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Terrestrial sources and sinks of carbon (fluxes) can be estimated over very large areas by combining two types of information: spatial and temporal variations in atmospheric concentrations of CO2 and models of atmospheric mixing or transport. At present, there are roughly 100 sites routinely operated by a few air sampling networks. Most measurement sites are located in the marine boundary layer, however-far from the direct influence of continental ecosystems. Better sampling of the continental air sheds is critical to reduce current uncertainties about terrestrial carbon sources and sinks. One also must make measurements through the air column to integrate the fluxes of CO2 from ecosystems over a large spatial domain, typically between 100 and 1,000 km. CO2 measurement by very high television towers (Bakwin et al., 1995) and aircraft (Choularton et al., 1995) appear to represent the most practical solutions to pursue that goal.
Atmospheric transport models are based on wind fields that are derived from numerical climate models or weather forecast models. Large-scale mathematical inversions of CO2 data with these models provide an estimate of the net flux of carbon from all surface processes. These models cannot be applied to determine the carbon exchange for projects, and, in general, they cannot be used to attribute fluxes to specific activities.
Recent analyses show conflicting results in apportioning CO2 fluxes over the continents. A debated issue is the partitioning of the Northern Hemisphere terrestrial CO2 flux among North America, Europe, and Asia. For example, a study by Fan et al. (1998) found most of the northern mid-latitude sink in North America, whereas Bousquet et al. (1999) found most of it in Asia. The high estimate for North America is also inconsistent with analyses that are based on land-use change and forest inventories (Houghton et al., 1999).
Current inversion analyses depend to some extent on a priori knowledge about surface fluxes and patterns of atmospheric transport. Thus, one must verify carefully that the solution is mostly constrained by atmospheric observations via the atmospheric transport, not by any of the model's artifacts. The current generation of inverse models of the global carbon cycle can generate intriguing hypotheses regarding the possible location of regional carbon fluxes, but these models are insufficiently constrained to be reliable for specific activities or inventory purposes. This situation is unlikely to change dramatically within the next decade, but in the longer term, inverse modeling in conjunction with large-scale flux measurements may pass from the research into the operational arena.
In summary, there are a range of flux measurement and flux estimation methodologies under active development that can be expected to improve the ability to determine average net surface fluxes over a range of spatial scales for CO2 and non-CO2 GHGs. In some cases, isotopic information can be used to partition fluxes between different source types (e.g., fossil fuel vs. biotic sources). Methods that are applicable at regional and larger scales are not expected to provide a basis for CO2 flux estimation that is useful for the purposes of the Kyoto Protocol in the first commitment period. Nevertheless, information from these larger scale methods can provide constraints on total surface fluxes that may be useful for full carbon accounting and for validating process-based models of GHG fluxes.
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