The uncertain, but changing, potential of the terrestrial biosphere as a future carbon sink

I am aware that the studies used in this post are not directly linked to the South American continent as such but I consider it interesting to cover the topics due to the large proportion of the global terrestrial carbon sink lying within South America.

As described in my previous post, forests have and had the potential to store large amounts of carbon. In fact many studies now agree (e.g. A, B,) that carbon uptake by natural sinks has increased over time. The reason for this is a physiological vegetation response to CO2 fertilization in the atmosphere, that increases plants' primary productivity and thus increases the carbon stored per hectare of intact forest. Ballantyne et al (2012) use a global scale CO2 mass balance analysis to show the evolution of the carbon budget 1959-2010. 

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While fossil fuel emissions have been pumping CO2 in the atmosphere at an alarming rate, there is a clear increase in CO2 from the atmosphere to global sinks over the same course of time. It mirrors the atmospheric concentration - it is a dynamic response, which means that "terrestrial ecosystem carbon fluxes both respond to and strongly influence the atmospheric CO2 increase and climate change". In a modelling study by Cao and Woodward (1998), we can see that we are still in a period in which the CO2 fertilization effect is strong enough to uphold a net positive relationship between emissions and sinks, however in the future as the effect becomes saturated a leveling off of potential terrestrial sinks is expected. Cox et al (2000) even suspect that post 2050 climate change will have such altering effects that terrestrial sinks will turn to sources (and this projection does not even take anthropogenic deforestation into account yet but is purely based on climate change effects!)

Now going back to land cover change directly: a carbon dioxide removal (CDR) technique of geoengineering is the afforestation of previously converted landscapes. This way, carbon is sequestrated from the atmosphere and stored in the new-grown forests. Fellow blogger Maria Christofi has reviewed the potential for this method in elevating dangerous levels of CO2 in the atmosphere at: http://geoengineeringinquiries.blogspot.co.uk/. A question that arises to me now is whether the negative feedback loop that has been described by Cox et al (2000) renders this method useless in the future? The uncertainty regarding the vegetation-climate responses of the future definitely adds to the factors needed to consider before intervening further with land-cover changes.