Australia was associated with a four-fold increase in the sensitivity of continental net carbon uptake to precipitation."

What it means
In the words of Poulter et al., "our findings suggest that the higher turnover rates of carbon pools in semi-arid biomes are an increasingly important driver of global carbon cycle inter-annual variability and that tropical rainforests may become less relevant drivers in the future." And so it would appear that semi-arid biomes are "where the action is" when it comes to anticipating the near-future state of the planet's terrestrial carbon sink.

References
Ballantyne, A.P., Alden, C.B., Miller, J.B., Tans, P.P. and White, J.W.C. 2012. Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years. Nature 488: 70-72.

Cox, P., Pearson, D., Booth, B.B., Friedlingstein, P. Huntingford, C., Jones, C.D. and Luke C.M. 2013. Sensitivity of tropical carbon to climate change constrained by carbon dioxide variability. Nature 494: 341-344.

Le Quere, C., Andres, R.J., Boden, T., Conway, T., Houghton, R.A., House, J.I., Marland, G., Peters, G.P., van der Werf, G., Ahlström, A., Andrew, R.M., Bopp, L., Canadell, J.G., Ciais, P., Doney, S.C., Enright, C., Friedlingstein, P., Huntingford, C., Jain, A.K., Jourdain, C., Kato, E., Keeling, R.F., Klein, K., Goldewijk, K.K., Levis, S., Levy, P., Lomas, M., Poulter, B., Raupach, M.R., Schwinger, J., Sitch, S., Stocker, B.D., Viovy, N., Zaehle, S. and Zeng, N. 2013. The global carbon budget 1959-2011. Earth System Science Data 5: 1107-1157.