A New Model Search for the "Missing Sink" of Anthropogenic CO₂

In introducing their study of the missing carbon sink, Esser et al. (2011) note that anthropogenic carbon (C) emissions to the atmosphere have totaled about 499 Pg C since preindustrial times, while 216 Pg C was stored in the atmosphere (raising its CO₂ concentration from 280 to 385 ppm in 2008), and while uptake into the oceanic buffer system was about 118 Pg C. The difference between these sources and sinks is 165 Pg C; and they write that "it is commonly assumed that this carbon is stored somewhere in the terrestrial biosphere." And since that "somewhere" has not been conclusively identified, it is typically referred to as the missing carbon sink.

In an attempt to hone in a little better on where the missing carbon might be, the three scientists note that efforts to explain the missing sink in recent years have included the role of nitrogen as an important constraint for biospheric carbon fluxes. And, hence, they used the Nitrogen Carbon Interaction Model (NCIM) of Esser (2007) "to investigate the carbon and nitrogen storage in the terrestrial biosphere, as influenced by the rising atmospheric CO₂ concentration together with different combinations of the minor nitrogen exchange fluxes," concentrating on two different time intervals: the historical period of 1860-2002 and the projected future period of 2002 to 2100.

In pursuing this course of action, Esser et al. found that nitrogen fertilization of the biosphere in the absence of an increase in the air's CO₂ concentration "would result in only minor additional carbon accumulation in plant biomass," while rising CO₂ alone, without consideration of the nitrogen cycle, would bind roughly half of the carbon in the postulated carbon sink. And in the most realistic situation of all, they determined that "a complete ensemble of rising atmospheric CO₂ and N₂ fixation, denitrification, and leaching is necessary to achieve the 160 Pg C bound in the terrestrial biosphere between 1860 and 2002 as required by the missing sink concept."

A challenge to these results of their modeling efforts, however, is raised by the progressive nitrogen limitation hypothesis, which posits that low concentrations of soil nitrogen will curtail the ability of the productivity-enhancing effect of atmospheric CO₂ enrichment to maintain increased plant growth and ecosystem carbon sequestration rates over the long term, which phenomenon is not observed in the NCIM. Esser et al.'s response is that the naturally slow rise of the air's CO₂ content in the model simulations is responsible for the model's success, because in contrast to the sudden huge increase in the atmosphere's CO₂ content that occurs in free-air CO₂ enrichment and open-top chamber experiments (and that requires large amounts of nitrogen in order to fully capitalize upon the aerial fertilization effect of that instantaneous CO₂ increase), "the naturally slow rise of CO₂ used in the model experiments" does not have this immediate need for huge amounts of nitrogen. Thus, there is a very good chance that the aerial fertilization effect of atmospheric CO₂ enrichment may well be doing a whole lot more for the planet in terms of sequestering carbon than what most people have long assumed.