Methods of Estimating Terrestrial Carbon Fluxes

Here we review some of the methods used to determine the size and geographic locations of terrestrial carbon fluxes. This is a cursory overview of the topic. In a recent paper, House et al. (2003) provides a more complete review of the various methods of estimating terrestrial carbon sources and sinks.

Methods for estimating the size and geographic pattern of the terrestrial carbon sink first arose from the so-called atmospheric inversion technique of flux estimation. In an inversion method, terrestrial carbon fluxes are inferred by having to fulfill the requirement that the global carbon budget has to be balanced. Thus, knowing the fossil-fuel and land use sources of carbon and the amount of carbon stored in the atmosphere, one can estimate the terrestrial and oceanic sources or sinks by difference. Further, the ocean and terrestrial fluxes can be partitioned by one of three methods: (1) simultaneous measurements of atmospheric CO2 and O2; (2) observations of atmospheric 813C; or (3) oceanic uptake as estimated by an ocean carbon cycle model. The inverse modeling approach has the advantage of being global in scale and of implicitly accounting for all the processes influencing the global carbon cycle. It has the disadvantage, however, of not being able to isolate the individual contributions of the various processes controlling the carbon cycle. Furthermore, while inversion methods are able to provide reasonably accurate estimates of global sources and sinks of carbon, and even sufficiently accurate estimates of the latitudinal north-south partitioning of the fluxes, they do not provide accurate longitudinal breakdown of the fluxes and of different regional fluxes.

Although inversion methods are useful, they are not sufficient to understand the functioning of the terrestrial carbon budget. More direct methods of observing the terrestrial sources and sinks of carbon have been developed. One such observational approach measures terrestrial carbon fluxes at the atmospheric boundary layer in flux towers using the "eddy covariance" technique. This technique takes advantage of the fact that transport in the boundary layer is dominated by turbulent eddies, and it uses turbulence theory and sophisticated instruments to measure vertical fluxes of carbon dioxide. Although flux measurements are useful to obtain terrestrial fluxes at local scales, they continue to be plagued by measurement errors when turbulence is low (such as at nighttime) and also have difficulty scaling up to regional levels and to decadal time scales.

Another method of estimating carbon fluxes directly is by using inventory methods. These methods are normally limited to observations of changes in aboveground biomass in forested ecosystems; from changes in biomass, sources or sinks of carbon can be inferred. The method has the advantage of comprehensively including all processes that affect an ecosystem but has the disadvantage of having limited consideration of below-ground processes and nonforested ecosystems.

Finally, various numerical models have been used to estimate terrestrial sources and sinks of carbon. These models include representations of the processes that are thought to affect terrestrial carbon fluxes. In particular, the models include controls such as atmospheric CO2 concentration, climate variability and change, atmospheric nitrogen deposition, and in a few cases anthropogenic land use and land cover change. The models have the advantage of being able to isolate the individual contributions of the various processes influencing the terrestrial carbon budget. The models are only as good, however, as our understanding of the processes, and moreover, they only include the processes that are currently hypothesized to influence the carbon budget.

In addition to all of these approaches to estimating present-day terrestrial carbon fluxes, many experimental approaches are in use to understand how terrestrial ecosystems might respond to changing atmospheric carbon dioxide concentrations and climate. In laboratories, greenhouses, and open-top chambers, plants are grown in conditions of increased (or decreased) ambient CO2 concentrations to evaluate their response. This method has been further extended to the plot or stand scale in the Free Air CO2 Enrichment (FACE) experiments, which aim to estimate the ecosystem-level response to increased CO2. Furthermore, many soil-warming experiments around the world attempt to measure the response of microbial respiration to increased soil temperatures.

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