Understanding the causes for the observed increase in atmospheric carbon dioxide requires that we can account for all the carbon dioxide entering and leaving the atmosphere from human and natural sources. Investigations of the global carbon cycle, in attempts to account for such fluxes, have been unable to account for a large "missing sink" of carbon, estimated to be about 2 petagrams per year (2 billion metric tonnes) (Sarmiento, 1993). This sink is thought to be in the Northern Hemisphere land mass (Ciais et al 1995; Keeling et al, 1996), but studies prior to 1998 had been unable to pin down the exact location of the sink or the processes responsible. Fan et al (1998) attempt to identify the location of this large sink by use of a combination of models and analysis of atmospheric carbon dioxide data, estimates of air-sea carbon dioxide fluxes, and emissions of carbon dioxide emissions from fossil fuel combustion. Their results indicate that this sink of previously unknown location is centered in the central US extending from the Great Lakes southward to the Gulf Coast. The total uptake of carbon in this North American sink is estimated to be about 1.7 petagrams. Fan et al (1998) state that this uptake is at least partially due to forest regrowth on abandoned farmland and previously logged forest land, enhanced growth due to anthropogenic nitrogen deposition (nitrogen compounds produced by combustion), and possibly global warming. Further study is needed to verify the location of this sink and identify with more precision the responsible processes.
References
Ciais P., P. P. Tans, M. Trolier, J. W. C. White, and R. J. Francy, 1995: A large Northern Hemisphere terrestrial CO2 sink indicated by the 13C/12C ratio of atmospheric CO2. Science, 269, 1098-1102.
Fan, S. M. Gloor, J. Mahlman, S. Pacala, J. Sarmiento, T. Takahashi, and P. Tans, 1998: A large terrestrial carbon sink in North America implied by atmospheric and oceanic carbon dioxide data and models. Science, 282, 442-446.
Keeling, R. F., S. C. Piper, and M. Heimann, 1996: Global and hemispheric CO2 sinks deduced from changes in atmospheric O2 concentration. Nature, 381, 218-221.
Sarmiento, J. L., 1993: Ocean Carbon Cycle. Chemical & Engineering News, 71, 30-43 (May 31).