Evaluating the tropical forest carbon sink.

Less than half of anthropogenic carbon emissions are accumulating in the atmosphere, due to large net fluxes into both the oceans and the land (Le Qu er e et al., 2012). The land sink in particular has increased markedly, doubling in strength since the 1960s, to reach 26 petagrams of carbon in the latest decade. However, the location and drivers of this large terrestrial sink are still relatively poorly constrained by atmospheric measurements (Ciais et al., 2013). Pan et al. (2011) recently utilized >1 million forest inventory plots to provide summaries of forest carbon stocks, and the first global bottom-up estimates of carbon fluxes for the world’s forest biomes for the period 1990–2007. One key result was that almost all the residual global terrestrial carbon sink (i.e. carbon uptake after accounting for land-use change), some 2.4 0.4 Pg of carbon per year, is located in the world’s established forests (Pan et al., 2011). The sink is distributed worldwide, with globally significant net fluxes into boreal and temperate forests, and a large sink in intact tropical forest, albeit with large uncertainty. Furthermore, Pan et al. (2011) showed that this tropical intact forest sink may have faded from an estimated annual 1.3 0.4 Pg C in the 1990s to 1.0 0.5 Pg C for 2000– 2007. The tropical intact forest sink is offset by a net land-use emission [1.5 Pg C yr 1 (1990–1999)] declining to [1.1 Pg C yr 1 (2000–2007)], and as a consequence aircraft measurements and inverse modelling studies indicate the tropics to be close to neutral in terms of net carbon fluxes (reviewed by Ciais et al., 2013). While the intact tropical forest sink values represent updates from similar values published previously (e.g. Lewis et al., 2009), the fact that almost the entire residual terrestrial carbon sink is accounted for by the forests of the world was a notable discovery. Evidence from the ground now points to established forests being a net sink across almost every major forest region, including all extra-tropical forest regions analysed. In a recent letter to this journal, Wright (2013) contested only the report of a sink in intact tropical forests. In so doing, he focused on only one aspect of the paper, ignoring the global result from Pan et al. (2011). Wright made three specific claims. Firstly, that the reported tropical forest sink is too large because of our definition of ‘tropical forest’. Secondly, that we ignored vital data in our analyses. Finally, he devotes most space to arguing that carbon dioxide fertilization cannot be substantially responsible for the global forest sink. We briefly address each in turn.